Trip 1: Early
EarlyProterozoic
Proterozoic Strata
Strata
of the Maquette
Marquette Iron
of
Range, Michigan
Ishpeming
Trip 22:: Archean
Arc1
Greenstone
Belt and
Ore,
G01d
Golf Mineralization
H-<o<a
3: Tilden
Trip 3:
Tilden and
and Empire
Empire A tc.*
~ c . \6
6
A\~
Mines
0f the Marquette
Mines of
Marpette
Iron Range, Michigan
Trip 4: Paleozoic andIGlacial
Glacial
Geology from1 Au Train to
to
Grand Marais,3, Michigan
I
on ~Lake
Institute on
ake
Superior Geology
45th
45th Annual
Annual Meeting
Meeting
Ramada
da IInn
nn
Rama
Marquette,,Michigan
Maquette
~ichigan
May 4-8,
4-8, 1999
1999
Sponsored by
Sponsored
Northern
Northern Michigan
Michigan University
and Michigan
University
and
Michigan Technological University
Proceedings Volume 45:
Field Trip Guidebook
Theodore J. Bornhorst, editor
45TH ANNUAL MEETING
SUPERIOR GEOLOGY
GEOLOGY
INSTITUTE ON LAKE SUPERIOR
Volume 45 consists of
Part
Part 1:
1:Program
Programand
and Abstracts
Abstracts
Part 2:
2: Field
FieldTrip
Trip Guidebook
Guidebook
Reference to material in this volume should follow the example below:
Holm, D., 1999,
1999, Characterization and timing constraints of post-Penokean
Ams, D. and Hoim,
meso-scale structures in the Watersmeet and Republic gneiss domes of northern
Michigan (abst.):
(ahst.): Institute
Institute on
on Lake
Lake Superior
Superior Geology Proceedings, 45th Annual
Meeting, Marquette, MI, v. 45, part 1,
1, p. 2.
2.
Volume 45 Published and distributed by
Institute on Lake Superior Geology
Mark
.G.
Mark Jirsa,
Jirsa, Secretary-Treasurer,
Secretary-Treasurer,I.L.S
I.L.S.G.
Minneosta Geological Survey
2642 University Avenue
St. Paul, MN USA 55114-1057
551 14-1057
(612)-627-4780
email:
jjrsaOOl
@tc.umn.edu
email: jirsaOOl @tc.umn.edu
ILSGwebsite
websitehtto://www.eeo.mtu.edu/ereat
http://www.geo.mtu.edu/great lakes/ilse/
lakes/ilsg/
ILSG
ISSN 1042-9964
1042-9964
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volumes are
are available for photocopying
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INSTITUTE ON
LAKE SUPERIOR
GEOLOGY
PROCEEDINGS
Volume 45
45
Part 2: Field Trip Guidebook
CONTENTS
PROCEEDINGS VOLUME
VOLUME 45
PROCEEDINGS
PART
2: FIELD
TRIP GUIDEBOOK
GUIDEBOOK
PART2:
FIELDTRIP
Editor: Theodore J. Bornhorst
Field Trip 1 :: Early Proterozoic Strata
Strata of the Marquette
Marquette Iron
Iron Range,
Range, Michigan
Leader: W.F.
W.F. Cannon
Early Proterozoic Strata
Strata of the Marquette Iron Range by W.F. Cannon ........................ 22
Field Trip
Trip 2:
2: Archean
Archean Ishpeming
Ishpeming Greenstone
GreenstoneBelt
Belt and
and Gold
Gold Mineralization,
Mineralization,Michigan
Michigan
Leaders: T.J. Bornhorst, D.J. Duskin, R.C. Johnson, R.A.
R.A. Mahin,
Mahin, T.O.
TO. Quigley,
Quigley, and
and G.W.
G.W.
Scott
and
An Introduction to the Archean Ishpeming Greenstone Belt, Michigan by R.C. Johnson and
12
T.J. Bornhorst ..........................................................................................
12
Introduction to the Ropes Gold Deposit in the Ishpeming Greenstone Belt, Michigan
abstracted from R.A. Brozdowski by T.J. Bornhorst, S.E. Bair, and G. W. Scott ............ 29
29
Geologic Field Excursion to the Ishpeming Greenstone Belt by R.C. Johnson and T.J.
T.J.
45
Bornhorst ...............................................................................................
Bomhorst
45
Shear Zones and Gold Mineralization in the South Half of the Ishpeming Greenstone
Belt, Michigan by D.J. Duskin and T.O. Quigley ................................................57
Examples of Gold Mineralization in the Northern Block of the Ishpeming Greenstone
97
Belt by T.O. Quigley and R.A.
R.A. Mahin ...............................................................
97
of the
the Marquette
Marquette Iron Range, Michigan
Field Trip 3: Tilden and Empire
Empire Mines of
Leaders: G.W. Scott, P.M. Nordstrom and H.M. Lukey
Geologic setting of the Tilden and Empire Mines, Michigan by T.J. Bornhorst ............ 108
108
Geologic Field Trip to the Tilden Mine by G.W. Scott and H.M. Lukey .................... 114
114
Geologic Field Trip to the Empire Mine by P.M. Nordstrom ..................................129
129
Trip 4: Paleozoic and
and Glacial
Glacial Strata
Strata from Au
Au Train
Train to Grand
Grand Marais,
Field Trip
Marais, Michigan
Michigan
Leaders: R.S. Regis
and
J.
Anderton
Regis and J. Anderton
Paleozoic and Glacial Geology from Au Train to Grand Marais, Michigan
by R.S. Regis and J.
J. Anderton ......................................................................136
136
Field Trip
Trip11
Early
Early Proterozoic
Proterozoic Strata
Strataof
of the
the
Marquette
MarquetteIron
IronRange,
Range,Michigan
Michigan
Leader:
Leader:W.F.
W.F. Cannon
Cannon
2
FIELD TRIP #1
EARLY PROTEROZOIC
PROTEROZOICSTRATA
STRATA OF
OF THE
THEMARQUETTE
MARQUETTEIRON
IRONRANGE
RANGE
LEADER: W.F.
GEOLOGICAL SURVEY
W.F.CANNON,
CANNON, U.S. GEOLOGICAL
INTRODUCTION
preserved in the Marquette Iron Range
Early Proterozoic strata of the Marquette Range Supergroup preserved
the south edge of the
the Archean Superior craton.
record the tectonic evolution of a continental margin along the
That evolution spans an imprecisely known period culminating in suturing with volcanic arcs to the south at
about 1855 Ma.
Ma. Lower
the Chocolay
Chocolay Group
Group(Stops
(Stops1,2,3,5),
1,2,3, 5), are mostly a sequence of
Lower parts
parts of the section, the
pure quartzite, dolomite, and shale believed to have formed in a stable cratonic setting. They
They were
were deposited
deposited
now most commonly a fault (Stop I).
unconformably on Archean greenstone and granite, but the contact is now
1).
A low angle unconformity separates the Chocolay Group from the overlying
overlying Menominee Group (Stop 5).
The Menominee Group consists of a basal quartzite-shale unit (Stop 5) overlain
overlain by the principal
principal ironproducing unit in the range, the Negaunee Iron-formation
The Menominee
Menominee Group
Groupwas
was
Iron-formation (Stops 6, 7, 10). The
deposited in actively subsiding grabens, probably on the rifting continental margin, although a foreland
researchers in
in the
the region.
region. Strata
basin setting is possible and is favored by some current researchers
Strata of
of the
the Baraga
Baraga
9, 11) are widely accepted
accepted as products
products of sedimentation
sedimentation in
in aa foreland
foreland basin
basin during
during
8,9,
Group (Stops 4, 8,
preserved in
in northern
northern Wisconsin.
Wisconsin. The
accretion of volcanic terranes now preserved
The Baraga
Baraga Group
Group which lies
lies with
with aa
prominent low angle unconformity
unconformity on rocks of the
the Menominee
Menominee Group
Group (Stop
(Stop 4),
4), is
is composed
composed mostly
mostly of a
prominent
turbidite sequence of graywacke and shale (stop 11),
and conglomerate
conglomerate (stop
(stop 4),
4), lesser
1I), quartzite and
lesser black
black shale
shale
(stop 8), and minor volcanic rocks
rocks (stop
(stop 9).
9).
The Marquette Iron Range
Range is
is aa complexly
complexly folded
folded and faulted,
faulted, gently
gently west-plunging
west-plunging synclinorium
synclinorium
composed of strata of the Marquette Range Supergroup and intrusive
intrusive diabase
diabase sheets.
sheets. Deformation
Deformation occurred
occurred
during the Penokean orogeny at about 1850
Ma.
The
synclinorium
is
bounded
on
both
the
north
and
1850 Ma, The synclinorium is bounded on both the north and south
south
by Archean volcanic and granitic rocks, which are most
most commonly in
in fault
fault contact
contact with
with the
the Early
Early
Proterozoic strata. The
Theintense
intense penetrative
penetrative deformation
deformation seen
seen in
in the
the Early
Early Proterozoic
Proterozoic strata
strata isis not
not present
present in
in
the Archean basement rocks, which were last penetratively
deformed in
in Lake
Lake Archean
Archean time.
time. This has led to
penetratively deformed
the interpretation that the Archean rocks behaved as relatively
relatively rigid,
rigid, fault-bounded
fault-bounded blocks
blocks during the
Penokean orogeny, and were structuraliy
highly deformed
deformed Early
Early Proterozoic
Proterozoic strata. By
structurally decoupled from the highly
By
that interpretation, the Marquette synclinorium is a graben with
with respect
respect to
to Archean
Archean basement
basement rocks.
rocks. The
synclinorium
synclinorium of
of Early
Early Proterozoic
Proterozoic rocks
rocks lies
lies within
within that
that graben.
graben.
localities that
that have
have been
been visited
visited by
by literally
literally
The Marquette area contains many classic geologic localities
thousands of geologists during the past century. The
TheInstitute
Institute on
on Lake
Lake Superior
SuperiorGeology
Geology has
has visited
visited the
the area
area
during several past meetings, but the
the Marquette
Marquette Range stratigraphy has
has not
not been
been examined
examined since
sincethe
the
Twenty-first Annual Meeting in 1975.
1975. This
Thisguidebook
guidebook isis based
based very
very largely
largely on
on the
the guidebook
guidebook prepared
prepared for
for
that meeting and other guidebooks
guidebooks (Cannon
(Cannon and
and others,
others, 1975;
1975; Prinz,
Prinz, and
and others,
others, 1975;
1975;Margeson,
Margeson,1989;
1989;
Morey, 1989).
1989). For
Forthose
thosewho
whohave
havetoured
toured the
the region
region before,
before, the
the stops
stops will
will be
be largely
largely the
the same
same and
and the
the rocks
rocks
familiar; only the interpretations have changed. The stops are arranged,
arranged, to
to the
the extent
extent practical,
practical, to
to view
view the
the
beginning at the base of the
the section
section and
and progressing
progressing up-section.
up-section. To streamline logistics and
stratigraphy beginning
minimize driving distances a few stops are out of proper stratigraphic order.
3
a
n
-
00
1
1
22
33
44
1
5 miles
5miles
4-
C;
frt?
+
C-
7p2
5
Stop 1. Enchantment
EnchantmentLake
LakeFormation
Formationand
andArchean
Archeanvolcanic
volcanic rocks.
rocks.
The Enchantment Lake Formation is
is a discontinuous unit at the base of
of the Marquette
Marquette Range
Range
Supergroup. ItIt consists
arkose, and
and slate.
slate. Most geologists who have studied it
Supergroup.
consists of
of conglomerate,
conglomerate, graywacke, arkose,
believe that itit is of
of glacial
glacial origin,
origin, although
although that
that interpretation
interpretation remains
remains in
in dispute.
dispute. It conformably underlies
underlies
hills immediately
immediately south
south of
of stop
stop 1,
I, but
the Mesnard Quartzite. The
The transition
transition to the Mesnard can be seen in hills
will not be visited on this trip.
hip.
Immediately north of the Old State
State Road are
are several exposures of
of basal
basal conglomerate
conglomerate of
of the
the
fragments of
of tonalitic
tonalitic gneiss
gneiss and
and greenstone.
greenstone. The matrix is
Enchantment Lake Formation, containing fragments
is rich
rich in
in
chlorite. The
The conglomerate
conglomerate can
can be
be seen
seen in
in sharp
sharp vertical
vertical contact
contact with slaty, highly sheared greenstone
greenstone of the
underlying Mona Schist (Archean). About
About 100
100 feet
feet north of the contact, the Mona becomes mostly massive
massive
greenstone. Both
Both the
the greenstone
greenstoneand
and conglomerate
conglomerate are
are strongly
strongly foliated
foliated near the contact and
and steeply
steeply
plunging to vertical lineations
lineations are
are common on
on foliation surfaces.
surfaces.
The contact of the Mona Schist and Enchantment Lake
Lake Formation
Formation has
has been
been interpreted
interpreted as
as aa fault. In
In
addition to the highly sheared rocks along
along the contact,
contact, the two units
units are
are stratigraphically
stratigraphically "back
"back to
to back".
back".
Pillows in the Mona Schist
Schist in
in nearby
nearby outcrops
outcrops are
are uniformly
uniformly north-facing,
north-facing, whereas
whereas the
the Enchantment
Enchantment Lake
Lake
and overlying formations are south-facing. The
present
attitude
of
the
rocks
is
probably
a
result
of
vertical
The present attitude of
rocks is probably a
of vertical
rise of aafault
faultblock
blockof
ofMona
MonaSchist
Schistand
and passive
passive draping
draping of
of the
the Enchantment
Enchantment Lake
Lake Formation
Formation over
over the block.
block.
Stop 2. Mesnard Quartzite
Quartzite and
and Jacohsvitle
Jacobsville Sandstone.
Sandstone.
In this lakeshore outcrop the Mesnard Quartzite, the basal orthoquartzite unit of
of the
the Chocolay
Chocolay
Group, is overlain with
with aa sharp
sharp angular
angular unconformity
unconformity by
by the
the Middle
Middle Proterozoic
Proterozoic Jacobsville
Jacobsville Sandstone.
Sandstone.
strikes eastward and
and dips
dips
White-weathering vitreous Mesnard Quartzite in beds 4 to 88 inches thick strikes
80°S
to vertically.
vertically. Cross
80's to
Cross beds
beds and
and ripple
ripple marks indicate top direction to the south, in conformance with the
position of the quartzite on the north limb
limb of the Marquette synclinorium. Reddish Jacobsvillle
Jacobsvillle Sandstone
Sandstone
dips 10-15°
10-15' eastward to southeastward and the contact with the Mesnard is visible at several places on
on the
the
outcrop. Patches of Jacobsville are plastered against the north side
of
the
quartzite
outcrop,
and
on
the
south
side
outcrop, and on the south
side the contact is steep. Evidently
Evidently during
during deposition
deposition of
of the
the Jacobsville,
Jacobsville, the Mesnard at this locality formed
formed
a low topographic ridge, much as it does now. Flat-lying
Flat-lying Jacobsville
Jacobsville is
is on the flank of Mt. Mesnard, about
about
3/4
miles west of here and 370 feet above current lake level, which indicates a pre-Jacobsville topographic
Vs
miles
relief and ruggedness approximately the same
same or
or greater than that adjacent to the present
present shore
shore of
of Lake
Lake
Superior.
Superior.
3. Kona
Kona Dolomite
Dolomite
Stop 3.
on the
the Mesnard
Mesnard Quartzite.
Quartzite. The formation is mostly dolomite,
The Kona Dolomite lies conformably on
fine- to
to medium-grained
medium-grained clastic
clastic rocks.
rocks. The outcrops on the north
siliceous dolomite, and lesser amounts of fineside of the highway show exceptionally
exceptionally well preserved sedimentary textures indicative of
of aa shallow
shallow water
water to
to
intratidal environment, probably open tidal
tidal flats at
at this
this stratigraphic
stratigraphic interval.
interval. Many beds are algal
carbonates with large stromatolites
stromatolites exceptionally well displayed and accentuated by differential
differential weathering.
weathering.
visitors. Other
Please help preserve these features for future visitors.
Other well preserved features
features include dessication
and rip-up
structures, ripple marks, and
rip-up beds
beds probably
probably produced
produced by tidal currents.
6
4. Goodrich Quartzite
Stop 4.
This stop, for the sake of economizing
economizing driving
drivingtime,
time, isisnot
notininproper
properstratigraphic
stratigraphicorder.
order. We
We have
have
temporarily skipped over the Menominee Group and will examine the basal conglomerate of the Baraga
Group. At
At this
this stop,
stop,just
just south
southof
ofthe
the village
village of
of Palmer
Palmer and
and near
near the
the old
old Isabella
Isabella Mine,
Mine, coarse
coarse conglomeratic
conglomeratic
Goodrich Quartzite are exposed.
exposed. The
rocks forming the lower part of the Goodrich
The Goodrich
Goodrich Quartzite,
Quartzite, the
the basal
basal unit
unit
of the Baraga Group, unconformably overlies the Negaunee Iron-formation. Here
Here aa basal
basal conglomerate
containing clasts of iron-formation
iron-formation as
as much as 0.5
0.5 m
m diameter can be seen.
The angular
angular clasts
clasts have
have features
features
seen. The
indicative of brittle fracturing, implying that the Negaunee
Negaunee was
was well
well lithified
lithified prior
prior to
to the
the time that
that these
clasts were eroded. Well
Wellrounded
rounded clasts
clastsof
ofvein
vein quartz
quartz are
are also
also common,
common, most
most likely
likely derived
derived from
from aa locally
locally
exposed basement
basement terrane.
terrane. The
lack
of
other
basement
rock
types
in
clasts
may
mean
that
the
surface
of
The lack of
in clasts may mean that the surface of
vein material
material persisted
persisted in coarse fragments. The
exposed basement rocks was deeply weathered so that only vein
The
exposures seen here are only a few feet above the unconformity and
and Negaunee
Negaunee Iron-formation
Iron-formation was mined
from small pits immediately
immediately south
south of
of the
the outcrop.
outcrop.
Conglomerate
Conglomerate units such as seen here are typically lenticular. They range up to a few hundred feet
thick locally but pinch out along strike so that quartzite commonly lies on the Negaunee with
with little or no
conglomerate. The
Thecoarse
coarseconglomerate
conglomeratelenses
lenses may
mayhave
have been
been alluvial
alluvial fans
fans along
along the
the scarps
scarpsof
offaults
faultsthat
that
were active during deposition. The
Thefeatures
featuresseen
seenhere
here suggest
suggestaa rather
rather long
long time
time interval
interval across
across the
the
unconformity and renewed deposition during reactivation
reactivation of basement faults. One
One interpretation
interpretation isisthat
that the
the
Menominee Group was deposited during continental
margin and that the Baraga Group,
continental breakup on a rifting margin
beginning with the Goodrich Quartzite, was deposited
deposited in
in aa foreland
foreland basin
basin at
at the
the outset
outset of
of collision
collision with
with
volcanic terranes now
now seen
seen in
in northern
northern Wisconsin.
Wisconsin.
5. Chocolay Group and lower
Group (Ajibik Quartzite
Quartzite and Siamo Slate)
lower part
part of
of Menominee Group
Stop 5.
The series of roadcuts along U.S. Highway
much of the
the stratigraphy
stratigraphy of the
the lower part of
Highway 41 exposes much
the Marquette Range Supergroup. The
Theeasternmost
easternmost cut
cut shows
shows aa few
few tens
tens of
of feet
feet of
of green
green chloritic
chloritic slate
slate
Formation. Here
mapped as the upper part of the Enchantment Lake Formation.
Here the
the Enchantment
Enchantment Lake
Lake differs
differs
significantly from that seen at stop I1 in
in being
being much
much thinner, probably not much thicker than the section
section
exposed here, and it lacks conglomerate beds. The
The Enchantment
Enchantment Lake
Lake grades
grades over
over aa few
few feet
feet into
into the
the
overlying vitreous quartzite of the Mesnard
Mesnard Quartzite, essentially similar
similar to
to that
that seen
seen at
at stop
stop 2.
2. The
The Ajibik
Ajibik
Quartzite is exposed in the next set of roadcuts to the west. Although
Although similar
similar in
in appearance
appearance to
to the
the Mesnard,
Mesnard,
the Ajibik in general is somewhat more feldspathic. The
The geology
geology near
near here
here was
was mapped
mapped in
in detail
detail by
by Willard
Willard
Puffett
Puffen to document the rdationship
relationshipbetween
between the
the two
two quartzite
quartzite units.
units. Puffett
Puffett was
was able
able to
to trace
trace aa distinctive
distinctive
granule bed, about 10
Ajibik and show that the
the Ajibik
Ajibik cuts across the
10 inches
inches thick near the base of the Ajibik
Mesnard at a low angle, demonstrating the angular unconformable nature of the
the contact.
contact. The
The thinness
thinnessof
ofthe
the
Chocolay Group here and the absence of the Kona Dolomite reflect
reflect the westward
westward beveling of the group by
the Ajibik. Within
Withinaamile
milewest
westof
ofhere
herethe
theChocolay
ChocolayGroup
Group isis absent
absent and
and the
the Ajibik
Ajibik rests
rests directly
directly on
on
Archean rocks.
The Ajibik grades upward into the Siamo Slate seen in
in roadcuts
roadcuts about
about 1000 feet
feet farther
farther west
west on the
south side ofthe
of the highway.
highway. The
The Siamo
Siamo is
is a lithologic
lithologic mixture of slate,
slate, arkosic
arkosic quartzite, and graywacke,
generally in beds from a few inches to a few feet thick.
thick. The
The rock
rock here
here has
has aa prominent
prominent cleavage
cleavage that
that dips
dips
southward more steeply than bedding, consistent with the location
location on the north limb
limb of
of the
the Marquette
Marquette
synclinorium. An
An interesting
interesting and
and controversial
controversial feature
feature of
of this outcrop
outcrop is
is lenses
lenses of graywacke that lie
lie within
7
the cleavage and transect bedding. Various
Variousauthors
authors have
have interpreted
interpreted these
these as
as clastic
clastic dikes
dikes or
or as
as lithified
lithified beds
beds
tectonically forced into
into cleavage
cleavage planes.
planes.
Stop 6.
6. Negaunee Iron-formation (carbonate fades)
facies)
This stop, near the old Athens Mine, shows carbonate
carbonate facies
facies iron-formation
iron-formation typical
typical of
of much
much of
of the
the lower
lower
part of the Negaunee Iron-formation.
Iron-formation. The
here consists
consists mainly
mainly of
of alternating
alternating laminae
laminae of
of
The iron-formation
iron-formation seen here
sideritic carbonate and
5% minnesotaite, which imparts a
and chert,
chert, but
but some
some layers
layers contain
contain as
as much
much as
as 115%
greenish color. Some
Somelayers
layerscontain
contain small
smallamounts
amounts of
of magnetite,
magnetite, but
but in
in general
general the
the rock
rock isis only
only weakly
weakly
foot thick, are
are also
also present
present in
in the exposure.
exposure.
magnetic. Thin
Thin sills
sillsof
ofaltered
altered metadiabase,
metadiabase, less
less than
than aa foot
This exposure provides striking
striking examples of the effects of
of surface
surface weathering
weathering and
and oxidation
oxidation and
and
replacement of sideritic iron-formation.
iron-formation. The
The weathering
weathering has
has taken place in
in the relatively few decades
decades since
since
the road cut (originally a railroad cut) was excavated. Oxidation
Oxidation has
has advanced
advanced inward
inward from
from cracks
cracks and
and
exposed surfaces, first converting
converting layers
layers rich
rich in
in carbonate
carbonate into
into yellow
yellow brown
brown to
to reddish
reddish brown
brown goethitic
goethiticiron
iron
laminae into iron
iron oxide.
oxide. These
oxide, and eventually changing even chert laminae
These surficial
surficial effects
effects are analogous
analogous to
to
those that formed the "soft ores" of the region. The
major
ore
bodies
in
the
eastern
end
of
the
iron
range,
The major ore bodies in the eastern end of the iron range,
including those in the immediate
immediate vicinity
vicinity of
of this stop,
stop, were replacement masses of
of iron
iron oxides
oxides and
and
hydroxides formed by low
low temperature alteration
alteration of
of carbonate
carbonate and
and silicate
silicate iron-formation
iron-formation accompanied
accompanied by
by
dissolution and replacement
replacement of
of chert.
chert.
Stop 7. Negaunee
Negaunee Iron-formation (hematite-jasper
(hematite-jasper fades)
facies)
Classic exposures
exposures of
of the jaspillitic upper
upper part
part of
of the
the Negaunee Iron-formation
Iron-formation are
are seen
seen on
on Jasper
Jasper Knob.
Knob.
These exposures
just north
north of
of the
the major
major fold
fold axis of the
exposures are in
in the central part of the Marquette synclinorium, just
district. The
Theknob
knob itself
itself isison
on aa west-plunging
west-plunging anticline
anticline so
so that the base of the Goodrich
Goodricb Quartzite wraps
around the west end of the knob and is
is exposed in
in some of the fenced mined areas
areas near
near the
the base
base of
of the
the west
west
slope. The
alternating thin
thin beds
beds of
of specular hematite and bright
The iron-formation
iron-formation here
here characteristically
characteristically has alternating
jasper. Specularite
red jasper.
Speculariteplates
plates are
are strongly
strongly oriented
oriented parallel
parallel to bedding in the hematitic layers.
Folds having drag folds on their limbs and themselves being drag folds on the limbs of still
still larger
larger
folds are strikingly shown
shown in
in many
many exposures.
exposures. Fold axes are horizontal
horizontal to
to gently
gently plunging.
plunging. Small breccia
zones are common and appear to lie roughly along axial
axial planes
planes of
of folds.
folds. In these zones fractured jasper beds
are surrounded by crystalline hematite.
hematite,
theories have
have been
been advanced
advancedfor
forthe
theorigin
originofofthejaspillite.
oxidation of
Two theories
the jaspillite. One advocates oxidation
siderite-chert iron-formation
iron-formation during the post-Negaunee erosional interval and its recrystallization
recrystallization to
to
Penokean deformation
deformation and
and metamorphism.
metamorphism. This theory accounts for patches of
specularite-jasper during Penokean
specularite-jasper within carbonate facies iron-formation near contacts between the two rock types,
types, as,
as, for
for
example, in the Cliffs Shaft mine about '/2
¼ mile
mile to
to the
the west.
west. The second theory advocates a primary origin
for the hematite and jasper in
in a more
more oxidizing
oxidizing depositional
depositional environment than that responsible
responsible for
for carbonate
carbonate
iron-formation. The
Theeven-bedded
even-bedded and
and thin-bedded
thin-bedded nature
nature of
of the jaspillite on Jasper Knob is very similar to
iron-formation. ItIt contrasts
the bedding characteristics of carbonate iron-formation.
contrasts sharply
sharply with wavy bedding and oolitic
structure common
common in
injaspillite
jaspillite elsewhere
elsewhere in the district.
district. At
At the
the Kloman
Kloman Mine, near Republic (stop 10),
lo), a
convincing case can be made
made for aa primary
primary depositional
depositionalorigin
originofjaspillite.
ofjaspillite. Here, at Jasper Knob, an origin
origin
by oxidation of carbonate iron-formation
iron-formation is more possible.
8
Stop 8. Michigamme
Michigamme Formation,
Formation, lower
lower slate
slate member.
member.
black sulfidic slate well
well represented by the
The lower part of the Michigamme Formation consists of black
outcrops along the south side of Highway 41. This
Thisoutcrop
outcropisisnear
near the
the axis
axisof
of the
the Marquette
Marquette synclinorium,
synclinorium.
Subtle beds show mostly low dips and gentle folds with nearly horizontal axes. A
A steep
steepaxial
axial plane
planecleavage
cleavage
is well developed. Very
Very fine-grained
fine-grained pyrite
pyrite is
is abundant in many beds. This
This black
black slate
slate facies
facies lies
lies on
on the
the
Goodrich Quartzite and is overlain by the Greenwood Iron-formation
Iron-formation Member, a lean silicate-magnetite
iron-formation.
iron-formation.
Stop 9. Michigamme
Michigamme Formation,
Formation, Clarksburg
ClarksburgVolcanics
Volcanics Member.
Member.
The Clarksburg Volcanics is
is a local unit resting on the Greenwood Iron-formation Member. The
volcanic rocks are restricted to the south limb of the Marquette synclinorium, where they are exposed along
length of about
about 15 miles
miles and
and range
range up
upto
toabout
about 1I mile
mile thick.
thick. The absence of volcanic rocks on the
a strike length
away, shows that
that deposition
deposition in
in the
the Marquette trough
trough
north limb of the synclinorium, only a few kilometers away,
was very assymetrical at this time and
and that
that the
the trough
trough was
was probably
probablymuch
muchdeeper
deeperon
onthe
thesouth.
south. Lenses of
lean iron-formation
horizons throughout
throughout the
the volcanic
volcanic section
section indicate that
that
iron-formation at numerous stratigraphic horizons
volcanism was mostly or
or entirely
entirely submarine.
submarine.
The volcanics are almost exclusively mafic and pyroclastic.
pyroclastic. This
This outcrop
outcrop along
alongthe
the north
north side
sideof
ofthe
the
which layering is shown
shown distinctly by
by variations in
abandoned railroad grade is representative of the unit in which
highly vesicular mafic rock,
rock, and
and the
the matrix is rich in
fragment size, most fragments are angular pieces of highly
carbonate minerals. Fragments
of
argillite
and
chert
are
also
present.
Rocks
here
are
Fragments of argillite and
Rocks here are metamorphosed
metamorphosed to
to
garnet grade and original mafic minerals are converted to amphibole and biotite.
biotite. In
In spite
spite of
ofthe
the
recrystallization, original igneous textures are very well preserved, even at microscopic
microscopic scales. Low
Low
outcrops south of the railroad grade show lean, impure iron-formation
typical of lenses that
that are widespread
iron-formation typical
in the
the Clarksburg Volcanics.
Volcanics. The rocks consist of
of alternating
alternating layers
layers of
of magnetic
magnetic biotite
biotite argillite
argillite and
and siliceous
in
beds with abundant
abundant fine-grained
fine-grained muscovite.
muscovite.
Stop 10. Negaunee Iron-formation at
a t Kioman
Kloman Mine.
Mine.
the Negaunee
Negaunee Iron-formation.
Iron-formation. Interbedded
Exposures here are of the uppermost units of the
Interbedded on
on aa scale
scale
iron-formation, aa wavy-bedded,
wavy-bedded, oolitic
oolitic jaspillite
jaspillite
of one to three feet are two contrasting lithologic types of iron-formation,
and even-bedded cherty grunerite-magnetite
gmnerite-magnetite iron-formation. The
The features
features seen
seen in
in this
this exposure
exposure were
were
instrumental in Hal James' formulation
formulation in
in the 1950's
1950'sof
ofhis
his concepts
concepts of
of sedimentary
sedimentary facies
facies of
ofiron-formation
iron-formation
mineral assemblages
assemblages during
during high
high grade
grade metamorphism.
metamorphism. A
as well as oxygen-buffering by contrasting mineral
A
critical feature
feature very well shown
shown here
here isis aa correspondence
correspondence between
between bedding
bedding characteristics
characteristics and
andoxidation
oxidation
state. Relatively
Relatively reduced
reduced rocks,
rocks, now
now grunerite-magnetite
grunerite-magnetite assemblages,
assemblages, are invariably
invariably in thin, even beds,
beds,
probably reflecting deposition in
in quiet,
quiet, poorly
poorly oxygenated
oxygenatedwater.
water. Interbeds ofjaspillite
ofjaspillite invariably
invariably show
show
irregular wavy
wavy beds and oolites are common,
common, especially
especially in
in thicker
thickerjasper
jasper beds.
beds. These
These units are
are believed
believed to
to
formed in agitated, more oxygenated
oxygenated water.
water. James
have formed
James used
used this correspondence of sedimentary
sedimentary features
features
and mineral assemblages to argue
argue that the different
different lithologies
lithologies and
and their oxidation
oxidation states,
states, although
although
metamorphosed to sillimanite grade
grade here,
here, still
still reflect
reflect differences
differences in
in primary
primary depositional
depositional settings.
settings.
9
Stop 11. Michigamme
Michigamme Formation.
the Michigamme
Michigamme Formation
Formation in
in the
the axis
axis of the
the
This roadcut shows very highly deformed
deformed schist of the
northwest-plunging Republic trough. The
The rock
rock here
here consists
consists of
of iron-rich
iron-rich metasediments, now
now consisting of
of
biotite-garnet-amphibole schist. The
The light-colored
light-colored bladed
bladed amphibole
amphibole is
is commonly
commonly mistaken for
for sillimanite,
sillimanite,
but although this exposure is
is within the sillimanite
sillimanite metamorphic zone, detailed petrographic
petrographic examination
examination
failed to find sillimanite in this outcrop. Thicker
has failed
Thicker beds,
beds, up
up to a few
few inches,
inches, of impure
impure quartzite form
form
readily traceable marker beds
and
reveal
very
tight
folds
with
amplitudes
many
times
greater
than
beds and reveal very
folds with amplitudes many times greater than
wavelength, and highly attenuated limbs. Fold
Fold axes,
axes, even
even in
in adjacent
adjacent folds,
folds, diverge
diverge by
by as
as much
much as
as 600.
60'. The
The
compression of
of the
the metasediments
metasediments between
between two
two blocks
blocks of
of
very tight folding is caused, in large part, by compression
Archean gneissic rocks that flank
flank the Republic
Republic trough on
on the northeast
northeast and
and southwest
southwest sides.
sides.
References
Cannon, W.F., Gair, J.E., Klasner, J.S., and Boyum, B.H., 1978, Marquette
Marquette Iron
Iron Range:
Range: Twenty-First
Twenty-First
Annual Institute on Lake Superior Geology, Field Trip Guidebook, p. 125-173.
125-173.
Prinz, W.C., Gair, J.E., and Cannon, W.F., 1975,
1975, Greenstone: Twenty-First Annual Institute
Institute on
on Lake
Lake
p.41-85.
Superior Geology, Field Trip Guidebook, p.
41-85.
and metal
metal occurrences,
occurrences, Michigan's
Michigan's upper peninsula: Society of
Margeson, G.B., 1989, Precambrian
Precambrian geology and
Economic Geologists
Geologists Fall Field Conference, 154
154 p.
Morey, G.B., 1989,
28"' International Geological
1989, Early Proterozoic rocks of the Great Lakes region: 28th
63 p.
Congress Field Trip Guidebook T145,
T145,63
10
Field
Field Trip
Trip22
Archean
ArcheanIshpeming
IshpemingGreenstone
GreenstoneBelt
Beltand
andGold
Gold
Mineralization,
Mineralization,Michigan
Michigan
Leaders:
Leaders:T.J.
T.J.Bornhorst,
Bornhorst,DJ.
D.J.Duskin,
Duskin,ftC.
R.C.Johnson,
Johnson,R.A.
R.A.
Mahin,
Mahin,T.O.
T.O.Quigley,
Quigley,and
andG.W.
G.W.Scott
Scott
11
An Introduction
Introduction to
to the
the Archean
Archean Ishpeming
Ishpeming Greenstone
GreenstoneBelt,
Belt,
Michigan
Michigan
Rodney
Johnson &
& Associates,
Associates, Inc., 1550
1550 Baldwin
Baldwin Ave.,
Ave., Negaunee,
Negaunee, MI
MI
Rodney C. Johnson, Rod Johnson
49866 Theodore
of Geological
Theodore J.
J. Bornhorst,
Bornhorst, Department
Department of
Geological Engineering
Engineering and
and Sciences,
Sciences,
Michigan Technological
Technological University,
University, Houghton,
Houghton, MI
MI 49931
49931
Introduction
Introduction
The Ishpeming
belt (IGB)
(1GB)covers
coversan
anarea
area of
of about
about 300
300 km2
km2 and
and is located
Ishpeming greenstone
greenstone belt
located in
Michigan's Upper
Upper Peninsula.
Peninsula. It is
is on
on the
the southernmost
southernmost edge of the Superior
Superior province and is
is
part of the
composed of
of tholeiitic
tholeiitic pillowed
pillowed basalt
basalt
the Wawa
Wawa subprovince
subprovince (Figure 1).
1). The belt is composed
flows and lesser amounts
to felsic volcanic
flows
amounts of intermediate
intermediate to
volcanic rocks
rocks and minor
minor clastic
clastic and
and
chemical sedimentary
sedimentary rocks.
rocks. The 1GB
IGB is about 2,700
2,700 Ma
Ma (Bornhorst
(Bornhorst and
and Johnson,
Johnson, 1998;
1998;
Hammond, 1978).
1978). The
The1GB
IGB is
is north
north of
of the
the Great
Great Lakes tectonic zone, an Archean
Archean continentcontinentcontinent
In Michigan,
the Archean
rocks south
Michigan, the
Archean rocks
south of the
the Great
Great Lake
Lake
continent suture
suture (Sims,
(Sims, 1991). In
tectonic
tectonic zone
zone are composed
composed of old
old Archean
Archean gneiss
gneiss and
and migmatite
migmatite intruded
intruded by younger
younger
Archean granites
granites and
and is
is part
part of
of the
the Minnesota
Minnesota River
River Valley
Valley Subprovince
Subprovince(Sims,
(Sims,1996).
1996).
Tholeiitic to caic-alkalic
calc-alkalic volcanism
volcanism of
of the
the 1GB
IGB belt resulted
resulted from
from north-directed
north-directed subduction
subduction
(Hoffman, 1989; Card, 1990)
Archean time. Successive Archean
Archean accretion resulted in
1990) during Archean
southward younging
younging of
of greenstone
greenstone belts
belts in
in the
the Superior
Superior province.
province. The Great Lakes tectonic
tectonic
zone (GLTZ)
km long
longArchean
Archean paleosuture
paleosuture (Sims,
(Sims, 1991),
1991), the
the most
most significant
significant
(GLTZ) is aa 1,000
1,000km
Archean
boundary within
within the
the U.S.A.
U.S.A. Sims
Archean tectonic
tectonic boundary
Sims (1991)
(1991) has
has studied
studied aasignificant
significant shear
shear
zone, just south
south of
of the
the 1GB,
IGB, that he has interpreted as the only exposed segment of
of the
the GLTZ
GLTZ
(Figure 1). The
buried. A
The Minnesota
Minnesota segment
segment of the GLTZ is buried.
A result
result of
of detailed
detailed and
andregional
regional
field studies,
studies, including
including structural
structural geology,
geology, is
is the
the synthesis
synthesis of
of the
the lithostratigraphy
lithostratigraphyof
ofthe
the1GB
IGB
in close
close proximity
proximity to
to the
the Great
Great Lakes
Lakes tectonic
tectonic zone.
zone.
Structure
Structure
stratigraphic correlation
correlation depends
depends on
on good
good understanding
understanding of
of deformation
deformationhistory.
history.
Accurate stratigraphic
studies have
have shown
shown that
that the
the rocks
rocks of
of the
the 1GB
IGB have been effected by six periods of
of
Recent studies
deformation (Table
(Table 1)(Johnson,
l)(Johnson, 1993;
1993;Nachatilo
Nachatilo and
and Bauer,
Bauer, 1993).
1993).D1
Dldeformation
deformation
Archean deformation
produced
axial-planar cleavage (Johnson
produced recumbent folds with a flat-lying penetrative, nearly axial-planar
and Bornhorst, 1991).
1991). D2
D; deformation
deformation produced large-scale strike-slip
strike-slip shear
shear zones
zones that
that
Dadeformation
deformation produced
produced upright
uprightfolds
folds
define the
the boundaries
boundaries of
of lithostratigraphic
lithostratigraphic blocks.
blocks. D3
define
axial-planar cleavage
cleavage that is
is developed best
best in
in felsic
felsic volcanic
volcanic
(Figure 2) with a near-vertical axial-planar
D4 deformation is associated with the intrusion of late
interflow sedimentary units. D4
and interfiow
trondjhemite-granite suite
suite plutons
plutons that
that reorient
reorient earlier
earlier fabrics.
fabrics. D5
Dsdeformation
deformation produced
produceddipdiptrondjhemite-granite
slip motion in shear
shear zones.
zones. D6
D6 deformation
deformation produced
produced crenulation cleavage,
cleavage, local
local kink
kink bands,
bands,
Archean rocks
rocks of
of the
the 1GB
IGB were not significantly deformed
TheArchean
and strike-slip shear zones. The
Orogeny (Nachatilo
(Nachatilo and
and Bauer,
Bauer, 1993).
1993).
during the Early Proterozoic Penokean Orogeny
strata of the southwest
southwest block (Figure
(Figure 2)
2) form
form aa steeply
steeply west-plunging synformal
synformal
The strata
This fold
foldisisdefined
defined by
by the
the orientation
orientation of bedding. Fieldwork
Fieldwork has
has identified
identified northnorthanticline. This
12
'
I
'00
700
I
I
700KM
Explanation
Explanation
Phanerozoic
Phanerozoic
111111
-
Precambrian
Precambrian
1----]
Middle Proterozoic
Proterozoic volcanic and sedimentary
sedimentary rocks
rocks
Early Proterozoic
Proterozoic sedimentary and volcanic rocks
V//A
-
Grenville front boundary
boundary fault
fault
Sedimentary rocks
rocks
--
Fault
Margins of Great Lakes
Lakes tectonic
tectonic zone;
zone;
short dashed where inferred
inferred or
or covered
covered
greenstone-granite complexes
Archean greenstone-granite
complexes
Archean gneiss
gneiss and
and local
local granitoid
granitoid rocks
rocks
Regional geologic setting of the Archean Ishpeming greenstone belt and Archean
Figure
Figure 1: Regional
Great Lakes tectonic zone.
13
____
87°30'
-
0
I
T.
50
2 Mi.
0 1 2 3 K m .
N
T.
49
N
++++++
T.
++++++
+++++++
48
N.
46°30
Explanation
Explanation
Mona
Mona Formation
Formation
Upper tuff unit
lapilli-tuff unit
unit and
and lapilli-tuff
IN '1 Basalt flow unit
unit
Kitchi
Formation
Kitchl Formation
Tuff unit and lahar
I- — — —l
lahar unit
unit
Basalt flow unit
Proterozoic
Proterozoic
Sedimentary
Sedimentary rocks
rocks
Archean
Archean
Intrusive Rocks
intrusive
Rocks
Granodiorite
k'."J Granodiorite
Ii Tonalite
Tonalite
Deer Lake
Lake Peridotite
Peridotite
P
Sedimentary Rocks
Rocks
£S&S1 Timiskaming-type
Timiskaming-type sedimentary
sedimentary rocks
Volcanic Rocks
Rocks
Lighthouse
Lighthouse Point
Point Basalt
Basalt
Basalt
Basalt
V//I Reany
Lake
pyroclastic
Reany
Lake
pyroclasticunit
unitand
and
Fire Center mine iron-formation
iron-formation unit
unit
PL.JLJ Nash Creek glomerophyric
glomerophyric basalt unit
unit
a
I\ /1
0
I
I
a
I
Bedding
Bedding
Overturned
Overturned bedding
bedding
Facing of pillows
pillows
'C Plunging anticline
anticline
syncline
'C Plunging syncline
Plunging overturned anticline
anticline
\ Shear zone
&
-
87
f
87+
A
%
Figure 2: Generalized
Figure
Generalized geologic
geologic map
map of
o f the
the Ishpeming
Ishpeminggreenstone
greenstone belt.
belt.
14
Table
Table1.1.Summary
Summaryofofdeformation
deformationsequence
sequenceand
andvolcanic/magmatic
volcanic/magmaticactivity
activity and
and constraining
constraining age
agedates
datesfor
forthe
the
Ishepming greenstone belt.
belt.
Event
Event
-
Characteristic
Characteristic
Volcanic/Magmatic Activity
Volcanic/Ma~matic
Activity
Age
Dates
Ace Dates
Undeformed post-tectonic
post-tectonic granite
granite
2585 Ma
Ma
D6
Kink
Kink bands
bands with
with north-easterly
north-easterly
striking kink
kmk planes and steeply
plunging hinges
D5
Ds
Dip slip motion along shear
shear
zones.
D4
D4
Folding associated
Folding
associated with
with
intrusion of plutons.
plutons.
D3
Upright
Upright folding.
folding.
Appinite suite
D2
Strike-slip
Strike-slip shearing.
shearing.
Trondjheniite-granite suite
Trondjhemite-granite
DI
Recumbent
Recumbent folding.
folding.
Tonalite suite
Extrusion of mafic and felsic
volcanic rocks.
15
2668.4+2.11-1.8 Ma
2668.4+2.1/-1.8
2705.8+/-1.6 Ma
northeast-lacing beds in tuffaceous
tuffaceous units on the northern
northern limb
limb of
of th'is
this D3
D3fold.
fold. Tuffs
Tuffs of
and northeast-facing
the southwest block have a well-developed axial planar foliation
foliation (S4.
(53). Pillow basalts of
of the
the
southeast block consistently
consistently young to the north and
and have
have aa poorly
poorly developed
developed cleavage.
cleavage.
Interfiow sedimentary rocks
rocks dip
dip steeply
steeply to
to the
the north.
north. Tuffs that overlie the basalts
basalts have two
Interflow
well-developed cleaviges
cleavages that defini
define a-moderately
a moderately plunging
plunging D3
D3synform.
synform. The
The strata of the
well-developed
sedimentary and felsic volcaniclastic
volcaniclastic rocks define a
north block have been re-folded, and sedimentary
pattern (Johnson
(Johnson and
and Bornhorst,
Bornhorst, 1991).
1991). The low density of
broad Z-shaped outcrop pattern
sedimentary rocks necessitates relying on pillows
pillows in pillow
pillow lava for defining younging
directions (Figure 2). AAwell-developed
well-developed foliation
foliation in
in amphibolite-facies
amphibolite-faciesbasalt
basalt was
was
subsequently refolded by D3 deformation.
deformation. D3
folds are
are upright,
upright, open
open to
to tight
tight and have
D3 folds
shallow to steeply plunging fold axes.
axes.
Stratified Rocks
Stratified
lithostratigraphic blocks that comprise
Bornhorst (1988) subdivided the 1GB
IGB into three lithostratigraphic
comprise
units bounded
boundedby
bydeformation
deformationzones
zones (Figure 3).
3). As
internally consistent stratigraphic units
As aa
whole, the belt is bounded to the south by Early Proterozoic
Proterozoic sedimentary rocks along the
Carp River Falls shear zone, a structure that was active from the Archean
Archean through the
Proterozoic (Puffett, 1974).
1974). The
Thesouthwest
southwest block
block isis bounded
bounded to
to the
the west
west by
by Archean
Archean
granitoid rocks. This
Thiscontact
contact may
may be
be aafault
fault since
since the
the boundary
boundary is
is not
not transitional
transitional lit-par-lit
lit-par-lit
as found near the intrusive greenstone-granitoid
contact of
of the
the north
north block
block of
of the greenstone
greenstone-granitoid contact
belt (Johnson and Bornhorst,
Bomhorst, 1991). The
The Carp
Carp River
River shear zone separates the southwest
interpreted to be a dextral shear zone with uncertain
block from the southeast
southeast block. It is interpreted
displacement. The
stratigraphic displacement.
The dextral
dextral Dead
Dead River
River shear
shear zone separates
separates the two southern
southern
blocks from the north block. The north block is bounded
bounded to the east, north
north and west by
Archean granitoid rocks that have intrusive relationships
relationships with
with the volcanic rocks
rocks (Wilkins
and Bornhorst, 1992;
1992; Johnson and Bomhorst,
Bornhorst, 1991).
1991).
Southwest Block
The southwest block is composed of mafic flows and intermediate to
to felsic volcanic and
volcaniclastic rocks of the Kitchi Formation which are subdivided into
vokaniclastic
into three
three informal
informal units:
units:
basalt flow, tuff, and lahar units (Figure 44 and 5). The
The basalt flow unit, at
at the base of
of the
the
Kitchi Formation is composed of 1,800
1,800 meters of dominantly pillowed basalt flows,
flows, but
but
includes massive basalt flows as well. The
The basalts
basalts are
are dominantly
dominantly magnesian tholeiites
tholeiites
composed of hornblende-actinolite-albite-chlorite-clinozoisite
hornblende-actinolite-albite-chlorite-clinozoisitewith minor amounts
amounts of
of sphene,
sphene,
magnetite, and (or) pyrite. The
The base
base of
of the
the basalt
basalt flow
flow unit
unit is
is in probable fault
fault contact
contact with
with
beds of
of chert and
and dacite
dacite to
to rhyolite
rhyolite tuff
tuff are
are interlayered
interlayered
Archean granitoid rocks. Several thin beds
basalt flow unit. Near
near the middle of the hasalt
Near the
the top
ton of
of the
the basalt flow
flow unit is
is aa 30-meter30-meterplagioclase phenocrysts
phenocrysts up
up to
to 22 cm
cm in
in diameter.
diameter. The
The
thick glomerophyric flow containing plagioclase
uppermost basalt flows intercalate with intermediate
volcanic
rocks
of
the
overlying
tuff
intermediate
overlying
unit.
- - tuff unit.
Formation is composed
composed of
of 5,000 meters
meters of
of tuff
tuff with
with lesser
lesser amounts
amounts
~Thei tuff
e unit of the Kitchi Formation
of interlayered lahar with the thicker beds of lahar separated
separated into
into the
the lahar
lahar unit.
unit. The
The tuff
tuff unit
unit
ash tuffs,
tuffs, dominantly
dominantly calccalcand lithic
lithic tuffs, with minor lapilli-ash and ash
consists of crystal and
alkalic andesite in composition. The
Therocks
rocks are
are schist
schist composed
composed of
of quartz-feldspar-sericitequartz-feldspar-sericitechlorite containing quartz and feldspar phenocrysts.
phenocrysts. Pillowed
Pillowed basalt
basalt flows
flows interfinger
interfinger with
with
16
T
50
N
T
49
N
T
48
N
4630
R2SW
R2JW
0
R26W
5
111111
R25W
10 KILOMETERS
Explanation
Explanation
Middle Proterozoic
Proterozoic (900-1,600
(900-1,600 Ma)
Ma)
Metavolcanic rocks
Metavolcanic
rocks
LAttJ ,Jacobsville
JacobsvilleSanstone
Sanstone(Midcontinent
(Midcontinentrift
rift system)
system)
Early Proterozoic
Proterozoic (1,600-2,500
(I
,600-2,500 Ma)
Ma)
I Sedimentav
Sedimentary rocks
0
rocks of
of Marquette
MarquetteRange
RangeSupergroup
Supergroup
I
Archean
--
- Contact
Contact
+f -
Shear zone
Approximate trace of
of axial
axial
plane of plunging
plunging anticline
anticline
+i
f-
Approximate trace of axial
axial
— plane
planeof
ofplunging
plungingsyncline
syncline
I÷++4 Granitoid
Granitoid rocks
rocks
Figure 3: Lithostratigraphic
Figure
Lithostratigraphicblocks
blocks of the
the Ishpeming
Ishpeminggreenstone
greenstone belt.
belt.
17
8r4
8fl0
AAA A Y
4t32'30"
66 A
AA
A Aa A A'St'/?s
—4
A
A A A A A A'S {/? z A
A A A A 2-
A A A A A%
M&io -
-
r4
2'
-
A A
'a
a FJA.Je5 A C - -
eq
-, -—
aa
,
V
0lshpemiig
e3cy
0
88 KILOMETERS
KILOMETERS
0
I
Explanation
Explanation
Early Proterozoic
Proterozoic
0Undifferentiated
Undifferentiated metasedimentary
metasedimentary rocks
rocks
I
LAAAAAJ
Tuff unit
Tuff
unit
----
Basalt flow
flow unit
unit
Basalt
+
App~ximate
Approximate trace of axial plane
of anticline
anttcl~ne
I
Archean
Archean
- Contact
Contact
Undifferentiated
Undifferentiatedrocks
rocks
—r
Metagabbro
Metagabbro
65
Strike and dip
dip of
of beds
beds
"- Shear
zone
Shear
zone
Deer Lake Peridotite
Peridotite
Kitchi Formation
Formation
Lahar unit
unit
#—
Note: All rocks are metamorphosed
metamorphosed
Figure 4:
4: Generalized geologic map of
southwest block
block of the Ishpming
n s t o n e belt.
Mi.
of the southwest
lshpeming p
greenstone
18
METERS
METE1
0
0
500
500]
1000
1000
Tuff unit
Lahar unit
unil
unit cc:
Lahar unit
J
Basalt flow
unit
Formation within the southwest block
Figure
Figure 5: Generalized stratigraphy of the Kitchi Formation
of the Ishpeming greenstone
greenstone belt.
belt.
19
the tuff unit near the top and base of the unit. The
Thelahar
lahar unit
unit of
of the
the Kitchi
Kitchi Formation
Formation isis
composed of
of beds and
and lenses
lenses of
of polymictic volcanic conglomerate containing clasts of thinly
laminated,
laminated, porphyritic,
porphyritic, and
and holocrystaline
holocrystaline rocks that are calc-alkalic andesite to dacite
dacite in
in
composition. The
polymict volcanic
volcanic conglomerates
conglomerates are
are more
more common near the base of the
Thepolymict
tuff unit
unit and
and are
are interpreted
interpreted as
as lahar
lahar deposits
deposits (Bornhorst
(Bomhorst and
andJohnson,
Johnson,1993).
1993).
Southeast
Southeast Block
Block
The southeast
southeast block is
is composed of basalt flows and felsic volcanic and volcaniclastic rocks
of the Mona Formation, which
which is subdivided
subdivided into informal units: basalt flow, lapiffi-tuff, and
and
upper tuff units (Figure 6). The
Thebasalt
basalt flow
flow unit,
unit, at
at the
the base of the Mona Formation, is a
5,500-meter-thick
5,500-meter-thick sequence of pillowed with minor massive basalt flows. The basalts are
dominantly magnesian tholeiite basalt flows. Within the basalt flow unit are thin interfiow
interflow
chert-magnetite iron formations and schistose
schistose quartz-sericite
quartz-sericiterocks
rocks (tuffs).
(tuffs). The most notable
occurrence of interflow sedimentary
sedimentary rocks in the Mona Formation occurs north of the Pine
Hill
Hill Quarry
Quarry (Figure
(Figure 6).
6). At
Atthis
thislocation,
location,chert-magnetite
chert-magnetiteand
andchert-pyrite
chert-pyriteiron-formation
iron-formationare
are
intercalated with graphite and quartz-sericite
schist (tuffs)
(tuffs) as
as interflow
interfiow units.
units. Diamond
quartz-sericite schist
drilling
drilling along
along this interval
interval has indicated that these units have a combined thickness of
approximately
approximately 10
10 meters.
meters. The
Thelapilli-tuff
lapilli-tuffunit
unitoverlies
overliesthe
thebasalt
basalt flow
flowunit
unit (Figure
(Figure7)
7)and
andisis
composed of
calc-alkalic
rhyolite
lapilli
in
a
quartz-sericite-chlorite
matrix
supporting
quartz
of calc-alkalic
quartz-sericite-chlorite
and, less commonly, feldspar phenocrysts. This unit is interpreted as subaqueous pyroclastic
flow and fall
fall deposits
deposits (Bornhorst
(Bornhorst and Johnson, 1993).
1993). The
Thelapilli-tuff
lapilli-tuff unit
unit grades
gradeslaterally
laterally
and vertically into the upper tuff unit. This
quartz-feldspar-sericiteThisunit
unitisiscomposed
composedof
of quartz-feldspar-sericitechlorite
chlorite schist
schist with
with aa bulk
bulk calc-alkalic
calc-alkalic dacite
dacite composition.
composition. It is interpreted as interbedded
tuffs
tuffs and
and volcanic
volcanic derived
derived graywacke (Bomhorst and Johnson, 1993). The
The combined
combined
thickness
thickness of
of these
these felsic
felsicvolcanic
volcanic and
andvolcaniclastic
volcaniclastic units
units isis900
900meters.
meters.
North
North Block
Block
The
The north
north block
block isis composed
composed of
of the
the 3,100-meter-thick
3,100-meter-thick Lighthouse Point Basalt (Figure 8).
8).
The
Lighthouse
Point
Basalt
is
dominantly
pillowed
tholeiitic-magnesian
basalt
flows
with
The Lighthouse Point Basalt is dominantly
with
minor
minor massive
massive flows.
flows. AAglomerophyric
glomerophyric(Nash
(NashCreek)
Creek)basalt
basaltflow
flowoccurs
occursnear
nearthe
thebase
base
(Figure 9).
9). Within
Withinthe
theLighthouse
LighthousePoint
PointBasalt,
Basalt,the
theReany
ReanyLake
Lakepyroclastic
pyroclastic unit
unitisiscomposed
composed
(Figure
of interbedded
interbedded ash
ash to
to lapilli
lapilli ash
ash tuffs
tuffs of
of caic-alkalic
calc-alkalic dacite composition. ItItisisininturn
tumoverlain
overlain
of
by
thinly bedded chert-magnetite
chert-magnetiteiron-formation
iron-formation
by the
the Fire
FireCenter
Centermine
mineiron-formation
iron-formationunit
unit—- aa thinly
with
with lesser
lesseramounts
amountsof
ofassociated
associatedchlorite-magnetite
chlorite-magnetite schist
schistand
andchert-pyrite
chert-pyrite iron-formation.
iron-formation.
Over-lying
Over-lyingthe
the iron-formation
iron-formation isisintercalated
intercalated chlorite-carbonate-feldspar
chlorite-carbonate-feldspar and
andquartz-chlorite
quartz-chlorite
schist
schist (tuffs).
(tuffs). Above
Aboveand
andbelow
belowthe
theiron-formation
iron-formationare
areisolated
isolatedlenses
lensesand
andpods
podsof
ofgraphitegraphitepyrite
pyrite schist.
schist. This
Thisinterval
internalofofsedimentary
sedimentaryrocks
rocksisispoorly
poorlyexposed,
exposed,but
buthas
hasan
anestimated
estimated
thickness
thickness of
of approximately
approximately 100
100meters.
meters. The
TheReany
ReanyLake
Lakepyroclastic
pyroclastic unit
unitalso
alsocrops
cropsout
outinin
the
the vicinity
vicinity of
of Hills
Hills Lakes
Lakes where
where itit is
is composed
composed of lapilli and ash tuffs of calc-alkalic dacite
to
to rhyolite
rhyolite composition.
composition. The
TheLighthouse
LighthousePoint
PointBasalt
Basaltrepresents
representsaasuccession
successionofofsubaqueous
subaqueous
tholehite
tholeliitebasalt
basalt flows
flowswith
with intercalated
intercalated distal
distal to
to nearly
nearly proximal
proximal felsic
felsic volcanic
volcanic rocks
rocks
(Johnson
(Johnson and
andBornhorst,
Bornhorst,1991).
1991).
20
3fl5
8740
a
4
___
-A
t
.........................
A A A A A A £e/Lt,
——
A A fr - &
DeadRiver
StorageBasin
AA
8t25
87%
87'30
\D
A A.
-- - A A A Ah
—---" A A A A A SMarquette
-_
4632 30 - - - -C4
•
- - - - - - - "s/i
CARP RIVER FALLS SHEAR ZONE
0Negaunee
10lshpeming
4630
10 KILOMETERS
KILOMETERS
j0
5
0
1
Explanation
Explanation
0Undifferentiated
Undifferentiated metasedimentary
rnetasedimentary rocks
rocks
I
I
V//A
Lapilli-tuff unit
unit
Basalt flow unit
unit
ARCH EAN
ARCHEAN
- --
L1 Undifferentiated
rocks
Undifferentiated
rocks
-
Metagabbro
Metagabbro
Mona Formation
Formation
'4-+
unit
LA.AI Upper tuff unit
—
Contact--Dashed where concealed
concealed
Shear zone
Approximate trace of axial
axial plane
plane of
of
plunging syncline
plunging
syncline
Note: All rocks
rocks are metamorphosed
metamorphosed
Figure 6: Generalized geolgic map of
off the Ishpeming greenstone
greenstone belt.
o f the southeast block o
21
Upper
upper
tuff unit
*tuff
unit
Lapilli-tuff
Lapilli-tuff
unit
unit
METERS
METERS
0
Basalt
Basalt
* flow
flow
unit
unit
500
1000
Figure 7:
7: Generalized
Generalizedstratigraphy
stratigraphy of
of the
the Mona
Mona Formation
Formation within
within the
the southeast
southeast block
block
Figure
the Ishpeming
Ishpeming greenstone
greenstone belt.
belt.
of the
22
_____
______
______
0
10
5
Explanation
Explanation
0Sedimentav
rocks of Middle and EaW
Proterozoic ages
ages
Sedimentary rocks of Middle and Early
I
Fire Center
center mine iron-formation and
and
Reany Lake pyroclastic
pyroclastic units
units
I
ARCHEAN
ARCHEAN
a
to a a]
Nash
Nash Creek glomerophyric
glotnerophyric basalt
basalt unit
unit
where inferred
- Fault—Dashed
Fault-Dashed where
inferred
---
Reany Creek
Reany
Creek Formation
Formation
Granodiorite near Rocking
Granodiorite
Rocking Chair Lakes
Lakes
Gneissic granite
Gneissic
granite to
to tonalite
tonalite
Strike and dip of overturned
overturned bedding
bedding
%5
+f I..
Lighthouse Point
Lighthouse
Point Basalt
Basalt
Hills Lakes
Hills
Lakes pyroclastic
pyrodastic unit
unit
Shear
Shear zone
zone
—
'-{T -
Approximate
Approximate trace of axial plane
plane of
plunging
plunging anticline
anticline
Approximate
Approximate trace
trace of
of axial
axial plane
planeof
of
plunging syncline
plunging
syncline
Figure 8: Generalized geologic map
map of
of the north block of
of the Ishpeming
Ishpeming greenstone
greenstone belt.
belt.
23
Local unconformil
unconformity
+
Hills Lakes pyroclastic unit
unit
Main lithology of
Lighthouse Point Basalt
Fire Center mine iron-formation unit
Reany Lake pyroclastic
pyroclastic unit
unit
Nash Creek glomerphyric basalt unit
Base unknown
unknown
Figure 9: Generalized stratigraphy of the Lighthouse Point Basalt within the north block
of the Ishpeming greenstone belt.
24
Stratigraphic Relationships
Stratigraphic
Relationships Among
Among Lithostratigraphic Blocks
Blocks
Stratigraphic
correlations made across major
maior shear zones are tenuous. The folded rocks in
in the
the
southwest block comprise a steep westward plunging synformal anticline, with tops to
bedding on the north limb facing northward (Figure
(Figure 2).
2). Pillows
Pillows in
in the
the southeast
southeast block
block
consistently
consist&tly young to the north, thus strata
strata of the southeast block could be
be interpreted
interpreted as
as
younger than and overlying strata of the southwest
southwest block.
block. Displacement
Displacement along
along the
the Carp
Carp
River shear zone may have eliminated
eliminated some of
of the
the stratigraphic
stratigraphic section.
section. However, itit could
could
also be argued that the stratigraphy of the southwest and southeast blocks may have been
juxtaposed by thrusting along the Carp River shear zone making stratigraphic relationships
between them indeterminate but evidence for thrusting is lacking.
&
The stratigraphic
stratigraphic units of the north block and the southeast block face each other
other across
across the
the
Dead River Basin,
Basin, but
but are separated
separated by
bythe
theDead
DeadRiver
Rivershear
shearzone
zone (Figure
(Figure 2).
2). The strata
strata of
the southeast block consist of pillowed and massive tholeiitic basalt flows overlain
calcoverlain by
by calcalkalic tuffs and lapilli-tuff.
lapilli-tuff. The
alkalic
The Lighthouse
Lighthouse Point Basalt and the Mona Formation may
represent the opposing limbs of a synform. This
This is
is much
much more
more speculative
speculative than the
stratigraphic correlation between the southeast
southeast and
and southwest
southwest block.
block.
If each block is stratigraphically unique and represents distinct and separate
separate strata,
strata, the
the total
total
stratigraphic thickness is 16,000
16,000 meters. If all three blocks are correlatable with one another,
the cumulative
cumulative thickness
thichess isis 13,000
13.000meters.
meters.
Intrusive
Intrusive Rocks
Rocks
have been
been intmded
intruded by
by multiple
multiple episodes
episodes of
of Archean
Archean mafic and felsic
The rocks of the 1GB
IGB have
magmas. The
and peridotite
peridotite sills.
sills. The gabbros
Theoldest
oldest intrusive
intmsive rocks
rocks are synvolcanic gabbro and
are chemically indistinguishable from the surrounding basalt flows and likely represent
subvolcanic intmsions
intrusions (Bornhorst
(Bornhorst and
and Johnson,
Johnson, 1993).
1993). The Deer Lake Peridotite is intmded
intruded
subvolcanic
between the basalt and tuff units of the Kitchi Formation.
Formation. Contacts with surrounding
volcanic rocks are conformable,
conformable, but lack of quench textures (spinifex), pillow structures,
structures, or
or
other features typical
tpical of ultramafic flows lead to the interpretation of the peridotite body as
as aa
(Bornhorst and
and Johnson,
Johnson, 1993;
1993; Brozdowski,
Brozdowski, 1990,
1990,Bornhorst
Bomhorstet
et al.,
al., 1986).
1986). The
sill complex (Bornhorst
peridotite is lherzolite
lherzolite to harzburgite in composition (Rossell, 1983)
1983) and similar to ultramafic
ultramafk
flows and sills
from
other
Archean
greenstone
belts.
sills from other Archean greenstone belts.
The oldest felsic intrusive
intmsive bodies are foliated diorite to granite gneiss batholiths that intrude
intmde
the margins of the greenstone belt. These
These intrusions
intmsions post-date
post-date mafic volcanism in the
greenstone belt and were deformed along with volcanic rocks during the earliest stage of
quartz-feldspar porphyry dikes cut
deformation. Archean granodiorite to syenite stocks and quartz-feldspar
greenstone belt and
Dzthrough
through D4
D4deformation
deformation
the greenstone
and are
are interpreted
interpreted to
tobe
besynchronous
synchronouswith
withD2
(Wilkin and Bornhorst,
Bornhorst, 1992).
1992).
25
Discussion
Structural
Stmctural studies
studies in the 1GB
IGB have led to a more accurate picture of stratigraphic
thickness within lithostratigraphic
Iithostratigraphic blocks and
and possible
possible correlations
correlations between
between blocks.
blocks.
Geological studies prior to 1988
1988 (Bornhorst)
(Bomhorst) did not recognize the stratigraphic significance
of lithostratigraphic
Iithostratigraphic blocks. The
Therecognition
recognition of
of the
the synformal
synfonnal anticline
anticline in
in the
the southwest
southwest
block allows the true thickness of the volcanic strata of the Kitchi
Kitchi Formation to be estimated
1,500 meter thickness estimated by Morgan
at 6,800
6,800 meters, a four fold increase
increase over
over the
the ++1,500
and DeCristoforo (1980).
polyphase deformation in the Lighthouse Point
(1980). The recognition of polyphase
Basalt of the north block lead to a thickness estimate of 3,000 meters, compared to an earlier
estimate of 4,500 meters (Morgan
(Morgan and DeCristoforo,
DeCristoforo, 1980).
1980).
The stratigraphy of the 1GB
can be
be compared
compared to
tothat
thatof
ofthe
theAbitibi
Abitibigreenstone
greenstonebelt.
belt. In
IGB can
particular, the volcanic stratigraphy
can
be
compared
to
the
Tisdale
Group,
which
stratigraphy
compared the Tisdale Group, which isis
composed of komatiites and basaltic komatiites at the base overlain
overlain by tholeiitic basalts,
calc-alkalic volcaniclastic
volcaniclastic rocks
rocks (Pyke,
(Pyke, 1982). The Tisdale
subsequently overlain by calc-alkalic
Tisdale Group
Group
has a total thickness of from 950 to 4,000 meters. The
The 1GB
IGB has a similar volcanic
stratigraphy: tholeiitic basalt (the basalt
basalt flow units
units of
of the
the Kitchi
Kitchi and
and Mona
Mona Formations,
Formations, and
the Lighthouse Point Basalt) at the base, succeeded by calc-alkalic volcaniclastic rocks
rocks (the
(the
tuff unit of the Kitchi
Gtchi and
and the upper
upper tuff
tuff unit
unit of the Mona
Mona Formations,
Formations, and
and the
the Reany
Reany Lake
Lake
pyroclastic unit of the Lighthouse Point
Point Basalt).
Basalt). The stratigraphy is even more comparable ifif
the Deer Lake
Lake Peridotite
Peridotite represents
represents subvolcanic
subvolcanic sills
sills analogous
analogous with
with komatiitic
komatiitic volcanism.
volcanism.
Thus, in terms of lithology
lithology and
and thickness
thickness the
the stratigraphy
stratigraphy of
of the
the 1GB
IGB is directly comparable
comparable
to the Abitibi greenstone belt. The
The1GB
IGB represents an Archean volcanic arc assemblage as
characterized
characterized by Thurston
Thurston and
and Chievers
Chievers (1990).
(1990).
The environment of deposition is important in identifying
identifying potential
potential ore deposits
deposits with'in
within the
JOB. Significant occurrences of volcanogenic massive sulfide deposits are not known in the
IGB.
the
1GB.
Chalcopyrite and sphalerite associated with chert in the Reany Lake pyroclastic unit of
IGB. Chalcopyrite
the Lighthouse Point Basalt was interpreted by Johnson and Bomhorst
Bornhorst (1991) as a massive
sulfide deposit incorporated
incorporated in the pyroclastic
pyroclastic flow.
flow. Typically,
Typically, volcanogenic massive
greenstone belts
belts that
that have
have aa greater
greater than
than average
average volume
volume of
of
sulfides are hosted in greenstone
volcanic and sedimentary
sedimentary rocks
rocks (Franklin,
(Franklin, 1990). The
intermediate volcanic, felsic volcanic
The JOB
IGB is
favorable for VMS deposits because of its considerable
considerable stratigraphic
stratigraphic thickness.
thickness. The
The Ropes
Ropes
18901s,and more recently from 1985
1985 to
to 1989,
1989,is
is
gold mine, which was in production in the 1890's,
1990).
controlled by a shear zone (Bornhorst
(Bomhorst et al., 1986;
1986; Brozdowski, 1990).
Card (1990) suggested that the Superior province grew by Archean accretion,
accretion, and
and that
that
of north directed
directed subduction.
subduction. A similar sequence of
Archean volcanism was the result of
of events
events
could explain the Archean geology of the IGB.
1GB. The mafic to felsic volcanism could have
subduction. Extra-belt
been the result of northward directed subduction.
Extra-belt tonalite
tonalite batholiths,
batholiths, intra-belt
intra-belt
granodiorite stocks, and appinite stocks intrude
intrude the
the greenstone
greenstone belt.
belt. The transition
transition from
from
subduction to collision is
is preserved by the volcanic rocks (subduction
(subduction related arc)
arc) and
and by
by
intrusive rocks as described by Wilkin and
and Bornhorst (1992)
(1992) (transition from
from subduction
subduction to
to
collision).
26
1GBcan
canbe
beplaced
placedininan
ananalogous
analogousmodem
modemenvironment.
environment. As
As a
The volcanic strata of the IGB
directed subduction
subduction thick
thick accumulations
accumulations of pillowed tholeiitic
tholeiitic basalt
basalt flows
flows
result of north directed
(basalt flow units of the Kitchi and Mona Formations and the Lighthouse Point Basalt) were
were
Subsequently, dome
dome complexes
complexes developed
developed on
on the
the mafic
mafic
erupted from shield volcanoes. Subsequently,
shield volcanoes. Subaqueous pyroclastic and lahar flows (tuff and lahar units of the Kitchi
Kitchi
Formation, upper tuff and lapilli-tuff units of the Mona
Mona Formation,
Formation, and
and pyroclastic
pyroclastic units
units of
of
the Lighthouse Point Basalt) erupted
erupted from the dome complexes. The
Therocks
rocksofofthe
the1GB
IGB
Continent-continent collision
collision caused
caused the
the
represent coalescing volcanoes in an arc complex.
complex. Continent-continent
cessation of volcanism
volcanism and
and complex
complex post-deposition
post-deposition deformation.
deformation.
Summary
Summary
The Tshpeming
Ishpeming greenstone belt consists of subaqueous tholeiitic basalt to calc-alkalic
calc-alkalic
rhyolite, 10-15
!an in total thickness.
thichess. U-Pb
U-Pb zircon
zircon data
data establish
establish the
the age
age of
of volcanism
volcanism at
at
10-15 km
2705.8
+1—i .6 Ma
Ma (Bornhorst
Johnson, 1998). The
2705.8+/-1.6
(Bomhorst and Johnson,
The volcanic rocks were enveloped by
tonalite suite
suite plutons just before
before and
and during the beginning of recumbent folding
folding (Table
(Table 1).
1).
Strike-slip
Stnke-slip shear
shear zones
zones and
and the
the deposition
deposition of clastic sediments into pull-apart basins followed
followed
recumbent folding. The
folded. A
The belt
belt rocks
rocks were
were then upright folded.
A trondhjemite
trondhjemite to granite suite
suite
intruded the belt and folded earlier fabrics. The
timing
of
deformation
is
constrained
with
UThe timing of deformation is
with Udata yielding
yielding an
an age
age of
of 2668.4+2.1/-1.8
2668.4+2.1/-1.8 Ma for an intrusion in this suite
suite (Bornhorst
(Bornhorst
Pb zircon data
and Johnson, 1998).
1998). Intrusions
Intrusions of
of hornblendite
homblendite to
to syenite
syenite (appinite
(appinite suite)
suite) and
and continued
continued
movement along shear zones represent
represent the end of
of deformation.
deformation. An undeformed post-tectonic
granite
(Sims,1991).
1991).
granite intrusion
intrusion has
has an
an age
age of
of 2585
2585 (Sims,
These events are readily interpreted in a modem
modern plate
plate tectonic
tectonic context.
context. North-directed
arc. Subduction-related
subduction produced a volcanic arc.
Subduction-related tonalite intruded the arc just prior to
collision of a small
km long
long
small continent
continent from
from the south (southern complex) along the 1000
1000 kin
Great Lakes tectonic zone (suture). Collision
Collision occurred
occurred over
over an
an extended period of time with
multiple deformation and magmatic events.
events. The
The appinite
appinite suite
suite is characteristic of modem
collision-related magmatism.
magmatism. Continuing
Continuingstudies
studiesof
ofthe
the1GB
IGB and
and vicinity
vicinity will
will help
help to
to unravel
unravel
the tectonic evolution
evolution of
of the
the southern
southem edge
edge of
of the
the Superior
SuperiorProvince.
Province.
Acknowledgments
Acknowledgments
We thank
thank Shannon
Shannon E.
E. Bair
Bair for
forcomputer
computerdrafting
drafting of
of the
thefigures.
figures.
We
References
References
T.J., 1988,
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Geological overview
overview of
of the Marquette greenstone
greenstone belt, Michigan
Michigan ThirtyThirtyBornhorst, T.J.,
annual meeting,
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Books, part
part 2,
2, p.
p,
fourth annual
Al-A31.
ALA3 1.
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Bornhorst, T.J., and Johnson, R.C., 1998,
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and Johnson, R.C., 1993,
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1904-P, l3p.
13p.
Ishpeming greenstone belt, Michigan: U.S.
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Bornhorst, T.J., Shepeck,
Shepeck, A.W., and Rossell, D.M., 1986,
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County, Michigan, U.S.A.-- an Archean hosted lode gold deposit: MacDonald, A.J. (ed),
(ed.),
Proceedings of Gold '86, an International Symposium on the Geology of Gold, Toronto,
p.213-227.
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Brozdowski, WA.,
R.A., 1990,
1990, Ropes pyritic gold deposit in a dilational bend,
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Card, K.D., 1990,
1990, A review of the Superior Province of the Canadian Shield, a product of
of Archean
Archean
accretion: Precambrian
56.
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v.48,p.99-1
p.99-156.
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Shield. Geoscience
Geoscience Canada,
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v.13, p.5-13.
Franklin, J.M., 1990,
massive sulphide
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deposits,inin Ho,
Ho, S.E.,
SE., Robert,
1990, Volcanic-associated massive
Robert, F.,
F., and
and
Groves D.I. (eds.), Gold and
and Base-Metal
Base-Metal Mineralization in the Abitibi
Ahitibi Subprovince,
Subprovince, Canada,
Canada,
With Emphasis
1-242.
Emphasis on
on the
the Quebec
Quebec Segment,
Segment,Short
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CourseNotes,
Notes,p.21
p.211-242.
Hammond, R.D., 1978,
1978, Geochronology and origin of Archean rocks in Marquette County, Upper
Upper
Michigan: University
of
Kansas,
Lawrence,
Kansas,
M.S.
thesis,
69
p.
University Kansas, Lawrence, Kansas, M.S. thesis, 69 p.
Hoffman, P.F., 1989,
1989, Precambrian
Precambrian geology and tectonic
tectonic history
history of North America,
America, in
in Bally,
Bally, A.W.,
A.W.,
and Palmer, A.R. (eds.),
(eds.), The geology
geology of North
North America:
America: an
an overview:
overview: Boulder,
Boulder, Colorado,
Colorado,
Geological society
society of America, The Geology
Geology of North America,
America, v.4,
v.4, p.447-512.
p.447-512.
Johnson,
R.C., 1993,
structural, tectonic,
tectonic, and
and economic
economic studies
studies of the
Archean
Johnson, R.C.,
1993, Stratigraphic,
Stratigraphic, structural,
the Archean
Ispheming greenstone belt, Marquette County, Michigan: Michigan Technological University,
University,
Houghton, Ph.D. Dissertation,
Dissertation, 119
119 p.
Johnson, R.C. and Bornhorst,
of the
the Ishpeming
Ishpeming
Bomhorst, T.J., 1991,
1991, Archean geology of the northern block of
Bulletin 1904-F, 20 p.
greenstone belt, Marquette County, Michigan: U.S. Geological Survey Bulletin
Morgan, J.P.,
J.P., and
and DeCristoforo,
DeCristoforo, D.T., 1980,
1980, Geological
Geological evolution of the
the Ishpeming
Ishpeming Greenstone
Greenstone
Belt, Michigan,
Michigan, U.S.A.:
U.S.A.: Precambrian Research,
Research, v.11,
v.11, p.23-41.
p.23-41.
Nachatilo,
Nachatilo, S.A., and Bauer, R.L., 1993,
1993, Structural
Structural analyses
analyses of Archean
Archean rocks
rocks in
in the
theNegaunee
Negaunee
area, Michigan ---- constraints
constraints on Archean versus
versus Proterozoic
Proterozoic deformation:
deformation: U.S.
U.S. Geological
Geological
Survey
Survey Bulletin
Bulletin 1904-0.
1904-0.
Puffett, W. p.,
U.S.
P., 1974,
1974, Geology of the Negaunee Quadrangle, Marquette County, Michigan: U
S.
Geol. Survey
Professional
Paper
788,
53
p.
Survey Professional Paper 788,53
Pyke, D.R., 1982,
Timmins area, District of Cochrane: Ontario Geological Survey
1982, Geology of the Tirnrnins
Report 219,
219, l4lp.
141p.
Rossell,
Rossell, D.M., 1983,
1983, Alteration of the Deer Lake peridotite in the vicinity
vicinity of the
the Ropes
Ropes mine,
mine,
Marquette County,
Michigan:
Michigan
Technological
University,
Houghton,
Michigan,
County, Michigan: Michigan Technological University, Houghton, Michigan, M.S.
M.S.
thesis,
thesis, 83
83 p.
p.
Subprovince (Archean
(Archean Gneiss
Gneiss Terrane):
Terrane): in
in Archean
Archean and
and
Sims, P.K., 1996,
1996, Minnesota River Valley Suhprovince
Proterozoic
Proterozoic Geology of the Lake Superior
Superior Region, U.S.A.,
U.S.A., 1993:
1993: U.S.
U.S. Geological
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Professional
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Paper 1556,
1556,p.
p. 14-23.
14-23.
Sims, P.K., 1991, Great Lakes
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tectonic zone
zonein
inMarquette
Marquettearea,
area,Michigan
Michigan--implications
- implications for
for
Archean tectonics in north-central United States: U.S.
US. Geological
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Bulletin1904-E, 77 p.
p.
Thurston, P.C.,
and
Chievers,
K.M.,
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Secular
variations
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sequence
development,
P.C.,
Chievers,
1990, Secular variations greenstone sequence development,
emphasizing
emphasizing Superior
Superior Province,
Province, Canada:
Canada: Precambrian Research,
Research, v.46,
v.46, p. 21-58.
21-58.
Geology and
and geochemistry
geochemistry of
of granitoid rocks in the
T.3., 1992, Geology
Wilkin, R.T.,
R.T., and Bornhorst,
Bornhorst, 713.,
Archean Northern complex,
complex, Michigan,
Michigan, U.S.A.:
U.S.A.: Can. Journal of Earth Science,
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v. 29,
29, p. 167416741685.
1685.
28
INTRODUCTION
INTRODUCTION TO THE
THE ROPES GOLD DEPOSIT
DEPOSIT IN THE
THE
ISHPEMING GREENSTONE
GREENSTONE BELT, MICHIGAN
ABSTRACTED
H PERMISSION FROM GEOLOGY OF
ABSTRACTED WIT
WITH
OF THE ROPES
ROPES GOLD
GOLD
DEPOSIT BY R.A. BROZDOWSKI
BROZDOWSKI (1988)
(1988)
Theodore J. Bornhorst and Shannon E. Bair, Department of Geological Engineering
Engineering and
and
Sciences, Michigan Technological University, Houghton, MI 49931;
4993 1; and Glenn
Glenn Scott,
Scott, Tilden
Tilden
Mining Company,
company, Ishpeming, MI 49849
Introduction
Introduction
The southwest part of the greenstone belt (Figure 1)
1) consists of,
of, from west
west to
to east:
east:
pillowed and massive basalt, hypabyssal gabbro, dacite tuff and subordinate tuff breccia,
breccia,
fine-grained peridotite
peridotite of
of the
the Deer
Deer Lake
Lake Peridotite.
Peridotite. Volcanic rocks strike
serpentinitic, fine-grained
northeast, with a major bend to the east northeast at the Bjork-Lundeen prospect, 1.5
1.5km
km west
west
of the Ropes deposit. Rocks face consistently to the southeast, based on pillows and
graded
and graded
bedding
The Ropes deposit is southeast of a transition from dominantly basalt to the
the northwest
northwest
to dominantly dacite tuft to the southeast (Figure
(Figure 2).
2). Meter-thick banded iron formation
formation and
and
several meter thick greywacke are within the dacite up section from the interlayered basalt
basalt
and dacite. Sericite
Sericiteprogressively
progressively overprints
overprints and
and finally
finally totally pesudomorphs feldspar
feldspar
phenocrysts in dacite tuff as the Ropes gold deposit is approached from the west. The
deposit is in quartz-sericite-chlorite
quartz-sericite-chloriterock which is interpreted as quartz-sericite altered
altered dacite
dacite
tuff.
tuff.
Major Rock Types within
Major
within the
the Ropes
Ropes Deposit
The four major rock types in or immediately bounding the Ropes deposit are
are (Figures
(Figures
quartz-sericite-chlorite rock which encloses ore, strikes 070'
070°
3 and 4): 1.) fine grained quartz-serieite-chlorite
overall, but 080'
080° where
where it hosts the
the Ropes deposit,
deposit, and
and dips
dips steeply
steeply south
south or vertical. This
This
rock type is bounded north and south by 2.) fine grained carbonate-quartz-chlorite rock
which is massive to compositionally layered on a scale of several millimeters. It and the
the
quartz-sericite-chlorite rock are locally complexly interlayered, particularly in the west part
quartz-sericite-chlorite
of the deposit, and have
have generally
generally sharp
sharp contacts.
contacts. Carbonate-quartz-chlorite rock is flanked
successively, on both the north and south sides by, 3.) fine grained, massive to moderately
rock, and 4.)
4.) fine
foliated, carbonate-talc rock,
fine grained,
grained, serpentinitic peridotite which commonly
original olivine
olivine and
and lesser
lesserpyroxene.
pyroxene. This is variably
has pesudomorph texture after original
with carbonate-talc
carbonate-talc rock.
rock. Contacts between carbonatecarbonate-bearing near its contacts with
quartz-chlorite rock and carbonate-talc rock and between the carbonate-talc rock and the
serpentinitic peridotite are generally gradational over one to several meters.
Quartz-Sericite-Chlorite
Quartz-Sericite-ChloriteRock
Rock
The main host for gold is light green, massive to slightly foliated, fine grained,
quartz-sericite-chlorite
thick on the 800
quartz-sericite-chlorite rock. This
This rock
rock is
is aa 080°-striking
080'-striking layer up to 40 m thick
mine level,
but
narrows
toward
the
surface
and
toward
the
east
to
less
than
6 m thick. It is
is
level.
29
0
km
2
EXPLANATION
EXPLANATION
PROTEROZOIC
PROTEROZOIC
1 graywacke and slate
2 quartalte
ARCH LAN
3 serpentinillc peridotila
4 volcanIc conglomerate, subordinate daclte tuti
5 banded Iron lorniatton
6 daclle tutt, lull breccta and ltows
7 tonallIe and granodlorlte
a gabbro
9 basalt
10 gtornerophyrlc basalt
Figure1:1:Geologic
Geologicsetting
setting
theRopes
Ropesmine
mineininthe
thesouthwest
southwestblock
blockofofthe
theIshpeming
Ishpeming
Figure
ofofthe
greenstone
greenstonebelt.
belt.
30
,
EXPLANI4
TION
EXPLANA JION
0
0
0
ii
GRAYWACKE
GRAYWACKE
10
QUARTZITE
QUARTZITE
w
F-
0
a-
SERPENTINITIC
PERIDOTITE
LJ SERPENTINI11C
PERIDOTITE
EIJ CARBONATE
RICH,
SERPENnNITIC
CARBONATE
RICH,
SERPENTINITICPERIDOTITE
PERIDOTITE
1
a
-
CARBONATE TALCROCK
ROCK
ED CARBONATE-TALC
COMPOSITIONALLY
LAYERED TO MASSIVE
COMPOSKIONALLY LAYERED
MASSIVE
CARBONATE
QUARTZ - CHLORITE
CHLORITE ROCK
ROCK
CARBONATE- QUARTZ
-
-
TUFF, TUFF
TUFF BRECCIA,
BRECCIA, AND FLOWS;
FLOWS;
DACITE TIJFF,
(ALTERED TO QUARTZQUARTZ-SERICITESERICITE- CHLORITE
CHLORITE
ROCK IN
INROPES
ROPESDEPOSE!)
DEPOSIn
S
"
=
-
ED BANDED
BANDEDQUARTZ
QUARTZ - MAGNETrrE
MAGNETITE IRON FORMATION
FORMATION
GRAYWACKE
ANDSILTSTONE
SILTSTONE
ED GRAYWACKE
AND
E1J FINE
GRAINED
FINE
GRAINEDGABBRO
GABBRO
1'
L
S
BASALT (INCLUDES
(INCLUDES PILLOWED
PILLOWED AND
GLOMEROPHYRIC VARIETIES)
VARIETIES)
PROJECTION TO
SURFACE OF
OF OREBODIES
OREBODIES
UPDIP PROJECTION
TO SURFACE
A = main
main ore
ore zone
zone
northwest ore
B = northwest
ore zone
zone
Figure 2: Surface
Surfacegeologic
geologic map
mapof
o f the
the Ropes
Ropes mine
m i n e and vicinity.
31
1
300 level
1152 level
200 Ft
CI
I
61 M
I
N
200E
400E
6006
BODE
I000E
12006
EXPLANATION
EXPLANATION
I
i
I
I
2
I
i
3
i
a
[5
a
Quartz-Sericite
SerpentiniticFeridotite
Peridotite L41 Quartz-Seridte
chlorite Rock -.
Serpentinitic
Chlorite
Rock
locally carbonate
carbonate rich
rich
locally
Carbonate-Talc
Carbonate-Talc Rock
Rock
Carbonate-Quartz -Carbonate-Quadz
Chlorite Rock
Rock
Chlorite
I
Orebody
Orebody
>2
2
, g /tonne
,tonne Au
Au
Figure 3: Geologic
Geologicplan
planmap
mapof
ofthe
the300
300and
and1152
1152levels
levels of
of the
the Ropes
Ropes mine.
mine.
Figure
32
ROPES
R O P E S MINE
MINE
600 E
E Cross-Section
Cross-Section
Looking
LookingS80°W
S80°
i
Elevation
Elevation(ft)
(
1
Serpentinitic Peridotite
Peridotite
Serpentinitic
locallycarbonate
carbonaterich
rich
locally
I
a
121 Carbonate-Talc
Carbonate-TalcRock
Rock
1400
1401
I
1200
1201
Carbonate-Quaflz -Carbonate-Quartz
Chlorite Rock
Rock
Chlorite
3
4
i
Quartz-Sericite
Quartz-Sericite ChloriteRock
Rock
Chlorite
Orebody
151 Orebody
gltonneAu
Au
>>22g/tonne
1000
1001
C
0mine
minelevels
levels
800
80C
Scale
0
600
600
100
200ff
61 m
100
400
40C
200 ft
61 m
200
200
Sea
SeaLevel
Level
-200
-200
Figure
Figure4:4:Geologic
Geologiccross
crosssection
sectionofofthe
theRopes
Ropesmine
minelooking
loohngS80°W
S80° along
alongthe
the600E
600Esection.
section,
33
complexly interlayered with carbonate-quartz-chlorite rock, especially west of the Ropes
main ore zone and in the mine. Randomly
Randomlyoriented
orientedto
tomoderately
moderately aligned,
aligned, 11to
to22 mm,
mm,
angular, rectangular mats of felted sericite up to 2 mm in length, comprised of individual
sericite grains less than 10
10 microns in size, and minor quartz, are enveloped in this matrix
and have an external habit identical to plagioclase
In dacite
dacite tuff
tuff immediately
immediately
plagioclasephenocryst.
phenocryst. In
west of the deposit there are plagioclase phenocrysts. The
Therectangular
rectangular mats
matsare
areinterpreted
interpreted as
as
pseudomorphs after the plagioclase phenocrysts.
phenocrysts. The rock is layered locally with 5 mm thick,
lens-like, sericite-rich
sericite-rich and
and alternating
alternating more
more chlorite-rich
chlorite-rich laminae.
laminae.
Sericite
quartz-sericite-chloriterock
rock decreases
decreases
Sericite in
in relict
relict feldspar
feldspar and
and lithic
lithic clasts
clasts in
in the
the quartz-sericite-chlorite
gradationally westward from the Ropes main ore zone over a distance of
of several hundred
meters into aa large
large body of
of dacite
dacite tuff
tuff that
that has
has moderately
moderately aligned
aligned twinned
twinned plagioclase
plagioclase
phenocrysts
phenocrysts with
with only
only very
very minor
minor sericite
sericite internal to the phenocrysts, bipyramidal quartz, and
volcanic
volcanic rock
rock fragments
fragments in
in an
an aphanitic
aphanitic quartz-sericite feldspar-chlorite matrix. The
Thedacite
dacite
tuff
outside
of
the
Ropes
deposit,
proper,
has
local
lapilli-sized
fragments
as
well
as
tuff
tuff outside of the Ropes
breccia
breccia layers.
layers.
Gold abundance
abundance is generally greatest where very fine to aphanitic pyrite is 5 to 8%
8% of
the
the rock,
rock, quartz
quartz and
and sericite
sericite are
are most
most abundant, and chlorite is least abundant. However,
However,this
this
generality
generality does
does not
not apply
apply to
to the
the chioritic,
chloritic, pyritic,
pyritic, eastern
eastern most
most rim
rim of
of the
the Ropes
Ropesmain
mainore
ore
zone.
zone. Anastomosing
Anastomosingquartz
quartzveinlets
veinletsless
lessthan
than11mm
mmthick
thickmake
makeup
upseveral
severalpercent
percentof
ofthe
the
rock.
rock, Rock
Rockoutside
outsideofofthe
theore
orezones
zonescontains
containsless
lessthan
than2%
2%very
very fine
finepyrite.
pyrite.
The
The overall
overall 070°
quartz-sericite-chlorite rock
rock which
which hosts
hosts the
theRopes
Ropes
070' striking
striking trend
trend of
of quartz-sericite-chlorite
main
main and
and northwest
northwest ore
ore zones
zones interfingers
interfingers with
with carbonate-quartz
carbonate-quartz chlorite
chloriterock
rockwest
westof
ofthe
the
deposit,
and
does
not
continue
west
of
the
northwest
ore
zone
(Figure
2).
East
of
the
deposit,
deposit, and does not continue west of the northwest ore zone (Figure 2). East of the deposit,
the
the trend
trend of
of quartz-sericite-chlorite
quartz-sericite-chlorite rock does not continue at surface beyond 300 m east of
the
the Ropes
Ropes main
main ore
ore zone.
zone. A
A zone
zone of
of carbonate-talc
carbonate-talc rock
rock within
within the
the serpentinitic
serpentinitic peridotite
peridotite
marks
marks the
the east-northeast
east-northeastprojection
projection of
ofthe
thetrend.
trend.
Carbonate-Quartz-Chlorite
Rock
Carbonate-Quartz-Chlorite
Rock
Carbonate-quartz-chlorite
Carbonate-quartz-chloriterock
rock envelopes
envelopes the
the quartz-sericite-chlorite
quartz-sericite-chloriterock
rockand
andisis
complexly
complexly interlayered
interlayered with
with it,
it, particularly
particularly toward
toward the
the west
west end
end of
of the
the Ropes
Ropesdeposit.
aeposit.
Carbonate-quartz-chlorite rock
rock isis restricted
restricted largely
largely to
to the
the deposit
deposit although
although layers
layersup
uptoto22mm
Carbonate-quartz-chlorite
thick
thick occur
occur up
up to
to 400
400 m
m west of the Ropes main ore zone. The
The thickest
thickest layers
layersof
of carbonatecarbonatetalc
talc rock
rock are
are immediately
immediately north
north and
and south
south of
of the
the deposit;
deposit; however,
however, thin
thin layers
layersof
oftalc-rich
talc-rich
rock
rock are
are at
at numerous
numerous localities
localities atat the
the contacts
contacts of,
of, and
and also
also within
within serpentinitic
serpentiniticperidotite
peridotite
along
along the
the northwest
northwest contact
contact of
of the
the peridotite
peridotite up
up to
to 500
500 m
m away
away from
from the
the deposit.
deposit.
Carbonate-quartz-chlorite
Carbonate-quartz-chlorite rock
rock contains
contains abundant
abundant ferroan
ferroan dolomite,
dolomite, and
andlesser
lesserquartz
quartz
and
and chlorite.
chlorite. Minor
Minorsericite
sericiteisisininsome
someparts
partsand
andvariable
variable amounts
amounts of
of talc
talc in
inothers,
others,but
butthese
these
two
quartz-sericite-chloriterock
rock
two minerals
minerals are
are generally
generally mutually exclusive. Contacts
Contactswith
with quartz-sericite-chlorite
are
are sharp
sharp in
in aa direction
direction across
acrossthe
the strike
strikeof
of the
the rock
rock type,
type, but
but are
arelocally
locallygradational
gradationalalong
along
strike.
strike. Barren
Barrenmilky
milkyquartz
quartzveins
veinsup
uptoto10
10cm
cmthick
thickoccur
occurlocally
locallyand
andare
areconformable
conformabletotothe
the
contact.
contact. Minor
Minorcarbonate
carbonateminerals
mineralsare
arepresent
presentininthe
thequartzquartz-sericite-chlorite
sericite-chloriterock
rocknear
nearits
its
contacts
contacts with
with the
the carbonate-quartz-chlorite
carbonate-quartz-chloriterock,
rock, and
and minor
minor sericite
sericiteisispresent
presentininthe
the
carbonate-quartz-chlorite
carbonate-quartz-chlorite rock
rock near
near its
itscontacts
contactswith
with quartz-sericitequartz-sericite-chlorite
chloriterock.
rock.
Carbonate-quartz-chlorite rock
rock has
has up
up to
to150
150ppb
ppb Au,
Au, and
and quartz
quartzrich
richparts
partsof
ofthe
thecarbonatecarbonateCarbonate-quartz-chlorite
34
quartz-chlorite rock only very locally contain several percent pyrite and up to 11 ppm Au.
Locally the carbonate-quartz-chlorite rock is brecciated, with angular fragments of carbonate
carbonate
veined carbonate-quartz-chlorite rock in a chlorite rich matrix particularly at the south
south side
side of
of
the deposit.
Carbonate-quartz-chlorite rock is generally compositionally layered on a scale of
several millimeters, but
but locally
locally is
is massive.
massive. Layering is defined by
hy fine to medium grained
white cxbonate
carbonate lenses, chlorite foliation,
foliation, and
and quartz-chlorite
quartz-chloriterich
richlaminae.
laminae. The rock is
white
increasingly talcose toward the contact with carbonate-talc rock. Coarse grained, barren,
white dolomite veins up to 5 cm thick cut across all other features and are several percent of
rock. Carbonatethe rock.
Carbonate- quartz-chlorite
quartz-chlorite rock
rock is
is not only within the deposit but also in a zone up
to 60 m thick on its north side, where it contains thin layers of the other three main rock
types. Carbonate-quartz-chlorite
types.
Carbonate-quartz-chlorite rock
rock is
is in local lenses along the northeast striking contact of
and northwest
northwest of
of the
the Ropes
Ropesdeposit.
deposit. The carbonatethe Deer Lake Peridotite, southwest and
quartz-chlorite rock is interpreted as an alteration product of a serpentinitic peridotite
protolith.
Serpentinitic Peridotite and Carbonate-Talc
Serpenrinitic
Carbonate-Talc Rock
Rock
quartz-sericite-chlorite rock and carbonate-quartz-chlorite rocks are
The interlayered quartz-sericite-ihlorite
are
bounded on north and south by elongate masses of serpentinitic peridotite. There are also
several smaller serpentinitic peridotite bodies enclosed within the carbonate-quartz-chlorite
of the Ropes
Ropes deposit.
deposit. The largest of these bodies is the north wall of
rock on the north side of
the Ropes deposit. The
The rock
rock is
is dark
dark gray
gray to green,
green, mostly fine grained, with major
serpentine, subordinate talc and carbonate,
carhonate, minor chlorite, and accessory chromite and
magnetite. The
The serpentine
serpentine isis aa mixture
mixture of
of felted
felted chrysotile and oriented plates of antigorite.
Commonly it has a relict texture of 11 to 3 mm serpentine pseudomorphs after major olivine
pyroxene. The
and subordinate pyroxene.
The pseudomorphs
pseudomorphs are surrounded by rims of talc, carbonate
and accessory
accessory fine-grained
fine-grained magnetite
magnetiteand
andchromite.
chromite. Locally the
the rock
rock is
is dark green,
minerals and
felted textured serpentine which
which lacks
lacks aa pseudomorphic
pseudomorphic texture.
texture. The serpentinitic peridotite
is increasingly carbonate-rich toward its borders, although the relict texture is commonly
preserved. Fibrous
continuously preserved.
Fibrous chrysotile
chrysotile veinlets in the serpentinite are locally
pseudomorphed by carbonate. Au abundance is typically less than 30 ppb, even in close
proximity to the deposit.
deposit.
Massive to well foliated, gray to dark green, very fine grained carbonate-talc rock
margins of
of serpentinitic
serpentinitic peridotite.
peridotite. Chlorite, serpentine, magnetite and
occurs around the margins
pyrite are minor phases.
phases. The
carbonate-talc
The carbonate-talc rock has gradational contacts over one to several
meters with serpentinitic peridotite and with talc-rich parts of the carbonate-quartz-chlorite
subequal. The carbonate
rock. Talc
Talc and
and carbonate
carbonate contents
contents are generally
generally subequal.
carhonate mineral in the
ferroan dolomite
dolomite with
with minor
minormagnesite.
magnesite. Au abundance is
carbonate-talc rock is dominantly ferroan
from less than 30 ppb to approximately 100
100 ppb locally in the carbonate-talc rock.
The Gold Ore
g/ton Au, including production and
The Ropes deposit is 2.8 million tonnes with 3.24 glton
reserves. There
Ninety five percent
percent of the ore
all categories of reserves.
There are
are two
two main types of ore: 1.)
1.) Ninety
dispersed pyrite
pyrite in
in quartz-sericite-chlorite
quartz-sericite-chloriterock.
rock. This ore has several subtypes
is gold with dispersed
proportions of
of quartz,
quartz, sericite,
sericite, chlorite,
chlorite, and
andpyrite.
pyrite. 2.) Five percent of the ore is
with varying proportions
35
auriferous quartz veins with saccharoidal
saccharoidal texture and tetrahedrite, pyrite, galena,
galena, and
and
chalcopyrite. ItItisisgenerally
generallyatator
ornear
nearthe
thesouth
southside
sideof
of the
the deposit.
deposit.
Dispersed Pyrite in Quartz-Sericite-Chlorite
Gold With Dispersed
Quartz-Sericite-ChloriteRock
Rock
The Ropes main ore zone is steeply dipping, 335 m in maximum strike length, 12
12 m
in average thickness, and 600 m in known down dip extent (Figures 4). It is within the
quartz-sericite-chlorite rock, and gold is associated
associated with dispersed pyrite with only minor
quartz veins.
veins.
The Ropes main ore zone can be divided longitudinally into three ore subtypes
section. Subtype
(Figure 5). These
These subtypes
subtypes are,
are, from west to east, in long section.
Subtype 11 is
is light
light gray
gray to
to
pale green, siliceous quartz-sericite rock, containing minor lenses of
of light gray cherty quartz
rock with pyrite, tetrahedrite, chalcopyrite and galena.
galena. Pyrite
Pyrite abundance
abundance does
does not
not correlate
correlate
with gold grade. Subtype
quartz-sericite-chlorite
Subtype11has
hasan
aninterdigitating
interdigitatingcontact
contact with
with barren
barren quartz-sericite-chlorite
and carbonate-quartz-chlorite rock to the west, and a gradational contact to the east with
is 6.8
6.8 glton.
g/ton. Subtype
subtype 2. The average gold concentration is
Subtype 2 is
is light green
green quartzquartzsericite-chlorite-pyrite rock with a positive correlation between fine-grained pyrite
pyrite content
content
and Au abundance. Subtype 2 is the thickest part of the ore zone and, therefore,
therefore, the most
volumetrically important. ItIt has
with subtype 3. The
has aa relatively
relatively sharp contact to the east with
The
average Au concentration ranges from 2.6 glton,
g/ton, rarely up to 6.2 glton.
g/ton. Subtype
dark
Subtype33isisdark
green quartz-chlorite-pyrite
consistently large fine-grained
fine-grained pyrite
pyrite content. ItIt has
quartz-chlorite-pyrite rock with a consistently
has
barren quartz-sericite-chlorite rock
rock to
to the
the east.
east. The
a sharp contact with barren
The average
average Au
Au
concentration is
is 10.3
10.3g/ton.
glton.
Zones of greater than 4 g/ton
glton Au abundance, defined by assay data, are
are contained
contained
centrally within the Ropes main ore zone. The
middle
parts
of
these
greater
than
The middle parts of these greater than 44 g/ton
glton Au
Au
zones strike
echelon fashion at approximately
strike across the 080°
080' trend of the orebody in en echelon
055°,
055', whereas their extremities have a nearly 070°
070' strike, thus forming
forming a low
low angle
angle sigmoidal
sigmoidal
pattern across the ore
ore zone.
zone. This
This pattern
pattern is
is most
most pronounced
pronounced at
at and
and below
below the
the 1152
1152level
level
toward the east end of
of the
the Ropes
Ropes main
main ore
ore zone
zone (Figure
(Figure 6).
6).
Above the 650 level, ore is separated from carbonate-quartz-chlorite rock on
on the
the north
north
by slightly auriferous quartz-sericite-chlorite rock, but
but below
below the
the 650 level
level the east end of the
ore is in contact with carbonate-quartz-chlorite
carbonate-quartz-chlorite rock (Figure 3).
3). The north side
side of
of the
the ore
ore
strikes 070°
070" and dips very steeply north whereas the contact between quartz-sericite-chlorite
quartz-sericite-chlorite
rock and the carbonate-quartz-chlorite rock strikes 080°
080Âwithin the deposit and dips
dips steeply
steeply
south. Therefore,
Therefore, atatdepth,
depth, the
the ore
ore abuts
abuts the
the carbonate-quartz-chlorite
carbonate-quartz-chlorite rock contact
contact on
on the
the
north, with the east limit of the Ropes main ore zone plunging to the west at
at deeper
deeper levels
levels
along the line defined by the intersection of the planes of the orebody and the
the carbonatecarbonatequartz-chlorite rock contact on the north. Below
Below the
the 1284
1284 level, the 25
25 to 40 m
m thick quartzquartzsericite-chlorite rock thins markedly to the east and interfingers along strike
strike to
to the
the east
east with
with
quartz-sericite-chlorite rock
rock plunges
plunges
carbonate-quartz-chlorite rock. The
The thickest
thickest part
part of
of the
the quartz-sericite-chlorite
approximately 45°
45' east and is coincident with the bend in strike of the trend of quartz-sericite-chlorite rock from 080°
070° east of the main ore zone.
080' within the deposit back to 070'
zone.
There are several other gold bearing bodies
bodies near the
the main
main ore
ore zone
zone (Figure
(Figure 5). The
The
northwest ore zone is a small ore zone of 101,000 tons
tons with
with 4.6
4.6 glton
g/ton Au,
Au, 170 m
m northwest
northwest of
of
the Ropes main ore zone, and is dominantly ore subtype 2.
2. Auriferous,
Auriferous, pyritic
pyritic quartzquartz36
_____
_____
long section
section of
of the
the Ropes
Ropes deposit in
in the
the
Vertical long
N80° plane, with gold
gold ore
ore subtypes:
subtypes:
N80°E
1
1
2
3
-
QUARTZ SERICITE
SERICITE ROCK,
ROCK,
QUARTZWITH
WITH GRAY
GRAY QUARTZ
QUARTZ LENSES
LENSES
QUARTZ
SERICITE -- CHLORITE
CHLORITE ROCK,
ROCK,
QUARTZ -- SERICITE
GOOD Au
AU :: PYRITE CORRELATION
CORRELATION
-
QUARTZ
CHLORITE - PYRITE
PYRITE ROCK
ROCK
QUARTZ -- CHLORITE
FigureS:
Figure 5 : Vertical
Vertical long
long section
section of
of the
the Ropes
Ropes mine
mine in
in the
the N80°E
N8O0E showing gold
gold ore
ore subtypes.
subtypes.
37
37
0
50
meters
meters
ROPESMINE
MINE 1152
1152 LEVEL
LEVELPLAN
PLAN
ROPES
tonne Au
Au ore
orezones
zones
>> 44 gg I1tonne
I
0quartz
quartz-- sericite
sericite-- chlorite
chloriterock
rock
1
Figure6:6:Geologic
Geologicplan
planmap
mapofofthe
the1152
1152level
levelofofthe
theRopes
Ropesmine
mineshowing
showinghigh
highgrade
gradeore
orezones.
zones
Figure
38
Ropes
Ropes Mine
Mine
Level
1620 Level
Geologic Plan
Geologic
Plan
IN
ul
m
C
L,
+
cu
"1"
50
50
m
EXPLANATION
EXPLANATION
serpentinitic peridotite
a serpenthitic
peridotite
C >2g/tonneAu
0
> 2 g I tonne Au
carbonate - talc rock
C] >4g/tonneAu
0
> 4 g I tonne Au
a carbonate - quartz - chlorite rock
rock
-
A,,
Au
LIII quartz
rock
quartz -- sericite -- chlorite rock
auriferous quartz vein
(Lower Secondary
Secondary Drift
Drift Vein)
Vein)
LI microcrystalline
a
rnicrocrystallinecarbonate
carbonate - quartz rock
rock
Figure 7: Geologic
Geologic plan
plan map
map of
of the
the 1620
1620 level of the Ropes mine.
mine
39
south
south of
of the
the central
central part
part of
of the
the main
main ore
ore zone.
zone,
Non-Aunferous
Non-Aur$erous Veins
Veins
Milky white,
white, vitreous,
vitreous, medium to coarse-grained quartz veins commonly cut the east
northeast foliation in the quartz-sericite-chlorite
quartz-sericite-chlorite rock at a high angle. Some
Somedip
dipsteeply,
steeply,
whereas others are shallowly dipping veins offset in north side up stepwise fashion along
foliation parallel structures. The
The veins
veins are
are not
not auriferous
auriferous and are commonly up to 10cm
10 cm
thick and several meters in length. They are approximately 1% of the quartz-sericite-chlorite
quartz-sericite-chlorite
rock.
rock. Quartz
Quartzininthe
theveins
veinshas
hasfull
fullextinction
extinctionunder
undercrossed
crossedpolars,
polars,simple
simplegrain
grainboundaries,
boundaries,
and locally
locally has
has minor
minor pyrite.
pyrite.
Banded dolomite veins commonly strike 015'
015° and dip nearly vertical.
vertical. They cut the
auriferous quartz veins. Massive, planar sided, coarse crystalline white
white dolomite veins up to
10
10 cm thick
thick cut
cut all
all other
other veins
veins and
andare
areundeformed.
undefonned.
Structure
Structure
An 065°
065' striking
striking near vertical foliation is defined by planar alignment of sericite and
chlorite
chlorite in
in the
the quartz-sericite-chlorite
quartz-sericite-chlorite rock. This
Thisfoliation
foliationisisinterpreted
interpreted as
astectonic
tectonic
schistosity
schistosity or
or SS fabric
fabricbecause
because the
the sericite
sericite and
and chlorite
chloriteare
aresecondary
secondaryminerals
mineralsthat
thatcut
cutand
and
locally
locally displace
displace relict
relict primary
primary magmatic features, such as domains of former feldspar
phenocrysts,
phenocrysts, and
and lithic
lithic fragments.
fragments. Locally
LocallyS-C
S-Cfabric
fabricisisdeveloped,
developed, especially
especiallyin
incarbonatecarbonatetalc
talc and
and carbonate-quartz-chlorite
carbonate-quartz-chlorite rocks.
rocks.
Contacts
Contacts between
between the
the quartz-sericite-chlorite
quartz-sericite-chlorite rock
rock and
and rocks
rocks derived
derived from
fromaaperidotite
peridotite
protolith
protolith defined
defined planes
planes of
of weakness
weakness that favored development of a shear zone. The
The
orientation
orientation of
of reactivated
reactivated zones
zones of
of weakness such as contacts between rock types are not
strictly
strictly governed
governed by
by the
the exact
exact orientation
orientation of
of the
the stress
stress that
that causes
causesthe
the movement.
movement.
Ore
Ore Related
Related Structure
Structure
The
The Ropes
Ropes main
main ore
orezone
zone trends
trends 080°
080' overall,
overall, but
but is
is internally
internally comprised
comprised of
of 055°
055' to
to
065°
065O trending
trending zones
zones of
of greater
greater Au concentration,
concentration, which host most of the gold. These
Theseoccur
occur
in
glton Au
Au concentration
concentrationwithin
within ore
ore
in low
low angle
angle sigmoidal
sigmoidalzones
zones of
of greater
greater than
than 44 g/ton
characterized
characterized by
by dispersed
dispersedpyrite
pyrite or
or in
in auriferous
auriferous quartz-tetrahedrite
quartz-tetrahedrite oblique
obliqueshear
shearveins
veinswith
with
similar
similar geometry
geometryand
andorientation.
orientation.
Zones
Zones with
withgreater
greaterthan
than44g/ton
@on gold
goldconcentration1
concentration, defined
defined by
by assay
assaydata,
data,are
aremore
more
centrally
centrally within
within the
the Ropes
Ropes main
main ore zone (Figures 6 and 7). The
Themiddle
middle parts
partsof
ofthese
thesehigher
higher
grade
grade zones
zones strike
strikeen
en echelon
echelon across
acrossthe
the overall
overall 080°
080" trend
trend of
of the
the ore
orebody
bodyatatapproximately
approximately
055" whereas their extremities
extremities nearer
nearer the contacts
contacts with the
the carbonate-quartz-chlorite
carbonate-quartz-chlorite rock
rock
0550
strike
strike approximately
approximately070°,
070°forming
formingaalow
low angle
angle sigmoidal
sigmoidal pattern
pattern across
acrossthe
thezone,
zone,which
whichisis
most
most prominent
prominent on
onthe
theeast
eastend
endof
ofthe
the1152
1152level
level (Figure
(Figure6).
6).This
Thisgeometry
geometryisisconsistent
consistentwith
with
greater movement
movementbeing
beingaccommodated
accommodated near
nearthe
themargin
margin of
ofmore
moreductile
ductiledeforming
deforming
greater
carbonate-quartz-chlorite
carbonate-quartz-chloriterock,
rock,where
wherethe
theoriginally
originally formed
formed 055°
055' to
to065°
065' schistosity
schistositywas
was
flattened
080' plane
plane of
of shear,
shear, represented
represented by
by the
the contact
contact between
between contrasting
contrastingrock
rock
flattened to
tothe
thelocal
local080°
types.
types.
40
sericite-chiorite
sericite-chlorite rock
rock was
was encountered
encountered in
in deep
deep drilling
drilling at
at the
the Ropes
Ropes deposit,
deposit, in
in aa zone
zone
separate
separate from,
from, and
and deeper
deeper to
to the
the east
east of,
of, the
the Ropes
Ropes main
main ore
ore zone.
zone.
Ore
Ore Minerals
Minerals and
and Textures
Textures and
and Metal
Metal Distribution
Distribution
Gold is with pyrite
pyrite less
less than
than 100
100 microns
microns in
in diameter
diameter dispersed
dispersed throughout
throughout the
the mass
mass of
of the
the
quartz-sericite-chlorite
quartz-sericite-chloriterock,
rock, and
and with
with this
this pyrite
pyrite on
on fractures,
fractures, foliations,
foliations,and
and within
within and
and
along the margins of millimeter-thick quartz veinlets.
veinlets. Native
Native gold,
gold, of
of variable
variable fineness,
fineness, isis 11
to
to 10
10micron
micron grains
grains isis attached
attached to
to the
the surface
surface of
of the
the fine
fine grained
grained pyrite,
pyrite, included
included as
asround
round
blebs
hlebs within
within fine
fine grained
grained pyrite,
pyrite, on
on fractures
fractures within
within fine
fine grained
grained pyrite,
pyrite, and
andatatgrain
grain
boundaries
boundaries of
of fine
fine grained
grained quartz
quartz and
and sericite. Minor
Minorcoarse
coarsegold
goldisison
onfractures
fracturesin
inquartz
quartz
veins.
veins. Silver
Silver isis in
in electrum,
electrum, native
native silver
silver isis with
with fine
fine pyrite,
pyrite, argentiferous
argentiferous tetrahedrite,
tetrahedrite,
argentiferous
argentiferousgalena,
galena,and
andrare
raredyscrasite.
dyscrasite.
Pyrite
Argentiferous galena,
galena,
Pyrite is
is 97%
97% of
of the
the metallic
metallic minerals followed by 1% chalcopyrite. Argentiferous
argentiferous
tetrahedrite,
tetrahedrite,
sphalerite,
millerite,
bravoite,
magnetite,
and
argentiferous tetrahedrite, tetrahedrite, sphalerite, millerite, bravoite, magnetite, and rutile
rutile are
are
present
present in
in trace
trace amounts.
amounts. The
The gold
gold is
is silver
silver bearing and metallic silver is gold bearing.
bearing, The
The
low
low bulk
bulk Au/Ag
AulAg of
of 0.65
0.65 for
for the
the Ropes
Ropes gold
gold deposit
deposit differs
differs from
from ratios
ratios reported
reported for
for
Precambrian
Precambrian gold-quartz
gold-quartz veins
veins and
and lodes
lodes (Boyle,
(Boyle, 1979),
19791,which
which range
range from
from1.37
1.37to
to12.5
12.5and
and
average
average about
about 4.2.
4.2. Both
BothAu
Auand
andAg
Aghave
havecontinuous
continuouspositively
positivelyskewed
skewed log
log normal
normal
distributions,
distributions, each
each with
with aa single
singlemaximum,
maximum, interpreted
interpretedasasindicating
indicatingonly
onlyone
oneperiod
periodof
of
mineralization.
mineralization.
Distribution.
Distribution.Geometry
Geometryand
andAu
Au Concentration
ConcentrationofofAur?ferous
Auriferous Quartz
QuartzVeins
Veins
Auriferous
Auriferous quartz
quartz veins
veins are
are concentrated
concentrated at the south side of the deposit. Individual
Individual
veins
average
20
cm
thick
and
extend
12
m
horizontally
and
15
m
vertically,
but
collectively
veins average 20 cm thick and extend 12 m horizontally and 15 m vertically, but collectively
these
these veins
veins are
aregrouped
groupeden
enechelon,
echelon,and
andlocally
locally they
they form
formore
oreshoots
shoots88to
to11
11mmthick,
thick,with
withthe
the
quartz
quartz-sericite-chlorite rock. The
Theveins
veinsstrike
strikeen
en
quartz veins
veins separated
separated by
by auriferous
auriferous foliated
foliated quartz-sericite-chlorite
echelon
echelonatat055°
055' to
to065°
065' across
acrossthe
the080°
080' trend
trend of
of the
the quartz-sericite-chlorite
quartz-sericite-chloriterock
rocktype,
type,dip
dip
vertically
geometry. They
vertically to
to 85°
85' south,
south, and
and have a slight sigmoidal geometry.
They occur
occur progressively
progressively
further
further east
east in
in successively
successively deeper
deeper mine
mine levels. Eight
Eightmajor
majorshoots
shootsmade
madeup
upof
of these
these veins
veins
were
were mined
mined in
in the
the 1800's
1800'sand
and 1890's
1890's above
above the 800 level. Broderick
Broderick (1945)
(1945)implied
implied that
that
single
single large
largequartz
quartz veins
veinsranged
ranged up
up to
to11
11m
m thick,
thick, 60
60m
m in
in horizontal
horizontal extent,
extent,and
and76
76m
m
vertical
vertical extent,
extent,but
but examination
examinationof
of the
the old
old mine
mine workings
workings concurrent
concurrent with
with development
developmentof
ofthe
the
modern
modern Ropes
Ropes mine,
mine, and
anddetailed
detailedhistoric
historic descriptions
descriptions indicate
indicate the
the old
old stopes
stopeswere
were ore
oreshoots
shoots
made
made up
upof
of aaseries
seriesof
ofsmaller
smallerindividual
individualquartz
quartz veins
veins and
and intervening
interveningauriferous
auriferousfoliated
foliated
quartz-sericite-chlorite
quartz-sericite-chlorite rock.
rock. The
Theveins
veinsare
aresaccharoidal
saccharoidalwhite
white to
tolight
lightgray
grayquartz,
quartz,with
with
pyrite
pyrite and
andminor
minorargentiferous
argentiferoustetrahedrite,
tetrahedrite, galena,
galena, and
and chalcopyrite;
chalcopyrite;and
and rare
rare free
freegold,
gold,
molybdenite,
molybdenite, dyscrasite,
dyscrasite, tourmaline,
tourmaline, and
and native silver. Overall,
Overall,metal
metal concentrations
concentrations in
inthe
the
veins
veins range
rangefrom
from20
20to
to75
75g/ton
gltonAu
Au with
with Au/Ag
AulAgratio
ratio from
from 0.08
0.08 to
to 1.76.
1.76.
Thequartz
quartzveins
veinshave
havegreater
greaterconcentrations
concentrationsof
ofgold
goldthan
thanmost
mostof
ofthe
thegold
goldwith
withdispersed
dispersed
The
quartz-sericite-chloriterock,
rock,but
butthe
thezones
zonesof
ofgreater
greaterthan
than44g/tonne
gltonneAu
Au
pyriteininquartz-sericite-chlorite
pyrite
concentrationin
inthe
thequartz-sericite-chlorite
quartz-sericite-chloriterock
rock are
are not
not generally
generally zoned
zoned about
about the
theveins,
veins,
concentration
rather,they
they occur
occurmore
morecentrally
centrallywithin
within the
the quartz-sericite-chlorite
quartz-sericite-chloriterock
rock north
northof
ofthe
theveins.
veins.In
In
rather,
some
instances,
Au
concentrations
of
greater
than
4
glton
in
the
gold
with
dispersed
pyrite
in
some instances, Au concentrations of greater than 4 g/ton in the gold with dispersed pyrite in
quartz-sericite- chlorite
chloriterock
rockextend
extendalong
alongthe
the same
same055°
055' to
to 065°
065' trend
trend eastward
eastwardfrom
fromthe
the
quartz-sericitewest end
endof
ofold
oldstopes
stopeswhich
which mark
markconcentrations
concentrations of
of quartz
quartz veins,
veins,particularly
particularlyfor
forold
oldstopes
stopes
west
41
westward plunging
plunging auriferous
auriferous quartz-tetrahedrite
quartz-tetrahedrite veins
Vertically dipping, steeply westward
occur progressively further east in successively deeper levels of the mine. These banded,
auriferous, tetrahedrite-bearing
tetrahedrite-bearing quartz veins with aligned schistose inclusions of sericitequartz-chlorite wall rock are at or near the south side of the main
main ore zone and have an
orientation the same as the zones of schistosity parallel gold concentration with dispersed
pyrite, which are more centrally within
within the
the trend
trend of
of quartz-sericite-chlorite
quartz-sericite-chloriterock.
rock. The veins
are oriented nearly parallel to the schistosity, and therefore
therefore nearly
nearly perpendicular to the
inferred maximum principal
principal stress. They
They contain schistose septa identical in composition to
the quartz-sericite-chlorlte wall rock and have quartz with undulose
undulose extinction and subgrain
development textures indicating grain size reduction.
reduction. These characteristics are compatible
with an origin as oblique shear veins formed in the direction of
of shear movement
movement (Hodgson,
1989). Oblique
shears
Oblique shear
shear veins
veins are
are inclined
inclined to
to shear
shear zone
zone margins
margins in the same
same way as
as PP shears
and show the same sense of en echelon stepping as the sense of
of movement
movement across the zone
containing them (Hodgson, 1989). In
In the case of the Ropes deposit, this indicates a dextral
component in plan view. An
An origin
origin for
for the
the quartz-tetrahedrite
quartz-tetrahedrite veins,
veins, and
and the
the en
en echelon
echelon
zones of Au concentration with dispersed pyrite,
pyrite, as tension
tension shears, extension
extension veins, or Riedel
shears in a sinistral shear zone is not indicated
indicated because the
the low
low angle orientation of the tips of
the veins with respect to the shear zone margin,
margin, and the similar orientation of
of the ends of the
en echelon zones of Au concentration with dispersed
dispersed pyrite,
pyrite, is not
not consistent with the sense
of rotation expected in sinistral
sinistral shear.
shear.
The sigmoidal zones of gold concentration with dispersed
dispersed pyrite
pyrite and the oblique shear
veins have orientations nearly parallel
parallel to the S fabric,
fabric, consistent with
with an origin as P shears. PP
shears commonly form at approximately 10°
10' to the shear
shear zone
zone boundaries,
boundaries, after
after peak
peak
deformation, when resistance to shear is decreased
decreased by
by the
the presence
presence of
of well
well developed planar
fabric (Tchalenko,
(Tchalenko, 1970).
1970).
Inferred Direction of Shear
Movement direction in aa shear zone is commonly considered
considered to
to be
he in
in the
the line
line
perpendicular to the intersection of
of the shear zone margin with the plane
plane of
of the
the oblique
obliqueSS
fabric, and the sense of movement is inferred from the
the sense of
of obliquity
obliquity of
of the
the fabric
fabric to the
shear zone margin. In
In aaductile
ductile environment
environmentthe
the acute
acute angle
angle between
between the
the SS fabric
fabric and
and the
the
shear plane faces away from the direction of movement. The
The sigmoidal
sigmoidal zones
zones of
of greater
greater gold
gold
concentration with dispersed pyrite follow an SS fabric,
fabric, interpreted as
as aa largely
largely compressive,
compressive,
ductile feature. Contacts
Contactswith
with nonfoliated
nonfoliated serpentinitic
serpentinitic peridotite
peridotite north
north and
and south
south of
of the
the
Ropes deposit are considered
considered shear planes parallel to the C
C direction.
direction. The
The orientation
orientation of
of the
the SS
fabric relative to the C direction indicates a dextral
dextral component
component of
of movement
movement when
when viewed in
the horizontal plane, with the south
south side
side of
of the zone moving west southwest
southwest and
and the
the north
north
side moving east northeast. (Figure 8). Offsets
Offsets along
along northeast
northeast trending
trending faults
faults in
in the
the deposit,
deposit,
and the Z sense of warping of layering between
between the
the rock
rock types
types at
at the
the south
south side of
of the
the deposit
(Figure 7)
7) also support
support a dextral component of movement.
Localization of the Deposit Within
Within a Dilational Jog
quartz-sericite-chlorite rock strikes
strikes 070°
070' overall
overall but
but 080°
080" where
where itithosts
hosts
The trend of quartz-sericite-chlorite
the main ore zone (Figure 8). This
This080°
080Âpart
part of
of the
the trend
trend acted
acted as
as aa dilational
dilational jog or
or releasing
releasing
of movement
movement in
in plan view
view (Figure
(Figure 8).
8).
bend during the indicated dextral component of
Releasing bends are
are commonly
commonly sites
sites for
for ore
ore deposits,
deposits, even
even if
if the
the main
main fault
fault trend
trend isis barren
barren
42
0
1
km
r
N
2
3
1
1
Ropes deposit
quartz vein prospect
- 18 svucture
anspg, s'rn obliqu, m, v.i,,.,
nd.s nht Loan 01 gvSr Au conc.,Wi,flon
slTh .p.nnpjfl,
Numbered quadrants mathed by dashed lines are
ranges of possible orientations of horizontal
components of maximum principal stresses which
could account for dextral components of strain at:
I. Bjork -- Lundeen
prospect
1.
Lundeen prospect
2.
2. I-I -18
18Structure
Structure
3. Ropes
Ropesdeposit
deposit
4.
range
4 Possible
Poss~ole
rangeof
01 horizontal
horizontalcomponent
component
01 maximum
maxfmumprincipal
pr,nc~paI
stress which
wnlcn could
co~ld
of
stress
s ~ m ~ l t a n e o usatisfy
sat~sfv
s l ~ requirements
requmvnentsfor
lor
simultaneously
sites
2, and
sqles 1,
1.2.
and33 above
above
Figure 8:
8: Structural
Structuralmodel
model for
for the
the Ropes
Ropes mine,
mine.
43
(Sibson, 1987). Both
Both the
the sigmoidal
sigmoidal zones
zones of
of Au
Au concentration
concentration with
with dispersed
dispersed pyrite,
pyrite, and
and the
the
auriferous quartz veins are interpreted as indicating local extension within aa dilational
dilational jog.
jog.
There is a component of positive volume change in the shear zone, in an otherwise
compressive tectonic regime. Increased
Increased pore
pore pressure
pressure within
within the
the shear
shear zone
zone as
as
microfractures filled with fluid, combined with the extensional effect
effect of
of the dilational jog,
jog,
local extension.
extension. Slip
jogs involves extensional
may have contributed to local
Slip transfer in dilational jogs
fracture openings localized in the jog,
jog, or step over. Formation
Formation of
of dilational
dilational jogs involves
involves
local loss of cohesion, or brittle failure, in an
an otherwise ductile
ductile environment.
environment.
The trend of quartz-sericite-chlorite rock which hosts
hosts the Ropes main ore zone
narrows to the east of the deposit, as well as toward
toward surface up
up dip from the deposit, where it
70° south dip at shallow
shallow levels.
levels. This
bends from a near vertical dip at depth to a 70"
This change
change in
in
thickenss and attitude may have caused barrier
barrier conditions
conditions for ascending hydrothermal fluids,
thickens
addition to
to the
the dilational jog
jog controlling
resulting in another control on localization of ore in addition
of the overall setting
setting of
of the
the deposit.
deposit.
References
References
Anderson,E.M.,
Anderson, E.M., 1951,
1951,The
The Dynamics
Dynamics of
of Faulting:
Faulting: London, England. Oliver and Boyd,
Boyd, 191
191 p.
p,
Geochemistry of Gold and Its Deposits:
Deposits: Geological
Geological Survey of
Boyle, R.W., 1979,
1979, The Geochemistry
Canada Bulletin 280,
584 p.
280,584
p.
Broderick, T.M., 1945,
1945, Geology
Geology of the Ropes
Ropes Gold Mine,
Mine, Marquette
Marquette County,
County, Michigan:
Michigan:
Economic Geology,
Geology, v.40,
v.40, p.115-128.
p.115-128.
of Economic
Brozdowski, R.A., 1989,
1989, Geology of The Ropes Mine, Society of
Economic Geologists
Geologists Fall
Fall
Field Conference Guidebook,
Guidebook, Precambrian Geology and
and Metal
Metal Occurrences,
Occurrences,
Michigan's
Michigan's Upper Peninsula Field Conference,
Conference, p. 38-75.
Groshong, R.H., Jr., 1988,
1988, Low Temperature
Temperature Deformation Mechanisms
Mechanisms and
and Their
Their
v. 100, p. 1329-1360.
1329-1360.
Interpretation: Geological Society of America Bulletin, v.100,
Hodgson, C.J., 1989,
1989, Patterns
Patterns of
of Mineralization, Chapter
Chapter 3,
3, in:
in:
Mineralization and Shear Zones, Geological Association of Notes v.6, Montreal,
Quebec, May 12-14,
12-14, p.51-88.
Lisle, R.J., 1989,
Dike Sets: Geol.
1989, Paleostress Analysis from Sheared Dike
Geol. Soc.
Soc. Am.
Am. Bulletin,
Bulletin, v.
v.
101, p. 968-972.
Sibson, R.H.,1987, Earthquake Rupturing as a Mineralizing Agent in Hydrothermal Systems:
Systems:
Geology, v.15,
v. 15, p 701-704.
701-704.
Tchalenko, J.S., 1970,
1970, Similarities
Similarities Between Shear
Shear Zones of Different Magnitudes:
Magnitudes:
625-1640.
Geological Society
Society of
of America
America Bulletin,
Bulletin,v.81,
v.81,p.1
p.1625-1640.
44
Geologic Field
Field Excursion
Excursion to
to the
the Ispheming Greenstone
Greenstone Belt
Rodney C. Johnson,
Johnson, Rod
Rod Johnson
Johnson &
& Associates,
Associates, Inc.,
Inc., Negaunee, MI
MI 49866
49866
Theodore J. Bornhorst, Department
Department of Geological
Geological Engineering
Engineering and
and Sciences,
Sciences, Michigan
Michigan
Technological
Technological University,
University, Houghton,
Houghton,MI
MI49931
4993 1
Introduction
Introduction
This geologic field excursion to the Ishpeming
Ishpeming greenstone belt consists
consists of
of 88 selected
selected stops
stops
(Figure 1). These
These stops
stops are
are designed
designed to
to cover
cover the major varieties of volcanic lithologies
found in the belt and several localities of structural
structural significance.
significance. Johnson and
and Bornhorst
Bomhorst
(this volume) provide an overview of the greenstone belt and explanation of stratigraphic
stratigraphic
terminology used in this guide. AA detailed
detailed topographic
topographic map
map is
is provided
provided to
to identify
identify the
the
location of the stops,
however
users
should
refer
to
larger
scale
maps
in
order
to
navigate
stops,
order to navigate
roads to the stops.
stops.
Trip Stops
Field Trip
Stops
Stop
Stop A -- Lighthouse Point
Purpose: View:
View:1)1)amphibolite
amphibolite schist
schist of
of Lighthouse
Lighthouse Point
Point Basalt,
Basalt, 2)
2) Dead
Dead River Shear
Shear Zone,
Zone,
and 3) cross-cutting relationships of Archean and Proterozoic rocks (Figure 2).
Note: You
Guard to
to visit
visit this
this locality.
locality. No rock
You must
must obtain
obtain permission
permission from
from the U.S. Coast Guard
rock
hammers or specimen collecting are allowed at this
this stop. Park
Park in
in the
the parking lot across
across from
from
the Coast
Coast Guard
Guard office.
office.
At this stop excellent shoreline exposures on Lighthouse Point and along the adjacent beach
provide a view of Archean and Proterozoic
Proterozoic rocks
rocks (Figure
(Figure 3).
3). Gair
Gair and
and Thaden (1968)
(1968)
mapped these rocks in detail.
detail.
Part I. The
Theoutcrop
outcrop in
in the
the middle
middle of
of the
the beach
beach near the parking lot is composed of Archean
thinly layered, amphibolite schist (basalt). The basalt here is part of the Lighthouse Point
generally strike E-W
E-W and
and dip
dip 70°N
70°N. The layers in
Basalt. The
The layers
layers on
on Lighthouse
Lighthouse Point generally
highly flattened
flattened pillows.
pillows. The flattening is interpreted to be a
the basalt are interpreted as highly
result of north verging recumbent folding (Johnson and Bomhorst, 1991).
1991).
Part 11.
II. The
Theoutcrop
outcropon
on the
the southwest
southwest end
end of
of the beach is composed of chlorite-sericite
schist, again part of the Lighthouse Point Basalt (see Johnson and Bornhorst,
Bomhorst, this volume).
The chlorite-sericite schist is interpreted to be deformed pillow basalt and are
Kinematic indicators
indicators (S-C fabric in particular)
metamorphosed to greenschist facies.
metamorphosed
facies. Kinematic
indicate a dominantly left-lateral shear sense, but on close inspection right-lateral and northnorthindicators are also present.
present. This locality is interpreted to be the eastern extension of
side-up indicators
block of
of the Ispheming greenstone
the Dead River Shear Zone that separates the northern block
belt from the southern
1).
southern two
two blocks
blocks (Figure
(Figure 1).
45
____
T.
50
N
T.
49
N
T.
48
N.
46°30'
R.28W.
R.27W.
R.26W.
R.25W.
Explanation
Explanation
Proterozoic
Proterozoic
Sedimentary rocks
0Sedimentary
rocks
I
Mona
Mona Formation
Formation
Upper tuff unit
unit and lapilli-tuff
lapilli-tuff unit
unit
Basalt
Basalt flow unit
unit
Kitchi Formation
Formation
Kitchi
Tuff unit and lahar unit
Basalt
unit
Basalt flow unit
l\ /1
I
Archean
Intrusive
Intrusive Rocks
Rocks
k''-4 Granodiorite
Granodiorite
177771 Tonalite
Tonalite
1 Deer
Lake
Peridotite
Deer
Lake
Peridotite
Sedimentary Rocks
Sedimentary
Rocks
K"Xl Timiskaming-type
Timiskaming-typesedimentary
sedimentaryrocks
rocks
Volcanic Rocks
Rocks
Lighthouse
Lighthouse Point
Point Basalt
Basalt
Basalt
Basalt
V/A Reany
Reany Lake pyroclastic
pyroclastic unit
unit and
and
Fire Center mine
mine iron-formation
iron-formation unit
unit
HP_Li Nash
NashCreek
Creek glomerophyric
glomerophyric basalt
basalt unit
unit
3
L\1
m
Bedding
Bedding
87
f Overturned bedding
bedding
87+
Facing of pillows
A Facing
pillows
Plunging anticline
Plunging
anticline
Plunging syncline
syncline
'C Plunging
overturned anticline
Plunging overturned
Shear
zone
1
@ Location
Location of stops
stops
&
-
A
%
Figure 1:
1: Generalized geologic
geologic map of the Ishpeming greenstone belt showing location
location
of stops.
stops.
46
R. 25W.
T. 48 N.
Figure 2:
2: Stop
StopAA- Lighthouse
Lighthouse Point
Point
Figure
47
100
0
200m
0
50rn
I
Explanation
Middle
Middle Proterozoic
Proterozoic
/,%j Keweenawan
Keweenawandiabase
diabasedike
dike
Early
Early Proterozoic
Proterozoic
'dt*S Metadiabase
Metadiabasedike
dike
Archean
Archean
I
Intrusive
IntrusiveRocks
Rocks
Rhyolite
Rhyolite
0
I
Lighthouse
Lighthouse Point
PointBasalt
Basalt
Basalt,
Basalt,outcrops
outcropsinindarker
darkershade
shade
Figure
Figure3:
3:Geologic
Geologicmap
mapof
of Lighthouse
LighthousePoint
Point (modified
(modifiedfrom
fromGair,
Gair,1959).
1959).
48
I
Within the basalt dominated outcrop there are lenticular pods of quartz-chert (hematite
(hematite
The interpretation of the
the quartz-chert
stained) and small texturally different
different quartz
quartz veins.
veins. The
with deposition
deposition of
of the
the basalts.
basalts. Small
Small quartz
quartz
pods is not clear, they could be synchronous with
veins are found in rocks throughout
throughout the
the Ishpeming
Ishpeming greenstone
greenstone belt.
belt. Most of
of these were
were
Archean deformation
deformation and
and metamorphism.
metamorphism. However, at
at least some
some
likely emplaced during Archean
1998).
were emplaced during the Proterozoic (Bornhorst
(Bomhorst and others, 1998).
Part III.
111. The main outcrops of Lighthouse Point are northeast of the first two localities.
localities.
This trip will only
only visit
visit the
the shorline
shorline ouctrops
ouctrops on
on the
the northeast end of
of the
the beach,
beach, near
near aa house.
house.
Here, Archean thinly layered, amphibolite schist (basalt) is cut by a tabular, porphyritic
rhyolite dike trending roughly parallel to the layering. The layered basalt is part of the
the
Lighthouse Point Basalt and the rhyolite is interpreted as synchronous with the synorogenic
late trondjhemite-granite suite plutons that reorient earlier fabrics.
are
The Archean rocks are cut by two N-S trending Proterozoic diabase dikes that are
metamorphosed to greenschist facies as compared to the surrounding Archean rocks that are
are
metamorphosed to amphibolite facies. The
The crest
crest of the ridge (not visited today) is underlain
by a relatively unmetamorphosed E-W
E-W trending diabase dike
dike of Keweenawan
Keweenawan age.
age.
White Bear Drive
Stop B -- White
Drive
Purpose: View
Viewthe
theupper
uppertuff
tuffunit
unitof
ofthe
theMona
Mona Formation
Formation and
andS3
S3crenulation cleavage
cleavage
(Figure
(Figure 4).
4).
The rocks at this stop are part
part of
of the
the upper
upper tuff
tuff unit
unit of
of the
the Mona
MonaFormation.
Formation. The rocks of the
quartz-feldspar-sericite-chlorite schist.
schist. Thin primary layering subupper tuff unit consist of quartz-feldspar-sericite-chlorite
parallel to S
S,foliation
foliationcan
canbe
beobserved
observed in
in some
some of the outcrop in this area. Based
Based on
on the
the
presence of sand-sized,
sand-sized, lenticular
lenticular volcanic
volcanic fragments, and lack of sand-sized quartz
quartz and
and
feldspar in an intact framework, most of these schist beds are interpreted as tuff.
On the north side of the outcrop nearest White Bear Drive we can see the hinge of a
fold. This allows us to see S3 crenulation
crenulationcleavage.
cleavage. On the limbs of
westward plunging F3 fold.
S and
Bornhorst, this volume,
andS3
S3are
are parallel
parallel and
and indistinguishable (see Johnson and Bomhorst,
the fold S,
Table 1).
1).
Stop C - Tower
Purpose: View
Viewthe
thelapilli-tuff
lapilli-tuff unit
unitof
ofthe
theMona
MonaFormation
Formation and
andS3
S3crenulation cleavage
cleavage
(Figure 5).
5).
Note: Park
Parkatatthe
thecommunication
communicationtower
towerand
andwalk
walkabout
about lOOm
loom downhill in a southerly
direction to aa series
series of
of outcrops
outcrops alongside
alongside the
the hill.
hill.
The rocks at this stop are part of the lapilli-tuff
lapilli-tuff unit
unit of
of the
the Mona
MonaFormation.
Formation. The rock is
composed of calc-alkalic rhyolite lapilli in a quartz-sericite-chlorite matrix with quartz and
and
49
R. 26W.
T. 48 N.
Figure 4: Stop
Stop B-White
B-White Bear Drive
Drive
R. 26W.
T. 48 N.
Figure
Figure 5:
5: Stop
Stop C-Tower
C-Tower
50
R. 26W.
1.48 N.
Figure
Figure 6:
6:Stop
Stop D-Shunk
D-Shunk Furniture
Furniture
R. 26W.
T. 48 N.
StopE-Tuff
E-Tuff Unit
Unitof
ofthe
theKitchi
KitchiFormation
Formation
Figure7:7:Stop
Figure
51
less commonly, feldspar phenocrysts. Flattened
Flattened apple
apple green
green lapilli
lapilli (muscovite
(muscoviteand
and chlorite;
chlorite;
Puffet, 1974) are also common at this stop. This
This unit
unit isis interpreted
interpreted as
as subaqueous
subaqueous
pyroclastic flow and fall deposits.
At this stop an earlier shallowly easterly dipping foliation (S1)
(Si) is crenulated by a later E-W
foliation and
and elongated
elongated in the
(S3). The
The lapilli
lapilli are
are flattened along the S1
S, foliation
trending foliation (53).
direction of the intersection of Si and S3.
The
Dead
River
Shear
zone
(D2,
D4) is interpreted
83. The Dead River Shear zone (Dz,D4)
to be located on the north side of the tower hill.
Stop D - Shunk Furniture
Purpose: View
Viewpillowed
pillowedbasalt
basalt of
ofthe
theMona
MonaFormation
Formation (Figure
(Figure 6)
6)
Note: Park
Parkalongside
alongsideof
ofthe
theroad
roadacross
acrossfrom
fromShunk
ShunkFurniture,
Furniture, but
but be
be very
very careful
careful of
oftraffic.
traffic.
There are excellent views of relatively undeformed pillowed
pillowed tholeiitic basalt in outcrops
pillowed basalts are part
part of
of the
the basalt
basalt flow
flow unit
unit of
of the
the Mona
Mona
alongside of U.S. The pillowed
30 cm
cm X
X 1.5
1.5 m;
m;
Formation. The
Thepillows
pillows are
are ellipsoidal
ellipsoidal in
in shape
shape with
with dimensions
dimensions about
about 30
E-W and
and dips
dips 85'
85° N.
N. Cusps on pillows indicate
attitude of bedding strikes approximately E-W
that stratigraphic top is to the north. There
There are
are characteristic
characteristic well-developed
well-developed pillow
pillow rinds,
rinds,
radial fractures, and interpillow void spaces filled with quartz and carbonate.
carbonate. Overall,
Overall,
pillowed and massive tholeiitic basalt is the dominant lithology in the Ishpeming greenstone
belt.
Stop EE—
Kitchi Formation
Stop
- Tuff Unit of the Kitchi
Purpose: View
View tuff
tuff unit
unit of
of the
the Kitchi
Kitchi Formation
Formation and
and evidence
evidence for
for superposed
superposed folding
folding
(Figure 7).
7).
Kitchi
At this stop we will see some of the lithologic variation within the tuff unit of
of the
the Kitchi
folding. Cross-bedding,
Formation and evidence for large scale folding.
Cross-bedding, graded
graded bedding, and
and cut
cut and
and
Kitchi Formation,
Formation, but
but in
in this
this area
area clearly
clearly indicate
indicate tops
tops to
to
fill structures are uncommon in the Kitchi
the east. AA foliation
foliation atatright
right angles
angles to
to bedding
bedding indicates
indicates this
this area
area is
is near aa vertically
vertically
plunging fold nose.
Stop F - Teal
Teal Lake
Purpose: View
View the
theCarp
CarpRiver
River Falls
Falls shear
shear zone,
zone, tuff
tuff unit
unit of
of the Kitchi
Kitchi Formation and
and Ajibik
Ajibik
8).
Quartzite of the Menominee Group of the Marquette Range Supergroup (Figure
(Figure 8).
At this stop we will cross the Carp
Carp River Falls shear zone from Lower Proterozoic
metasedimentary rocks into Archean
We will
will begin the
the traverse in
in the
the
Archean metavolcanic
metavolcanic rocks.
rocks. We
steeply dipping Ajibik Quartzite of the Lower Proterozoic Marquette
Marquette
relatively undeformed steeply
TheCarp
CarpRiver
River Falls
Falls shear
shear zone
zone is
is aa high
high strain
strain zone
zone indicated
indicated by
by
Range Supergroup. The
The foliation
foliation in
in the
the schist
schist is
is near
near vertical
vertical and
and has
has aa
quartz-sericite-feldspar-chlorite schist. The
steep mineral lineation. As
As the
the shear
shearzone
zone isis traversed
traversed there
there is
is aa noticeable
noticeable increase
increase in
in the
the
52
R. 27W.
T. 48 N.
Figure 8:
8: Stop
Stop F-Teal
F-Teal Lake
Lake
R. 27W.
T. 48 N.
Figure
Figure 9:
9: Stop
Stop G-Alorig
G-Along Deer
Deer Lake
Lake
53
amount of chlorite in the schist. On
Onthe
thenorth
north side
side of
of the
the Carp
Carp River
River Falls
Falls shear
shear zone
zone isis
quartz-feldspar-sericite-chlorite schist
schist of
of the
the tuff
tuff unit
unit of
of the
the Kitchi
Kitchi Formation.
Formation.
Stop G --Along
Along Deer Lake
Lake
Purpose: View
View lahar
laharunit
unitof
ofthe
theKitchi
Kitchi Formation
Formation (Figure
(Figure 9).
9).
At this stop polymictic volcanic conglomerate is exposed in
in a glacially
glacially polished
polished outcrop
along the north side the road
road towards
towards Deer
Deer Lake.
Lake. Park alongside of the road.
road. This
This volcanic
volcanic
part of the
the lahar
lahar unit
unit of
of the
the Kitchi
KitchiFormation.
Formation. On the polished
polished surface, the
conglomerate is part
volcanic clasts vary from 25 cm X 35 cm in cross section,
section, down
down to
to the
the size of the matrix
grains. Larger
1.2mminincross
crosssection,
section, are
are present
present in
in the
the nearby
nearby lake
lake
Larger clasts,
clasts, up
up to
to 60cm
60 cmXX1.2
shore outcrops. Clasts
Clastsgreater
greater than
than 11cm
cm make
make up
up about
about 20
20 %
% of
of the
the rock
rock and
and there
there isis an
an
obvious stratification in clast
clast size. Shape
very angular to
to rounded.
rounded. The
Shape of clasts varies from very
The
volcanic clasts are caic-alkalic
calc-alkalic andesite
andesite to dacite
dacite in composition with
with most
most clasts
clasts at
at this
this
locality being near the andesite to dacite division
division with
with small
small chemical
chemical variation
variation (Bomhorst
(Bornhorst
unpublished
holocrystalline. The
unpublished data). Most
Most clasts
clasts are
are porphyritic and some are holocrystalline.
The volcanic
volcanic
conglomerate is interpreted as a subaqueous lahar deposit.
deposit.
Stop H - Shoreline of Deer Lake
Purpose: View
ViewDeer
DeerLake
LakePeridotite
Peridotite (Figure
(Figure10).
10).
road. Park
Note: There
There isis aa small
small pullover
pullover on
on the east side of the road.
Park and
and walk
walk along
along the
the road
road to
to
the north where you can find a small
small trail that leads through the woods to
to the
the tip
tip of
of the
the point.
point.
Outcrops occur along the shoreline of this small point into
into Deer Lake. At this
this stop
stop
serpentized peridotite of the Deer Lake Peridotite is exposed along the shore
shore of
of Deer
Deer Lake.
Lake.
Elongate pods of massive serpentized peridotite with closely spaced
spaced fractures
fractures are
are surrounded
surrounded
by 2 to 5 cm wide ribbons of 11 to 22 mm subparallel veins of cross
cross cross-fiber serpentine
serpentine
(Bornhorst
fine(Bomhorst and others, 1986).
1986). The
Thedark
dark grey
grey to
to green
green massive
massive serpentized
serpentized peridotite
peridotite is
is fineto medium-grained and lacks obvious foliation. lit
In hand sample one can find well-developed
well-developed
1 to
to 55 mm
mm pseudomorphs
pseudomorphs after olivine and
and pyroxene
pyroxene outlined
outlined by
by thin
thin selvages of white
carbonate or talc. No
No spinifex
spinifex or
or pyroxene "string beef'
beef' textures
textures indicative
indicative of
of komatiite
komatiite
flows have been found. The
Thechemical
chemical composition
composition indicates
indicates an
an olivine
olivine to
to pyroxene ratio
ratio
from about 0.25 to 2 and most samples
samples are
are in the harzburgite field and
and some
some are
are in
in the
the
Iherzolite
lherzolite field. The
The Deer
Deer Lake
Lake Peridotite
Peridotite is
is interpreted
interpreted as
as a sill complex.
This locality is only a few 100
Ropes gold
gold mine.
mine. The
100 meters from the Ropes
The shear
shear zone
zone that
that hosts
hosts
the Ropes gold mine projects immediately
immediately north
north of
of this
this small
small point.
point. The
The serpentized
serpentized
peridotite has a strong,
veinlets and
and streaks
streaks
strong, contorted
contorted foliation with magnetite as irregular
irregular veinlets
along the foliation
foliation planes indicating the shear zone that hosts gold at
at the Ropes
~ o ~mine
mine
e s
continues into the Deer Lake Peridotite. Adjacent
Adjacent to
to the
the gold
gold ore
ore body
body the
the serpentized
serpentized
peridotite is altered
altered to a talc-carbonate rock (Bornhorst
(Bornhorst and
and others, this
this volume).
volume).
54
R. 26W.
T. 48. N.
Figure 10:
10:Stop
Stop HH- Shoreline
Shoreline of
of Deer
Deer Lake
Lake
Figure
R. 26W.
T. 48 N.
Figure 11:
11:Stop
StopI-I-County
CountyRoad
Road510
510
Figure
55
StopiStop
I -County
County Road
Road 510
Purpose: View
Purpose:
View fire
fire Center
Center Mine
Mine iron-formation
iron-formation of the Lighthouse Point Basalt (Figure
(Figure 11).
11).
Note: Park
Parkalongside
alongsideof
ofthe
theroad,
road, but
but be
be careful
careful to
to watch
watch for
for logging
logging trucks
trucks moving
moving at
at high
high
speeds.
There are several outcrops
outcrops on both sides of County Road 510
510 at
at this stop.
stop. The
The particular
particular
outcrop of interest is near the top of the slope
slope on the east side of the road, itit may
may take
take aa little
little
effort to find the iron-formation.
iron-formation. At
At this
this stop
stop the
the Fire
Fire Center
Center Mine
Mine iron-formation
iron-formation unit
unit of
of the
the
Lighthouse Point Basalt is overlain and underlain by massive tholeiitic basalt of
of the
the
Lighthouse Point Basalt. The
Theiron-formation
iron-formation isis approximately
approximately 1.5
1.5 m
m thick
thick and
and strikes
strikes
N30°W.
N30¡WItIt isis composed
composed of
of fine,
fine, black
black chert
chert that contains scattered magnetite grains and
pyrite. To
Tothe
thewest
west of
of this
thislocality,
locality, the
the iron-formation
iron-formation is
is layered
layered with 1.5
1.5 cm
cm thick chert
chert and
and
magnetite bands. Despite
Despitevariable
variable magnetite
magnetite content,
content, this
this iron-formation
iron-formation produces
produces aa strong
strong
magnetic signature
signature and can be traced from north to south in this stratigraphic
stratigraphic block
block of
of the
the
greenstone belt. This
marker. The
This iron-formation
iron-formation is an excellent stratigraphic marker.
The basalt is dark
dark
amphibolite
green to black, fine-grained and relatively non-foliated and metamorphosed to amphibolite
grade.
grade.
Acknowledgments
We thank Shannon E. Bair for computer drafting of the figures.
References
Bomhorst T.J.,
T.J., Shepeck,
Shepeck, A.W.,
A.W., and
and Rossell,
Rossell, D.M.,
D.M., 1986,
1986, The
The Ropes
Ropes gold
gold mine,
mine, Marquette
Marquette
County, Michigan,
gold deposit:
deposit: MacDonald,
- an Archean hosted lode gold
MacDonald, E.J.
E.J.
Michigan, U.S.A -(ed.), Proceedings of Gold '86, an International Symposium on the Geology of Gold,
Toronto, p.
13-227.
Toronto,
p. 2213-227.
Bornhorst,
Bomhorst, T.J., Thorpe, R.I.,
R.I., and Johnson, R.C., 1998,
1998, Lead isotope study of veins in
in the
the
Archean Ishpeming greenstone belt, Michigan: Econ. Geol., v. 93, pp. 102-107.
102-107.
Gair, J.E. and Thaden, R.E., 1968,
1968, Geology of the Marquette and Sands
Sands Quadrangles,
Quadrangles,
Marquette County, Michigan: U.S. Geol. Survey Professional Paper397, 77 p.
Johnson,
R.C. and Bornhorst,
T.J., 1991, Archean
Archean geology
geology of
of the
the northern
Johnson, R.C.
Bomhorst, T.J.,
northern block
block of the
the
Ishpeming
belt, Marquette
Ishpeming greenstone
greenstone belt,
Marquette County,
County, Michigan:
Michigan: U.S.
U S . Geological
Geological Survey
Survey
Bulletin 1904-F,
1904-F, 20
20 p.
p.
Puffett, W. P., 1974,
1974, Geology of the Negaunee Quadrangle, Marquette County, Michigan:
U.S.
53 p.
U S . Geol. Survey
Survey Professional
Professional Paper
Paper 788,
788,53
p.
56
Shear
Shear Zones
Zones and
and Gold
Gold Mineralization
Mineralization
in the
the South
South Half
Half of
of the
the Ishpeming
Ishpeming Greenstone
GreenstoneBelt,
Belt,
Michigan
Michigan
by
D.J. Duskin
Duskin
Consulting
Consulting Geologist
Geologist
210
21 0 Union Street
Street
Camden, SC 29020
29020
and
and
T.O.
T.O. Quigley
President
Minerals
Minerals Processing
Processing Corporation
Corporation
4547
4547 County
County Road
Road 601
60 1
Champion, MI 49814
49814
57
Foreword
Foreword
Much of this field trip is devoted to gold occurrences.
occunences. At
Atmost
moststops,
stops,the
theguidebook
guidebookdescriptions
descriptions
will include the
the results
results of
of sampling
sampling and
and assaying
assayingfor
forgold.
gold. The term "grab sample" means a small
reconnaissance-style sample deliberately chosen
chosen by
by an
an exploration
exploration geologist to have, in his experience, the
maximum chance of yielding gold in the assay because of the presence of alteration, sulfides,
sulfides, gossan,
gossan, etc..
etc..
If our reported result contains
contains gold,
gold, this
this doesn't
doesn't necessarily mean
mean that
that someone
someone else's
else's subsequent sample
the same value.
value. This phase of sampling is deliberately subjective, and
and gold
gold distribution
distribution is
is far
far
will give the
from uniform in Archean deposits,
deposits, varying
varying greatly
greatlywithin
withinsmall
smalldomains,
domains,even
evenatatproducing
producingmines.
mines. More
representative sampling will
will be
be discussed at some of the field trip stops as either "channel" or "chip"
samples across an exposure.
exposure. Channel
front diamond blade rock saw cuts or hand-cut
Channel samples,
samples, taken either from
by hammer and moil, are meant to be volumetrically equivalent to a ¼
'/2 split of BQ (1.5-inch
(1 S-inch diameter)
drill core. Chip
Chipsamples
samples are
are more
more representative
representative of the exposure than a grab sample, but not as rigorously
taken as a channel sample.
sample.
Unless otherwise specifically noted, all gold values reported herein are from samples
samples crushed,
ground, split and assayed at the former Callahan Mining Corporation assay lab in the Humboldt (Ropes)
Gold Mill, now owned and operated by Minerals Processing
Processing Corporation
Corporation (MPC).
(MPC). MPC's
MPC's assays
assays were
were all
all
done by the fire
fife assay-gravimetric finish
finish method,
method, using
usingaa 1-assay
I-assay ton
ton (29.167
(29.167 gram)
gram) subsplit.
subsplit. MPC's
MPC's
lower detection limit by
by this
this technique
technique isis ,003
.003 odton
oz/ton (103
(103 ppb)
ppb) Au.
Au. The terms "no gold",
gold, or
or "below
"below
detection" refer to this limit. In
Insome
somecases
caseswhere
wherewe
we felt
feltititimportant
importantto
toknow
knowthe
thepresence
presenceof
ofeven
evenlower
lower
Au values, separate subsplits were sent to Chemex Labs in Vancouver, B.C. for analysis by a lower
detection limit technique, noted herein where so done. This
This technique
technique is
is the
the fire
fire assay
assay -- atomic absorption
finish
fmish process (FA-AA),
(FA-AA), and has
has aa lower
lower detection
detection limit
limit of
of 10
10ppb.
ppb.
Chemex Labs also performed whole rock
rock major
major oxide analyses.
analyses. Results
Results from samples
samples taken at
(Grunsky, 1983; Jensen, 1976)
some of the field trip stops are shown on Jensen cation plots (Gnmsky,
1976) that have been
routinely used for characterizing the lithochemisby
lithochemistry of
of the
the rocks
rocks from
from this
this region (Bomhorst
(Bornhorst and Johnson,
1993). Figure lisa
1993).
1 is atemplate
templateJensen
Jensenplot.
plot.The
TheJensen
Jensenplots
plotswere
weredone
doneby
byconsulting
consulting geologist
geologist R.A.
Campbell, Burlington, Ontario, using "Newpet"
"Newpet" software developed and distributed by the Earth Sciences
Sciences
Department of Memorial
Memorial University
Universityof
ofNewfoundland
Newfoundland.. Mr.
Mr. Campbell's
Campbell's assistance
assistance is
is gratefully
acknowledged.
acknowledged.
58
Chemex
Chemexalso
alsodid
didaa32-trace
32-traceelement
elementanalysis
analysisusing
usingthe
theX-ray
X-rayfluorescence
fluorescencetechnique
techniquefor
forall
all
samples
samplessubmitted
submittedfor
forassay;
assay;results
resultsare
arediscussed
discussedwhere
wheresignificant.
significant.
All
thinsections
sectionsexamined
examinedwith
withthe
theassistance
assistanceofofR.L.
R.L.
Allpetrographic
petrographicdescriptions
descriptionsare
arefrom
frompolished
polishedthin
Bamett
BamettofofBameti
BamettGeological
GeologicalConsulting,
Consulting,London,
London,Ontario.
Ontario. Mineral
Mineralcompositions
compositionswere
weredetermined
determined
using
JEOLModel
Model733
733five
fivespectrometer
spectrometerscanning
scanningelectron
electronmicroprobe.
microprobe.Mineral
Mineral
usingBaniett
BamettGeological's
Geological'sJEOL
percentages
percentagesare
arevisual
visualestimates;
estimates;no
no point
point counts
counts were done. Mr.
Mr.Bamett's
Barnett'svaluable
valuablehelp
helpisisalso
alsogratefldly
gratefully
acknowledged.
acknowledged.
FeO* + 1102
Cation %
Jensen (1976)
PJ203
A1203
Mgo
MgO
Figure
1: 1 :Fields
Plot
Figure
Fieldsfor
forthe
theJensen
JensenCation
Cation
Plot
595 9
I
CONTOUR INTERVAL 20 FEET
0.. )
20
Zr.C
.5
r
CQ
0
t%Z
0
0
Stop SC-I:
SG1:
Variably
basalt of
ol'Mona
Mona Formation
Formation near
near M
Marquette
Harbor
Variably sheared
sheared pillow basalt
a q u e t t e Harbor
(Location
2).
(Location map, Figure
Figure 2).
Gair and
(1968)
and Thaden
Thaden (1
968) last mapped this locality and assigned it to the lower member of the
Mona Schist. Recently
(1993)have
have revised
revised the
the stratigraphic
stratigraphic nomenclature
nomenclature of
of the
the
Recently Bornhorst
Bomhorst and
and Johnson
Johnson (1993)
lshpeming
Ishpeming Greenstone Belt; this locality would now be assigned to the informal basalt flow of the Mona
lowermost unit
unit.. Pillowed
Formation, its lowermost
Pillowedmetabasalts
metahasaltsof
ofgreenschist
greenschistfacies
facies are
are the
the predominant Ethology
lithology
in this
this unit.
unit.
The exposures here are best seen in a three-part sequence. After
After parking in
in the
the abandoned
abandoned railroad
railroad
yard, head east and take the trail along the south side of
of Whetstone
Whetstone Brook
Brook to
to the
the lakeshore
lakeshoreto
tosee
seeglacially
glacially
polished outcrops of pillow lavas. The
Theoutcrops
outcropsdisplay
displaywell
wellpreserved
preservedglacial
glacial grooves
grooves and
andstriae
striaetrending
trending
198°;
19S0; some of the grooves contain
contain classic
classic examples
examples of
of crescent-shaped
crescent-shaped chaffer
chatter marks pointing down-ice.
down-ice.
The two outcrops here show
show a layer of massive
massive non-pillowed
non-pillowed basalt
basalt conformably
conformably sandwiched
sandwiched
between two pillowed
pillowed layers.
layers. The layering shown by
by this "sandwich"
"sandwich" trends
trends approximately
approximately 305'.
305°. The
The
pillows are pristine and
and undeformed,
undeformed, some
some of
of them
them up
up to
to six
six feet
feet across.
across. They display internal cooling
cooling
fractures, well preserved rinds, and cusps
cusps consistently
consistently showing
showing tops to
to the
the north.
north.
The next point of interest is about
about 100 feet
feet south
south on
on the
the rocky
rockypoint
pointof
of the
thelakeshore.
lakeshore. Please he
be
careful while making your wav
way southward over
over this
this slioperv
slippery outcro~.
outcrop. On
On the
the south
south side
side of
of this
this rocky
rocky point
point
are pillowed basalt and thin discontinuous lenses of chert interbedded
interbedded with the pillows,
pillows, some
someforming
forming
pillow selvages. Some
brecciated red
Someof
ofthe
thechert
chertcontains
contains patches
patches of
ofbrecciated
red jasper; this
this texture is
is probably the
the
deformation. None of
result of soft sediment deformation.
of these cherts is magnetic. A
A great
great deal of
of excess calcite is
also present with the pillows. This
This probably
probably contributed
contributed to
to the
the development
development of
of shearing,
shearing, which
which can
can be
be
to be increasing rapidly
rapidly in
in intensity
intensity southward
southwardin
inthis
thisoutcrop.
outcrop. We think that the calcite and
clearly seen to
the result
result of
of degassing
degassing and
and seawater
seawaterprecipitation
precipitationininand
andnear
nearaaflow
flowtop.
top. In our experience
chert lenses are the
such interpillow zones containing discontinuous thin cherts are very common.
path from the beach
beach westward
westward back
back to
to the
the railroad
railroadyard
yard the
the progressive
progressive deformation
deformation of
of
Taking the path
best seen.
seen. Entering
pillow basalt here is perhaps best
Entering the yard, the outcrop exposure on the right (north) again
marks the north edge of the developing shearing. Here
Here the rock is moderately and variably sheared but the
outlines of pillows can still be seen
seen despite a penetrative
penetrative foliation striking
striking 290°,
290°85°N
85'N (photo,
(photo,Figure
Figure3).
3).
The pillow rinds are pinkish weathering calcite, and the pillows here are much smaller than those earlier
seen to the north, again a possible contributing
contributing factor to the development
development of
of shearing
shearing here.
here.
61
Figure
northwest
Figure3.3.Looking
Looking
northwestatatsheared
shearedgreenstone
greenstoneoutcrop
outcropononeast
eastside
sideofofrail
railyard,
yard,Stop
StopSG-1.
SG-1.
Hammer
Hammerpoint
pointtouching
touchingcalcareous
calcareousflattened
flattenedpillow
pillowrind.
rind.
Moving
is is
itsitswest
Movingwest
westacross
acrossthe
therailroad
railroadyard,
yard,the
thethird
thirdpart
partofofthis
thisstop
stop
westwall.
wall.The
The
features
here
features
here
50yards
yardswest
westofofhere
hereand
andabove
aboveususininthe
theU.S.
U.S.Highway
Highway4141road
roadcut;
cut;
aremore
morecontinuously
continuouslyexposed
exposedabout
about50
are
that
vexydangerous
dangeroushigh
highspeed
speedIraffic
trafficlocation
locationand
andwe've
we'vechosen
chosentotoshow
showthe
thesame
sameunits
unitsinindetail
detailatat
thatisisa avery
this
instead.
thisrail
railyard
yardexposure
exposure
instead.
62
'V
p/I/sawed
4.ilr
1
n.bbl caicgreo4Jt —-—---—
ZontS
ii
4n14
a
71
Careen
/1/1
Th— r--sheand 2
i4/ pt/al- p//Iota c,_.rinds
—'-c
re/ic4..
p1/ala
0
20
—'-
Peer
7R2-2
- -4--
/1/in
Jo.sper £he-rt
71
P/I/a
Ec/kt p//loots
v
Shea rtd
X
Acsay .capIe
®
u/kale- rock
mj#trtJ ,tiaiQa
d1 te.
e-'-- ,-._,
_iCoeapsss and Tape.
ao
Figure
Figure4.
4. Compass
Compassand
andtape
tapemap
mapofofwest
westwall
wall of
of railroad
railroad yard, Stop SG-l.
SG-1
63
Ca inple
-Thea
£
—A...
....LL__
C
-at
ny
Figure 4 is a map of the rail
rail yard
yard exposure.
exposure. From
From north
north to
to south,
south, the
the first exposed
exposed unit
unit is
is dark
blackish green non-amygdaloidal
non-aniygdaloidal pillowed
pillowed basalt
basalt with
with calcareous
calcareousepidote-rich
epidote-richpillow
pillowrinds.
rinds. The pillows
are quite large, some up to four feet across. Blobs
Blobs of
ofcalcite
calcite and
and calcite-cemented
calcite-cemented breccia fill some
some of the
pillow interstices. Within
Withinthis
thisunit
unitthere
thereare
areshallow
shallownorth-dipping
north-dipping rubbly
rubbly zones
zonesthat
thatmay
may represent
represent
irregular flow tops. Many
Many of
ofthe
thepillow
pillowcusps
cuspsindicate
indicatefacing
facingto
to the
thenorth,
north, but
butthis
thisisiseasily
easilyarguable
arguableatatthis
this
outcrop. The
Thepillows
pillows themselves
themselvesare
arenot
not megascopically
megascopically foliated.
foliated.
Moving southward, there is an 8-foot covered interval critical to the relationship between the
pillowed basalt and the next unit.
unit, In
In the
the Highway
Highway 41 exposure
exposure to the west, this zone is occupied by a
sheared-off lens of gray chert and highly sheared rusty weathering chlorite
chlorite rock
rock with
with abundant
abundant oxidized
oxidized
pyrite along
along shear
shearplanes.
planes. (grab
(grab sample
sample HIWAY-1,
HIWAY-l, .013
fine cubic pyrite
,013 ozlton Au). The
The remainder
remainder of the unit,
exposed past the covered interval here, is highly sheared and light gray-green, with
with aa strong Si
S foliation
foliation
striking 269",
269°, 8O0N,
80°N, defined by chloritic slip planes with vertically plunging lineations. Oblique
Obliqueto
toS1
Sl and
dipping more shallowly to the north are discontinuous blobs of white calcite and feathery seams of light
brow!] ankerite.
ankerite. These appear to
to be flattened and
and sheared-out
sheared-out pillow
pillowrinds.
rinds. Both textural features are cut
brown
S1,striking
strikingapproximately
approximately300'
300°and
and dipping
dipping 30'-4O0N,
30°-40°N, defined by
by a tensional cleavage conjugate to Si,
seamlets. There
thin fibrous calcite searnlets.
There are
are also
also wide
wide spaced
spaced flat-dipping
flat-dipping relief
relief fractures
fractures with
with fibrous
fibrouscalcite.
calcite.
At first sight this
this rock
rock might
mightbe
betaken
takenfor
foraamafic
mafictuff.
tuft However,
However, itit lacks definitive features of
such a protolith, i.e., lapilli, crystals, lithic fragments or bedding. Instead,
Instead,itit appears
appearsto
tohave
have been
been derived
derived
from shearing of massive rock, save for the relict pillow rinds, which here indicate that the pillows were
quite small and probably more susceptible to shearing than the more massive large pillows to the north.
Southward there is another
another 10-foot covered
covered interval.
interval, in
In the Highway 41 exposure to the west, a
highly sheared rusty sericite-chlorite-ankerite
sericite-chlorite-ankeritezone with traces of oxidized cubic
cubic pyrite
pyrite occupies
occupies this.
this.
(grab sample HIWAY-2, .009 odton
ozlton Au). Past
Past this
this covered
covered interval, the exposure is of moderately
sheared gray-green rock.
rock. SSifoliation
vertical;
foliationisisapproximately
approximately270°,
270°
vertical;the
theshallow
shallowdipping
dipping cleavage
cleavage isis
weakly developed. Blobs
Blobsof
ofchlorite
chlorite in
in the
the plane
plane of SSlshow
showvertical
verticalalignment,
alignment,and
and the
the rock
rock carries
carries
disseminated coarse cubic pyrite with vertically aligned
aligned calcite
calcite pressure
pressure shadows;
shadows; in
in thin
thin section
section both
both of
of
these exhibit porphyroblastic development, rather than a pre-existing textural fabric. This
Thischlorite-spotted
chlorite-spotted
unit has a higher MgO content than the adjacent rocks, plotting in
in the
the komatiitic field
field (see
(seeJensen
Jensencation
cation
plot, Figure
Figure 5).
5).
Further south within this sheared rock
rock is a 1.5 -foot bed of non-magnetic jasper chert that almost
285°, dipping 73'N.
73°N. The
surely
surely defmes
defines an
an original
originallayering
layering(So).
(So). It strikes 285',
Thepatchy
patchy bright
bright red
red jasper zones
zones
64
in this chert are brecciated in
in places, as seen
seen in
in the
the outcrop
outcrop east
east across
across the
the railroad
railroad tracks.
tracks. The chert is
is not
foliated, but carries a trace of fine cubic pyrite on late white quartz fractures. A
A grab
grab sample
sample here
here yielded
yielded
oziton gold
gold;;FA-AA
0.003
0 3 odton
FA-AA re-analysis
re-analysis by
by Chemex
Chernex gave
gave 51
5 1ppb
ppb Au.
Au.
Good examples of relict pillows are a couple of feet north of the hanging wall of the chert.
This sheared lithology
southern contact
contact is
is
hthoáogy grades
grades southward
southward into
intonon-foliated
non-foliatedpillowed
pillowedbasalt.
basalt. The basalt's southern
This
a thin discontinuous 0-4"
0-4" nonmagnetic
non-magnetic chert-jasper breccia
breccia that
that gave
gave no
no gold
gold in
in aa grab
grab sample.
sample. This
Beyond its
its cherty
cherty southern
southern contact
contact the
the
foliation-resistant pillow basalt unit is about 15 feet thick. Beyond
remainder of the exposure consists
consists of
of highly
highly sheared,
sheared, light
light gray-green
gray-green rock with arguable
arguable relict
relict pillow
pillow
"vein"
S i at 260°,
260°70°N.
70°NWithin this southernmost part of the exposure is a 2-foot calcareous "vein"
textures, S1
oblique to S,,
S, strike
foliation. A grab sample
strike 315°,
3 15O, 72°N.
72'N. ItItexhibits
exhibitsslickensides
slickensides and
and drag of the S1
S, foliation.
yielded no gold. In
In thin
thin section
sectionthis
this rock
rock consists
consists of
of alternating
alternating parallel bands of
of fine
fine grained
grained calcite and
and
plagioclase aggregates in nearly equal amounts, both carrying interstitial high magnesian chlorite that
constitutes about 10%
10% of the rock. The
Thecalcite
calcitebands
bands contain
contain lozenge-shaped
lozenge-shaped ankeritic
ankeritic domains.
Accessory minerals include apatite, zinc-rich cbromite
chromite and abundant chromian titanornagnetite
titanomagnetite commonly
displaying atoll textures, a feature of
of ultramafic rocks.
rocks. Other
Other indications
indications of
of an
an ultramafic
ultramafic parentage
parentage are
are the
the
high magnesian chlorite and a trace element analysis that includes 296 ppm Ni, an unusually high result in
comparison to the nickel content found in
in the dozens
dozens of
of other
other rocks
rocks analyzed
analyzed in
in the
the study
studyfor
forthis
thisfield
field
trip. We
altered ultramafic
ultramafic dike.
dike. Gair and Thaden (1968,
Wethus
thus consider
consider this
this "vein" to be
be a hydrotherrnally
hydrothemally altered
pl. 1) showed
southernmost end of
of what
what was
was then
then the
the
p1.1)
showed an
an outcrop
outcrop of
of "hornblende
"hornblende lamprophyre" at the southernmost
Highway 41 road cut. Their
Their outcrop
outcrop cannot
cannot be
be found
found today,
today, but
hut this
this dike
dike may
may represent
represent its
its eastward
eastward
extension.
extension.
Marked on Figure 4 are five samples taken for
for whole rock major oxide
oxide and
and 32-element
32-element trace
trace
analysis. Two
Two are
are from
from unfoliated
unfoliated pillow
pillow basalts,
basalts, two are
are from
from the
the sheared
sheared lithologies, and the fifth is
The whole
whole rock
rock results
results are
are shown
shown on Figure 5.
from the ultramafic dike. The
Discussion
In this exposure chemically very similar rock
rock units
unitsexhibit
exhibit shearing
shearingthat
that isispreferentially
preferentially
developed in weaker units (zones of small pillows, lava interfiow
interflow beds, more
more magnesian
magnesian layers).
layers).
to shearing. The
Intervening more massive buttress units are resistant to
The sense
sense of
of shearing
shearingisis nearly
nearly vertical.
vertical
65
FeO* + 1102
SAMPLES
Cation %
Cation
%
Jensen
Jensen(1
(1 976)
976)
S
RRCVN
•
PR-i
A
V
RR-2
RR-3
•
RR-4
S
BK
V
A1203
MgO
Figure 5:
Stop SG-1
Figure
5: Stop
SG-1
66
Gait
outcrop on
on their
their map
map of
of the
the Marquette
Marquette quadrangle,
quadrangle,but
but didn't
didn't
Gair and Thaden (1968) noted this outcrop
indicate shearing or
or slaty lithology
lithology here.
here. A couple of
of miles
miles to
to the
the southwest
southwest they did describe
describe east-west
shearing at the southern contact of the Mona greenstones and the overlying
overlying Proterozoic
Proterozoic Enchantment
EnchantmentLake
Lake
conglomerate. Mapping
Fails
Mapping the
the next
next quadrangle
quadrangle west, Puffett (1974, p.43) recognized the Carp River Falls
shear zone near that same contact and correlated it with the shearing in the Marquette quadrangle
quadrangle
by Gair and Thaden. Both
previously noted by
Both authors
authors pointed
pointed out
out that
that the
the sense
sense of
of shearing
shearing isis strongly
strongly
vertical. Puffett
and noted
noted (p.
(p. 45),
45), "The
"The
Puffett (1974)
(1974) also
also recognized
recognized and named the Dead River shear zone and
projection of the shear zone to the southeast
southeast across the Marquette quadrangle
quadrangle meets
meets aa conspicuous
conspicuous rereentrant in the shoreline of
of Lake
Lake Superior."
Superior." Thus
Thus neither
neither of
of these
these two
two major
major later-recognized
later-recognized block
boundary shear zones is actually
actually shown
shown on
onGair
Gait and
andThaden's
Thaden's 1968 map.
map. This
This was
was left
left to
to later
later authors of
maps at various smaller scales, who usually place the Dead River shear
shear zone
zone as
as aa west-northwest
west-northwest trend
trend
meeting the lakeshore at the approximate contact with the Mona formation basalt
basalt flow unit of the southern
unit of
of the
the northern
northern block
block of
of the
the Ishpeming
lshpeming Greenstone Belt. This
block and the Lighthouse Point basalt unit
This
the Dead
Dead River
River shear
shear zone
zoneabout
about'/<
¾mile
miledue
duenorth
northof
ofthis
thisfield
fieldtrip
tripstop.
stop. (Figure 2)
2) The
would place the
The zone
seen here is a probable parallel, sympathetic
sympathetic structure.
Discussing the structural framework of the Ishpeming Greenstone Belt, Bornhorst
Bomhorst and
and Johnson
Johnson
(1993,
event (Dl)
(Di) was
was recumbent
recumbent folding.
folding. This was followed by
(1993, p. 4) stated,
stated, "The oldest recognized event
axes (Dz).
(D2). Shear zones are interpreted to
to have formed during and
upright folding about east-west oriented axes
later than 1)2
andmay
mayhave
havebeen
beenreactivated
reactivatedduring
duringthe
theearly
earlyProterozoic
Proterozoic Penokean
Penokean deformation."
deformation." They
D; and
They
"Rock units in all blocks are also cut by
by minor shear zones.
zones. These
went on to point out,
out, "Rock
These high-strain
high-strain zones
zones
are characterized by
by rocks that
that are more
more intensely
intensely foliated
foliatedthan
thanthe
thesurrounding
surroundingrocks."
rocks." We agree with
these interpretations and offer the exposure at this stop as a case in point.
point. Whether
Whether or
or not
not this
this particular
particular
major or minor one has yet
yet to
to be
be determined. To
shear zone is a major
To the
the west
west itit projects
projects into
into an
an urbanized
urbanized area
area
with no outcrop.
outcrop.
The sheared lithologies at this locality exhibit varying levels of propylitic alteration (pyrite,
carbonate, chlorite); significant
significant trace amounts of gold can
can be found where pyrite andlor carbonate are
are well
developed. As
Asaapractical
practicalmatter,
matter, further
further exploration
exploration for
for gold
gold in
in this
this shear
shear zone
zone isis ruled
ruled out
out by
by its
its
location.
67
56-2:
Stop SG2:
Dead
with gold
gold mineralization
mineralization at
at Marquette
Marquette Mall (Location map,
Dead River Shear Zone with
Figure 2).
Figure
2).
After parking at the head of the ski trail in the southeast comer
corner of the Marquette Mall parking lot,
walk about 500 feet ESE up the trail to an outcrop knob
knob of undeformed metabasalt. This
This greenstone
greenstone has
unusually high
high number
number of
of blocky
blocky hematitic
hematiticfractures.
fractures. The northern limit
limit of
of this
this outcrop
outcrop is
isaa cliff
an unusually
cliff face
face
that is controlled by
by foliation
foliation planes
planes striking
striking280°
280°,85%
85°S. This cliff
cliff face
face represents
representsthe
thesouthern
southern margin
margin of
of
was exposed
exposed in man-made outcrops in the bank of the Holiday Inn parking lot until it was
shearing that was
recently covered
covered by
by landscaping.
landscaping. R. Brozdowski (personal communication, 1988) reported anomalous
recently
gold values from Callahan's
Callahan's sampling
sampling of
of that
that sheared
sheared rock.
rock.
Returning about 400 feet westward down the trail, a makeshift wooden bridge crosses
crosses the stream
leading to the motel's duck pond. Intermittently
Intermittentlyexposed
exposed along
along the
the stream
stream are
are small outcrops
outcrops of sheared
chiorite-sericite rock
rock with
with brown
brown carbonate
carbonate weathering
weathering rinds
rinds and
and oblique
oblique quartz
quartz veinlets.
veinlets. Some
chlorite-sericite
Some of
of this
this
phenocrysts, indicating aa crystal
crystal tuff
tuff protolith.
protolith. Of
rock has small gray quartz phenocrysts,
Of three
threewidely
widely spaced
spacedgrab
grab
took here,
here, one
one gave
gave ,023
.023 odton
or/ton Au, the
the other
other two
two were
were below
below MPC's
MPC's detection limits. The
samples we took
The
sample that yielded the gold value was taken aa couple of feet below the little wooden bridge.
Mail parking lot and moving west along its south bank,
bank, there is a 650 foot
Back at the Marquette Mall
rock. Bornhorst
long exposure of highly sheared rock.
Bomhorst and Johnson (1993, p. 9)
9) gave aa brief description of this
"..... tuffaceous rock and a graphitic zone crop out."
outcrop: ".
rock is badly
badly slumped northward towards the
the parking
parking lot throughout
throughout
This sheared phyllonitic rock
much of its length. At
At its
its eastern
easternend
end itit is
is more
more hematitic
hematitic than elsewhere,
elsewhere, with reddish
reddish foliation
foliation planes
planes
in the
the relatively
relatively underformed
underformed greenstone
greenstone to
to the
the east.
east. Throughout
resembling the fractures in
Throughout its
its entirety
entirety there
on foliation planes,
planes, with
with much
much of the rock exhibiting a
is a strong presence of chlorite and sporadic sericite on
color due
due to
to weathering
weathering of
of chlorite
chloriteand
andferroan
ferroancarbonate.
carbonate. Where not
not slumped,
slumped, foliation
maroon-reddish color
measurements
measurements cluster
cluster closely
closely about
about 280°,
280°vertical.
vertical.
probable tuff
There are some zones of chlorite-poor rock with small quartz eyes indicating aa probable
protolith, similar to that found in the stream to the east. Near
Near the
the eastern
eastern end
end of
of the
the outcrop
outcropisisan
an irregular
irregular
to 2 feet layer of nearly pure graphite. Our
Our grab sample of the graphite gave
foliation-parallel two inches to
no Au. One
Oneorortwo
twofeet
feetacross
acrossfoliation
foliationfrom
fromthe
thegraphite
graphiteisisaathin
thinsemi-parallel
semi-parallel layer
layer of
of rusty
rusty to
to black
gossan with selvages of massive and semi-massive
semi-massive pyrite carbonate
carbonate breccia.
breccia. From
From the
the pyrite-gossan
pyrite-gossanzone,
zone,
Gleason (1986) reported values
values up
up to
to 22 gmltonne
gmltonne (.06
(.06 ozlton)
or/ton) Au.
Au. Our
Our grab
grab sample
sample of
of the
the gossan
g o w n gave
gave
.037
gave .028
.028 odton
or/ton Au.
Au. A
0 3 7 or/ton
odton Au,
Au, while our sample of pyrite breccia selvages gave
A polished
polished thin
thin section
section
sample of the pyrite breccia shows
shows that there are
are two generations
generations of
of pyrite
pyrite in
in aa quartz-calcite-chlorite
quartz-calcite-chlorite
68
gangue, with the calcite late
late in
in the
the paragenetic
paragenetic sequence.
sequence. The first generation pyrite is large and often
framboidal, corroded and
and partially
partially replaced
replaced by
by quartz
quartz and
and second
secondgeneration
generationpyrite
pyrite in
in smaller,
smaller,euliedral
euhedral
crystals. Heavy
as inclusions
inclusions in the second generation pyrite and as tiny
Heavy traces
traces of
of covellite
covellite (CuS)
(CuS) occur
occur as
gangue. Other
free grains in the gangue.
Other inclusions
inclusionswithin
within pyrite
pyrite found
found by
by the
the microprobe
microprobe include
include chalcopyrite
and sphalerite as
as lesser trace sulfides. Accessory
Accessory minerals
minerals found
found in
in this
this breccia
breccia are
are monazite
monazite and
and ilmenite
ilmenite
rutile, a common
hydrothermal alteration.
alteration. No grains
common result of hydrothermal
partially altered to mtile,
grains of
of free gold were found
in this thin section sample. Besides
Besides the
the highly anomalous gold in
in the corresponding
corresponding assay
assay grab
grab sample,
sample,
anomalous trace
trace elements
elementswere
werepresent
present(ppm):
(ppm): Ag-3;
Ag-3; As-252;
As-252; Cu-1700;
Cu-l700; Pb-68; Zn-124; Uthe following anomalous
10.
10.
About 150
150 feet from the western end of this exposure is a 2-foot complexly brecciated chloritecalcite-quartz-graphite vein
vein with
with strong
strong vertical
verticalhematitic
hematiticslickensides
slickensidesand
andminor
minorcopper
copperstain.
stain. The
slickensided hangingwall of
of this
this vein
vein strikes,
strikes, 290'
290° 75'N.
75°N. Our
Our grab
grab sample
sample yielded .005
.005 ozlton
odton Au,
Au, as
aswell
well
as 244 pprn
ppm Cu and 100 ppm Zn. Study
Study of
ofaa corresponding
corresponding polished thin section did not fmd any gold
grains but confirmed the presence of
of very fine grained
grained chalcopyrite
chalcopyrite and
and covellite
covellite in
in irregular
irregular trains
trains
following late fractures and rimming graphite. This
This vein
vein appears to be cut
cut off
off by
by aa northerly
northerly cross
cross fault.
fault.
About 75 feet from the western end of the exposure is a thin discontinuous vein
vein of similar composition;
our grab sample
sample yielded
yielded no
no Au.
Au.
About 100
100 yards southward and uphill
uphill from the west end off the Mall exposure is a slumped-in
and partially flooded small open
open cut
cut next
next to
to the
the north-flowing creek. In
to the
the south
south wall of
of this old mine pit
there is an exposed patch of
of rusty
rusty weathering,
weathering, ankeritic phyllonite similar to the Mall exposure
exposure lithology.
lithology
of the
the ore
ore mined.
mined. This
There are no dumps or other present evidence of
This is
is the
the Eureka
Eureka mine
mine (Williams,
(Williams, 1890).
1890).
Gair and Thaden (1968, p. 69) provided the following: "The
"Theprincipal
principaloccurrence
occurrenceof
ofiron
ironininthe
the
Marquette-Sands area was at the old Eureka mine ...
... Perhaps
Perhaps as much as several hundred tons of earthy
goethitic iron ore was mined from
from aa shear
shear zone
zone about
about 55 feet
feet wide
wide in
in chloritic
chloritic slaty
slatygreenstone
greenstone (Mona
(Mona
Schist). Secondary
Secondaryquartz
quartz and
and iron-bearing
iron-bearing carbonate
carbonate were deposited along the shear zone in places, prior
to formation of
of the iron ore. The
Theore
oreresulted
resultedfrom
fromoxidation
oxidationof
ofcarbonate
carbonateand
andchlorite,
chlorite,the
theconcentration
concentration
of iron,
iron, and
and the
the removal
removal of
of silica,
silica, MgO,
MgO, CaO
CaOand
and alumina
alumina by
by ground
groundwater
watercirculating
circulatingalong
alongthe
theshear
shear
zone. Virtually
Virtually all
all mining
mining was
was done
done prior to
to 1880,
1880,and
and whatever
whatever ore
ore was extracted was shipped
shipped to
to local
smelters. There were several unsuccessful attempts to
to renew
renew mining
mining in
in the
the 1880's and
charcoal-burning smelters.
three test holes were drilled at the site in the 1920's".
1920's".
69
Discussion
Discussion
This
(1968, P1,
1). Their
Thislocality
locality was
was last
last mapped by (3air
Gair and Thaden (1968,
PI. 1).
Their map shows
shows only
only aa couple
couple
of
members of
of small
small exposures
exposureshere,
here, marking
marking the contact between their Lower and Lighthouse Point members
of the
the
Mona
MonaSchist.
Schist.We
Webelieve
believethat
thatthe
theoutcrops
outcropshere
hererepresent
representthe
thesouthern
southernmargin
marginof
ofthe
theDead
DeadRiver
Rivershear
shear
zone.
zone. The
Thepresence
presenceofofgold
goldand
andbase
basemetals
metalsininthe
theMaIl
Mallexposure
exposureisisdirect
directevidence
evidence of
of their
their localization
localization by
by
major
major shear
shearzones,
zones, and
and we
we suspect
suspect that some
some of
of the
the ore
ore produced
produced by
by the
the Eureka
Eureka mine
mine may have
have been,
been, in
in
aftersuluides.
sulfides.
part,gossan
gossanafter
part,
The
The trend
trend of
of shearing
shearingprojects
projects from
from here
here westward
westward along
along the
the valley of an E-W creek.
creek. No
No
exposures
exposures could
could be
be found
found along
along itit except
except aa few
few outcrops
outcropsof
of aanarrow
narrowE-W
E-W magnetic
magnetic diabase
diabase dike
dikeon
on its
its
south
4,200feet
feetwest
west of
ofhere,
here, probably
probably emplaced
emplaced atator
orclose
closeto
tothe
thesouthern
southernmargin
margin of
of the
theshear
shearzone.
zone.
southside
side4,200
Thatdiabase
diabaseisisnot
noton
onany
anypublished
publishedmap.
map.
That
StopSG—3:
SG3:
Stop
Archean metavolcanics
metavolcanics (Location
map,
Keweenawan diabase in hornfelsed, foliated Archean
(Location map,
Figure
Figure2).
2).
Loojeuvs &4.cr
'4
C
-c
-a
C
U
'4
'7
sphero,Sc
'a
0"
S'a 6atc
h. h b It
pg
S
-c
1
-z
Lj
:W40/e t-k/Occay scnsp.'e
SM
0
U.
a
m rr
AFPCg
m&>C
o%
.
reE
Figure6.
6. Sketch
Sketch cross
cross section of outcrop
outcrop at
at Stop
StopSG-3,
SG-3,looking
lookingeast.
east.
Figure
70
This outcrop was last mapped by
by Gair
Gair and
and Thaden
Thaden (1968,
(1968,P1.
PI. 1)
1) as
as Late
Late Precambrian
Precambrian diabase
diabase with
with
a small adjoining patch of
site" belonging to their lower unit of the Lighthouse Point Member
of "fel
"felsite"
Member of
of the
the
Mona Schist. There
There is no outcrop description in their text. This
This one
one is
is part of
of a series
series of outcrops
outcrops of
of a
long narrow diabase dike that Gair and
and Thaden traced eastward
eastward into
into Lake
Lake Superior
Superior at
atLighhouse
Lighhouse Point.
Point.
Here it strikes 250°
250°, dip 8O0N.
80°N. ItIt appears
not
appears to
to be
be about
about 50
50 feet
feet thick,
thick, although its southern contact is not
exposed. The
Thediabase
diabaseisisnon-foliated
non-foliatedand
andisismagnetic
magneticthroughout.
throughout.
Figure 6 isisaa sketch
sketch cross
cross section
section of this exposure. At
Atthe
thesouth
south end
end the
the diabase
diabase is
is medium
medium
jointing
grained, has a felty texture and contains rare vugs lined with drusy calcite.
calcite. It shows
shows rectangular
rectangularjointing
that controls incipient spheroidal
spheroidal weathering
weathering at the erosion surface;
surface; the joint planes
planes are
are hematitic
hematitic and
and
calcareous. The
The diabase
diabase grain
grain size
size lessens
lessens northward to a fme
fine grained but still magnetic chilled margin at
the host rock contact. This
Thiscontact
contactisisstrongly
strongly foliated
foliated parallel
parallel to
to the
the dike
dike margin
margin for
for aa few
few inches.
inches.
Beyond that the immediate
immediate host
host rock
rock is
is aablack,
black,non-magnetic,
non-magnetic,aphanitic
aphanitichomfels
homfelswith
with strong
strong foliation
foliation
defined mostly by prominent pink K-spar-quartz laminae. This
Thisrock
rock carries
carries aa trace
trace of
of disseminated
disseminated
extremely fine-grained pyrite, and some
some discontinous
discontinous foliation-parallel
foliation-parallel lenses
lenses of
of coarsely
coarselyciystallized
crystallized
quartz.
greenish gray.
gray. Outward to the north, the host rock becomes rusty weathering,
quartz. It weathers light greenish
"baked"-lookthg, all the while exhibiting strong foliation planes closely
slaty and less "baked"-looking,
closely clustered
clustered around
around
280°, 80%.
80°N. The source
280"
source of
of the rusty color appears to be numerous foliation partings of ferroan carbonate;
no pyrite is visible on a fresh surface. An
An irregular
irregular 4-6
4-6 inch
inch foliation-parallel
foliation-parallel quartz-iron oxide
oxide vein marks
marks
the transition northward to non-thennally
non-thermally altered
altered grainy-light
grainy-light greenish-gray
greenish-gray sheared
sheared rock
rock with
with rusty
rusty
275°, 85¡
85°N continuing
continuing to the northern limit of the exposure.
exposure.
foliation planes at 275¡
Four grab samples of the host rock are shown on the sketch
sketch cross
cross section.
section. Two
Two of
of these
these were
were
clearly below
below MPC's
MPC's detection limit for gold (samples DBS-2
DBS-2 and
and 3).
3). The other two returned values at or
near the detection limit and were re-analyzed by Chemex using FA-AA, returning weakly anomalous gold
values: DBS-l,
DBS-1,51
5 1ppb;
ppb;HFEL,
HFEL,33
33ppb.
ppb.No
Nounusual
unusualtrace
traceelement
elementsignatures
signatureswere
werepresent
presentininany
anyof
ofthese
these
samples.
samples.
Study of a polished thin section of a sample corresponding to assay sample HFEL
HEEL confirmed that
that
the pink bands consist of quartz phenocrysts
phenoctysts in a K-spar groundmass. The
The darker-colored
darker-colored domains
domains of
of this
this
phenociysts/ciystals of albite-oligoclase,
rock have a porphyritic texture with scattered phenocrysts/crystals
albite-oligoclase, quartz
quartz (often
(often in
in
with pyrite),
pyrite), and
and composite
composite large
large grains
grains or
or fragments
fragments of
of quartzquartz- plagioclase.
plagioclase. The dark
biminerallic grains with
color appears to be mainly due to biotization
biotization of
of numerous
numerous foliation-defining
foliation-defining chlorite-quartz-plagioclase
chlorite-quartz-plagioclase
to ruhle.
rutile. The
laminae and to an overall content of about 5% opaques: pyrite and ilmenite altered partially to
The
nature of the coarse grained components suggests that
that this
this rock
rock was
was originally
originallyaa crystal
crystallithic
lithictuff.
tuff. The
71
thermal metamorphic effects
effects near the contact of the diabase
diabase dike
dike appear
appear to be limited
limited to
to prograde
prograde
alteration of chlorite to biotite
biotite and recrystallization
recrystallization of quartz and feldspar.
feldspar. Figure 7 is a Jensen cation
plot of
of the
the whole rock analyses
analyses of the four
four samples
samples taken at this stop.
stop.
Discussion
Discussion
The foliation in the weakly hornfelsed country rock here is at an oblique angle to the diabase
diabase dike,
can best
best be
be seen
seen at
at the
the top
top of
of the
the outcrop
outcropknob).
knob). Because of this, we
(which can
webelieve
believethis
thisfoliation
foliationprepreexisted the emplacement of the dike. We
We also
also believe
believe that much of the foliation
foliation in the host rock here is the
result of shearing,
shearing, and that this outcrop lies near the middle of the Dead River
River shear
shear zone. The
The southern
southern
boundary of this shear zone would lie
lie near
near the
the Marquette
Marquette Mall
Mall outcrop
outcropatatStop
Stop SG-2.
SG-2. The emplacement of
this diabase
diabase dike
dike within such
such aa large
large zone
zone of
of weakness
weaknesswould
would not
notbe
beunexpected.
unexpected.
Gaff and
and Thaden
Thaden (1968),
(1968), though noting shearing in
in their
their text
text from
from place
place to
to place in the Archean
Gair
Quadrangle, did
did not
not emphasize
emphasize its
its importance,
importance,nor
norillustrate
illustrateititon
ontheir
theirmap.
map. They
They
rocks of the Marquette Quadrangle,
also found very few outcrops of their lower unit of the Lighthouse Point Member, most of which are
are
shown as "felsite".
"felsite". About
About 1200
1200feet
feet NNW
NNW of
ofhere,
here, theft
their map
map shows
showsaa cluster
cluster of
of 44 small
small outcrops
outcrops on
on aa low
low
bill,
"felsite". The
hill, marked as ""felsite".
TheWestwood
WestwoodMall
Mall and
and its
its parking
parking lots
lots now cover
cover them, but we suspect that
have exhibited
exhibited shearing
shearingsimilar
similartotohere.
here. About 500 feet further north in
in the
the woods
woods
these outcrops would have
and sewer line
line were
were being
being constructed
constructed in
in the
the fall
fall of
of 1998. The
behind the Mall, a new east-west street and
The
construction equipment tore up the tops of several small subcrops of sheared rock whose composition
chioritic to nearly pure
pure massive
massive sericite. ItIt was
was not
not possible
possible to
to obtain
obtain dips, but the strike
varies from highly chloritic
of the shearing is east-west.
east-west. These exposures lie
he near the probable northern contact
contact of
of the
the Dead
Dead River
River
shear zone. Since
Since it's
it's unlikely
unlikely that
that they'll
they'll be
bestill
still available
availablein
in the
the spring
spring of
of 1999,
1999, we
we offer
offer Stop
Stop SG-4
SG-4 as
as
an alternative.
alternative.
72
FeO* + 1102
SAMPLES
•
•
Cation
Cation %
%
Jensen (1976)
Jensen
(1 976)
A
DBS-1
DBS-2
DBS-3
v HEEL
BK
A1203
A1203
MgO
Figure 7:
Stop SG-3
Figure
7; Stop
SG-3
73
Stop
Stop SG-4:
SG4:
margin of Dead River Shear
Northern margin
Shear Zone on Wright
Wright Street
Street (Location map,
Figure
Figure 2).
2).
This is an optional
optional stop
stop that
that is
is included
included here
here for
for those
those interested
interestedin
in seeing
seeing more
moreof
of the
theDead
DeadRiver
River
Zone in
in the
the Marquette
Marquette area.
area. This exposure
be
Shear Zone
exposure is very close to a busy highway, and care should be
exercised when
when visiting
visitingit.
it. About
About 65
65 feet
feet of
of sheared
sheared greenstone
greenstone is exposed here on the east side of Wright
Street. The
The lower
lower level
level of
of the
the exposure
exposurewas
wasrecently
recently created
created by
by widening
wideningthe
the road,
road, and
andwas
wasnot
notavailable
available
to Gair and Thaden (1968), who showed
showed the
the upper
upper part
part of
of the
the exposure
exposureas
as aa lithology
lithologylabeled
labeled"interlayered
"interlayered
)
massive basalt(?)" on
on their
their map
map(P1.
(PI. I1).
The degree of shearing is variable here,
here, but
but much
much of
of itit is
is intense.
intense. Foliation
The
Foliation becomes generally
more strongly developed from north to south, and strikes E-W,
E-W, nearly
nearly vertical.
vertical, hi
In the
the small
small cut above
above the
forms similar to
to the
the sheared
shearedpillows
pillows at
at Stop
StopSG-1
SO-i are present. Figure
lower ledge, curvilinear sheared forms
Figure 88 isis
a photograph of pencil cleavage
cleavage developed
developed further
further south
south as
as shearing
shearing intensifies.
intensifies. There
Thereisisweak
weakcarbonate
carbonate
alteration in some of the most heavily sheared rock, but it is not pyritic; we didn't sample
samplefor
forgold
goldhere.
here.
Figure 8.
Figure
8. Looking
Lookingdue
dueeast
eastatatpencil
pencilcleavage
cleavagedeveloped
developedby
by shearing
shearingin
in greenstone
greenstonein
in low
low
roadcut, east side
side of
of Wright
Wright Street,
Street, Stop
StopSG-4.
SG-4.
74
The northern part of this outcrop consists of nearly chaotic chloritic foliation that appears to result
from the contact with a shear-resistant dike of non-magnetic gabbro that is only weakly foliated. Only
Onlyaa
couple of feet of this gabbro
gabbro are exposed
exposed in the roadcut, but a larger, glacially smoothed outcrop
outcrop of
of itit lies
lies
above
above and
and aa few
few yards
yards east
east of
of here.
here.
ii
D is c u S 5
Discusssion
About 400 feet to the north, on the east side of
of the road
road is an
an outcrop
outcrop of
of relatively undeformed
metabasalt; glacially polished outcrops
outcrops of similar rock are exposed
exposed in the south side
side of the gravel pit being
development aa couple
couple of
of hundred
hundred yards
yards further
furtherENE.
ENE. These rocks appear
appear to
to be
be
converted to a housing development
Point basalt
basalt typical
typical of
of this
this area
area(Johnson
(Johnsonand
andBomhorst,
Bornborst,1991).
1991). So we would
would place
place the
the
Lighthouse Point
northern boundary of the Dead River Shear Zone, if not at this outcrop, then no more than a few hundred
feet north of here. Figure
Figure22shows
showswhat
what we
we think
think are
are the
the approximate
approximatelimits
limits of
of the
the shear
shear zone,
zone, which
which
corresponds roughly to the lower unit
imit of the Lighthouse Point
Point Member of
of the
the Mona
Mona Schist of
of Gair and
corresponds
Thaden (1968),
(1968), in turn corresponding roughly to the "Eureka Group" of Williams
Williams (1890), nomenclature
nomenclature
that was discarded by Van Hise and Bayley (1897). This
Thiswould
wouldmake
makethe
theDead
DeadRiver
RiverShear
ShearZone
Zone3,000
3,000
feet wide at this point. Puffett
Puffett (1974,
(1974, p.
p. 44)
44) noted
noted that
that the
the Dead
Dead River
River Shear
Shear Zone four miles west-
northwest of here is about 3,400
3,400 feet
feet wide.
wide.
To better define
define and
and understand
understand the
the Dead
Dead River
River Shear
ShearZone
Zone in
in this
this important
importanthinge
hinge area
areabetween
between
the northern and southern blocks of the Ishpeming Greenstone Belt,
Belt, remapping
re-mapping and
and structural study
study of
of
existing and newly
newly created
created outcrops
outcrops are
areneeded,
needed,before
beforeit's
it's too
too late.
late. The ex-urban growth around
Marquette,
Marquette, although presently
presently creating
creating new outcrops,
outcrops, will
will eventually
eventually result in a net loss
loss of
of exposures.
exposures.
75
I
I
I
Figure 9. Location map for Stops SG-5,6 and?
76
Stop SG-5:
Stop
SG5:
Progressively shearedlaltered
sheared/altered metavolcanics
metavolcanics in Carp River Falls Shear
Shear Zone.
9).
(Location map, Figure
Figure 9).
Puffett
Puffett (1974)
(1974) last mapped this
this locality
locality as
as "undifferentiated
"undifferentiated greenstone"
greenstone" within
within the Carp River
Falls shear zone. Dealing
Dealing with
with this
this exposure
exposurespecifically,
specifically,he
he included
included aa photograph
photograph of
ofits
itsmost
mostsheared
sheared
part, and described it as follows (p. 43): "Sericite,
and altered part.
"Sericite,chlorite,
chlorite,carbonate,
carbonate,and
andleucoxene
leucoxeneare
arethe
the
products. Weathered
brown from
from oxidized
oxidized
most conspicuous alteration products.
Weathered surfaces are commonly stained brown
minerals. Quartz-carbonate
iron minerals.
Quartz-carbonate veinlets fomi
form an
an anastomosing network in some rock. Copper
Copper minerals
minerals
are present locally. Analytical
Analytical data
datasuggest
suggestthat
that the
the altered
altered rocks
rocks are
are enriched
enriched in CaO
CaO and Co2
Co2(Table 12)."
value, 90
90 ppm.
ppm. W.A.
Puffett's analysis from here gave an anomalous copper value,
W.A. Bodwell
Bodwell (personal
(personal
communication, 1998)
1998) reports having noted trace chalcopyrite and copper staining here, hut
but they are not
not
easily found.
found.
This exposure was a 1988
follows (Bornhorst
(Bomhorst and
1988 I.L.S.G. field trip stop
stop described,
described, in part, as follows
others, 1988):
1988): "The
"The rocks
rocksin
in this
this roadcut
roadcut have
have aa well developed close-spaced foliation (N75-88W, dip 7089°S) which produces
produces a slate-like appearance.
appearance. These
89OS)
These rocks
rocks are
are within
within the Archean Carp River Falls Shear
Zone. There
two lithologies
lithologiesin
in this outcrop: chlorite
chloriteschist
schistfrom
fromaabasaltic
basaltic parent,
parent, and
and quartz-sericite
quartz-sericite
Thereare
are two
schist from a rhyolitic parent."
Bornhorst
Bomhorst and others (1988) also pointed out that near the very eastern end of the outcrop are
probable relict pillow rinds. As
probable
Asseen
seenelsewhere
elsewhereon
onthis
thisfield
fieldtrip,
trip,these
these are
are gradually
gradually obliterated
obliterated by
by
shearing as one moves westward
westward along the outcrop.
outcrop. The
The chioritic
chloritic sheared
sheared metabasalt
metabasalt gives way
progressively to more and
and more
more rusty
rusty weathering
weathering carbonate-sericite
carbonate-sericitealtered
altered rock;
rock; we
we believe
believe itit represents
represents aa
progressively
progressive change from
from propylitic
propylitic alteration
alterationto
to weak
weak potassic
potassicalteration
alterationin
in aa rock
rock of
ofthe
thesame
sameparentage.
parentage.
progressive
lighter colored
colored carhonate-sericite
carbonate-sericitealtered
alteredrock
rocknear
nearthe
thewestern
westernend
endthere
thereisisaa 6-inch
6-inch pyritic
In the lighter
pyritic
carbonate zone. Our
Our grab
grab sample
sample of
of this
this gave .005 oz/ton
odton Au (sample STOP 3). AA70-foot
70-footchip
chip sample
sample
across
across the entire
entire sericite-carbonate
sericite-carbonate altered
altered zone here (not including
including the pyritic
pyritic grab
grab sample)
sample) gave
gave
undetectable AU
Au (sample STOP-3A)
STOP-3A)..An
AnFA-AA
FA-AAre-analysis
re-analysisby
by Chemex
Chemex of
of the
the pyritic
pyritic grab sample gave a
value of 90 ppb Au,
Au, confirming MPC's
MPC's results. We
Wealso
alsotook
tookwhole
wholerock
rocksamples
samplesfrom
fromthe
thevery
veryeastern
eastern
exposure in the pillowed
pillowed greenstone
greenstone(sample
(sample 5-1),
5-l), one
onefrom
frommoderately
moderatelysheared
shearedrock
rocknear
nearthe
the
end of the exposure
(5-I), and one at the western end of the outcrop in approximately the same spot sampled and
middle (5-1),
photographed by Puffett (our sample 5-3). Whole
Whole rock analyses
analyseswere
were also
also done
done on
on the
the two samples
samples taken
for gold assay; sample STOP-3A is the
themost
mostrepresentative
representativeof
of the
the light
light colored
colored sheared
sheared rock at
at the
western end of the exposure,
exposure, since
since itit was aa composite
composite chip
chip sample
sampleacross
across 70
70 feet
feetof
ofit.
it. Figure 10
10 is a
77
Jensen plot showing the
Note that
that sample
sample STOP
STOP33 is
is anomalously
anomalously high
high in
in iron
iron because
becauseititwas
was
Jensen
the results.
results. ((Note
chosen
chosen for
for best
bestpyrite
pyritecontent).
content).
+
FeO*
FeO* + 1102
Ti02
Cation%
%
Cation
Jensen (1976)
(1 976)
Jensen
SAMPLES
SAMPLES
• 5-1
5-1
• 5-2
5-2
5-3
5-3
"v STOP3
STOP 3
A
A
• STOP-3A
STOP-3A
PJ203
MgO
Figure
Figure 10:
10: Stop
Stop SG-5
SG-5
78
A polished thin section was made of a specimen corresponding
corresponding to
to pyntic
pyritic grab
grab sample
sample STOP
STOP 33
that yielded anomalous gold. Megascopically
Megascopicallythis
thisspecimen
specimenisisaa chlorite-sericite
chlorite-sericiteschist
schist hosting
hostingaa foliationfoliation¼-inch veinlet
veinlet of
of carbonate.
carbonate. Under
conformable %-inch
Under the microscope
microscope and electron microprobe the host rock is
1%tiny
tiny
a fine grained mixture of 50% quartz, 30%
30% chlorite
chlorite and 20%
20% niuscovite/sericite,
muscovite/sericite, containing
containing about
about 1%
rutile grains. The
grains of disseminated chalcopyrite, as well as trace amounts of tiny disseminated mtile
The
dolomite with high-manganese cores, with about
carbonate veinlet is a mosaic of coarse grains of zoned dolomite
10%
high-iron chlorite.
chlorite. This
10% interstitial high-iron
This vein
vein assemblage
assemblage hosts
hosts schlieren texturally similar to the host rock
except that these fragments have no quartz.
quartz, consist primarily of zoned ankerite with dolomitic cores and
contain about 25% opaques, mostly Mn and Fe oxides, but also abundant fine
fine grained euhedral pyrite and
subordinate chalcopyrite. One
Oneof
of the
thechalcopyrite
chalcopyritegrains
grains was
was found
found to
to have
have inclusions
inclusions of
of nonnonand one 3-micron grain of
of high-gold
high-gold electrum
electrum (photo,
(photo, Figure 11). This
argentiferous tetrahedrite and
Thiswas
was the
the
only gold grain found in
in this
this sample.
sampie Arsenopyrite
Arsenopyritewas
wasnot
notfound,
found,but
but trace
traceelement
elementanalysis
analysis of
of the
the
corresponding assay sample yielded 170
170 ppm As.
'I,
3e+
Figure
3, White
Figure II.
11.Polaroid
Polaroidphoto
photoofofbackscattered
backscatteredelectron
electrondetector
detectorimage,
image, sample
sample STOP 3.
White
vertical scale bar at lower left is 10
10 microns. Bright 3-micron grain of
of high gold
gold electrum
electrum in
in
(teØ. Black
chalcopyrite (cpy) that also contains larger inclusions of tetrahedrite (tet).
Black areas
areasare
are gangue.
gangue.
79
Discussion
Puffett (1974,
(1974, P.
p. 43-44)
43-44) noted
noted copper
copper minerals
minerals at
at more
more than one locality in the Carp
Carp River Falls
shear zone, an encouraging base metal association from
kom the gold exploration
explorationpoint
point of
ofview,
view,since
sincegold
gold
mineral here.
here. This
occurs directly with a copper mineral
This outcrop
outcrop is
is close
close to
to the
the north
notth margin
margin of the shear
shear zone.
zone.
margin appears to be marked by bluffs that are coneolled
controlled by
by E-W foliated greenstone, across
across
The south margin
behind Ball Moving and Storage, makiig
making the
the zone
zone about
about 400
400 feet wide
wide at
at this
this point
point, mostly
mostly
the highway behind
covered by U.S.Highway 441.
1. We
Wewill
will cross
crossthe
the highway
highway here to
to see
see more
more evidence
evidence of
of gold
gold
mineralization.
mineralization.
Stop SG-6:
Stop
SG&
Gold-bearing
gtacial erratic
erratic (Bill
Bodwell'sBoulder)
Boulder).. Location
Gold-bearing glacial
(Bii Bodwell's
Location map,
map, Figure
Figure 9.
first noticed
noticed and
and sampled
sampled by
by Bill
BillBodwell
Bodwellininthe
the1980's.
1980's. His
This was fust
His attempts
attempts to find
fmd a bedrock
source in the immediate
immediate vicinity were unsuccessfid.
unsuccessful. However,
However, judging
judging by
byits
itslarge
largesize
sizeand
andangularity,
angularity,this
this
boulder hasn't traveled
traveled very
very far.
far. The
Theeastern
easternend
endof
ofthe
theboulder
bouldershows
showsthe
thepreserved
preservedremains
remainsof
ofan
an
original bedrock surface.
snrface.
This rectangular 5 by 10
I0 foot
foot block
block was
was unearthed
unearthed in
in sandy
sandy glacio-fluvial
glacio-fluvial sediments
sedimentsduring
during site
site
clearing for the moving and storage facilities, and
and has
has lain
lain in
inits
itspresent
presentposition
positionfor
forabout
about 30
30years.
years. It
up to
to one
one foot thick.
thick. The
consists of about
about 50%
50% close-spaced,
close-spaced, sub-parallel
sub-parallelen
en echelon white quartz veins up
veins contain l0%-20%
10%-20%light
lightbrown
brownweathering
weathehgferroan
ferroancarbonate
carbonateininblobs
blobsand
andseamlets,
seamlets,lesser
lessersmall
small
masses of dark green euhedral
euhedral chlorite,
chlorite, and rare
rare small
small blobs
blobs of
of very
very late
late stage
stage black tourmaline.
tourmaline.
Interstitial to the quartz veins,
veins, the
the other 50%
50% of
of the
the boulder
boulder is
is made
made up
up of
of highly
highly foliated
foliated "horses"
"horses" of
of
chloritic greenstone, showing an impressive
impressive internal
internal continuity
continuity of
of foliation throughout
carbonate-altered cblontic
with oblique tensional development in a
boulder. The
vein g", consistent with
the boulder.
The style
style of veining is "ladder veining",
sheared
sheared rock.
rock.
chalcopyrite, and abundant malachite
The quartz veins contain
contain patchy blobs of pyrite with lesser chalcopyrite,
staining. Bill
Bill Bodwell's
Bodwell'sinitial
initialgrab
g a bsample
sampleof
ofthe
thesulfide-bearing
sulfide-bearingquartz
quartz gave
gave .267
,267oz/ton
odton Au;
Au; two later
later
10,000 ppm Cu,
samples
odton and .006 01/ton,
odton, respectively. All
All of
of these
these samples
samples yielded ++lO,OOO
samples yielded .036 oz/ton
Ch
showing that copper, although obviously mineralogically associated with gold here, does not necessarily
have a direct assay correlation (Bodwell, 1988, written
written communication).
communication). Our grab sample
sample of the quartz.006 odton
oz/ton Au.
Au. Our
greenstone yielded 005
sulfide gave ,006
Our sample
sample of
of the
the barren-looking host greenstone
,005o1/ton
odton Au.
Au.
80
This
This wide range of gold
gold assay results demonstrates the "nuggety" distribution
dihbution of gold
gold within
within this
this boulder,
boulder,
aa phenomenon
phenomenon that
that requires
requires special
special assaying
assaying techniques
techniquesto
toobtain
obtainaarepresentative
representativevalue.
value.
However,
Carp
However, the
the main point of
of this
this field
fieldtrip
trip stop
stopisisthat
that this
this boulder
boulderlikely
lkeiy caine
cameeither
eitherfrom
from the
the Carp
River
from a similar zone nearby. It's
River Falls
Falls shear zone or &om
It'sinteresting
interestingto
to note
notethat
that glacial
glacialstriae
striae on
on the
the
pillowed greenstone
greenstone near
near the USGS
USGS benchmark
benchmark at
at the eastern
eastern end
end of
of the
theStop
StopSG-5
SG-5outcrop
outcrop across
across the
the
pillowed
highway
highway are
are nearly
nearly east-west,
east-west, paralleling the shear zone. The
Thestrongest
strongestinference
inferencefrom
h m the
theevidence
evidenceatat
both
both this
this stop
stopand
and the
the preceding
preceding one
one isis that
that the
the Carp
CarpRiver
River Falls
Fallsshear
shear zone
zone isis in
in itself
itself aagold
goldexploration
exploration
target.
target.
Stop
StopSG-7:
SG7:
Basal
Basal Proterozoic
Proterozoic Enchantment
EnchantmentLake
Lakeconglomerate
conglomeratein
in contact
contact with Carp
Carp River
River
Falls
Falls shear
shear zone
zone in
in Archean
Archean metabasalt.
metabasalt. (Location
(Locationmap,
map,Figure
Figure9).
9).
This
This spot
spot has
has been
been aa classic
classic field
field trip
trip stop
stop for
for seeing
seeingthe
the features
features of
of the
the Mesnard
Mesnard and
and Ajibik
Ajibk
quartzites
U.S 41
41roadcut.
roadcut.But
Butour
ourinterest
interestlies
liesthither
furtherdown-section
down-sectionininthe
theProterozoic,
Proterozoic,atatits
its
quartzites in the US.
contact
contact with
with the Archean. This
Thislocality
localitywas
waslast
lastmapped
mappedby
byPuffett
hffett(1974),
(1974),who
whoshowed
showedthe
theArchean
Archean
rocks
rockshere
here as
as Mona
Mona Schist,
Schist, now
now included
included in
in the
the informal
informal lower
lower basalt flow
flow unit
unit of
of the Mona
Mona Formation
Formation of
of
Bornhorst
Bomhorst and
and Johnson
Johnson (1993).
(1993).Puffett's
Puffett'smap
map(P1.
(PI.1)1)shows
showsthe
theCarp
CarpRiver
RiverFalls
Fallsshear
shearzone
zoneasasaaline
line
passing
passingabout
ahoutaa half
half mile
mile north
north of
ofhere,
here, but
but he
hestated
statedin
in his
his text
text (p.43),
(p.431, "The
"The zone
zoneisis indicated
indicatedon
on the
the map
map
as
asaanarrow,
narrow,well
well defmed
defmedstructure,
structure,but
but the
the symbol
symbolmarks
marks only
only the
the north
north limit
limit of
of aa broad
broad zone
zoneof
of sheared
sheared
and
andaltered
alteredlower
lowerPrecambrian
Precambrianrocks."
rocks."
After
M e rparking
parking at
atthe
the intersection
intersectionof
of U.S.
U.S. 41
4 1and
andChippewa
ChippewaDrive,
Drive, walk
walk about
about100
100feet
feetnorthwest
northwest
past
pastthe
thebillboard
billboardto
to aaseries
seriesof
oflow
lowoutcrops
outcropsininthe
thewoods.
woods.The
Thefirst
fistone
oneencountered
encounteredisisglacially
glaciallypolished,
polishe4
and
andexposes
exposesthe
theCarp
CarpRiver
RiverFalls
Fallsshear
shearzone
zoneinincontact
contactwith
withthe
theoverlying
overlyingbasal
basalconglomerate
conglomerateofofthe
the
Enchantment
EnchantmentLake
L&e Formation.
Formation.Both
Botheast
eastand
andwest
westofofhere
herethere
thereare
arenumerous
numerousnearby
nearbyexposures
exposuresofofthe
the
shear
shearzone
zonelying
lyingnorth
north of
of this
this contact.
contact. AAwidth
widthofofabout
about200
200feet
feetofofthe
theshear
shearzone
zoneisisexposed
exposedininthe
the
immediate
immediatevicinity.
vicinity. Shearing
Shearingintensity
intensity isis variable;
variable; relict
relict pillow
pillow textures
texturesare
are present
present in
in exposures
exposuresabout
about
150
150yards
yardsto
to the
the east.
east. Here
Hereatatthis
thisexposure,
exposure,the
theArchean
Archeangreenstone
greenstoneshows
showsstrong
strongfoliation
foliationstriking
striking280°,
280°
85°
85' S.
S.AAparallel
parallelfoliation
foliationalso
alsoaffects
a f k t sthe
thelowermost
lowermostpart
partofofthe
theoverlying
overlyingconglomerate.
conglomerate.
The
Theconglomerate
conglomerateisisabout
about55feet
feetthick,
thickbut
butisissomewhat
somewhatpatchy
patchyand
anddiscontinuous,
discontinuous,grading
grading
intermittently
intermittentlyboth
bothupwards
upwardsand
andlaterally
laterallyinto
intocoarse
coarsegrained
gainedquartzite
quartzitecontaining
containingonly
onlyoccasional
occasionalsmall
small
81
cobbles or pebbles. The
Theentire
entireunit,
unit, including
includingthe
the quartzite,
quartzite,isis20-50
20-50 feet
feetthick,
thick forming
formingan
an erosionally
erosionally
resistant E-W ledge. South
South of
of it,
it, aa 100-foot-wide
100-foot-widelow
lowswale
swaleoccupies
occupiesthe
theinterval
interval presumed
presumed to
to represent
represent
erosionally recessive sericite slate that constitutes the upper part of
erosionally
of the
the Enchantment
Enchantment Lake Formation
Fornation in
in
this area (Puffett, 1974);
1974); the very uppermost
uppermost part
part of
of this
thislithology
lithologycan
canbe
beseen
seenbelow
belowthe
theMesnard
Mesnardquartzite
quartzite
back at the
the highway.
highway.
The basal conglomerate
conglomerate can be traced
traced westward
westward from
fromhere
here at
at the
the initial
initial outcrop
outcrop for
forabout
about250
250feet.
feet.
layers of
of itit consist
consist of
of +80%
The coarsest layers
+8O% white vein qquartz
u a cobbles. ItIt bears
bears aa striking
striking physical
resemblance to the low-pyrite varieties of the Witwatersrand
resemblance
Witwatersrand conglomerate
conglomerate of
of South
South Africa,
Afiica, and
and occurs
occurs in
in
a similar geologic setting.
setting. We
We took
tookthree
threegrab
grabsamples
samplesof
of conglomerate ffom
from these
these outcrops.
outcrops. Two of
them retumed
returned detectable gold:
gold: ,003
.003 odton
ozlton and
and ,005
.005 odton.
or/ton. These
These two
two were
were re-analyzed by Chemex and
returned
retumed 65 ppb and 37 ppb Au, respectively.
respectively, These
Theseresults
resultssuggest
suggestaa problem
problem with
with nugget
nugget effect
effect at the
low end of the detectability scale, which would not be surprising
surprising in the case of placer gold. No sulfides
sulfides are
are
megascopically visible in this rock, nor were any found
found under
under the
the binocular
binocular microscope
microscope in
in an
an examination
examination
crushed rejects
rejects from
from our
our grab
grabsamples,
samples. We
of the cmshed
Wehave
havenot
not done
doneany
any further
further testing to ddetermine
e t d e the
internal domains hosting these low but anomalous gold values.
intemal
Discussion
Diicussion
Although it may tum
turn out that the
the traces
traces of gold
gold in this rock are caused solely by the foliation event
it, aa paleoplacer
paleoplacer origin
originseems
seemstotous
ustotobe
be much more
more likely.
likely. For economic concentrations of
of
that affects it,
paleoplacer gold to be found
found in
in the
the Enchantment
Enchantment Lake
Lake conglomerate,
conglomerate, the first
fust consideration
consideration would be
be
deposits in the Archean tmane
terrane were actually in existence prior
prior to
to the erosional events
whether or not gold deposits
deposition of
of the
the conglomerate.
conglomerate. The
in
that led to the deposition
Theonly
onlyknown
known significant
significant gold mineralization in
region occws
occurs in veins at the Champion iron mine; the
Proterozoic rocks in this region
the Proterozoic
Proterozoicisis otherwise
otherwise
notable for its paucity of gold showings. Bornhorst
Bomhorst and
and others
others(1998)
(1998) have
have attempted
attempted to constrain
constrain the
dates for some
some of the mineralization in the Archean rocks by lead isotope studies, obtaining
obtaining a model
model age
age
2,455 to
to 2,510
2,s 10Ma
Ma for
forgalena
galenathat
thatoccurs
occurswith
with gold
goldin
inthe
theRopes
Ropesmine,
mine, but
but also
alsomentioning
mentioningstructural
stn~ctural
of 2,455
evidence suggesting that
that the
the Ropes
Ropes gold
gold was
was deposited
deposited at
at about
about 2,690
2,690 Ma.
Ma. They also pointed out that ages
are poorly constrained for the timing
timing of the
the rifting that
that initiated
initiated the earliest Marquette
MarquetteRange
RangeSupergroup
Supergroup
sedimentation that this conglomerate represents.
Whatever the absolute age(s) of the
thegold-bearing
g o l d - b e ~veins
gveinsin
in the
the Archean
Archean rocks
rocks of
of the
the Ishpeming
Ishpeming
belt, this
this conglomerate
conglomerateprobably
probablypost-dates
post-datesthem.
them. Evidence for this is
is the
the high
high preponderance
preponderance
greenstone belt,
82
of vein quartz within it at this
this locality,
locality, the
the presence
presenceof
of anomalous
anomalous gold
gold in
in it,
it, and its apparently
unconformable relationship with the Carp River Falls shear zone, which was demonskated
demonstrated to be goldpaleoplacer-typegold
golddeposit
depositwithin
withinthis
this
mineralized at Stop SG-5
Whether or
or not
not a paleoplacer-type
SO-S of
of this field
field hip.
trip. Whether
for exploration
exploration is
is another
another matter. There
Thereare
arevery
veryfew
fewexposures
exposuresofofthe
theEnchaniment
Enchantment
unit is a prime target for
Lake Formation, and especially few
hin conglomeratic
few of
of this
this tthin
conglomeraticphase.
phase. The many samples needed
needed for
for
assaying, provenance
provenancestudies
studiesand
andthe
thelike
likewould
would nearly
nearly all
all have
have to
to be
be obtained
obtainedby
byexpensive
expensivesubsurface
subsurface
assaying,
means
means over
over aa wide
wide area.
area.
Stop
Stop SC-8:
SG8:
Lithology-seleetive
Lithology-selectiveductile
ductile shearing
s h e a ~ with
g
boudinage i
in
n the Carp
Carp River
River Falls
F a h shear
shear
12).
zone. (Location
(Locationmap,
map,Figure
Figure12).
This
This locality
locality was last mapped
mapped by Cannon and
and Klasner
Klasner (1977),
(1977), shown
shown as
as part
part of
of their
their Precambrian
Precambrian
mafic-uleamafic complex.
complex. The
Therevised
revisedstratigraphy
stratigraphyof
ofthe
theArchean
Archeanrocks
rocksof
ofthe
theIshpeming
Ishpeminggreenstone
greenstone
W mafic-ultramafic
belt (Bomhorst
(Bomhorst and
and Johnson,
Johnson, 1993)
1993)assigns
assignsitit to
to the
the informal
informal basalt
basalt flow
flow unit
unit of
of the
the Kitchi
Kitchi Formation.
Formation.
Beginning about us
118 mile
mile east
east of
of here, Cannon and Klasner mapped a large area
area of
of Precambrian W
serpentinite,
serpentinite, now considered
considered part of the Deer Lake peridotite. About
About1/4
I14 mile to the northeast are a number
of outcrops
outcrops of
of aa very
very fresh
fiesh looking,
looking, nearly
nearly unfoliated biotite
biotite tonalite
tonalite intrusion
inhusion that we informally refer to
as the "Mockler tonalite", named
named for
for an
an adjacent
adjacent small gold
gold prospect; Cannon
Cannon and
and Klasner
Klasner mapped
mapped that
as
Only a few hundred feet south of here Cannon and Klasner's
outcrop area as Precambrian W felsic rocks.
rocks. Only
Klasner's
map shows
shows a fault
fault contact
contact between the Archean and the Proterozoic. Bornhorst
Bomhorstand
andJohnson
Johnson(1993)
(1993)
included
included this
this in the Carp
Carp River
River falls
falls shear
shear zone.
zone.
This
expowe was
was not
not available
available to previous workers. ItItwas
wascreated
createdonly
onlyvery
very recently,
recently, when
when the
This exposure
Marquette
Marquette County
County Road
RoadCommission
Commissionstraightened
straightenedthe
the approach
approachto
tothe
therailway
railwayunderpass
underpassimmediately
immediately
south of
of us. The
TheRoad
RoadCommission
Commissiondumped
dumpedaavery
very large
large pile
pile of
of excess
excess blasted
blasted rock from
fiom this new
south
roadcut
roadcut in aa gravel pit that lies a couple of hundred
hundredyards
yardssoutheast.
southeast.
Figure 13
13 is a detailed
detailed map of
of this
this exposure.
exposure. ItItshows
showsaawide
widevariety
varietyof
ofrock
rocktypes,
types,which
which are
are
affected
affected by
by an
an east-west
east-west zone
zoneof
ofductile
ductileshearing
shearingthat
that varies
varies from
fiom weakly
weakly to
to intensely
intenselydeveloped,
developed,
dependiig on the rock type. There
Thereisisevidence
evidencehere
here for
formultiple
multiple deformation:
deformation: aa nearly
nearly horizontal
horizontal 52
S2
depending
crenulation
crenulationcleavage
cleavageisis superimposed
superimposedon
onaavertical
verticalS1
S, cleavage, and
and is
is mimicked
mimickedatatall
allscales
scalesby
bynearnear-
horizontal
horizontal boudins
boudinsof
offoliation-resistant
foliation-resistantveins
veinsand
androck
rockunits
units (see
(seephoto,
photo,Figure
Figure15).
15).
83
___
_____
Figure 12. Location map for Stops SG-8, 9 and 10.
32'30'
-______
16/8
I
ii
I
."
•- -:
r
a
H
o
?&
0
"N 7r"X)r
0
002
—C S
0
1000
-
60
-
—-——
2000
0
—
,g)
45
3000
I
-5
I—
-
2/±
-
—.
T. 48 N.
T. 47 W
1
MILE
3
601)0
7000 FEET
i
KILOHErER
CONTOUR INTERVAL 20 FEET
47'JO'
GREENWOOD 1.2 ML I,rrrnlOB—GE0LOGICAL SUSVEV - WASHING TON, Oj._ISSS—NS
SI 5,3565
84
46030)
87°45'
I
Figure
Figure 13.
13. Plan
Planmap
map of
of Marquette
Marquette County Highway
8.
Higbway "CL" roadcut, Stop SGSO-S.
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85
Figure IS.
50-8. Near
Figure
15.Looking
Lookingeastward
eastwardat
at roadcut
roadcut exposure, Stop SG-8.
Nearcenter
centerof
of photo
photo is
is
mega-boudined blocky tonalite porphyry with
with brecciated south
south margin
margin of
of tonalite
tonalite fragments,
fragments,
quartz, chlorite and tourmaline.
tounnaline. Tonalite
Tonalitemega-boudin
mega-boudinisisenveloped
envelopedin
in highly
highly sheared
sheared basalt
basalt that
also hosts horizontally
horizontally boudined
boudined quartz-carbonate
quartz-carbonatevein
veinnear
nearleft
leftside
sideof
ofphoto.
photo. Highly
sheared talcose ultramafic
ultramafic is
is at far
far right.
right.
This exposure exhibits lithologic and structural features too numerous to be described
described in detail in
will be
be briefly
briefly mentioned.
mentioned. Near the south end,
end, a 15-foot
this paper; the highlights will
%foot layer of virtually
piliowed metabasalt and
undeformed graywacke and minor pink chert lies between weakly foliated pillowed
moderately sheared metabasalt.
rnetabasalt The
not clear.
clear. The southern
Thegraywacke
grayacke has graded bedding, but tops are not
contact of the sediments
sediments with the basalt appears to be conformable,
conformable, but the northern contact
contact with the
by pyritic
pyritic slickensides.
slickensides. Moving northward past a mbbly
rubbly interval of
moderately sheared basalt is marked by
near-outcropping sheared
sheared basalt, shearing becomes progressively more intense, and seems to be centered
on a 4-foot
none can be
be found at the
4-foot zone of talcose.
talcose,papery-thin
papery-thin sheared
sheared ultrarnafic
ultramafic rock. Although none
outcrop itself, aapiece
pieceof
ofthis
thisrock
rock found
foundin
in the
the gravel
gravel pit
pit waste
waste pile
pile contained
contained cross-fiber
cross-fiber chrysotile. On
On
the Jensen
Jensen cation
cation plot
plotof
ofour
ourwhole
wholerock
rockanalyses
analysesfrom
fromthis
thisexposure,
exposure,the
thesample
samplefrom
from this talcose material
material
86
plots in the peridotitic komatiite field (see Figure 14). This
This rock
rock isis either
either an apophysis
apophysis of the Deer Lake
peridotite, or a tectonically emplaced slice
slice of it;
it; it exhibits
exhibits vertical
vertical rodding
rodding but
but horizontal
horizontal slickensides.
slickensides.
Because of the structural deformation here, it's problematic
problematicwhether
whether the
the boudined
bondined tonalite
tonalite
porphyry is a dike or a sill. This
Thisisisaafme
fmegrained
grained dark
dark pinkish
pinkish gray
gray rock with up to about 20% small
phenoctysts, which
which in
in thin
thin section
sectionare
areall
allhighly
highlysericitized
sericitizedalbite-oligoclase.
albite-oligoclasa The
feldspar phenocrysts,
The groundmass
composition is: a few
few percent opaques,
opaques, (about
(about ¾
% rutile
rude pseudomorphing
pseudomorphing ilmenite
ilmenite and ¼
'/4 porphyroblastic
porphyroblastic
cubic pyrite); one third anhedral quartz;
quartz; one
one third
third sericitized
sericitized albitic
albitic plagioclase;
plagioclase;and
andone
onethird
thirdinterstitial
interstitial
chlorite, sometimes as pressure shadows of pyrite, defining a clear thin section-scale foliation. This
This isis an
an
albitic rock that yielded
yielded 6.04%
6.04% Na20,
Na20, the
study. A
the highest
highest of any of the whole rock analyses done for this study.
A
K20 content of 1.39%
K20
1.39% is
is probably
probably entirely
entirely due
due to
to sericite;
sericite; no
no K-spar
K-spar was
was found.
found.
A zone of
of interest
interest in
in the
the sheared
sheared metabasalt
metabasaltand
and marked
marked on
onthe
themap
mapisisaathin
thininterval
intervalrich
richinin
disseminated
disseminated coarse
coarse magnetite
magnetite coexisting
coexisting with randomly
randomly oriented
oriented coarse
coarsecubic
cubicpyrite
pyrite that
thatappears
appearsto
tohave
have
developed porhyroblastically
porhyroblastically without pressure shadows in a low strain environment. Another
Another isis aa few
few feet
feet
of sheared
sheared greenstone
greenstone with minor
minor iron
iron carbonate,
carbonate, aa trace
trace of
of fine
finedisseminated
disseminatedpyrite
pyriteand
andrare
raremalachite
malachite
spotting.
spotting.
This exposure has been sampled for gold more than once by MPC;
MPC; the locations
locations of
of the latest three
grab samples are shown on the map. None
None of
of the
the samples
samples yielded
yielded gold above
above detection limits. Despite
Despite
the favorable-looking
favorable-looking shearing,
shearing,the
the lack
lack of
of significant
significantaccompanying
accompanyingcarbonate
carbonatealteration
alterationmay
maybe
hean
an
important reason for the lack of
of detectable
detectablegold
gold in
in it,
it, even
even where
where copper
copperminerals
mineralsare
arepresent.
present.
Figure 14,
14, the Jensen
Jensen plot
plot for
for the
the whole
whole rock
rock samples
samplestaken
takenfrom
fromthis
thisexposure,
exposure,illustrates
illustratesthat
thatthe
the
greenstone samples,
samples,regardless
regardless of
of degree
degreeof
of shearing,
shearing, plot
plot closely
closely together
together in
in the
the high iron tholeiite
tholeiite field.
Our sample of the tonalite porphyry from here is shown repeated on the Jensen plot of Figure 16 for the
purpose of comparison
comparison with
with two
two other
other felsic
felsic intrusive
intrusive bodies
bodies in
in this
this vicinity,
vicinity, the
the Mockler
Mockler tonalite
tonalite(sample
(sample
"MOCK")
mine (sample
(sample"PG-1").
"PG-I"). (Note:
"MOCK) and
andthe
the tonalite
tonalite porphyry at the Peninsula
Peninsula mine
(Note: Accessory
Accessory zircon
zircon
was noted in thin sections
sections of all of
of these
these rocks).
rocks).
The shearing
be traced westward only
shearing seen here cannot be Iraced
traced eastward, and can he
only a few
few hundred
feet, where it is partially exposed in the old highway
highway cut. Beyond
Beyond that
that point,
point, scattered
scatteredoutcrops
outcrops in
in the
the hills
to the west are of unfoliated metabasalt. The
Theductile
ductileshearing
shearingseen
seen at
at this
this stop
stop might
might represent
represent only
only a
branching splay of the Carp
Carp River
River Falls
Falls shear
shear zone, whose
whose main expression
expression may
may well
well lie
lie south
southof
of here
here at
at
the Proterozoic
Proterozoiccontact.
contact.
87
FeO* + 1102
SAMPLES
Cation %
Cation
YO
Jensen
Jensen (1976)
(1 976)
• PC-B-i
• PC-B-2
£ PC-8-3
V PC-UM
•
PC-PFWO
AJ203
MgO
Figure 114:
4: Stop
Stop SG-8
SG-8
Fe0*
FeO*++1102
m02
Cation
Cation %
%
Jensen
Jensen (1976)
( 1 976)
.
w
SAMPLES
• MOCK
K C K
• RC-PI-WO
m
K-RtNO
£
PJ203
A203
PG-I
MgO
Figure
6
Figure116
88
Stop SG9:
SG.9: Peninsula
Peninsula gold
gold deposit (Location
(Location map,
map, Figure
Figure 12).
12).
In
In contrast to the broad anastomosing
anastomosing ductile shear
shear zones seell
seen so far on this field trip, the
Peninsula gold deposit occurs in a discrete brittle-ductile structure. Also
Alsoin
in contrast
contrast to
to the
the nearly
nearly east
east -west regional shear zones seen so
so far, the Peninsula
Peninsula structure
structure strikes
strikes north
north -- northeast.
northeast. This structure
structure cuts a
by Cannon
Cannou
variety of host
hostrocks,
rocks, regionally
regionally mapped
mapped as
as Precambrian
Precambrian W massive
massive and layered felsic rocks by
and Klasner (1977) and subsequently
subsequently assigned to the informal basalt flow
flow unit of the Kitchi Formation
(Bornhorst
1993). At
(Bomhorst and Jobnson(
Johnson(l993).
At aa scale
scale useful
usefbl for exploration,
exploration, the
the geology
geology is
is quite
quite complex.
complex.
History
Gold was discovered at the Peninsula in the 1880's,
1 8 8 0 ' but
~but
~ there
there was
was never
never any
any commercial
commercial
production. All
Allinformation
informationon
onthis
thisperiod
periodconsists
consistsof
ofold
oldnewspaper
newspaperarticles
articleswith
withvery
verylittle
littleuseful
useful
technical data except the probable depths of the two old main shafts on the property, 69 feet for the North
shaft and 80 feet for the South Shaft (Fountain,
(Fountain, 1992).
1992). There
Therewas
wasno
nomodem-day
modem-dayactivity
activityat
at the
the Peninsula
Peninsula
by Callahan Miing
Mining Corp. in 1981. MPC
until it was purchased by
MPCpurchased
purchasedthe
thetracts
tractsencompassing
encompassingthe
the
Peninsula and the nearby Michigan gold mine
mine from
from Callahan
Callaban in
in 1995;
1995;included
includedin
inthis
thisacquisition
acquisitionwere
wereall
all
reports on these properties.
properties. The following
of Callahan's
Callahan's drill
d d l core,
core, exploration data
data and internal rqorts
following synopsis
part of a
of the work done by Callahan is based on an analysis and summary of this acquired data in p~
detailed report for MPC (Duskin,
(Duskin, 1997).
1997).
detailed
mapping, rock sampling and
and
Between 1981
1981 and 1984
1984 Callahan conducted surface reconnaisssance
recomaisssance mapping
geophysics. An
An early
early phase,
phase,called
calledthe
the "granitoid
"granitoid program"
program'' was
wasfocused
focusedon
onintensive
intensivechip
chipand
and channel
channel
sampling of the tonalite porphyry, thought to be a pluglike intrusion
with potential for large tonnage,
tonnage, open
inmsion with
open
pittable gold mineralization. Disappointing
pittahle
Disappointingresults
resultsled
led to
to aare-focus
re-focus on
on the
the steeply
steeplydipping
dipping"siliceous
"siliceous
exhalite" originally tested by the oldtimers, termed
tenned the "Target
exbalite"
"Target Zone" by Callahan.
Callahan. The
The subsequent
subsequent testing
No detailed mapping or trenching
Irenching were
were done.
done. Three
program consisted entirely of angle core drilling. No
mee
shallow test holes under the old workings
workings in 1984
1984returned
returned sub-ore
sub-oregrade
gradeassay
assayresults
resultsbut
but encouraging
encouraging
Shaft, returned
returned 15.6 slant feet
feet grading
grading .216
thicknesses. In
In 1985
1985the
the fourth
fourth hole, under the old South Shak
,216
oz/ton
odton Au. Intensive
Intensiveclose
close spaced
spaced drilling
drillingwas
wasthen
then carried
carried out
out through
through 1987,
1987, resulting
resulting in about 20,000
feet of core in a total of 37
37 holes.
boles. During
D U Mthe
the
~ course
courseof
ofthis
thisdrilling
drillingaapresumed
presumedblind
bliid en
enechelon
echelon
termed the
the "Back Zone", was
was discovered.
stratabound zone
zone of
of mineralization,
mineralization, tenned
The purpose
purpose of
of the
the drilling
drilling was
was to
to quickly
quickly prove
prove up
up aa ramp-minable
ramp-minableshallow
shallowtonnage
tonnageof
ofhigh
higb
grade
grade ore to augment
augment or eventually
eventually replace the ore being fed to the mill by the Callahan's mining operation
89
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t.
mafic intrusive
200 feet
Geology
Peninsula Gold Project
Minaals Processing
Corporation
100
Trench
Outcrop
Fault
Gold—bearing zone
Laminated felsic tuf I
Greenstone
Metagabbro
Magnetic
Figure 17: Stop SG-9
0
r
Tonalite porphyry
at the
the Ropes
Ropes deposit.
deposit. There
Therewas
wasno
noattempt
attemptatatstepout
stepoutdrilling,
drilling,and
andthe
the deposit
deposit remains
remains open
open at
at depth
depth and
and
at
in both
both directions
directions along
along strike.
strike. Callahan
Callahan eventually
eventually calculated
calculated a reserve of 51,700
5 1,700tons
tons grading
grading .141
,141 01/ton
odton
in
in
in the
the outcropping
outcropping Target
Target Zone
Zone segment
segment of
of the
the structure,
structure, and
and an
an additional
additional 41,200
41,200 tons
tonsgrading
grading.082
,082
oz/ton
asfar
far as
as obtaining
obtaining aa mining
mining permit in 1988
1988 from
from the first
fust
odton in
in the
the Back Zone. Mining
Miingplans
planswent
went as
level
level of
of authority,
authority, Marquette County. But
Butthe
thedeclining
decliningfortunes
fortunesof
of the
the Ropes
Ropes mine
mine and
and ofofthe
the
corporationitself
itself led
ledtotoaasuspension
suspensionofofactivity
activityatatthe
thePeninsula
Peninsulafrom
fromthen
thenuntil
untilthe
thesale
saletotoMPC.
MPC.
corporation
Taking
in the
the early
early
Taking aa new
new approach
approach not influenced
influencedby
bythe
thestratabound
strataboundgold
goldmodels
modelsin
in vogue
vogue in
1980's, MPC
MPCbegan
began its
its re-evaluation
re-evaluation of
of the
the property
property by rock sampling
sampling and mapping
mappingon
onaagrid
gridat
atl"=lOO'
l"=lOO'
1980's,
in
in 1996.
1996.This
Thisled
ledtotothe
thediscovery
d i s w v wof
ofthe
theBack
BackZone
Zoneininoutcrop
outcrop(now
(nowrenamed
renamed the
the "B
"B Zone"),
Zone"), and
and the
the
recognition
recognition that
that itit is
is simply
simply aa fault
fault offset
offset of
of the
the Peninsula
Peninsula structure.
structure. Further
Furtherwork
workincluded
includeddigging
diggingand
and
channel
"=20' mapping
channel sampling
sampling 11
11backhoe
backhoe trenches,
trenches, detailed
detailed11"=20'
mappingalong
alongthe
theentire
entireexposed
exposedstructure,
strncture,
petrographic
petrographic and polished section studies
studies of mineralized core intervals, complete re-logging of many of
the
thecore
coreholes,
holes,and
andcorrelation
correlationof
ofthe
therevised
revisedcore
corelog
loggeology
geologywith
withthe
thesurface
surfacemapping.
mapping. Figure
Figure17
17isisaa
geologic
geologicmap
mapcompiled
compiledfrom
fromthis
thiswork.
work.
Lithology
Lithology
The
The oldest
oldest of
of the
the rocks
rocks hosting
hosting the
the deposit
deposit area
area appear
appear to
to be
be an
an interlayered
interlayered sequence
sequence of
of
greenstones
greenstones and a laminated felsic
felsic tuft'fragmental
tuWfiagmental unit.
unit. The
Thetuff'fragmental
tuWEapentd isisnon-magnetic
non-magnetic and
and
erosionally
one very
v q small outcrop
outcrop just east of the North Shaft
Shaft can be found
found on
on surface;
snrface; it
erosionallyrecessive.
recessive. Only one
was
wasalso
alsoencountered
encounteredin
in Trench
Trench 77 northwest
northwest of
of there.
there. InIndrill
drillcore
corethis
thisgray
grayrock
rockisisextremely
extremelywell
wellbanded
banded
and
andoften
oftencarries
canies coarsely
coarselyfragmental
kagmental heterolithic
heterolithic layers.
layers. ItItexhibits
exhibitsgraded
gradedbedding,
bedding,but
buttops
topsand
andbottoms
bottoms
in
inthis
thissequence
sequencehave
have not
not yet
yet been worked
worked out.
out. The
Thegreenstones
greenstonesare
aretypical
typicalmetabasalts
metabasaltsand
andoften
oftenexhibit
exhibit
amygdules
amygdulesin
in drill
drill core.
core. The
Themany
manystructural
structuraldislocations
dislocationsininthe
theimmediate
immediatePeninsula
Peninsulavicinity
vicinitymake
makeitit
difficult
difficultto
todetermine
determineaastrike
striketrend
aendfor
forthese
theseunits,
nnits,but
butaadistinctive
distinctivebedded
beddedchert
cherthorizon
horizonwithin
withinthis
this
sequence
sequencethat
that outcrops
outcropsabout
about1000
1000feet
feetnorthwest
northwestgives
givesan
anattitude
attitudeof
of035°,
035",72°SE.
72"SE.
The
Thecoarse
coarsegrained
grainedmetagabbro
metagabbrowest
westofofthe
theNorth
NorthShaft
Shaftappears
appearstotobe
beaasmall
smallplug
plugororsill
sillintruding
inin~diig
the
thegreenstone;
greenstone;itit contains
containslarge
large angular
angular fragments
Eagments of
of greenstone.
greenstone.This
Thismetagabbro
metagabbroisisnon-magnetic
non-magneticand
and
completely
completelyamphibolitized,
amphibolitizedwith
withpseudomorphs
pseudomorphsofofcalcic
calcicamphibole
amphiboleafter
afterorthopyroxene
orthopyroxenefaintly
faintlypreserved.
presewed.
ItIt contains
contains 2-5%
2-5Yo coarse
coarse cubic
cubic porphyroblastic
porphyroblastic pyrite
pyrite that
that often
often hosts microscopic inclusions
inclusions of
of
6
chalcopyrite;
chalcopyite; tiny
tinyfree
Eeegrains
grainsof
ofchalcopyrite
chalcopyriteare
arealso
also present
present in
in traces. The
Therock
rockcontains
containsabout
about5%
5%
chlorite
chloritereplacing
replacing amphibole,
amphibole, indicating
indicatingretrograde
retrogrademetamorphism.
metamorphism.
91
The
porphyry. This
The most
most erosionally
erosionally resistant
resistant of
of the
the Peninsula
Peninsula structure
structure host rocks is the tonalite porphy~~.
This
rock
rock has
has been
been affected
affectedby
by both
both metamorphism
metamorphismand
andhydrothermal
hydrothermalprocesses,
processes,especially
especiallywhere
whereititlies
liesclose
close
to the
the auriferous
auriferous structure,
structure, where gold values ranging from
fiom a few to a few hundred ppb can often be found
to
in
init.
it. In
In mapping
mapping near
near this
this structure,
structure, itit is
is often
often difficult
diff~cultto
todistinguish
distinguishaadivision
divisionbetween
between the
the two
two alteration
alteration
effects,
effects, but
but one
one of
of the
the principal
principal hallmarks
hallmarks of
of the
thehydrothermal
hydrothermalalteration
alterationisis an
an increase
increasein
in chlorite
chloriteas
as
replacements,
replacements, seamlets,
s e d e t s ,enhanced
enhanced foliation
foliation partings
p d g s and
and slickensides. Other
Other such
such effects
effects are
are elevated
elevated
content
content of
of fme
fine grained
grained disseminated
disseminatedpyrite,
pyrite,carbonate
carbonatereplacement
replacementof
ofthe
thegroundmass,
groundmass,and
andwhite
whitequartz
quark
veining.
veining. Strong
Strongalteration
alterationseldom
seldompersists
persistsfor
formore
morethan
thanaafew
fewfeet
f& from
fromstructural
structuralcontacts.
contacts.
The
Theporphyritic
porphyriticnature
natureof
ofthis
thisrock
rock isisoften
oftentoo
toosubtle
subtletotoreadily
readilydiscern
discernininhand
handspecimens,
specimens,even
even
in
in relatively
relatively fresh
fiesh samples. In
Inthin
thinsections
sectionsof
ofthese,
these,the
thephenocryst
phenocrystcontent
contentvaries
variesfrom
from5%
5%to
to 30%,
30%, and
and
these
these are
are all
all heavily sericitized,
sericitized, corroded albite-oligoclase
albite-oligoclase crystals. The
The groundmass
groundmasshas
has about
about 20%
20%
anhedral
anhedral quartz,
quark, within
within aa very
very fme
fme grained
grained mixture
mixtureof
ofbiotite
biotitealtering
alteringto
to chlorite,
chlorite, chlorite,
chlorite,albite,
albite,
muscovite/sericite,
primarily of
of rutile
rutile and
and pyrite. Very
muscovitelsericite, and opaques consisting primarily
Very rare
rare tiny
tiny grains
grains of
of
probable
thin
probablemolybdenite
molybde~tehave been found in a few hand specimens. In
In one polished t
hin section of altered
altered
tonalite,
tonalite,microscopic
microscopicinclusions
inclusionsof
ofmixed
mixedchalcopyrite-pyrrhotite
chalwpyrite-pymhotitewere
werefound
foundininpyrite.
pyrite.
Because
Becauseof
ofits
itsrelative
relativeerosional
erosionalresistance,
resistance,this
thisunit
unit has
hasbeen
been "overmapped"
"overmapped"ininthe
thepast,
past,and
andisis
probably
probablyalso
alsosomewhat
somewhatovermapped
overmappedas
asdepicted
depictedon
onFigure
Figure 17.
17. InInseveral
severaldrill
drillholes
holespassing
passingunder
under
outcrops
outcropsof
oftonalite,
tonalite, none
none was
was encountered
encountered in the subsurface.
subsnrface. In
Insome
someof
ofthe
thetrenches
trenchesitit was
was found
foundthat
that
low
by outcrops
outcropsof
oftonalite
tonalitewere
wereunderlain
underlamby
bysubcrop
subaopof
ofthe
theolder
oldergreenstone-tuff
greenstone-tuff
lowareas
areassurrounded
surroundedby
sequence
sequencethat
thatthis
thishypabyssal
hypabyssalintrusive
intrusiveinvades.
invades. In
In drill
drillcore
coreintercepts,
intercepts, its
itscontacts
contactswith
withthis
thissequence
sequence
are
areusually
usuallybrecciated
brecciated and/or
and/or slickensided,
slickensided, but
butaafew
fewof
ofthem
themshow
showinjections
injectionsof
ofthe
thetonalite
tonaliteinto
intothe
the
greenstone.
greenstone. The
Thetonalite
tonaliteporphyry
porphyryisisnot
notaamassive
massiveplug,
plug,but
butrather
ratheraacluster
cluster of
of dikes/sills.
diiedsills.
The
Theyoungest
youngestrock
rock unit
unitisismapped
mappedas
as"magnetic
"magneticmafic
mafic intrusive",
intrusive", but
butititisisprobably
probablyaametadiabase.
metadiabase.
This
Thisblack
blackrock
rockisisvery
veryweakly
weaklyfoliated
foliated(except
(exceptimmediately
immediatelyadjacent
adjacenttotocontacts),
contacts),exhibits
exhibitsan
an equigranular
equigranular
texture,
texture,and
andcontains
containsmegascopic
megascopicfresh
fieshbiotite.
biotite. ItItcarries
carriesaafew
fewpercent
percentcoarse
coarseeuhedral
euhedralpyrite,
pyrite,asaswell
wellasas
ubiquitousdisseminated
disseminated magnetite,
magnetite, sometimes
sometimes up
up to
to 10%.
10Y0.InIndrill
drillcore
coreititcontains
containstrains
bainsand
andscreens
screensof
of
ubiquitous
angular
fragmentsofofthe
thetonalite.
tonalite.
angularfragments
92
Structure
Mineralization
Structure and
and Mineralization
The Peninsula
Peninsula fault
fault structure
structure affects
affects all of
of the
the rock
rock types,
types, inclusions
inclusionsof
ofwhich
whichcan
canbe
be found
foundwithin
within
it. We
Werecognize
recognize two
two basic
basic lithotypes within the structure: quartz-carbonate
quartz-carbonate ("q-c"),
("q-c"), and chlorite("ccpy"). Angular
carbonate-pyrite ("copy").
Angular breccias are common in the q-c lithotype. Host
Hostrock
rock fragments
fragments are
are
less abundant, but sometimes
ccpy lithotype, usually as boudins. The
sometimes present
present in the copy
Theq-c
q-clithology
lithology
predominates
Target Zone;
Zone; itit exhibits
predominates where the fault
fault structure
structure intersects tonalite, primarily in the Target
exhibitsmostly
mostly
brittle
brittle texures and has abundant
abundant albite as veinlets and breccia matrix. The
Theprimary
primary carbonate
carbonate is
is dolomite,
dolomite,
with lesser ankerite, rhodochrosite and
and late
late calcite.
calcite. Silica content is highly variable.
variable. The q-c usually
contains
chioritic slip planes
planes,,and
contains chloritic
and sometimes
sometimes lenses
lenses or intetfingering
interfmgering layers of copy. Megascopic
Megascopic metallic
minerals in the q-c are pyrite, very minor chalcopyrite,
minerals
chalcopyrite, and,
and, rarely, visible
visible gold.
gold. There
Thereisis aa strong
strong
conelation
correlationof
ofgold
gold assays
assays with
with pyrite, but it is not 1:1.
1:1. In
Inmicroscope
microscopeexamination
examinationof
of high
high gold
gold -- high
pyrite samples, gold
gold is
is found
found as
as tiny
tiny inclusions in pyrite, either alone or with sphalerite. Other
Otherinclusions
inclusions
in pyrite
samples with visible gold, the
pyrite are
are argentiferous
argentiferous galena,
galena- pyrrhotite
pyrrhotite and
and chalcopyrite.
chalcopyrite. In low pyrite samples
gold appears
appears to
to be
be later
later than
than pyrite,
pyrite, occurring
occurring as
as free
freegrains
grains in
in chlorite
chloriteand
and carbonate,
carbonate,associated
associatedwith
with
chalcopyrite.
chalcopyrite.
characteristically displays
The dominant mineral in the copy
ccpy lithology is chlorite, and this rock characteristically
ductile textures. This
Thislithology
lithologyseems
seemsto
to dominate
dominatethe
the fault
fault zone
zone where
where itit intersects
intersects greenstone or
and in
in the
the deeper
deeper levels
levels of
of the
the Target
Target Zone
Zone segment.
segment. There is
metadiabase, both in the B Zone segment and
albite in the matrix of this rock,
rock, but
but virtually
virtuallyno
noquartz.
quartz. In the B Zone,
Zone, most
most of which
which involves
involves faulting
adjacent to the metadiabase, much of the chlorite can be
be seen in thin
thin section to
to be
be replacing
replacing biotite.
biotite. The
The
ccpy is generally much higher in pyrite than the q-c, and also contains
copy
contains several
several percent
percent magnetite in the B
Zone segment, probably
probably derived
derived directly from the metadiabase. In
Inpolished
polished section
sectionmagnetite
magnetite and
and pyrite
pyrite
exhibit coexisting mutual
mutual textures.
textures. Chalocopyrite
in this
thislithology.
lithology. Gold,
Chalocopyrite also occurs as aa minor sulfide in
where found so far, occurs
occurs as
as inclusions in pyrite, and in quartz and
and carbonate
carbonate veinlets.
that the
the composition
composition of
of the
the Peninsula
Peninsula gold
gold structure's
structure's gangue is
is very
very host
host
The clear impression is that
rock-dependent. ItIt appears
appearsthat
that aa great
great deal
deal of
of host
host rock has
has been
been assimilated
assimilated into
into the structure, with the
the metatonalite
metatonalite contributing
contributingto
tothe
thequartz
quartzin
inthe
theq-c
q-clithology.
lithology. Similarly, the
the
availability of free silica in the
of ferromagnesians
fenomagnesians in more mafic host rocks promoted the
lack of free silica and the abundance of
rocks were
were intersected.
intersected. The abundance of dolomite in the q-c and of
development of chlorite where those rocks
chlorite
cepy indicates
introduction of magnesium
chlorite in the copy
indicates a very strong introduction
magnesium into
into the
the hydrothermal system
system that
that
operated within the
operated
the Peninsula
Peninsula fault
fault structure.
structure.
93
Zone strikes 020'
020° and dips steeply SE. On
The Target Zone
Onthe
thesouthwest
southwestititisisoffset
offsetby
by aa later
laterhigh
high
angle fault which also rotates it into
Zone).
into an
an orientation
orientation trending
trending 050°,
050°still
stilldipping
dippingsteeply
steeplySE
SE(the
(theBBZone).
Vertical movement on
on this
this cross
crossfault
faulthas
has exposed
exposedthe
thegold
goldstructure
structurehere
hereatataadifferent
differentdepth
depthlevel,
level,
porphyry. Where
involving mostly
mostly the
the magnetic
magnetic metadiabase in its footwall instead of the tonalite porphyry.
Where
intersected
of breccia. These
intersected in drilling, late faults such as this one consist of narrow zones of
Thesefaults
faults form
form an
an
interconnecting pattern. Their
interconnecting
Their traces
traces in
in some
somecases
cases yield
yield anomalous
anomalous gold
gold in
in grab
grab sampling,
sampling, but it's not
not
clear whether this represents a later
later mineralizing
mineralizing event or mineralization
mineralization remobilized
remobilized from
from the
the Peninsula
Peninsula
structure.
structure.
SG-1O: Michigan
Michigan Gold Mine
Mine (Location
Stop SG10:
(Location map,
map, Figure
Figure12).
12).
MPC has done only a limited amount of work so far on this gold occurrence,
MFC
occurrence. ItItisisincluded
included as
as an
an
optional field trip stop
stop if time permits. There
Thereare
arevery
veryfew
fewoutcrops
outcropsin
inthe
theimmediate
immediatemine
mine vicinity;
vicinity; most
most
of them are metabasalts
metabasalts of the
the Kitchi
Kitchi Formation
Formation informal
informal basalt
basalt flow
flowunit,
unit, cut
cutby
by tonalite
tonaliteporphyry
porphyrysimilar
similar
to that at the Peninsula mine. These
Theserocks
rocksare
are cut
cut by
by at
at least
least five
five parallel, east-west, nearly vertical quartz
veins. Compared
Comparedto
toother
othergold
golddeposits
depositsininthis
thisarea,
area, the
theMichigan
Michigan veins
veins are
are very
very low
low in
in pyrite, and
represent a low sulfide
sulfide hydrothermal system. There
Thereisis no
no particular
particular correlation
correlation of
of gold
gold with
with pyrite, and
and
this deposit represents a case of severe nugget
nugget effect.
effect. These
These veins
veins are
are famous
famous for
for carrying
carrying free
free gold,
which can only be
be found today
today through
through very
verydiligent
diligenteffort
efforton
onthe
theold
oldmine
minedump.
dump. Examination of
of the
the
dump rocks and three core holes drilled by Callahan in the
the 1980's
1980's shows
shows that the
theveins
veinsrepresent
representbrittle
brittle
structures. Angular
are open
open space
space filling
filling textures. Black
Blacktourmaline
tourmalineisisaacommon
common
Angular breccia
breccia is
is common,
common, as
as are
vein mineral here; other exotic
exotic minerals
minerals found
found here include
include coarse
coarse molybdenite,
molybdenite, powellite,
powellite, scheelite,
scheelite,
chalcopyrite
chalcopyrite and a bismuth telluride. This
Thismineralogy
mineralogy is
is typical of
of the
the igneous
igneous assemblage often found in
the roots
roots of
of porphyry
porphyry copper
copper deposits.
deposits.
Recorded 19th
19&Century production from
from here was 856
856 ounces
ounces of gold. Most
Most of
ofthis
this appears
appears to
have come
come from
from the
the No.
No. 66 Shaft
Shafton
onthe
the "main"
"main"vein,
vein,which
whichwas
wasmined
minedfrom
fromthe
the55-foot
55-footlevel
levelupwards.
upwards.
the property
property by
by MPC,
MPC, nothing
According to records inherited with the
nothing happened at
at this
this mine again until the
1930's, when aa financially
financially struggling
strugglingsmall local stock company
company deepened
deepened the
the No.
No. 66to
to 250
250feet
feetand
and
drifted laterally east and west along the
the vein.
vein. The blocked-out ore was never mined except for bulk
sample
sample tests.
tests. It is not clear exactly
exactlywhere
where in
in the
the mine the bulk samples came from.
from. The
Thereason
reasonthat
thatthe
the
bulk tests were done
done is that the company's
company's underground
underground grab
grab and
and channel
channelsamples
samplesall
allshowed
showedextreme
extreme
94
assay variability.
variability. Two
Two bulk tests,
tests, first
first of
of aa 1-ton
1-ton sample
sample and
and then of a 10-ton
10-ton sample, were done in
Floughton by
by Michigan
Michigan Tech.,
Tech., which
which determined
determined aa grade
grade of
of approximately
approximately .3 odton
oz/ton Au.
Au. A
Houghton
A third
third test
test was
was
done in 1937 at a flotation
flotation mill
SI! on-site
Tech.. This
on-site under
under the
the supervision
supervision of Michigan Tech..
This also indicated a
oziton. At
Atthis
this point
point the
the stock
stock company
company tried
tried to
to pay
pay its
its debts
debts and move to commercial mining
grade of .3 odton.
by floating a new stock issue in a difficult fmancial
financial and regulatory environment. The
The various
variousrecords
records and
and
correspondence in MPC's files
files show
showthat
that this
this company
companywas
was still
still frying
trying to get approval to market its stock
the summer of
of 1941. As
began, the
the War
War Production
Production Board
Board prohibited all
as late as the
Assoon
soonas
asWorld
WorldWar
War11
TI began,
gold
nothing apparently
apparently ever
ever happened
happenedat
atthe
the Michigan
Michigan after
afterthat
thatexcept
exceptthe
thethree
threecore
core
gold mining in the U.S.; nothing
holes
by Callahan.
Callahan.
holes drilled
drilled by
95
References
References Cited
Cited
Bornhorst,
Bornhorst, T.J.,
T.J., Thorpe,
Thorpe, R.I.
R.I. and
and Johnson,
Johnson,R.C.,
R.C., 1998,
1998,Lead
LeadIsotope
Isotopestudy
studyof
ofveins
veinsin
inthe
theArchean
Archean
Ishpeming
Ishpeming(}reenstone
GreenstoneBelt,
Belt,Michigan:
Michigan:Economic
EconomicGeology,
Geology,vol.93,
vol.93,p.102-107.
p.102-107.
Cannon,W.F.,
W.F., and
and Klasner,
Klasner, J.S.,
J.S., 1977,
1977,Bedrock
BedrockGeologic
Geologicmap
mapof
ofthe
thesouthern
southernpart
part of
ofthe
theDiorite
Diorite
Cannon,
and
Champion
7
Vz-minute
quadrangles,
Marquette
County,
Michigan:
U.S.
Geological
and Champion 7 %-minute quadrangles,
Geological
Survey
Survey Miscellaneous
MiscellaneousInvestigations
InvestigationsSeries
SeriesMap
Map1-1058,
1-1058,scale
scale1:24,000.
1:24,000.
Duskin, D.J.,
D.J.,1997,
1997,Peninsula
Peninsula gold
gold prospect:
prospect: private
privatereport
reportfor
forMinerals
MineralsProcessing
Processing Corporation,
Corporation,
Duskin,
21p.
2lp.
in the
the Upper
Upper Peninsula:
Peninsula: Lake
Lake Superior
Superior Port
Port Cities
Cities
Fountain, W., 1992,
1992,Michigan
Michigan gold
gold mining
mining in
Fountain,
Press,
Press, l68p.
l68p.
Gair,
Gair, J.E., and
and Thaden,
Thaden, R.E.,
R.E., 1968,
1968,Geology
Geology of
of the
the Marquette
Marquetteand
and Sands
SandsQuadrangles,
Quadrangles, Marquette
Marquette
County,
Tip., scale
County, Michigan:
Michigan: U.S.
US. Geological
GeologicalSurvey
SurveyProfessional
Professionalpaper
paper 397,
397,77p.,
scale1:24,000.
1:24,000.
Gleason,
Gleason, R.J.,
R.J., 1986,
1986,Extended
Extended Phase
Phase 11Exploration
Explorationof
of the
the Dead
Dead RiverRiver- Ishpeming
Ishpeming Greenstone
Greenstone
Belt,
CallahanMining
Mining Corp.
Corp. private
privatereport,
report, 57p.
57p.
Belt,Marquette
Marquette County,
County, Michigan:
Michigan:Callahan
Grunsky, E.C., 1981,
1981, No.16No. 16- An algorithm
algorithm for
for the classification
classification of subalkalic
subalkalic volcanic rocks
Grunsky,
using
1using the
the Jensen
Jensen cation
cationplot:
plot: Ontario
OntarioGeological
GeologicalSurvey
SurveyMiscellaneous
MiscellaneousPaper
Paper100,
100,p.6
p.6165.
65.
subalkalicvolcanic
volcanicrocks:
rocks: Ontario
Ontario
1976, A new cation plot for classifying
Jensen, L.S., 1976,
c1assi'ing subalkalic
Division
66,22p.
22p.
Division of
of Mines
Mines Misc.
Misc.Publication
Publication##66,
Johnson,
Johnson, R.C.,
R.C., and
and Bornhorst,
Bomhorst, T.J. 1991,
1991,Archean
Archean geology
geology of
of the
the northern
northern block of
of the Ishpeming
Ishpeming
greenstone
belt,
Marquette
County,
Michigan:
U.S.
Geological
Survey
Bulletin
1904-F,
greenstone belt, Marquette County, Michigan: US. Geological Survey Bulletin 1904-F,
20p.
20p.
Puffet,
Puffet, W.P.,
W.P., 1974,
1974,Geology
Geologyof
ofthe
theNegaunee
NegauneeQuadrangle,
Quadrangle,Marquette
MarquetteCounty,
County,Michigan:
Michigan:U.S.
U.S
Geological
Geological Survey
Survey Professional
Professional Paper
Paper 788,
788,53p.,
53p.,1:24,000.
1:24,000.
Van
Van Hise,
Hise, C.R.,
C.R., and
and Bayley,
Bayley ,W.S.,
W.S.,1897,
1897,The
TheMarquette
Marquetteironiron-bearing
bearingdistrict
districtof
ofMichigan:
Michigan:U.S.
U.S.
Geological
6O8p.
Geological Survey
SurveyMonograph
Monograph28,
28,608~.
Williams, G.H., 1890,
1890, The greenstone
greenstone schist areas of the Menominee and Marquette regions of
Michigan:
62, 2'llp.
Michigan: U.S. Geological Survey Bulletin 62,241~.
96
Examples of
of Gold
Gold Mineralization
in the N
orthern
Mineralization in
Northern
Block of
of the
the Isbpeming
Ishpeming Greenstonc
Greenstone Belt
Thomas 0.
0. Quigley and
A. Mnhin
Mahin
and Robert
Robed A.
Minerals
Minerals Processing Corporation,
Champion MI
Corporation, Champion
97
Minerals Processing Corporation's High
High Point,
Point., Silver
Silver Creek West, and Gold Bluff
Bluff gold properties
properties
the in
in the
the northern
northern m
arm of
of the
the Ishpeming
Jshpeminggreenstone
greenstonebelt.
belt. The field trip stops are intended to
to
all lie in the
illudrate
Host lithologies, structure,
illustrate the
the various
various styles
styles of
of gold
gold mineralization
mineralization found
found in
in this
thispart
part of
of the
the belt.
belt. Host
mineralization, and alteration
discus^ and
and compared.
compared.
controls of mineralizatioq
alteration will be discussed
High Point Gold Prospect
The High Point prospect was discovered
diswvered by
by MPC
MPC during
duringreconnaissance
rewnnaissanceexploration
explorationactivities
activities
from Callahan
Callahan Mining Corp's
working out fiom
Corp's Silver
SilverCreek
CreekWest
Westoccurrence
occurrencelocated
located1500
1500ft.
ft.totothe
thesoutheast.
southeast,
High grade surface mineralization at High Point is hosted by aa prominent knob
knob of
of intrusive
intrusive felsic
felsic porphyry
porphy~~
escarpment of the
the Dead
Dead River Shear Zone
Zone (DRSZ).
(DRSZ). Subsequent
along the topographic escaynent
Subsequent exploration
activities at the prospect included detailed mapping and
and sampling,
sampling, geochemical surveys, trenching, and
diamond drilling. Figures
Figures 11and 22 show
show the
the geology
geology of
of the
the High
High Point
Point prospect.
prospect.
Lithology
Rock types
rocks,
types at the High Point
Point prospect
prospect consist
wnsist of
of felsic,
felsic, porphyritic
porphy5ticintrusive
in-ive
rocks, within
within aa sheared
sheared
of matic
mafic flows and tuffacwus
tuffaceous rocks and minor interfiow
sequence of
interflow sediments
sediments corresponding
correspondingto
to the
the
Lighthouse Point Basalt of Johnson and Bomhorst
Bornhorst (1991). The
TheHigh
High Point
Point intrusive
intrusiveis
is aa fme
fme to
to medium
medium
grained, massive to weakly foliated
foliated feldspar
feldspar porphyry
porphyq stock
stock consisting
consisting of
of small
small phenocrysts
phenoaysts of
of albite
albitein
in aa
matrix of quartz, albite, potassium feldspar and carbonate. In
thin
In addition
additionto
to the
the main
main stock,
stock, numerous
numerous thin
stringers of pporphyry
o ~ h y r yof similar composition
composition are
are present
present in the surrounding
mounding volcanics. These mafic
volcanic
volcanic rocks consist of fine
h e grained, chlorite,
chlorite, sericite,
sericite, and
and amphibole
amphibole schists,
schists, and
and are
are probably
probably altered
altered
basalts and related kff"wus
tuffaceous rocks that have been variably sheared and deformed.
Structure
types
defonnation related
All rock t
ypes at the High Point prospect have been subjected to shearing and deformation
related to
to
the DRSZ. Mafic
units
exhibit
steeply
dipping
shear
related
foliations
striking
NW-SE,
parallel
to
the
Mafk units exhibit steeply dipping shem
foliations
the
ancillary shearing oblique
obliqueto
tothe
themain
main DRSZ
DRSZtrend.
trend. This shearing is
main DRSZ, as well as E -- W striking ancillaq~
through 33 (see Figure 21,
2), which expose the flanking volcanic d
section
well exposed in backhoe trenches 11 &rough
o n
to the High
High Point
Point iintrusive.
and dikes exhibit
exhibit
adjacent to
n h i v e . The
The High
High Point
Point Intrusive
I n h i v e body and related sills and
intense brittle deformation
deformation manifested
manifested by irregular stockwork fracturing, quartz -- carbonate
carbonate veining,
veining, and
and
brecciation. Irregular
Irregularcontacts
contactsof
ofthese
these felsic
felsicunits
unitsobserved
observedin
indrill
drillcore,
core,as
aswell
weU as
as aa lack
lackof
ofcontinuity
continuity
between pporphyry
units have been
o r p h y ~units
~ in outcrops, drill holes, and trenches,
kenches, suggest that these felsic units
extensively disrupted and possibly rotated as
as large competent
wmpetent blocks within this portion of the DRSZ.
Alteration
Alteration
Ubiquitous
Ubiquitous carbonate
carbonate alteration
alteration consisting
consisling of
of calcite
calcite and dolomite-ankerite is present throughout the
the High
High Point
Point P
Prospect,
pervasive replacement
replacement of
of rock
rock matrix, and as
sheared section at the
r o s p q occurring as a pmasive
discrete
fillings in both mafic and felsic
muscovite, as
discrete veins and fracture
fiacture mlings
felsic units. Coarse, secondary muswvite,
as well as
black shiny carbon mineral are wmmonly
commonly associated with
with carbonate
carbonate veining
veining in
in the
the High
High
an unknown black
Point area. Examples of the carbon mineral
mineralcan be readily
readily found in the talus slope below the
the mineralized
that
mineral is present
present throughout
throughout the
the
ha! this mineral
outcrops of High Point intrusive,
intrusive, and drilling has revealed t
98
________
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CORPORATION
Geology of the High
Point Prospect
ala
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Hiab Point Intrusive
Porpbyiy, Slice to madimu pained, reddish brown porphyriuo rock
p,mpo.ed of 1 to 5 = sibite pbgiooryat. in a matrix of elba.. K—feitpar.
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ten-can dolornito. ankorita. end en action black carbon minaret.
liii j4j4j
}li.h Point Intrusive
Bericit. end cerbonata alias-ed porph1ry, cut by bieguler etockwork Veina and
mesas of gray quartz. iron carbonates, minor pyrita, minor guiana, bismuth
tafluride., and gold.
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Scala In S'bet
Basalt. Indesita
EJ
-
Fine grelnad chlorite, earlclt carbonate ecbi.te. Massive to roustad end
fractured with atockwork carbonate — carbon mineral yams and
dissemlatatad pyrtte where .haared, Locaily containing gray quarts veining
ecd gold inlnarellsauon, Sequence contains minor mntarfior shexty. pyritln
sediments-
I
Figure
Figure 1.
1. Geology of High
High Point
Point
0
0
'4
+
4
4
4
4
4
4-4-4
MPC Trench
in ppb.
7 4+4 444
1
+
+
÷
+
-1-
-4-
±
+4-
—E
'Intt.
4
4
4
——3—
4.
±
Gold Values (ppb)
CORPORATION
Oda\E.ctMc
Psat Geel
0
25
0
IT
I
Scale in Feet
0
Channel Chip and Trench
Samples
Figure 2. Detailed Geology of High Point
4
44 tt* 4+4444
4 44444444 * 4
rTTI Channel sample with gold value
Basalt, Andesite
4
4
+t.tt.t-,-ttttt4
444 4-4+4 4
4
T:] High Point Intrusive
IThrfl
vfl
C
4+44
Trench 1
50
MINERALS PROCESSING
__
flanking volcanic section as well. Within
Withinthe
themineralized
mineralizedportion
portionof
ofthe
theHigh
HighPoint
Pointintnisive,
intmsive,microprobe
microprobe
analysis has shown that zoned
zoned dolomite-ankerite
dolomite-ankeriteis
isintimately
intimatelyintergrown
intergrownwith
with barium
bariumbearing
bearingK-feldspar
K-feldspar
Tourmaline is also locally present in quartz veins, in the matrix
matrix of brecciated porphyry,
and muscovite. Tourmaline
and intergrown
intergrown with chlorite
chlorite along
along fracture
ffacture surfaces
surfacesin
in sheared
sheared volcanic
volcanicrocks.
rocks.
Mineralization
Mineralization
gray
Surface mineralization at High Point is confmed to the porphyry, and consists of irregular gray
quartz veining in altered porphyry
porphyry on
on the
the southeast
southeast side
sideof
ofthe
themain
mainknob.
knob. Here channel chip
chip sampling
sampling of
of
exposed
exposed outcrops
outcrops (Chip
(Chip Samples
Samples A,
A, B,
B, and
and C)
C) returned
retumed significant
significant gold
gold values
values including 28 ft.
&. of
of .09
.09
ozlton,
.35 oz/ton
odton including
including 4 ft of 1.25
1.25 ozlton
odton (Figure 2). The
Themineralized
mineralized quartz
quartz from
ffom these
odton, and 14
14 ft. of .35
exposures
gay, cut
cut by
by iron
ironcarbonate
carbonateveinlets,
veinlets, and
andcontains
containssmall
smallamounts
amountsof
ofpyrite,
pyite, chalcopyrite,
chalcopyrite,
exposures is dark
dark gray,
galena, sphalerite,
galenq
sphalerite, and electrum
electnun as
as well
well as
as an
an exotic
exoticsuite
suiteof
of bismuth
bismuth tellurides,
tellurides,lead
leadbismuth
bismuth tellurides,
tellurides,
and an unknown lead-bismuth-silver sulfide. The
Thegeometry
geometryof
ofthis
thismineralized
mineralizedzone
zonehas
has not
not yet
yet been
been
defined,
and
defmed, although
although its
its extent
extent has
has been
been partially
patially confined
confmedby
by trenching
~~enching
andwide
widespaced
spaceddrilling.
drilling.
Silver Creek
Creek West
Fiver
Silver Creek West is a gold occurrence
variably altered and
and foliated
foliated mafic
mafic rocks.
rocks. In
occmnce within variably
In 1988
1988
million tons @O.O37optAu
@0.037optAu for Silver
I. 1 &ion
Silver Creek
Callahan Mining Corporation calculated a resource of 1.1
controls. Surface
West but never exploited it because of unresolved concerns over gold continuity and coneols.
Surface
trenching exposes
exposes gold mineralization
eenching
mineralidon in
in ductile
ductile sheared
sheared and
and carbonate-chlorite-sericite-pyrite
carbonate-chlonte-sericite-pyritealtered
altered
basalt near the margin of a felsic porphyry. In
Indrill
drillhole
hole intersections,
intersection&mineralization
mineralizationappears
appears to
to be
stratabound
seatabound within 170
170 foot
foot section
section of
ofmoderately
moderatelycarbonate
carbonatealtered,
altered,massive
massivemedium
mediumgrained
grainedmafics.
mafics,
Mineralization in both trenches
eenches and
and drill
drill core
core will be
be observed
observed at
at this
this stop.
stop.The
Thegeneral
general geology
geologyof
ofSilver
Silver
Creek West is
3.
is shown
shown in
in Figure
Figure3.
Lithology
The lithologies in the immediate vicinity of the Silver Creek
Creek West
West prospect
prospect consist
consist of
of aa number
number of
of
generally
generally northwest striking,
striking steeply dipping
dipping units, largely
largely of the Archean Lighthouse
Lighthouse Point Basalt
(Johnson and Bornhorst,
largeexpanse
expanseof
offine
finegrained,
graind
Bomhorst, 1991).
1991).To
Tothe
thenortheast
northeastof
ofthe
theSCW
SCWprospect
prospectisisaalarge
pillowed basalt. Nearer
Nearerto
tothe
theprospect
prospectoccurs
occursaamedium
mediumto
tofine
finegrained
grainedequigranular
equigranularchiorite-albitechlorite-albitecarbonate
carbonate unit, variously interpreted
interpreted as
as an ophitic
ophitic basalt, aa sheared
sheared carbonated
carbonatedbasalt,
basalt, and
and aa hypabyssal
hypabysd
mafic
pyrrhotite and hosts gold
mafic intrusive
intrusive or sill. The
Theunit
unit isisgenerally
generally massive,
massive, contains
contains significant
significant pynhotite
mineralization
mineralization at depth in drill core. The
Thenext
nextunit
unit to
to the
the southwest
southwestis
is aa narrow
narrow band of
of chlorite-albitechlorite-albitephyllite interpreted
interpreted as
as aa ductile
ductile sheared
shearedbasalt.
basalt. This unit hosts
hosts gold mineralization
mineralization
sericite-carbonatephyllite
trenches. This
exposed in surface trenches.
Thisbasalt
basalt is
is bounded
bounded to
to the
the southwest
southwest along
along the cliff
cliff face
face by aa rhyodacite
rhyodacite
porphyry which
which consists
consists of
of quartz,
q w feldspar,
feldspar,and
andchlorite
chloritephenocrysts
phenocrystsin
in aa felsic
felsicgroundmass.
groundmass.Drill
Bill
intercepts show an additional sequence
sequence of chiorite-albite-sericite
chlonte-albite-sericite phyllite and quartz-feldspar
quartz-feldspar intrusive
intrusive to
the southwest. In
Inatatleast
least one
onehole
holethe
the latter
latter felsic
felsic is
is in
in fault
fault contact
contact with Proterozoic
Proterozoic sediments
sediments which
have
have been down-thrown
dom-thrown to form
form the
the basin
basin to
to the
the southwest.
southwest.
101
_
__
___
____
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VVVVVV
VVVV
MINERALS PROCESSING
CORPORATION
--vvwVvvVvVt
VVVnVflVVV
vVVVVVVVVVVV
VVVVVVVVVVVVV.
uqVVVVVVVVVVVVV
VVVV VV VV Vt Vt Vt V
VV
1
——
V V V V V V V Vt PVV VVV %
V V V V VVV VPVVV VVVV
Silver Creek West
— a'—.-
1,
1VVVVVVVV
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'VVVVVVVV
Dt V V V V V V V V V V V V V V V
'VVVVVVVVVVVVVV
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Geology
— ni-la
VVVVVVVVVVVVVW%R%i
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Me
Reversely polarized
0
a
N441 Sedimantan
rock.
S*cii4 Undifferentiated. Berega Group
V÷A FeolnaiRocks
I
Mafic Volcanic Rook.
lv
•
Hvnabvsai Vafic Intnstve Rocks
•
Enterwediate Vo4genãc ggçp
SedimoctmHoon
•
—
Fault ihowing
downthron aide
Outcrop
Shearing
Trench
—
200
0
Figure 3.
3. Geology
Geology of
of Silver
Silver Creek
Creek West
West
Figure
200
Scale
in feet
400
V Vi
Structure and mineralization
mineralization
At the surface, gold mineralization exposed
exposed in trenches by Callahan occurs within a ductile
ductile sheared
mafic
mafic volcanic
volcanic near the margin of
of aa felsic
felsic porphyry. This
Thisshearing
shearingstrikes
strikes300°
300' and
andmineralization
mineralization is
is
considered C-type replacement. In
Incontrast
contrastto
tosurface
surfaceexposures,
exposures,mineralized
mineralizedintervals
intervalsin
in drill
drill core
core are
are in
in
an unfbliated,
weakly altered
altered medium
medium grained
grained mafic
maficunit;
unit; there
thereisislittle
littleresemblance
resemblanceto
to mineralization
mineralizationin
in
unfoliated, weakly
depth to
to the
the easthortheast
east/northeast from
from
the sheared basalt. Gold-bearing intervals that in drill intercepts occur at depth
the trenched intervals
intervals do not project
project upwards
upwards along
alongobvious
obviousstructural
structuralplanes
planesto
tothe
thesurface
surfaceworkings.
workings.
The exact controls to the mineralization at
at depth
depth have
havenot
not been
beenproved.
proved. Intersection lineations
lineations of
of
plunge down to
to the east and offer
offer one possible explanation for joining
joining surface
various structural features plunge
and drill-hole gold (Johnson,
1991).
MF'C has
hasshown
shownthat
thatanomalous
anomalousmineralization
mineralization
fJohnson
19911Limited
Limitedwork
workby
bvMPC
.
occurs in outcrop
outcrop to
to the
the east
eastof
o fthe
thehigh
gradetrenches
trenchesatatthe
theup-dip
up-dip(along
(alongSSlfoliation)
projection of
of the
the
occurs
high grade
1 foliation) projection
mineralized drill core
mineralized
core intervals,
intervals, suggesting
suggesting that the trench and
and drill
drill core
core intercepts
interceptsare
are not connected.
connected.
Alteration
The
mineralized zone
The mineralized
zone is
is associated
associated with
with weakly
weakly disseminated
disseminatedcalcite+chlorite+sericite+pyrite
calcite+chlorite+sericite+pyrite
alteration. Locally,
Locally,minor
minorFe-carbonate
Fe-carbonateisisobserved
observedatatsurface
surfaceand
andin
infelsic
felsicunits
units encountered
encountered in
in drilling.
drilling.
Additionally,
Additionally, Callahan
Callahan geologists
geologists identified
identified an
an early
early epidote-pyrrhotite
epidote-pynhotite(spilite)
(spilite)event
eventin
in the
the basalts.
basalts.
Gold
Gold Bluff
BluffProspect
Prospect
Gold Bluff
Bluff is one of Minerals Processing Corporation's
Corporation's active gold properties
properties (see
(see Figure
Figure4).
4). Gold
Bluff
trending, 1000 foot wide shear zone in mafic
mafic volcanics.
volcanics. A
A 100 to
to 200 foot
Bluff is
is located
located within a 3000
300' trending,
wide
wide felsic
felsic porphyry
porphyry (the
(the GB
GBporphyry)
porphyry) intrudes
intrudesthe
theshear
shearzone.
zone.Within
Withinthe
thebounding
boundingshear
shearoccur
occureasteastwest and northeast striking mineralized subsidiary shear zones.
zones. Outcrop
Outcropsampling
sampling routinely yields
yields gold in
the
Oppm range.
range. Trenching
the lppm
lppmto
to1lOppm
1998 intersected
intersected significant
significant widths of disseminated
Trenching and drilling
drilling in 1998
low
low grade gold
gold mineralization
mineralization the GB porphyry and higher grade gold in mafic host rock. Mineralization
in both mafic
felsic hosts
hosts in
in different
different structural
structural orientations
orientations will
will be
be seen
seen at
at Gold
Gold Bluff.
Bluff.
mafic and felsic
Lithology
Three lithologies are found at Gold
Gold Bluff:
Bluff:fine-grained
fine-grained mafic
mafic volcanic;
volcanic; medium
medium to
tocoarse-grained
coarse-grained
mafic volcanic,
referred to as aa gabbro;
volcanic, referred
gabbro; and
andfelsic
felsicporphyry.
porphyry. The
The mafic
maficunits
units are
aremetamorphosed
metamorphosed to
to
facies and both
both can be massive
massive to
to strongly
strongly foliated.
foliated. They
greenschist fades
They belong
belong to
to Lighthouse
Lighthouse Point
Point Basalt
Basalt as
as
described by
by Johnson
and Bonthorst
(1991). The felsic unit
described
Johnson and
Bornhorst (1991).
unit isis aarhyodacite
rhyodacite porphyry
porphyry interpreted
interpreted as
as
intrusive. Where
unit that
that divides the
the two
two mafic
mafic units,
units, itit is
is dubbed
dubbed the
the GB
intrusive.
Where it appears
appears to be aa continuous
continuous unit
throughout the
theproperty.
property. Contacts
porphyry. Smaller
Smallerfelsic
felsic dikes
dikes are
are also observed in various orientations throughout
Contacts
strike northwest and have
have steep dips. A
Areversely
reverselypolarized
polarized dike
dikeof
ofKeeweenawan
Keeweenawan age
age is
is interpreted
interpreted from
from
data to lie just
just to the north of the outcropping rocks.
rocks. The
magnetic data
The only
only other
other unit
unit of
of significance
significance in
in the
the
area is the Reany Lake Pyroclastic (Johnson
(Johnson and
and Bomhorst,
Bomhorst, 1991) which
which occurs
occursjust
just off
off the Gold Bluff
property to the
the north and
and west
west in
in association
association with
with gabbro
gabbro flows.
flows.
103
104
Bluff Gold of Geology
—
.tgnlflcnt
I
thoy
Figure
4.
soda-
tat
wits. — .siclt.
flax. Isyaxat
a .aanta
acp.alUon. .od.duo to basaitia pobabi. of rook
mle4 and The.. u4—eet.d rse,. patoad. o.tdf to Ito.
Rnnko Vnlnin MaNe
Locally
33 Sec
28 Sec
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to coax..
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(t. t.rbcats S44h .lnd .04 1.t.d broesalad. fr.osr.d. locally qwtL
and/ot Mltpc or pb.toetyM. otth pakUo to SqWjnmiIar bO4a
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nedja to
vest
jeus.
Rocks Intrusive Feisic
palnb.d
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Ir'
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200
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GEOLOGY
Project Bluff Gold
Corporation
Processing Minerals
—.
S
WE
N
Structure
The orientation
orientation of
of brittle
brittle and
and ductile
ductile shear
shear structures,
structure&felsic
felsicporphyries,
porphyries, and
and quartz
quartzveins
veinsisis
with a system of
of conjugate shears
shears that
that control
control gold
gold mineralization,
mineralization. Bounding
geometrically consistent with
shear features, mostly penetrative foliation in mafic rocks
rocks and along felsic contacts,
contacts, strike
strike 300'
300°.
. Within
this, subsidiary stnctures
structures and a variety of
of mineralization
mineralization styles are
are observed.
observed. For
For example,
example, the
the GB
GB
trends
3200 and appears
porphyry m
d s 320'
appears to occupy an oblique shear
shear position (probably R-type Reidel).
Mineralization
300 °and
and is C-type
Mineralization in Trench 5 and DDH GB-3, and along felsic contacts, strikes 300'
replacement veining. R'-type
R'-typeshears
shearsare
arepresent
presentas
asnortheast
northeaststriking
strikingductile
ductilemicro-shears
micro-shears and
and as
as numerous
numerous
mineralized quartz veins. Numerous
Numerousductile
ductileshears
shearsand
and minor
minor felsic
felsic porphyries
porphyries that
that strike
strike 080°
080' to 1000
100'
or S-type
S-type structures.
structures.
may be either
either P or
quartz-carbonate-tounnaline veins
White quartz-carbonate-towmaline
veins with
with fibrous
fibrousstructure
structnreperpendicular
peqendicular to
to the
the vein
vein walls
walls are
are
interpreted
features. These
intqreted as extensional fmtures.
Thesetypically
typically dip
dip in
in the
the 30°
30' to
to40°
40' range
rangeand
andindicate
indicateaahigh
highangle
angle
component
veins. Gray
Gray quartz
quartz veins
veins are
are often
often mineralized,
component to the shearing event responsible for these veins.
defonned,
deformed, and
andcross-cut
cross-cutby
bywhite
whitequartz
quartzveins
veinsindicating
indicatingmultiple
multipleshearing
shearingand
andvein
veindeposition
depositionevents.
events.
Alteration
Alteration
A zoned carbonate alterationhalo
halo occurs
occursat
atGold
GoldBid.
Bluff Distal
Distalalteration
alterationin
in the
the mafic
ma& volcanics
consists
cousists of
of disseminated
disseminated calcite
calciteand
and weak
weak Fe-carbonate,
Fe-carbonate, (ankerite-dolomite).
(ankerite-dolomite). As
As the
the GB
GB porphyry
porphyryisis
approached, Fe-carbonate
approached,
Fe-carbonate increases
increases and can
can be pervasive to the point
point of
of flooding
floodingand
and the
theassemblage
assemblage
sericite+white quartz+pyrit&tourmaline
quartz+pyrite+tounnaline appears,
sencitewhite
appears, largely
largely in
in the
the felsics.
felsics. Lithologic
Lithologic textures
texturesare
are generally
generally
intact. Within
Withinthe
theGB
GBporphyry
poxphyryaazone
zoneof
ofstrong
saongsericite+pyrite+Fe-carbonate
sericite+pyrite+Fecarbonate occurs
occurswith
with porphyritic
porphyritic
sometimes completely
completely replaced by fine
textures sometimes
fine grained
gained sericite.
sericite.
Mineralization
Mineralization occurs in all three lithologies
lithologies and consists
consists of quartz+gold+pyrite+Ag&Bi-tellurides.
qm?z+gold+pyrite+Ag&Bi-tellurides.
mineralization (i.e.,
In the metabasalts C-type replacement mineralkalion
(i.e., highly silicified and sulfidized
sulfidized zones
wnes with
with
is generally
generally confined
confined to thin
thin ductile
ductile shears
shears at
at or
or
narrow foliation-parallel
foliation-parallel quartz
quartz veins, lenses
lenses and pods) is
proximal
proximal to
to felsic
felsic contacts.
contacts.Similar,
Similar, although
althoughoften
oftenhigher
higher grade,
grade, mineralization
minerahation occurs
occursat
atthe
thegabbro/felsic
gabbro/felsic
contact. Significant
SignificantC-type
C-typevein
vein mineralization
mineralizationnot associated
associated with a felsic contact also occurs within
sheared
gabbro
south
of
the
GB
porphyry
(S.7'@O.
188opt
optAu
Auand
and17'@0.077
l7'@0.077 opt
sheaxed
(S.T@O. 188
opt including
including
7.5'@0.1480pt w/VG).
wNG).
7.5'@0.l4Sopt
In
In the GB porphyry, mineralization
mineraIizatiou is
is found
found in
in quartz
quartz vein
vein stockwork
stockwork and
and tends
tends to
to be
be associated
associated
with disseminated pyrite and strong alteration. Gold-bearing
Gold-bearing veins
veins are gray
g a y colored compared
compared to
to barren
veins, although
although white
whitequartz
quartzveins
veinsare
arean
animportant
importantindicator.
indicator. One ggray
ay
white quartz-carbonate-tourmaline veins,
quartz vein 0.4 feet wide assays 20-50 opt Au and has yielded impressive
impressive visible
visible gold
gold specimens. Recent
Recent
drilling in the GB porphyry by MF'
MPC
significant m
mineralization
(224'@0.Ol8opt including
C intersected sigificant
i n e r h t i o n (224'@0.018opt
including
1l'@O,l78 and 3'@0.l22
3'@0.122 opt).
opt).
ll'@O.178
105
References
R.C., 1991.
proposal: Stage
Johnson, KC.,
1991. Silver Creek West drilling proposal:
Stage III.
111. Western
Western Mining
CorporationlCallahan
CorporatiodCallahanMining Corporation
Corporation unpublished
unpublished report,
report, February,
F e b m ~1991.
1991.
,
R.C., and Bodorst,
Bornhorst, T.J.,
Johnson, R.C.,
T.J., 1991,
1991, Archean geology
geology of
of the northern
northern block of
of the Ishpeming
Greenstone belt, Marquette
Marquette County,
County, Michigan:
Michigan:U.S.
U.S.Geological
GeologicalSurvey
SurveyBulletin
Bulletin1904-F,
1904-F,2Op.
20p.
106
Field
Field Trip
Trip33
Tilden
Marquette Iron
Tilden and
and Empire
EmpireMines
Mines of the Marquette
Iron
Range,
Range,Michigan
Michigan
Leaders:
Leaders:G.W.
G.W.Scott,
Scott, P.M.
P.M. Nordstrom
Nordstrom and
andH.M.
H.M. Lukey
Lukey
107
Setting of
of the
the Tilden
Tilden and
and Empire
Empire Mines, Michigan
Michigan
Geologic Setting
Theodore
Theodore J. Bornhorst,
Bornhorst, Department
Department of
of Geological
Geological Engineering
Engineering and
and Sciences,
Sciences, Michigan
Michigan
Technological
Technological University,
University, Houghton,
Houghton, MI
MI 49931
4993 1
Introduction
Introduction
The Tilden and Empire Mines are located in the Marquette Iron
hon Range
Range of Michigan's
Michigan's Upper
Upper
Peninsula (Figure
(Figure 1).
1). Mining
Miningbegan
beganatatthe
theEmpire
EmpireMine
Minein
in 1963
1963and
and at
at the
the Tilden
TildenMine
Minein
in
1974. This
This is
is the
the first
first Institute
Institute of
of Lake
Lake Superior
Superior Geology
Geology field trip to either of these iron
mines. The
TheMarquette
Marquette hon
IronRange
Rangehas
hashad
had aalong
long and
and varied
varied mining history (Boyum,
(Boyum, 1988).
1988).
reserves are
are sufficient
sufficient to
to sustain iron mining
Iron mining began in the mid-1800's and current reserves
until at
at least
least 2030.
2030.
Stratigraphic Setting
Stratigraphic
Marquette
Marquette Range
Range Supergroup
Supergroup
The Tilden and Empire Mines produce iron ores from the Early Proterozoic rocks of the
Marquette Range Supergroup (Figure 2). The
The Marquette Range Supergroup consists of
sedimentary
sedimentary rocks with minor
minor amounts
amounts of
cf volcanic
volcanic rocks and is
is subdivided
subdivided into
into three
three groups:
groups:
to youngest.
youngest. The
Chocolay, Menominee and Baraga Groups, from oldest to
The rocks
rocks of
of the
the
on older
older Archean
Archean basement.
basement.
Marquette Range Supergroup were unconformably deposited on
are separated
separated by
by minor
minor angular
angular
The three groups of the Marquette Range Supergroup are
and informal
informal units.
units. The
unconformities. The
The groups
groups are
are subdivided
subdivided into formal formations and
The
from the base upward, of conglomerate
Chocolay Group
Group consists,
consists, from
conglomerate (Enchantment
(EnchantmentLake
Lake
Formation), quartzite
quartzite (Mesnard
(Mesnard Quartzite),
Quartzite), dolomite
dolomite (Kona
(Kona Dolomite),
Dolomite),and
and slate
slate (Wewe
(Wewe
Slate).
Slate). The
TheMenominee
MenomineeGroup
Groupunconformably
unconformably overlies
overlies the
the Chocolay
Chocolay Group
Group and
and consists,
consists,
from the base upwards,
upwards, of local
local conglomerate,
conglomerate, quartzite
quartzite (Ajibik
(Ajibik Quartzite),
Quartzite),slate
slate (Siamo
(Siamo
Slate), and iron formation (Neguanee Iron
IronFormation).
Formation). The Baraga Group is
is the
the youngest
youngest of
of
Menominee Group.
Group. The
the three groups and unconformably overlies the Menominee
The Baraga
Baraga Group
Group
includes a variety of stratigraphic units. In
In the
the Marquette
Marquette area
area itit consists,
consists,from
from the
the base
base
upwards, of quartzite
quartzite (Goodrich
(Goodrich Quartzite),
Quartzite), slate
slate (Lower
(Lower Slate
Slate of the
the Michigamme
Formation), iron formation (Greenwood Iron
hon Formation),
Formation),
Formation), mafic pyroclastics (Clarksburg
(Clarksburg
Volcanics),
Volcanics), iron formation,
formation, slate
slate (Upper
(Upper Slate
Slate of the Michigamme
Michigamme Formation),
Formation),and
and iron
iron
formation (Bijiki Iron Formation). hon
Ironore
orehas
has been
been mined
mined from
from iron formations within the
Menominee and Baraga Groups. However,
However, most
most production
production has
has come
come from
from the
the Negaunee
Negaunee
hon Formation
Iron
Formation of
of the
the Menominee
Menominee Group,
Group, including the Tilden and Empire Mines.
Diabase Dikes
Dikes and
and Sills
Sills
stratified units of the Marquette
The stratified
Marquette Range Supergroup
Supergroup are commonly
commonly intruded
intruded by Early
Early
the form
form of
of dikes
dikes and
and sills.
sills. Early
Proterozoic diabase (metamorphosed) in the
Early Proterozoic
Proterozoic
diabase dikes and sills
sills are particularly
particularly common in vicinity of the Tilden and
and Empire
Empire Mines.
Mines.
Volcanics of
of the
the Baraga
Baraga Group,
Group, 10 to
to 20
20 km
km
Diabase is also associated with the Clarksburg Volcanics
108
%
LAKE
SUPERIOR
srow
or
EXPLANATION
EXPLANATION
Paleozoic
Paleozoic
Undifferentiated Paleozoic
Undifferentiated
Paleozoic Rocks
Rocks
Keweenawan
Keweenawan
UndifferentiatedKeweenawan
Keweenawan Rocks
Rocks
[----1 Undifferentiated
Early Proterozoic
Early_Proterozoic
ji Undifferentiated
UndifferentiitedEarly
EarlyProterozoic
ProterozoicRocks
Rocks
Archean
Archean
Undifferentiated
Undifferentiated Archean Rocks
Rocks
I
I
Marquette iron
Figure 1: Geologic setting of the Marquette
Figure
iron range.
range
109
Explanation
Explanation
Archean
Proterozoic
Proterozoic
Middle Proterozoic
Early Proterozoic
Archean
Metadiabase
Undifferentiated
Keweenawan
sandstone
ItLIMI Undifferentiated
Keweenawan sandstone
Metadlabase
I
and diabase
diabase
Marquette
Range
Supergroup
Marquette Range Supergroup
Baraga Group
Group
Baraga
rc::.i Undifferentiated
Undifferentiated
Menominee Group
Group
Menornmee
Negaunee Iron
Negaunee
Iron Formation
Formation
Siate
I>'">I Siamo Slate
Quartzite
LL.Jj.JI Ajibik Quartzite
Chocolay Group
Group
Chocolay
Undifferentiated
Undifferentiated
1++1
0
I
!1
Generalized geology in vicinity
Figure 2: Generalized
vicinity of the Tilden
Tilden and
and Empire
Empire Mines.
Mines.
110
west of the mines. Several
Severalgeologists
geologists have
have suggested
suggested that Early Proterozoic
Proterozoic diabase
diabase
intrusions
intrusions throughout the Marquette region are the same age as the Clarksburg
Clarksburg Volcanics
Volcanics of
of
younger than
than the
theNegaunee
NegauneeIron
hon Formation.
Formation. However, some of the
the Baraga Group and younger
diabase sills at the Tilden and Empire Mines may have been lava flows (Scott
(Scott and Lukey,
Lukey,
this volume). This
Thisinterpretation
suggestsmultiple
ages of Early Proterozoic
Proterozoic diabase
diabase and
interpretation suggests
multiple ages
of deposition of
of iron
iron formation.
formation. The Hemlock Formation,
mafic magmatism during the time of
Formation,
south of the Marquette area in the Iron
hon Mountain area, consists of mafic volcanics that is
(Morey, 1996). This supports
correlative with the Menominee Group (Morey,
supports the interpretation
interpretation of
mafic magmatism during the Menominee Group in the Marquette area.
metamorphosed) UKC>
dikes also
{UUL LLLCLUWL~LLUXAI,
uu cut
LUL the rocks
rocks
diabas~(not
Later Middle Proterozoic
Proterozoic Keweenawan diabase
and cut
cut the
the Negaunee
Negaunee Iron
hon Formation at the Tilden and
of the Marquette Range Supergroup and
Empire
Empire Mines.
Mines.
Negaunee
Fonnation
Negaunee Iron Formation
The Neguanee hon
Iron Formation
Formation consists
consists of
of aa variety of iron rich rocks in the Marquette
Marquette area
(Figure 2). The
Themajor
major types
types of
of iron
iron formation
formation are
are carbonate iron-formation (iron carbonate
carbonate
and chert with minor magnetite), oxide iron formation (hematite or magnetite and chert),
magnetite-banded iron formation (laminated magnetite and chert), hematite banded iron
formation (laminated hematite and chert), silicate iron formation (iron silicate minerals
formation
minerals and
types (Gair,
(Gair, 1975). The
TheNegaunee
NegauneeIron
hon Formation is
chert) and combinations of these types
described in detail by Scott and Lukey (this volume), Nordstrom (this volume), and Gair
(1975). The
Theorigin
originof
of the
the iron
iron minerals
minerals in
in the
the Negaunee hon
Iron Formation
Formation isis aa complex
complex
processes.
combination of primary sedimentary depositional, diagenetic, and metamorphic processes.
The
The maximum
maximum thickness
thickness of
of the
the Negaunee
Negaunee Iron Formation is about
about 1150
1150meters,
meters, although
although
to assess
assess (Gair,
(Gair, 1975).
1975). The Negaunee hon
folds and faults make thickness difficult to
Iron
Formation is thickest
thickest in
in vicinity
vicinity of the Tilden and Empire Mines where is occurs
occurs in
in the
the nose
nose
km across
across (Figure
(Figure2).
2).
of a regional west plunging
plunging syneline
synclineabout
about 66 km
Age of the Marquette Range Supergroup
Morey (1996) has provided a stratigraphic synthesis of the Marquette Range Supergroup.
Supergroup.
Supergroup is
Although the age of the onset of sedimentation of the Marquette Range Supergroup
2,219+1-4Ma.
Ma. The age
uncertain, he has suggested that the Chocolay Group is younger than 2,219+/-4
age
the Menominee
Menominee Group
Groupisis 1,910+/-10
1,910+/-10Ma.
Ma. Deposition of the
of volcanic rocks correlated with the
Baraga Group
Baraga
Group ended
endedby
by1,852+1-6
1,852+/-6Ma.
Ma.
Depositional and
and Tectonic
Tectonic Setting
Setting
The Marquette Range Supergroup is interpreted to have been deposited in several stages
(Morey, 1996; Schulz and
and others,
others, 1993).
1993). The
Thesediments
sedimentsof
of the
the Chocolay
Chocolay Group
Groupwere
were
deposited
or intrarift
intrarift stage
stage (Figure
(Figure3).
3). The sediments
deposited during a late intracratonic or
sediments of the
Menominee Group were deposited on a passive continental margin in normal listric fault
bounded rift basins (Cambray, 1991). The
The Negaunee
Negaunee hon
Iron Formation
Formation mined
mined at
at the
the Tilden
Tildenand
and
111
Late lntracratonic
Intracratonic or Intrarift
lntrarift Stage
Stage
Chocolay Group
Archean Basement
Basement
Rift Stage
Rift
Stage
Menominee Group
Group
Spreading
Spreading
Center
Archean Basement
Foredeep
Foredeep Basin
Basin Stage
Stage
Archean Basement
Continent-arc
Continent-arc
Collision
matic
Oceanic Crust
Figure
Figure 3: Depositional
Depositional and
and tectonic
tectonic setting of the Marquette
Marquette Range
Range Supergroup
Supergroup
(modified
(modified from
from Morey,
Morey, 1996).
1996).
112
Empire Mines was deposited
deposited during this stage
stage and some
some faults
faults at the mines may
may be
be related
related to
to
this stage (Scott and Lukey, this volume). Elsewhere,
Elsewhere, and
and perhaps
perhaps in
in the Marquette
Marquette area
area too,
too,
tholeiitic basaltic
basaltic magmatism also
also accumulated in these rift basins. The Negaunee
Negaunee Iron
Iron
Formation
basin(s) are
are located along the Great
Great Lakes tectonic
tectonic zone,
zone, aa late
late
Formation depositional
depositional basin(s)
others, 1980). Passive margin deposition transitioned into
Archean suture zone (Sims and others,
deposition
deposition of sediments
sediments and
and minor volcanic rocks in a foredeep
foredeep basin stage
stage that
that formed
formed as
as aa
result of Penokean collision
collision of the Wisconsin
Wisconsin magmatic arc with the edge
edge of the
the continent
continent
(Morey, 1996). The
The continent-arc
continent-arcsuture
suture zone
zone approximately
approximately follows the MichiganWisconsin border. The
TheBaraga
BaragaGroup
Groupwas
was mostly
mostly deposited
deposited during
during the foredeep
foredeep stage
stage as
as
turbdites.
turbdites.
The Marquette Range
t h s t belt of the 1850
1850Ma
Ma Penokean
Penokean
Range Supergroup
Supergroup is
is within the fold and thrust
orogen (Sims, 1996). The
The Tilden
Tilden and
and Empire
Empire Mines are located on the south limb of the
Marquette Synclinorium, an east-west trending, west plunging syncline that is about 6 km
km
across and 55 lun
km long
long parallel
parallel to
tothe
thefold
foldaxis.
axis. The synclinorium, like
likethe
theNegaunee
NegauneeIron
hon
Formation depositional basin(s), may have been controlled by the Archean Great Lakes
tectonic zone. The
Archean structures
structures were subsequently folded during
The iron
iron formation
formationand
and Archean
the Penokean Orogeny. The
The Penokean
Penokean orogenic
orogenic event produced complex folds and faults and
greenschist facies metamorphsim
metamorphsim of the rocks at the Tilden and Empire Mines
Acknowidegments
Acknowldegments
for comments
comments on
on aa version
versionof
ofthis
thispaper.
paper. Shannon Bair
I thank Rod Johnson and Glenn Scott for
and Chris Pascoe
Pascoe helped
helped with
with drafting
draftingof
of the
the figures.
figures.
References
References
Boyum, B.H., 1988,
1988, Marquette
Marquette Mineral District of Michigan Mining History and Geology:
Geology:
Meeting,
Institute on Lake
Lake Superior
Superior Geology
Geology Proceedings,
Proceedings, 34th
3 4 Annual
Annual
~
Meeting, Marquette,
Marquette, MI,
MI, v.
v.
34, part 2, p. B1-B33.
Bl-B33.
Cambray, F.W., Mancusco, J.J.,
J.J., and Slitor, W., 1991,
1991, Detachment faulting and the origin of
asymmetric depositional
depositional pattern of the Marquette trough (abstract): Institute on Lake
Clalre, WI, v. 37, part 1,
Superior
Superior Geology
Geology Proceedings,
Proceedings, 337th7 Annual
Annual
~
Meeting, Eau Claire,
1, p.
17-18.
17-18.
Gair, J.E.,
J.E., 1975,
1975, Bedrock
Bedrock geology and ore deposits of the Palmer Quadrangle,
Quadrangle, Marquette
Marquette
County, Michigan: U.S.
U S Geological Survey Professional Paper 769, l59p.
159p.
Continental Margin Assemblage:
Morey, G.B., 1996,
Assemblage:
1996, Early
Early Proterozoic
Proterozoic Penokean Orogen - Continental
U.S.
U S . Geological
Geological Survey
Survey Professional
Professional Paper
Paper 1556,
1556,p.
p. 30-44.
30-44.
Sims, P.K., Card, K.D., Morey, G.B., and Peterman, Z.E.,
Z.E., 1980, The Great Lakes tectonic
major crustal
crustal structure
structure in
in central
central North
North America: Geological Society
-A major
Society of
zone —A
v.91,
p.690-698.
America Bulletin, v.
91, Pt.
pt. 1, p.
690-698.
Structure of the Continental
Sims, P.K., 1996,
Continental Margin:
Sims,
1996,Early
Early Proterozoic
ProterozoicPenokean
Penokean Orogen
Orogen -- Structure
U.S.
p.44-51.
U
S . Geological Survey Professional Paper 1556, p.
44-51.
113
GEOLOGIC
GEOLOGIC FIELD TRIP
TRIP TO THE
THE TILDEN MINE
Engineering Department, Tilden Mining
Glenn W. Scott and Helene M. Lukey, Mine Engineering
Mining
Company, P.O. Box 2000, Ishpeniing,
MI
49849
Ishpeming,
Introduction
Introduction
This geologic
primary structural and
geologic field trip to the Tilden Mine is designed to show the primary
stratigraphic features of the iron ore bodies and the implication
implication of these geologic
stratigraphic
geologic features
features
in mine planning and ore control. The
The interaction
interaction of sedimentation
sedimentation and
and diagenesis
diagenesis within
within
and supergene oxidation
oxidation has
has resulted
resulted in
in aa
growth fault controlled basin, metamorphism and
complex suite of ore types, each with specific,
specific, if not entirely
entirely objective, blending
blending
characteristics
and
problems.
Although
the
mining
operation
tends
toward
characteristics
Although the mining operation tends toward "brute
"bruteforce",
force",
some of the blending concerns can be quite subtle and sensitive.
sensitive.
For the purpose of mine modeling, the deposit has been divided
divided into
into geologic
geologic domains
domains
3;Table
Table1)
1)based
based in
in part
part on
on lithology
lithology but, more importantly
importantly on the
(Figures 1,2
1 , 2and
and3;
bench tests
tests and
and in
in the
the processing
processing plant
plant (Table
(Table 2).
2). The
metallurgical response based on bench
The
primary
p r i m w ore/waste
orelwaste parameters are weight recovery (the percentage of each ton of ore that
is turned into concentrate
concentrate and pellets; i.e. the value of that ton of ore) and grade
grade (does
(does the
the
ore contain sufficient
sufficient iron and silica
silica grade). Other factors which can effect the plant
operation and pellet quality are mineralogy (as related to total oxides and loss on
ignition); trace chemical
ignition);
chemical composition (in particular phosphorous but also
also Mn and
and alkalis);
alkalis);
crude, magnetic and slime iron; and autogenous grinding feed. It should be noted that
essentially all based on rather involved bench tests that may not directly
these data are essentially
directly
reflect the plant response.
response.
Structural Geology
Geology
Ththis
thisregion,
region,the
the Marquette
Marquette Range
Range Supergroup
Supergroup is
is in
in aa narrow
narrow east
east west trending basin
In
basin
known as the Marquette Synclinorium. This
basin
lies
along
the
Great
Lakes
Tectonic
This
lies along
originally defined by Sims and others (1980) as a Late Archean crustal scale
Zone as originally
suture with Superior
Superior Province
Province granite-greenstone
granite-greenstone terrain to the north and older
older Southern
Southern
Complex gneiss
gneiss to the
the south.
south. The
The northern
northern margin displays a more or less complete
complete
stratigraphic
stratigraphic succession from the base of the Chocolay Group through to the top of the
Menominee Group. The
Thesouthern
southernmargin
margin is
is truncated;
truncate& the
the Negaunee Iron Formation
Formation at
Menominee group abuts directly against the high grade Archean Gneiss.
Gneiss.
the top of the Menominee
This sharp
sharp contact
contact is
is marked
marked by outcrops
outcrops of the Palmer Gneiss,
Gneiss, described
described as
as a
"comminuted, sericitised
sericitised and partly silicified phase of the lower Precambrian gneiss"
"comminuted,
gneiss"
use and
(Van Hise
andBayley,
Bayley, 1897).
1897).
since that time
time has exposed a large face (Stop2)
(Stop2) of the chioritic
chloritic schists
schists
Mining activity since
that form part of the Palmer Gneiss. ItIt displays
displays mega-shear
mega-shear bands, the spacing
spacing between
114
NORTH
INTRUSIVE
4--
-
35
I.A±J -,
—
—
-4'
-
HEI'l
F1
:
H
lN
1
rC
it.
4.
CDIII
PIT
MAGNETIU CD-Ill
I
420
L?1AIN
/3
1/
—
a
H 1420
/
/
H
DOOIAIN
vii Jat
2
7/
/
(_
CROSS SECN'
'N
iT
I OOP$A1S1
V
2
zi
IIAIH
'F
7-
CN-
N,
L
—
3
JF
-'
'H'
1
(i11T"fH
,-_
-!
-
IZO F
—
—
TILDEN MINE
Figure
1-21.000
DATE:
1
3/11/99
"
IDSnAINJ:
v1
''
-
.1
400
0
I
=
*ivwW
C::
g
-
000
I
)
.(
1
—
20
-
'
PIT
F
=
800
I
1000
C
w
OanoIa BoeIdorIts
::
flT
Me Tot
TILDEN HEMATITE
NORTH-SOUTH CROSS-SECTION -13,100
-13,10(
-
TILDEN MAGNETITE
MAGNETITE
EAST-WEST CROSS-SECTION
CROSS-SECTION -16,000 WES
WE'
—
________
_________
PIT
PIT
I
LOOKING WEST
WEST
LOOKING
NORTIOWEST DOMAIN
(500)
HANGINGWALL ZONE
(470)
L
MAGP11TITI DOMAIN
1(420)
x4 rooTwALi. ZONE
Li
ri
(410)
HEMATITh DOMAIN
$NOWAU. CONTACT 20111
DOMAIN
INTRUSOVE
(200) (230)(240)
I_____
L
CLAY ZONE
(350)
HEMATITE - MARTIn DOMAIN
(350)
MAIN PIT CARIONATE IRON FOMIATIOII
(340)
FOOTWALL CLASTICS
(480)
FOOTWALI. IRON FOM4ATIOII DOMAIN
—
I
(310)
FOOTWAU. AND MANOINOWALI.
(12Q(250)
200
400
600
800
L
FIGURI 2
CDIII PIT
PIT
CDIII
LT
LnqKING NORTH
NORTH
LOOKING
I
-.
-
ORIGINAL GR0tMD
r
I
CDIII HANGINGWALL AND FOOTWALL
E-:J (230) (250)
,/ HEMATITE
r'
/
(440)
1 INTRUSIVE
(460)
HAGNETITE
(420)
Figure 2
HANGING WALL ZONE
(470)
0
200
I
I
400
I
600
I
800
I
FIGuRE 5
000 FT.
1000 F
North
North
S
outh
South
coolly UT PIT
.tt I FT
NW UUVU
600 rT
EAST PIT
+1200 FT
a
DIAGRAMATIC STRATIGRAPHIC
STRATIGRAPHIC SECTION
SECTION OF
OF TILDEN
DIAGRAMATIC
T I L D E N MINE
MINE
West
Looking
Lookin9
est
- W
Refer
R
e f e r to
t o tcxbIe
t ~ k d e11
Figure 33
Mine. The
Table 1:
1: Geologic domains
domains and subdomains at the Tilden Mine.
The domains
domainsare
arebased
based on
on
lithology
and
metallurgical
response.
The
numbers
for
each
domain
correspond
to
those
lithology
metallurgical
The numbers for each domain correspond to those
shown on Figures 1,
2, and 3 and are used as drill core codes and in modeling of the deposit
1,2,
using MEDSYSTEM.
MEDSYSTEM.
NORTHWESTDOMAIN
DOMAIN
500 NORTHWEST
The Northwest Domain is stratigraphically above CD IIll/West
I W e s t pit hanging wall metadiabase
metadiabase
(250) and below North Intrusive
Intrusive (270);
(270); this
this domain
domain includes
includes numerous
numerous dikes
dikes and
and one
one
mappable igneous
igneous horizon,
horizon, the
the West Intrusive
Intrusive (260).
(260).
550 West Hematite
550
Hematitedomain,
domain,restricted
restrictedto
tothe
theFar
Far West
West Extension,
Extension, is dominantly
40% and
and variable
variable but
but elevated
elevated
hematite chert with mixed goethite, recoveries around 40%
and metallurgical
metallurgical
phosphorous. Contact
Contact with
with 530
530 domain is defined by a thin intrusive and
change.
change.
530 (Hematite)-Goethite
(Hematitel-Goethitedomain
domainincludes
includesflot
flot"ore"
"ore"(531)
(531)and
and WIF
WIF (532).
(532).
531 Plot
Flot"ore"
"ore"that
thatisisdominantly
dominantlygoethite-chert
goethite-chert with
with weight
weight recovery
recovery of
of iron
iron in
in the
the
low 30 to -mid 40
40 %;
%; variable, but generally
generally high silica
silica and
and phosphorous.
phosphorous.
532 WIF
WIF"ore"
"ore"isisoxidized
oxidizedmartite/goethite;
martitelgoethite;low
low weight
weight recovery,
recovery,high
high silica
silica and
and
phosphorous; low heads
indicate
original
iron
formation
may
have
been
carbonate(?).
heads
formation
have been carbonate(?).
Sulfates
Sulfates are
are locally
locally common
common in
in bench
bench faces.
faces.
520 Magnetite
horizons. This
Magnetitedomain
domainisisdominantly
dominantlymagnetite-carbonate
magnetite-carbonate with silicate horizons.
may be flot and/or
andor mag ore in part depending on liberation.
liberation.
510
Clastic
horizon
is
at
the
contact
with
top
of
5 10 Clastic horizon is at the contact with top of the
the CDffl/West
CDIIT/West pit
pit hanging
hanging wall (250)
syncline.This
This horizon
horizon isis in
in part
part flot
lot ore.
in a local ('1)
(?) syncline.
ore.
400 CDIII-WEST
CDIII-WESTPIT
PITDOMAIN
DOMAIN
This domain is stratigraphically
stratigraphically between CDIIJIWest
C D W e s t pit hanging wall metadiabase
metadiabase (250)
(250) and
and
Thisdomain
domainincludes
includesnumerous
numerous small
small dikes
dikes and
and sills,
sills, aa Keweenawan
Keweenawan
CDIII footwall (230). This
dike and the West Pit Marker horizon (240).
Footwallclastic
clasticzone
zonealong
alongMain
Mainpit
pit footwall
footwall (100).
(100). Consists of dominant martite
480 Footwall
clastics with coarse
coarse quartzite/conglomerate
quartzitelconglomerate and interbedded martite-hematite
martite-hematitechert.
chert.
470 Hanging
Hangingwall
wallzone
zonealong
alongbase
baseofofCDIII/West
CDIIIlWestpit
pithanging
hangingwall
wallmetadiabase
metadiabase(250).
(250).
WIF
Defined as W
F due to very fine grain size andlor
andor oxidization.
Dikedomain
domainisisdefined
definedasasaanortheast
northeasttrending
trendingzone
zone of
of chioritic
chloritic dikes
dikes and
and
460 Dike
associated oxidized
associated
oxidized and unoxidized iron formation.
450 South
South Hematite
Hematitedomain
domainoccurs
occursininthe
thesouth
southpart
part of
of CDffl
CDIII and
and the
the West pit. It
contains flot
oreof
of variable
variablemetallurgy
metallurgy and
and WIF and is dominantly thin bedded, fine
lot ore
grained hematite-martite chert although some zones may be oxidized
oxidized carbonate.
carbonate.
451 Goethite
Goethitezones
zoneswithin
withinhematite
hematitedomain.
domain.These
Thesezones
zonesare
areassociated
associatedwith
with folding
folding
and faulting. A
goethite
zone
along
the
footwall
is
present
but
not
broken
out
as
A goethite zone along the footwall
as aa
domain
domain at
at this
this time.
time.
452 Goethite
Goethitezone
zonealong
alongCDIII
CDIIIfootwall
footwallsouth
southof
ofKeweenawan
Keweenawan dike. This
This zone
zone
typically
typically has high slime
slime Fe,
Fe, may be oxidized
oxidized Carbonate
Carbonate (430)
(430) domain
domain along
along
intersection of dike
and
footwall.
dike
footwall.
118
440 North
Northhematite
hematitedomain
domainconsists
consistsofoffine
finegrained
grainedoxidized
oxidizedmartite-hematite
martite-hematite chert
with numerous dikes. The
Theboundary
boundary between
between this
this domain
domain and the Magnetite
Magnetite domain
domain
(420) trends northeast and dips steeply
Plot ore in part.
steeply south. Flot
Carbonatedomain
domainisiscarbonate
carbonateflot
flotore
orewith
withlow
lowmagnetite
magnetite content, high weight
430 Carbonate
recovery and low concentrate grade. Fault
Fault bounded
bounded on
on north and south
south but apparently
apparently
gradational down dip to the west into magnetite domain (420).
gradational
(420).
420 Magnetite
Magnetitedomain
domainconsists
consistsofofmagnetite-carbonate
magnetite-carbonateand
and magnetite
magnetite silicate-chert
silicate-chert
with variable
variable oxidation and grain size.
size. This domain has relatively sharp
sharp boundaries
boundaries with
with
other domains. Domain
Domaingenerally
generallydefined
defined by
by magnetite
magnetite content,
content, not ore type, so
so it
contains potential flot
contains
flot ore.
ore.
421 West
Westpit
pitmagnetite
magnetitedomain
domainisisananisolated(?)
isolated(?)zone
zoneof
ofhigh
highgrade
grademagnetite
magnetite in
the west pit. It is defined
defined by drilling
drilling and
and blast
blast pattern
pattern data.
data.
Footwallzone
zoneisisdefined
definedasasthe
themagnetite-silicate
magnetite-silicate horizon
horizon at
at the contact
contact with the
410 Footwall
CDIII Footwall metadiabase
metadiabase (230).
(230). It is typically waste or low grade
grade due to low
low
magnetite content
or
poor
liberation.
content or poor liberation.
MAIN PIT
PIT DOMAIN
DOMAIN
300 MAIN
The Main Pit
Pit Domain
Domain contains
contains iron
iron formation
formation units stratigraphically
stratigraphically below the
the CDffl
CDIII
footwall metadiabase (230) and/or the East pit hanging wall metadiabase (200).
(200). This domain
domain
includes numerous
includes
numerous small
small mafic
mafic intrusives.
intrusives.
370 Hanging
Hangingwall
wallcontact
contactincludes
includeszones
zonesof
oferratic
erratic metallurgy
metallurgy along
along the base of the
CDffl footwall
CDIII
footwall (230)
(230) or
or East
East pit
pit hangingwall
hangingwall(200).
(200).
360 Transition
Transitionzone
zonebetween
betweenCDffl
CDIIIfootwall
footwall(230)
(230)and
and East
East pit hanging wall (200).
This zone consists
consists of variably
variably oxidized
oxidized hematite
hematite iron formation
formation and mafic intrusives.
intrusives.
Restricted to north side
side of East
East pit.
350 Hematite-martite
Hematite-martitedomain
domainininEast
Eastpit
pitconsists
consistsofofvarious
varioustypes
types of
of martite
martite chert.
This domain includes
includes horizons of magnetite-carbonate iron formation and thin dikes.
Gradational transition over 20-50 feet to Carbonate iron formation (340).
Gradational
340 Carbonate
is is
stratigraphically
Carbonateiron
ironformation
formation
stratigraphicallybelow
belowthe
thehematite-martite
hematite-martitedomain
domain
It
consists
of
martite-carbonate-chert
with
variable
in
East
pit.
(350)
(350) East pit. consists martite-carbonate-chert
variable
magnetite/martitelFe silicate content. This
magnetite1martiteFe
This unit is defined by magnetic Fe, weight
recovery and total oxides. ItIt has
has lower
lower weight recovery and higher concentrate grade
than CDIII
CDIII carbonates
carbonates (430)
(430) and
and may
may be
be magnetite
magnetite ore
ore in
in part.
part.
330 Clay
Clayzone
zoneisisdefined
definedasasthe
thehorizons
horizonsof
of iron
ironformation
formationoutlined
outlinedas
as waste
waste due
due to
to high
high
silica from montmorillonite (or other) interference.
interference. This does not differentiate
differentiate
nonliberating hematite material. This zone may be stratigraphically
stratigraphically controlled.
controlled. Includes
nonliberating
Includes
some
some flot
flot ore
ore within
within boundaries.
boundaries.
320 East
East pit
pitclastics
clasticsare
aremixed
mixedsiliceous
siliceous clastics
clastics and iron formation. This
This unit
unit includes
includes
oxide and carbonate horizons. ItIt may
may be
be the
the stratigraphic
stratigraphic equivalent
equivalent of carbonate
domain (340)
down
plunge
to
the
west
(7).
(340) down plunge to the west (?).
321 Clay
similar
Clay zone
zone in
in clastics.is
c1astics.i~
similarto
to clay
clay zone
zone (330)
(330) but in clastic
clastic domain.
domain.
Footwalliron
ironformation
formationdomain
domainconsists
consistsofofvariably
variablyoxidized
oxidizedoxide
oxide iron
iron
310 Footwall
by erratic metallurgy.
formation and coarse clastics. This domain is typified by
311
31 1 Earthy
Earthyfines
finesare
arehigh
highgrade
grade (>50
(>50 weight recovery
recovev and >50 head Fe) oxidized
zones
zones are
are controlled
controlledby structures
structures within
within the
the footwall
footwall domain.
domain.
119
200 INTRUSIVE
INTRUSIVEDOMAINS
DOMAINS
These domains are used for correlations of the iron formation domains, and structural trends.
They appear to be conformable at the
the scale
scale of
of the
the ore
orebody.
body. These rocks are generally
which vary
vary from
from diabasic
diabasic to
to porphyritic
porphyritic
interpreted as intrusives, they consist of mafic rocks which
to aphanitic. All
horizons
appear
to
thin
to
the
west
and
south.
Contacts
tend
to
be
All horizons appear
Contacts tend to be sheared
sheared
if
and locally oxidized. Contact
Contact metamorphism
metamorphism of the iron formation is minimal and, if
present, results in finer grained iron
iron formation. Synclinal
structures
and
intersections
with
Synclinal
dikes have focused oxidation of the iron formation.
formation.
270 North
intrusive
is
a
poorly
defined
North intrusive is a poorly definedhorizon
horizonatatthe
thetop
topof
of the
the Northwest
Northwest zone
zone (500).
(500).
West intrusive
260 West
intrusiveisisaapoorly
poorlydefined
definedbut
but mappable
mappable horizon within the Northwest
zone
zone (500).
(500).
CDIIIIWest pit
250 CDIII/West
pithanging
hangingwall
wallisisaarelatively
relativelyeasily
easilymappable
mappablehorizon
horizon and
and along
along
principle horizons for stratigraphic
with the CDIII footwall
footwall (230)
(230) is one of the principle
stratigraphic
correlations between the CDffl
correlations
CDIII pit and the
the Main
Main pit.
240 West
Westpit
pitmarker
markerisisa athin
thinbut
butcontinuous
continuoushorizon
horizon within
within the
the CDIII/West
CDin/West pit
stratigraphy (300). ItIt isis interpreted
interpreted to
to extend
extend from
from the
the Foster
Foster Lake
Lake slot
slot through
through the
the West
West
pit.
230 Top
Topof
ofthe
theCDIII
CDIIIfootwall
footwalldefines
definesthe
thebase
baseofofthe
theCDIIIfWest
CDIO/West pit
pit domain
domain (400)
(400)
while the base defines the top of the Main pit east domain (300).
(300).
220 Chloritic
Chloriticand
anddiabase
diabasedikes
dikesand
andthin
thinsills
sillsoccur
occurininall
alldomains.
domains. This
Thisunit
unitincludes
includes
east-west trending 30+ foot thick Keweenawan
Keweenawan dike in CDIII.
200 East
Eastpit
pithanging
hangingwall
wallisisseparated
separatedfrom
fromthe
theCDffl
CDIIIfootwall
footwall (230)
(230) by the Transition
zone (360) iron formation. This
Thisunit
unit occurs
occurs along
along the
the north
north side
side of
of the
the East
East pit,
pit, the
the base
base
Main pit
pit East
East domain
domain (300)
(300) for
for mining
mining and
and planning
planning
of this horizon marks the top of the Main
purposes
PIT FOOT
WALL DOMAIN
100 MAIN
MAIN PIT
FOOTWALL
100
rocks which
which are
are separated
separated from
from the
the iron
iron
This domain consists of Archean metamorphic rocks
formation
formation domains
domains by an
an east-west
east-west trending, north-dipping
north-dipping high angle
angle fault.
fault.
121 Chloritic
Chloriticschist
schistisisthe
thedominant
dominantfootwall
footwallrock
rocktype
typeexposed
exposed within
within the
the pit
pit and
and in
in
of the
the CDIII
CDffl footwall horizon
horizon (230) within
the drill holes. This rock may the extension of
the fault zone.
zone.
111
111 Granite
Granitegneiss
gneissoccurs
occurssouth
southofofthe
thechloritic
chloriticschist
schist(121)
(121)but
but isisonly
onlypoorly
poorly exposed
exposed
in the pit. This
Thisdomain
domainhas
hasnot
not been
been used
used in
in the
the drill
drill hole
hole codes.
codes.
999 OVERBURDEN
OVERBURDENDOMAIN
DOMAIN
This domain
domain consists
consists of
of Quaternary
Quaternary overburden,
overburden, rock
rock fill
fill and
and broken
broken bench
bench material.
material.
120
Table
Table 2: Glossary
Glossary of terms
terms and
and abbreviations.
abbreviations,
Natural
The amount of material recovered from the material fed
Natural Weight
Weight Recovery
Recovery -- The
fed into
into the
the
concentrator
In other words,
words, it's the tons of concentrate
concentrate made
made (measured
(measuredas
as filter
filter
concentrator circuit.
circuit. In
cake) from tons of crude
crude ore
ore used (measured
(measured by #3 belt scale).
scale).
Metallurgical
Metallurgical Weight
Weight Recovery
Recovery (Met.
(Met. Wt.
Wt. Rec.)
Rec.) - Calculated
Calculated by comparing
comparing the iron losses
losses
(as tailings)
tailings) with the iron
iron content
content of the crude
crude ore fed into
into the concentrator
concentrator circuit
circuit (i.e.,
(i.e., the
the
head Fe). The formula
formula used for
for this
this calculation
calculation is called the iron balance
balance formula,
formula, or
or
sometimes
sometimes called
called the
the concentration
concentration formula.
formula.
(Head Fe - Tail Fe)
Wt. Rec. =
x 100
(Grade - Tail Fe)
Grade
Grade -- Also
Also called the concentrate grade, is a chemical measurement (assay) of the total
total
iron oxide
oxide of the concentrate. Iron oxide
oxide is found in iron minerals such
such as hematite
hematite (Fe203),
(Fe203),
magnetite
magnetite (Fe304),
(Fe304),geothite
geothite(Fe203
(Fez03** OH),
OH), and iron
iron carbonate
carbonate(FeCO3).
(FeC03).
Concentrate Silica Grade
Grade - The chemical
Concentrate
chemical measurement
measurement (assay)
(assay)of
of the
the %
% SiO2
Si02in the
the
concentrate.
concentrate. When
When aalower
lowerconcentrate
concentratesilica
silicagrade
grade is
is achieved,
achieved, the losses
losses in iron units
(tailings)
(tailings) increases.
increases.
content of the crude ore fed into the concentrator
concentrator circuit.
circuit,
Grade -- The assayed iron content
Head Grade
concentrator's ability
Iron
Iron Recovery
Recovery(Fe
(FeRec.)
Rec.) -- A calculation of the efficiency of the concentrator's
ability to
recover the iron available.
This
is
calculated
by
comparing
the
Met.
Wt.
Rec.,
at
some iron
available.
grade, with the head Fe of the crude
crude ore. For example,
(Met. Wt. Rec. x Grade
% Fe)
Grade %
%
= ..................................
% Fe Rec,
Rec. =
(Head
(Head % Fe)
Percent
Percent Magnetic
Magnetic Iron
Iron Recovered
Recovered (%
(% Mag. Fe Rec.) - The calculation
calculation of the efficiency of
recovering
recovering the magnetic
magnetic iron that was in the feed (crude ore). The Met. Wt. Rec.,
Rec., at
at some
some
iron grade, is compared
compared with the magnetic potential (i.e.
(i.e. head) of the crude
crude ore. For example,
example,
(Met. Wt.
WI Rec.
% Fe)
Rec. xx Grade
Grade %
Rec = ..................................
% Mag Fe Re
(Head Mag % Fe)
Tailings
Tailings -- The
The product lost in the process. Tailings always includes iron, because iron is
always associated with many other minerals (silica, phosphate, carbonate, etc.).
121
Flot (flotation) -- Flot ores are
are the martite,
martite, hematite,
hematite, geothite
geothite and
and carbonates,
carbonates that are
are treated
treated
The final stage of the
by selective chemical
chemical processes to achieve Fe and silica grade. The
magnetite process
magnetite
process is
is flotation
flotation to
to achieve
achieve target
target silica
silica grade.
grade.
to low
low weight
weight recovery
recovery andlor
and/or high
WIF (waste iron formation)
formation) -- hon
Iron formation that due to
silica cannot be treated in the plant to produce economic
economic concentrate.
concentrate.Rarely,
Rarely, phosphorous
phosphorous
levels are too high to be treated.
treated.
The percent of the crude iron that
that is concentrated
concentrated in
in the
the Davis
Davis Magnetic
Magnetic
Magnetic iron - The
Tube Test (DMTT).
(DMTT).
%MagFe
%MagFe == DMTT Wt. Rec.
Rec. x DMTT Grade
Grade
However, in
in the
the Tilden
Tilden ores
ores an
The assumption
assumption is that all of this occurs as magnetite. However,
appreciable
and is
is carried
carried into the DMTT
appreciable amount of hematite is up locked with the magnetite and
Fe content by
by 1-2% points
points and
concentrate. This tends to over estimate the magnetic Fe
therefore
therefore the over
over estimate
estimate weight
weight recovery.
Satmagan
susceptibility and
and is
is the
the
Satmagan - The satmagan magnetic iron content is measured using susceptibility
actual magnetite content
content of
of the
the crude
crude or
or concentrate.
concentrate.
Domain - The
The deposit
deposit is divided into volumes of rock with similar
similar metallurgical
metallurgicalresponse.
response.
These are usually stratigraphic
stratigraphichorizons
horizons but may be fault
fault bounded
bounded or
or nonconformable
nonconfonnable
alteration/oxidation
alteratiodoxidation zones. The
The domains
domains are
are the basis for
for the economic
economic and
and planning
planning models.
models.
122
CIstructures
structuresisison
onthe
theorder
orderofof2.5
2.5meters,
meters,most
mostcommonly
commonlysuch
suchstructures
structuresare
arehand
hand
Cl
specimen
specimentotomicroscopic
microscopicininscale.
scale.They
Theyindicate
indicatethe
thepresence
presenceofofa alarge,
large,reverse
reversedip
dipslip,
slip,
ductile
ductileshear
shearzone
zonewith
withaaNNE
NNEtotoSSW
SSWtransport
transportdirection.
direction.The
Thefact
factthat
thatthe
theNegaunee
Negaunee
hon
IronFormation
Formationisisthrust
thrustup
upon
ontotothe
theolder
olderArchean
ArcheanGneiss
Gneissindicates
indicatesthat
thatthere
theremust
musthave
have
been
beenprior
priorlocal
localsubsidence
subsidencehere
heretotoform
formthe
thesedimentary
sedimentarybasin,
basin,aaclassic
classicbasin
basininversion
inversion
pattern
pattern(Gillchrist
(Gillchristand
andothers,
others,1987).
1987).
Folds
Foldsmeasured
measuredwith
withininthe
theNegaunee
NegauneeIron
IronFormation
Formationhave
haveaxial
axialsurfaces
surfacesand
andhinge
hingelines
lines
that
WNW,indicating
indicatingaasimilar
similarstrain
strainenvironment
environmenttotothat
thatdetermined
determined
thattrend
trendeast-west
east-westtotoWNW,
from
the
shear
zone.
The
folding
and
shearing
are
thought
to
have
occurred
at
from the shear zone. The folding and shearing are thought to have occurred atthe
thesame
same
time.
time.
The
Theiron
ironformation
formationisismostly
mostlyaachemical
chemicalor
or biochemical
biochemical deposit.
deposit. The
Theclastic
clasticlenses
lensesthat
that
in
the
south,
thinning
and
occur
occurininthe
theformation
formationare
arethickest
thickestand
andmost
mostnumerous
numerous in the south, thinning and
becoming
becomingless
lessfrequent
frequentto
tothe
thenorth
north (Breithart,
(Breithart, 1983).
1983). The
Thepattern
patternsuggests
suggestsan
an
asymmetric
asymmetricrift
riftbasin
basinwith
withaabreakaway
breakawayfault
faulton
on the
thesouthern
southernmargin
margin and
andaarollover
rolloveron
onthe
the
north
side.
The
principal
source
of
clastic
sediments
would
have
been
derived
from
the
north side. The principal source of clastic sediments would have been derived from the
southern
southernmargin
marginthat
thatwas
wasbeing
being uplifted
uplifted along
along the
the fault.
fault. This
Thisepisode
episodeisisrelated
relatedto
tothe
the
development
of
a
passive
margin
during
the
early
Proterozoic.
Subsequent
compression
development of a passive margin during the early Proterozoic. Subsequent compression
during
duringthe
thePenokean
PenokeanOrogeny
Orogenyinverted
invertedthe
thebasin
basinand
andinitiated
initiatedreverse
reversefaulting
faultingthat
thatgave
gave
rise
to
the
shear
zone
(Cambray,
1978).
To
the
east,
in
the
Harvey
Quarry,
a
similar
rise to the shear zone (Cambray, 1978). To the east, in the Harvey Quarry, a similar
pattern
patternof
ofyounger
youngerover
overolder
olderthrusting
thrustingisisobserved
observedsuggesting
suggestingthat
that this
thisrelationship
relationshipisis
rift
and
later
thrusting
common
commonalong
alongthe
theSynclinorium
Synclinoriumand
andconfirms
confirmsthe
theearly
early rift and later thrustingpattern.
pattern.
Mining
Mining
Ore
50'x50'x45'
Orereserve
reserveand
and long
long range
range mine
mine planning is done in MEDSYSTEM using 5O'x50'x45'
blocks
based
primarily
on
development
drill
holes
which
are
on
a
nominal
300
blocks based primarily on development drill holes which are on a nominal 300foot
foot
spacing.
spacing.Weekly
Weeklyplans
plansand
anddaily
dailyore
orepredictions
predictionsare
arebased
basedon
onblast
blastpattern
patternsamples,
samples,
which
which are
areon
on about
about 90
90 foot
foot centers.
centers. Reconciliation
Reconciliationof
of ore,
ore,waste
wasteand
and pellet
pellet production
production
between
betweenthe
the'model"
"model"and
andactual
actualproduction
productionshows
showsless
lessthan
than1%
1%difference.
difference.
The
Theannual
annualmining
miningtask
taskisisto
tomove
moveapproximately
approximately38
38million
million long
longtons,
tons, about
about20
20million
million
long
longtons
tonsof
ofore
oreand
and18
18million
millionlong
longtons
tonsof
ofstripping
strippingwaste,
waste,to
to produce
produce 7.8
7.8million
millionlong
long
tons
tonsof
of flux
fluxpellets
pellets per
per year.
year. Production
Productionsince
since1974
1974has
hasbeen
been 126
126million
million long
longtons
tonsof
of
pellets
pellets from
from355
355million
million long
long tons
tons of
of ore
ore at
at aa stripping
stripping ratio of 0.94. As
As of
of January
January1,1,
1999,
1999,the
the proven
proven and
and probable
probablelong
longrange
range (30
(30year)
year) mine
mine plan
plan reserve
reservewas
was628
628million
million
long
long tons
tons of
of ore
ore containing
containing233
233 million
million long
long tons
tons of
of pellets
pellets at
at aa stripping
strippingratio
ratio of
of 0.77.
0.77.
The
P&H 2800
2800(38
(38 cubic
cubic yard)
yard) cable
cable shovels,
shovels, three
three P&H
P&H
Theloading
loadingfleet
fleetconsists
consistsof
of two
twoP&H
2100
2100 (l7cubic
(17cubicyard)
yard)cable
cableshovels
shovelsand
andtwo
two994
994(21
(21cubic
cubicyard)
yard)Caterpillar
Caterpillarwheel
wheel loaders.
loaders.
The
The hauling
haulingfleet
fleetconsists
consistsof
of three
three 793
793 (240
(240 ton)
ton) Caterpillar
Caterpillar trucks,
trucks, seven
seven 789
789 (190
(190 ton)
ton)
Caterpillar
trucks
and
six
Haulpack
630E
(170
ton)
trucks.
Drilling
is
done
with
one
trucks. Drilling is done with one
Caterpillar trucks
GD120
120drill.
drill.
P&H and
and one
oneGD
P&H
123
Field Trip
Trip Stops
Stops
- .
stops to be visited and their locations cannot he
be determined
determined until
until iust
just urior
prior to
The actual stoos
the field trip. The
generic
stop
descriptions
are
provided
in
this
guide
and
the
actual
The generic stop descriptions are provided in this guide and the actual
localities to be visited will be provided
provided to participants on
on the
the day
day of the
the field
field trip.
trip.
Stop 1 - Pit Service Building
Before beginning the tour of the Tilden pit, the mine safety rules will be reviewed and
and aa
short video of the Tilden
Tilden Mine
Mine operation
operation will be shown.
shown. The
The video, not
not particularly
particularly
geologic, describes
pelletizing processes that have
describes the mining cycle, milling and pelletizing
have aa major
major
impact on the ore quality determinations.
determinations.
Stop 2 -- Main
Main Pit Footwall Shear
Shear Zone
Zone
Chlorite schist defines the footwall of the Tilden ore body.
body. This
This chlorite
chlorite schist
schist is
is
described
described in the literature
literature (Van Hise and Bayley, 1897;
1897; Gair, 1975)
1975) as part of the Palmer
Palmer
Gneiss and marks the boundary between the Archean Compeau Creek gneiss
gneiss of the
the
Southern complex and the early Proterozoic sediments. This
This contact
contact extends
extends for
for
approximately seven miles east from the
the Tilden Mine. Gair
Gair and Simmons
Simmons (1968)
(1968) made
madeaa
complete
complete study
study of this rock and proposed three possible modes of origin: 1)
1)alteration
alterationand
and
shearing
shearing of Precambrian rock during
during faulting;
faulting; 2) migration of fluids
fluids along
along the contact
contact
between lower
and
middle
Precambrian
rocks;
and
3)
alteration
of
a
regolith
during
lower
Precambrian
alteration
regolith during
folding.
folding. At
Atthe
theTilden
TildenMine,
Mine,there
thereappear
appearto
to be
be two
two varieties
varieties of
of gneiss,
gneiss, aa southern
southern
sericitic phase related to a granitic protolith and a northern chloritic phase with a diabasic
diabasic
protolith.
protolith.
At this locality
surface cut by the
the more
more
locality one
one can see
see the dominant,
dominant, steep
steep north dipping
dipping S surface
gently north dipping Cl
4). The
C/ surface (Figure 4).
The intersection
intersection between the S and Cl
C/ plunges
plunges
WNW. The
gently to the WNW.
The movement
movement plane
plane on the shear
shear zone is at right angles to this
intersection and the intersection of this plane and the boundary of the shear zone
(determined from drill records) gives the direction of movement (marked with a filled
(determined
filled
triangle on Figure 5). The
Thepattern
patternof
of the
the SS surfaces
surfaces and
and shear
shear bands
bands give the sense
sense of
movement (Figure 4). Chemical
Chemical analyses
analyses indicate that the protolith for the shear zone
was most likely to have been one of the diabase bodies found interlayered with the
banded iron formation.
formation.
(-60') foliation
foliation in the chlorite schist creates a bench and slope
slope
The steep
steepnorth
northdipping
dipping(—60°)
stability concern. This
45° final wall slope with 35
Thisisis controlled
controlled by
by triple
triple benching to a 45"
35
foot catchers. Proper
Properclean
cleanup
up along
alongthe
the toe
toe line
line is
is critical
critical to avoid "undercutting" the
the
foliation.
foliation.
124
West
2.5
Shear Zone
(Palmer
IPaImrGness)
GneW
Figure
Figure 4:
4: Footwall
Footwallshear
shearzone,
zone,Tilden
TildenMine.
Mine.
Pole
Poleto
to mean
mean C-foliation
C'-foliation
S
*
U
Pole
poleto
tomean
meanS-foliation
~-fo~atim
Slip direction
Mean
~ e a S-C
S-C.
n intersection
intersection
S-C intersections
S-foliafion plane
Top of shear zone
'
.C-foliation
C'-foliation plane
p4ane
"Nonnal
intersection
\ N o m d totoS-C'
S-C/intersection
Figure5:
5: Stereo
Stereo projection
projection of
of shear
shearzone
zonestructures.
structures,
Figure
125
Stop 3 -- Martite and Main Pit
Pit Carbonate
CarbonateDomains
Domains
The Martite and main pit Carbonate
Carbonatedomains
domains lie
lie stratigraphically
stratigraphically below
below the
the CDffl
CDEI
Footwall and Main Pit Hangingwall intrusives. This
This is
is metallurgically
metallurgically "good
"good ore"
ore" and
and isis
typically characterized
characterized by high weight recovery (35-45+%),
(35-45÷%), low phosphorous, low slime
slime
iron and good grinding media. With
Withcrude
crudeFe
Fe around
around 35%,
35%, iron
iron recovery
recovev averages
averages over
over
70%. The
The'best'
'best'ore
oreisiscentered
centeredon
onthe
theMain
MainPit
Pitanticline
anticlineand
andconsists
consistsof
of thick
thick bedded
bedded (2(24 cm) chert with variable
variable martite
martite and magnetite
magnetite plus subordinate
subordinatecarbonates
carbonatesand
and silicates.
silicates.
The boundary between the Martite and Carbonate
domains
is
based
on
higher
magnetite
Carbonate domains
higher magnetite
content;
content; the Carbonate
Carbonate domain
domain contains
contains low calculated
calculated total oxides
oxides that
that may
may be
be due
due to
to
iron silicates.
silicates. The
Thevariation
variationin
inmagnetite
magnetitecontent
content may
may reflect
reflect either
either the
the environment
environment of
of
deposition
deposition or
or diagenesis;
diagenesis;itit is
is unlikely
unlikely to be due
due to supergene
supergene effects.
This domain comprises
reserve. The
mine plan
comprises about 56% of the ore reserve.
The~mine
plan attempts
attempts to
to control
control
the blend percentages to about 50% of this domain. This
This has
has been
been only
only partially
partially
successful due to pellet quality concerns when blending with other types of ore. The
successful
planned final
final depth
depth of
of the
the Main
Main pit
pit is
is about
about sea
sea level
level so
so the
the pit will
will be
be about
about 1000
1000feet
feet
deeper.
deeper.
As the base of the iron formation is not exposed at the Tilden Mine, the stratigraphic
stratigraphic
thickness is unknown. The
Thethickness
thickness exceeds
exceeds 1000
1000feet
feet and
and is probably underlain by and
is gradational into undifferentiated clastics and iron formation
formation of the Empire
Empire Mine.
"Intrusive" Igneous Horizons
Stop 4 - Mafic "Intrusive"
formation and the rapid
Due to the lack of clear stratigraphic markers within the iron formation
rapid
facies changes in the iron formation, several igneous horizons are used for stratigraphic
stratigraphic
and structural correlation
correlation(Figure
(Figure 1).
1).
The term 'intrusive'
'intrusive' or
or'dike'
'dike'isisused
usedfor
forgreen
greenand
andgray-green
gray-green mafic
mafic igneous
igneous rocks,
which vary from (semi)
(semi) conformable sill-like
sill-like horizons to obviously crosscutting
crosscutting bodies.
bodies.
Since the fabric varies from diabasic
diabasic to porphyritic to aphanitic, the conformable
conformable bodies
bodies
are usually interpreted as synsedimentary sills
sills but
but may
maybe
beflows.
flows. The sills range in
thickness
thickness from a few tens of feet to at least 600
600 feet with the thicker horizons
horizons thinning
thinning to
to
the south. Dikes,
typically 55 to
to 20
20 feet
feetthick.
thick. The
Dikes,obviously
obviously crosscutting
crosscutting bodies, are typically
dikes may be feeders.
feeders. The dikes and thinner parts of the sills are commonly altered
altered to
chlorite and carbonate
retain primary
primary igneous
igneoustextures.
textures. Contacts
cabonate while the thicker parts retain
Contacts
with the
iron
formation
are
variably
sheared
and
altered.
the iron formation are variably sheared and altered.
The principal
principal sills
sills are
are the
the Main pit
pit hangingwall,
hangingwall, CDffl
CDEI footwall
footwall and the CDffl
CDEI
hangingwall.
hangingwall. The
Thesills
sillsand
andmajority
majorityof
of the
the dikes
dikes appear
appear to be of early Proterozoic
Proterozoic age but,
there are several dikes that are interpreted to be of Keweenawan age based on the
and fracture
fracture zones.
zones. The igneous event
magnetic signature. Dikes
Dikes commonly fill faults and
Clarksburg,
that emplaced
emplaced the sills
sills and majority of the dikes may be related to the Clarksburg,
Hemlock and Emperor Volcanics.
126
Dilution from dikes, especially along shallowly dipping contacts, is the most common
internal waste.
waste. kcipient
type of internal
Incipientrecrystallization
recrystallization due
due to
to contact
contact metamorphism
metamo~hismand
hangingwall.
quartz overgrowing magnetite results in high silica waste along the CDIII hangingwall,
Stop 5 - West
West Pit/CDIJI
Pit/CDZZZ Hematite
Hematite Domain
The West Pit/CDffl
PivCDIII Hematite
Hematite domain
domain in
in the
the west pit is
is stratigraphically
stratigraphicallybetween
between the
the
CDffl
hangingwall
and
footwall.
This
domain
extends
from
CDffl
along
the
north
limb
CDIII
This domain extends
CDIII
pit anticline. In contrast to the martite domain, this is
and over the crest of the Main pit
metallurgically 'poor' ore
orewith
with higher
higher slime
slime iron,
iron, phosphorous and poor grinding.
Fe), due to the
the high crude Fe
Although weight recoveries are relatively good (about 36% Fe),
below 60%.
60%. Mineralogically
of 40%, the iron in this domain is below
Mineralogically and texturally, this
domain differs from the martite domain in being dominantly platey hematite with thin
(mm scale) chert laminae. Presumably, the bedding thickness reflects the original
sedimentary
sedimentary features but it is unclear as to the nature of the oxidation event that produced
the hematite
hematite (related
(related to
to deposition,
deposition,diagenesis
diagenesisor
or supergene
supergenealteration).
alteration).
With a thickness
thickness of about
about 600
600 feet, this domain contains
contains 27% of the ore reserve but, due
due
to blending concerns for phosphorous,
slimes
or
grinding,
it
is
often
difficult
to
maintain
phosphorous,
maintain
that percentage
percentage in
in the
the blend
blend going
going into
into the
the processing
processing plant.
plant.
Stop 66 -- Magnetite Domain
The Magnetite
PitJCDffl hematite
hematite
Magnetite domain
domain is the stratigraphically equivalent of the West PivCDIII
but is essentially
essentially restricted to the CDffl
CDIII syncline (basin?). Magnetite
Magnetiteore
orequality
qualityisis
determined
(magnetic
determined by the percentage of the crude iron that occurs as magnetic iron (mapetic
concentrate determined by the Davis Magnetic
Magnetic Tube
TubeTest).
Test). On average, the Magnetite
domain contains 27% magnetic iron with 37% total
total crude
crude iron.
iron. With about 90%
magnetic
magnetic iron recovery
recovery in the plant, weight recoveries are 34-36% iron. Mineralogically,
Mineralogically,
the ore consists
primarily
of
magnetite-siderite-chert
with
variable
hematite
and silicates.
consists primarily
silicates.
This
This mineralogy, along
along with the thin laminations,
laminations, appears to indicate a restricted basin
and a reduced environment
environment of deposition.
deposition.
respect to
to the
There is some scientific
scientific controversy as to the role of diagenesis with respect
hematite and
and siderite.
siderite. Within the Magnetite domain
relationship between magnetite, hematite
there is a faultldike-bounded
faulvdike-bounded zone of low magnetite siderite ore; it is uncertain whether
this is the protolith of the magnetite ore. The
The southern
southern extension of the CDffl
CDIII magnetite
into the hematite domain contains the highest
highest grades
grades (-30
(-30 magnetic iron). The
The sharp
sharp
contact, not structural,
structural, indicates
indicates that this is some sort of redox front but it is unknown
unknown
whether it is
is of
of diagenetic
diageneticor
or supergene
supergene origin.
origin.
Approximately
reserves are
are in
in magnetite.
magnetite. Development of the
Approximately 23% of the Tilden ore reserves
of pushbacks
pushbacks along
along the
thewest
westwall
wallininthe
theintrusive.
intrusive. The
The wall is
reserve involves a series of
planned
planned to move
move back about
about 800
800 feet and the pit is to be
he deepened about 600 feet.
127
Acknowledgments
Acknowledgments
We thank Bill Cambray
Cambray and Cheryl Webster for their contribution of the Structural
Stmctural
Geology section in the Introduction and structural
structural discussion in Stop
Stop 2.
References
References
Cambray, F. W., 1978,
1978, Plate Tectonics as a model for the environment
environment of deposition
deposition and
and
deformation
Northern Michigan:
Michigan:
deformation of the
the early
early Proterozoic
Proterozoic (Precambrian X) of Northern
Geological
7, p. 376.
376.
Geological Society
Society of America Abstracts with Programs,
Programs, v.10, no. 7,
Breithart, M. S.,
S., 1983,
1983,Significance
Significanceof
of the
the distribution
distribution of
of clastic
clastic lenses
lenses within
within the
the
Breithart,
Negaunee Iron Formation
Formation at the eastern end of the of the Palmer Basin, Marquette
Marquette
Synclinorium,
Synclinorium, Northern Michigan: Masters
Masters thesis, Michigan
Michigan State
State University.
University.
Gair, J.E.,
J.E., 1975,
1975,Bedrock
Bedrock geology
geology and ore deposits
deposits of the Palmer
Palmer Quadrangle,
Quadrangle, Marquette
Marquette
County, Michigan: U.S.
U S . Geological
Geological Survey
Survey Professional
Professional Paper 769, lS9p.
159p.
Gair, J. E. and
and G.
G. C.
C. Simmons,
Simmons, 1968,
1968,Palmer
Palmer Gneiss
Gneiss -- an
an example of retrograde
retrograde
metamorphism
metamorphism along
along and
and unconformity: U.S.
US. Geological
Geological Survey
Survey Professional
Professional Paper,
Paper,
600-D,
600-D, p. D186D186- D194.
D194.
Gillchrist,
Gillchrist, Ralph,
Ralph, Mike
Mike Coward
Coward and
and Jean-Louis Mugnier,
Mugnier, 1987,
1987,Structural
Structural inversion
inversionand
and
its controls:
controls: examples
examples from
from the
the French
French Alpine foreland
foreland and
and the
the French
French Alps:
Alps:
Geodinamica Acta (Paris) v.1, no. 1, p.
p.5-34.
5-34.
Sims, P.K., Card, K.D., Morey, G.B., and Peterman, Z.E., 1980, The Great Lakes tectonic
zone
A major crustal
cmstal structure
structure in central North America: Geological Society of
zone -- A
America bulletin, V.
V.91,
91, Pt.
pt. 1,
1, P.690-698.
p. 690-698.
Van Hise, C.R.,
and
C.R.,
and Bayley, W.W., 1897,
1897, The Marquette iron-bearing district of
Michigan:
Michigan: U.S.
U.S. Geological
GeologicalSurvey
Survey Monograph
Monograph 28,
2 8 ,608p.
608~.
128
Geologic Field
Field Trip
Trip to the Empire
Empire Mine
Mine
,
M. Nordstrom,
Nordstrom, Empire
Empire hon
IronMining
MiningPartnership,
Partnership,P.O.
P.O. Box
Box 38,
38, Palmer,
Palmer, MI
MI 49871
49871
Paul M.
Introduction
Introduction
Empire mine commenced operations in 1963;
1963; over the past thirty-five years the operation
The Empire
has produced nearly 200
200 million long tons of pelletized iron concentrates. The
TheEmpire
EmpireMine
Mine
represents
repr&ents a partnership
partnekhip of steel making
m G n g and iron
iion mining interests and is partially oowned
h e d and
operated by the Cleveland-Cliffs Iron
hon Company. The
Thereserve
reserveis
is about
about 600
600 million
million long
long tons
tons
of magnetite
magnetite ore.
ore. The
Thecurrent
currentannual
annualproduction
productionisis8.4
8.4million
million long
longtons
tonsof
of pellets
pelletsfrom
from27.5
27.5
million long tons of ore. Han
hon
Han (1975)
(1975)provides
provides aa detailed description of the Negaunee Iron
Formation at
at the
the Empire
Empire Mine.
Mine.
Stratigraphic
hon Formation
Stratigraphicrelationships
relationships within the Negaunee Iron
Formation in
in the
the vicinity
vicinity of
of the
the Empire
Empire
Mine
Mine are:
are:
• Upper
UpperUndifferentiated
UndifferentiatedSeries
Series(+
(+1500'):
1500'):intermixed
intermixedsequence
sequenceof
ofcarbonate,
carbonate,silicate,
silicate,
clastic
clastic and "lean"
"lean" carbonate
carbonateassemblages.
assemblages.
• Clastic
Clastic horizon (200-300');
(200-300'); graywacke
graywackeand
and feldspathic
feldspathicquartzite
quartzite interbedded
interbedded with
with
chert, iron carbonate
carbonate minerals
minerals and
and magnetite.
magnetite.
• Carbonate
Carbonate horizon
horizon (350-500'):
(350-500'): alternating
alternating bands
bands of
of magnetite-chert
magnetite-chert and
and chert,
chert, with
with
siderite
siderite or
or ankerite
ankerite distributed
distributedthroughout.
throughout. Local,
Local, minor
minor beds
beds of
of riebeckite
riebeckiteand
and aegerine-augite
aegerine-augite
are present
contact.
present near
near the
the upper
upper
contact.
..
• Silicate
Silicate horizon
horizon (400-500'): alternating
alternating laminae
laminae of
of magnetite,
magnetite, carbonate,
carbonate, greenish
greenish
microstilpnomelane),
and
chert.
micro- to
to sub-microscopic
sub-microscopiciron
iron silicate
silicateminerals
minerals(principally
(principally
stilpnomelane),
and
chert.
- .
• Lower
h w e r Undifferentiated
undkferentiated Series
Series (700-1500'):
(700-1500'): intermixed
intermixid sequence
sequence of carbonate,
carbonate,
silicate
silicate minerals and
and clastics
clastics (clastics
(clasticsincrease
increasetoward
toward base
base of
of interval).
internal).
The
The basal
basal contact
contact of
of the
the Lower
Lower Series
Series is
is transitional with the underlying Siamo Slate. The
The
Siamo
Slate
is
represented
in
the
mine
area
by
quartz-arkose
and
graywacke.
Siamo Slate is represented in the mine area
graywacke.
A
A geologic
geologicmap
map and
and section
sectionof
of the
theEmpire
EmpireMine
Mine(Figure
(Figure11and
and 2)
2) illustrate
illustratethe
therelationships
relationships
between
between the
the various
variouslithologic
lithologicunits.
units.
hon
Ironformation
formationatatthe
theEmpire
EmpireMine
Mineisistransected
transectedby
by aa number
number of
of near-vertical,
near-vertical, chioritized
chloritized
metadiabase
are
metadiabase dikes.
dikes.The
Thedikes
dikesoccupy
occupyfaults,
faults,believed
believed to
to be
be of
of early
earlyProterozoic
Proterozoicage,
age,which
which are
probably related
related to
to basin
basin development.
development. The
Thedikes
dikeshave
havehad
hadaasignificant
significantlocalized
localizedimpact
impact on
on ore
ore
probably
processes.
processes. The
Thefine
finegrain
grainsize
sizeofofmagnetite
magnetite(5
(5p.)
p)in
in iron
iron formation
formationadjacent
adjacentto
tothe
thedikes
dikes
represents
represents aa "selvage"
"selvage" which
which was
was apparently
apparently impervious
impewious to subsequent
subsequent processes that influenced
influenced
growth of
of the
the magnetite
magnetite grains
grains in
in other portions of the iron formation. Effective
Effectiveliberation
liberationof
of
growth
magnetite in
in these
these"fine-gralned"
"fine-grained"rocks
rockscan
canrarely
rarelybe
beachieved.
achieved.
magnetite
The iron
iron content
content of
of the
the iron
iron formation
formation at
at the Empire
Empire mine is fairly uniform, about 32-35%. The
The
The
amount
amount and
and grain-size
grain-size of
of magnetite
magnetite determines
determines ore quality. Ore-forming
Ore-formingprocesses
processescontributed
contributed
and remobilized
remobilizediron
ironby
bydiagenetic
diageneticor
orlow-grade
low-grademetamorphic
metamorphicreactions
reactionsof
of recrystallization
recrystallizationand
and
and
replacement. Oxidation
Oxidationofofsiderite
sideritehas
hasresulted
resultedininlarge-scale
large-scalereplacement
replacementby
by magnetite,
magnetite,but
but
replacement.
129
N
N
N
N
P1010ED
04/05/99
Formation Iron Fine—Grained
2000
Horizon Carbonate
UOji.OWtiOJ Iron
UOJI Oxidized
PdZlplXn
Formation
Horizon Ctastics
Series Undifferentiated Upper
Siamo
1
Figure
Series Undifferentiated Lower
Horizon Silicate
\
Dike Diabase
Keweenawan
SiLt/Dike Mc-taciiobase
-
LEGEND GEOLOGIC
—-
'9tfV.K
's.-.)
'
Eevation) Bench (+940
—22000
---r-
ç—1j
.
-
\(iJ l
—
-.
GEOLOGY MINE EMPIRE
—18000
(7bt
--
20
t
'tion
_J
'A'
--
—14000
Resource
IV CD
—10000
Cu
0
0
0
Li
00
0
nJ
o
o
o
Li
Li
EIIJ
of the
the magnetite
magnetite in
in these
these enriched
enriched
chert layers are also replaced by magnetite; the grain size of
zones is coarser
than
magnetite
in
zones
not
effected
by
replacement.
coarser
magnetite zones
effected replacement.
the Empire
Empire Mine.
Mine. NonSecondary oxidation has a profound impact on ore quality at the
Non- or
or weakly
weakly
characteristic of
of oxidized
oxidized zones
zones are
are typically
magnetic iron minerals (e.g., martite, hematite) characteristic
rejected during primary magnetic separation. Incipient
Incipient or
or partial
partial oxidation
oxidation of
of magnetite
magnetitegrains
grains
then in flotation
flotation stages.
may cause them also to be rejected, if not in magnetic separation, then
extent of
of oxidation
oxidation and
and predict
predict its
its impact
impact on
on plant
plant
However, it is often difficult to recognize the extent
recovery. The
Theoxidation
oxidationisisthought
thoughtto
to be
be largely,
largely,ifif not
not entirely,
entirely, the
the result
result of
of supergene
supergene
processes.
processes. Circulation
Circulationof
ofthe
theoxidizing
oxidizingfluids
fluidswas
wascontrolled
controlled by
by faults,
faults,dike
dikecontacts
contactsand
andclastic
clastic
beds and lenses. However,
the
boundaries
of
oxidized
zones
are
very
irregular
and
difficult
to
However, the boundaries of oxidized
define.
define.
all Empire
Empire iron
iron ores,
ores, including
including the
the
Due to the very fine grain size of magnetite in virtually all
carbonate ores, extremely
carbonate
extremely fine
fine grinding
grinding (93-95%
(93-95% passing 500
500 mesh) is
is required
required to
to achieve
achieve
sufficient
sufficient liberation of magnetite for product specifications. Process
Process control
control is
is dominated
dominated by
by the
the
specification for Si02
Si02 in the final product.
product. Smaller
Smalleraverage
average grain
grain size
size of magnetite
magnetite in
in silicatesilicateliberation. In
type ores means that they must be ground finer to achieve acceptable liberation.
In general,
general,the
the
becomes significantly
significantly more
finer grain size also implies increased work index so that grinding becomes
costly.
costly.
Field Trip
Trip Stops
Stops
be determined
determined until
untiljust
just prior
prior to the field
The actual stops to be visited and their locations cannot be
trip. The
Thegeneric
genericstop
stopdescriptions
descriptionsare
areprovided
provided in
in this
this guide
guideand
and the
the actual
actual localities
localities to
to be
be
visited will be provided to
to participants
participants on
on the
the day
day of the
the field
field trip.
trip.
Stop
Overlook
Stop 11 -- Dispatch Overlook
Mining development
pit, with
with significant contributions
contributions from
development at Empire has focused on the Main pit,
satellite
satellite pits to the north
north (CD-I),
(CD-I), east
east (Section
(Section 20)
20) and
and west (Southwest
(Southwest Extension),
Extension),and
and
expanding
expanding development
development to the northwest (CD-V). The
The bottom
bottom bench
bench of
of the
the Main
Main pit
pit is
is
approximately
pit will
will ultimately
ultimately be
be developed
approximately 1,000
1,000 feet below the dispatch overlook, and the pit
750 feet below the present mining
mining level.
level.
entirely in
in the
the Carbonate
Carbonate and
and Silicate
Silicate
The bottom bench of the Main pit is developed almost entirely
horizons.
horizons. The
The iron
iron formation
formation in
in this
this area
area strikes
strikes about
about N35°E
N35"E and
and dips
dips 30-35°
30-35' to
to the
the northwest.
northwest.
Stop 2 -- First Class Carbonate
Carbonate
The Carbonate
i)
Carbonate unit is gray to dark gray, relatively coarse-grained (magnetite grain size: 20-30 p)
and characterized
characterized by "thick" (5-25 mm), indistinct magnetite-rich and chert-rich layers with
subordinate
The term
term "carbonate"
"carbonate" is
is utilized
utilized at
at the
the Empire
Empire Mine
Mine to
to
subordinate siderite and ankerite. The
c 10)
10) with very good liberation characteristics (low concentrate of
indicate soft ores (work index <
Si02). These high-grade ores
ores have
havebeen
beenthe
the"life-blood"
life-blood' of
Si02).
of Empire's
Empire's production.
production.
132
Stop 33 -- First Class
Silicate
Class Silicate
Silicate iron
ironformation
formationconsists
consists
thin
laminae
(2 mm)
of magnetite
(minor
tominor
very minor
of of
thin
laminae
(2 mm)
of magnetite
(minor
to very
amounts
greenish or
or brownish-gray
brownish-gray mixtures
mixtures of
of
amounts of silicate, carbonate, chert, and clastics) and greenish
silicate minerals, siderite, magnetite, chert and clastics. The
The silicate
silicate minerals
minerals are
are dominantly
dominantly
stilpnomelane
stilpnomelane and
and minnesotaite. The
TheSilicate
Silicatehorizon
horizoncommonly
commonlyhas
has aa greater
greaterabundance
abundanceof
of
carbonate
p) is finer
carbonate minerals
minerals than the Carbonate
Carbonate zone. The
Thegrain
grainsize
sizeof
of the
the magnetite
magnetite(5-20
(5-20 L')
than in the Carbonate-type.
The
term
"silicate"
at
the
Empire
Mine
is
applied
to
ores
Carbonate-type. The term "silicate" at the Empire Mine is applied to oreswith
withwork
work
index>
10and
andliberation
liberationcharacteristics
characteristicsthat
thatare
areinferior
inferiorto
to carbonate
carbonateores;
ores; silicate
silicate or
or yield
yield
index >10
higher
higher concentrate
concentrate of
of Si02.
Si02.
Stop4Stop 4 -CD-V
CD-V
Future development
in two areas: the
the "pillar"
"pillar" between the
development of the mine will be principally focused in
Main
Main pit and
and the
the Section
Section 20
20 pit
pit (west
(west wall),
wall), and
and the
the north
north and
and northwest
northwest areas
areas of
of the
the Main
Main pit
pit
termed CD-V.
CD-V. Development
Developmentof
ofthe
thewest
westwall
wallincludes
includesrelocation
relocation of
of the
the existing
existing rail
rail lines
linesaround
around
the
the east side
side of the Section
Section 20 pit; the "pillar" area will then be excavated to expose ores in the
wall and,
and, more
more importantly,
importantly,below
below the
the bottom
bottom of
of the
the present
present pit.
pit.
The
The CD-V
CD-V pit
pit is
is being
being developed
developed in
in the
the Upper
Upper Series
Series horizon,
horizon, characterized
characterizedprincipally
principallyby
by
carbonate-type
ores
and
clastic
bands
and
lenses.
Much
of
the
yellowish
carbonate
material
carbonate-type
Much of the yellowish carbonate material
contains
contains very
very little
little magnetite
magnetite ("lean"
("lean" carbonate),
carbonate),consisting
consistingdominantly
dominantlyof
of siderite
sideriteand
andchert.
chert.
However, very
very rich
rich carbonate
carbonate ores
ores are
are also
also present
present in
in the
the push-back of
of the
the Main
Main pit
pit wall.
wall.
However,
Current
Current development
development is
is just below
below aa large
large diabase
diabase sill
sill that
that is
is 200-400
200-400 feet
feet thick.
thick. Several
Several of
of these
these
large, generally
generally conformable,
conformable, tabular
tabular diabase
diabase bodies
bodies intrude
intrude the
the Negaunee
Negaunee Iron
Iron Formation
Formation in
in this
this
large,
area of the mine. Predictably,
Predictably, "fine-grained"
"fine-grainer effects
effectsare
are extensive
extensiveadjacent
adjacentto
to these
these large
largeigneous
igneous
bodies.
bodies.
In
In the
the CD-V
CD-V area
areaof
of the
themine,
mine, iron
ironformation
formationand
and interlayered
interlayeredsills
sillshave
have been
been folded
foldedinto
intoseveral
several
smallsmall- to moderate-scale
moderate-scale anticlinal
anticlinal and synclinal features. Fold
Fold axes
axes plunge
plunge 10-20°
10-20' toward
toward the
the
northwest.
northwest.
Bounding
BoundingCD-V
CD-V to
tothe
thenorth
northisisaalarge
largearea
areaunderlain
underlain by
by magnetite-bearing
magnetite-bearingiron
iron formation
formationthat,
that,
because
because of
of its
its extreme
extremehardness
hardness (work
(workindex
index18-20)
18-20)and
and poor
poor liberating
liberatingqualities,
qualities,isispresently
presently
regarded
regarded as
as aa resource,
resource, not
not aa reserve.
reserve. The
Thenature
natureof
ofthe
themetallurgical
metallurgicalproblems
problemsin
in processing
processing
these
these ores
ores isis under
under investigation
investigationby
by Cleveland
ClevelandCliffs'
Cliffs'research
researchgroup.
group.
Stop 5- CD-I
CD-I
CD-I has
has been
been an
an important
importantsource
sourceof
of very
veryhigh
high quality
quality(high
(high weight
weight recovery,
recovery,low
lowconcentrate
concentrate
Si02)carbonate
carbonatematerial
materialfor
forblending
blendingwith
with lower
lowerquality
qualityores
ores from
from other
otherlocations
locationsin
in the
themine.
mine.
Si02)
Ironformation
formationininCD-I
CD-Istrikes
strikesnorth-south
north-southand
anddips
dipsabout
about30°
30' to
to the
the west.
west. Two
TwoKeweenawan
Keweenawan
hon
age (1.1
(1.1 Ga)
Ga) diabase
diabase dikes
dikes cross the pit in a westerly to southwesterly direction.
These two
two dikes
dikes
age
direction. These
are
are coarser-grained
coarser-grainedand
and fresher
fresherin
in appearance
appearancethan
than the
the other,
other, older
older early
early Proterozoic
Proterozoic diabase
diabase
dikes.
dikes.
133
The current phase of pit development
development in CD-I will be completed in 1999. High
High quality
qualitycarbonate
carbonate
but its potential
potential development
development is
is hindered
hindered by
by the
the large
large
ore extends to the west beneath the pit wall but
amount of stripping required. About
About 750
750feet
feet of
of overlying
overlying waste
waste rock
rock would
would have
have to
to be
be mined
mined in
in
order to access
access the ore.
ore.
References
References
Han, Tsu-Ming, 1975,
Petrology of
of Iron-Formation
hon-Formation at the Empire
1975, Lithology, Stratigraphy, and Petrology
Deposits of
of the
the Palmer
Palmer Quadrangle,
Mine, in Gair, Jacob E., Bedrock Geology and Ore Deposits
Marquette
Marquette County,
County, Michigan,
Michigan, U.S.G.S.
U.S.G.S. Prof.
Prof. Paper
Paper 769,
769, pp.
pp. 76-106.
76-106.
134
Field Trip
Trip 4
and Glacial Geology
Geology from
from Au
Au Train
Train to
Paleozoic and
Grand Marais,
Grand
Marais, Michigan
Leaders:
and J.
J. Anderton
Anderton
Leaders: R.S. Regis and
______
______
____
______
_____0144
____On
______________________________________
___________
____________________________
________
Pt(ISIOCENE NOMENCLATURE
SYSTEM
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---
CHART
CHART 11
1964
¶964
2071
em
ILSG Field
Field Tdp
Trip ##4from
1999 ZLSG
4 -Paleozoic
Paleozoic and
andPleistocene
Pleistocene Geology
Geologyfrom
Au Train
Train to Grand
Grand Marais, Michigan
Road log
log
Overview
Ovewiew
-
-
Au Train
Train channel
Profile of Au
Stop 11
stop
--
Au
Au Train
Train Falls
Falls
Stop
stop 22
-
-
Beach
Beach ridges and
and
Stop
stop 33
--
Au Train-Whitefish
Train-Whitefish Channel
Channel
Grand
Grand Island Overlook
Overlook
Stop
stop 44
--
Stop
stop 55
--
Stop 6
--
Stop 77
stop
--
Break
Stop 88
stop
--
Stop 9
stop
Stop 10
stop
10
Stop 11
stop
11
----
Stop 12
stop
12
--
US4
US41I S. (view to east at 2.3-3 ml.)
mi.)
continue 16.2
ml
to
M94,
F.
then
E.
16.2 mi
18.2 mi.
ml. to Forest
Forest Lk. Rd.
Rd.
Return to Forest Lk. Rd. N 7.4 mi. to
M28
M28
F. 2.5 mi. to H58, then to
M28 E.
Washington St.
St. (1.2 mi.), to
Munising
Munising Falls (0.55
(0.55 mi.)
mi.)
Munising
Munising Falls
Falls
Return to H58 (0.55 mi.), E to
Miners Castle Rd (3.7 mi.), then N to
Miners Castle
Castle (4.9 mi.)
mi.)
Miners
Miners Castle
Castle
Return to H58, continue
continue F
E to
to Bear
Bear
Trap Inn 9 (3.6 mi.),
mi.), keep
keep left
left and
and
continue on H58 (end of pavement
another 4.3 mi.) to Kingston Corner
Comer
(11.2
(1
I .2 mi. beyond pavement end)
Kingston
Kingston Outwash
Outwash Plains
Plains
N on H58 to 2-track on left (7.1 mi.)
Proceed on 2 track 0.1 mi.
& park
park
mi. &
Walk due W 400 yds (strenuous)
(strenuous)
Turtle
Return to H58,2.1
H58, 2.1 mi. to dropoff
Turtle Lake Kettle
into channel, @ 6.7 mi. is edge of
upper outwash terrace, @
@ 8.7 mi.
turn left on Log Slide
Slide Rd.. Continue
Continue
to Log Slide
Slide (0.75 ml.)
mi.)
Picnic area
area @
Grand Sable
@ NW side Grand
Sable Lk.
Log Slide
Slide
Return to
to H58.
H58. FE on
on H58
H58 to
to parking
lot for Sable Creek falls and Ghost
forest (5.6 mi.).
Ghost
Ghost Forest
Sable
Sable Falls
Falls
To Grand Marais (1.7
(1.7 mi.)
Grand
Harbor
Grand Marais Harbor
Nipissing Bluff
E-W
M77 to M28 (22.6
E-W channels
channels crossed
crossed by
by M77
M77
(22.6 mi.)
To Munising (22.4 mi.)
To Marquette (44 mi.)
137
Detailed
Detailed Road
Road Log.
Log.
of Au
Au Train
Train Whitefish Channel
Overview of
From the WashingtonfBaraga
Washingtonmaraga St.
St. intersection in downtown Marquette travel S on baraga St
St
US41). Along
(turns into US4l).
Along U.S.
U S . 41
41 at
at about
about 2-3
2-3 miles
miles south
south of Marquette, a view to the east
reveals a profile of channel cut by proglacial meltwater that flowed eastward along the margin of
the Marquette (Grand Marais) ice.
ice. The
the Onota
The feature
feature was referred to by Hughes (1971) as the
outlier. Water
Waterspilled
spilledinto
intothe
theAu
Au Train
TrainWhitefish
Whitefish channel
channel southward
southward to
to (present)
(present) Lake
Lake Michigan
after passing through this channel. Continue
on
U.S.
41
south
16.2
miles
to
M94,
turn east.
Continue
16.2
Travel to 29.9
29.9 mi. to
to Chatham,
Chatham, turn
turn SS at
at stopsign,
stopsign, travel
travel to 30.9
30.9 mi and
and turn FE on
on M94,
M94, continue
continue
to 34.4
34.4 mi and
and turn N
N on
on Forest
Forest lake
lake rd., travel 0.1 mi to dirt road on right, follow to
to gated
gated
parking area. Walk
Walkto
to falls.
falls.
Au Train
Train Formation
Formationand
andAu
AuTrain-Whitefish
Train-WhitefishChannel,
Channel,Beach
Beachridge
ridgecomplex
complex
Return to Forest
Forest lake
lake Rd.,
Rd., turn N. Follow
Follow Forest
Forest Lake Rd. At 6.3 miles and all the way to M28,
note the beach ridge complex. Turn
TurnEE on
on M28
M28 and
and travel
travel to
to the Grand Island overlook parking
lot on the N side
side of M28.
M28.
Grand Island
Island Overlook
Overlook
From the Grand Island overlook, return
return to M-28 and
and travel
travel east
east 2.5
2.5 miles
milesto
toMunising.
Munising. Turn on
Dogpatch restaurant
restaurant is on Ge
the right at
H-58 after passing the Holiday gas station on the left (the Dogpatch
the intersection...excellent
vittles! In
In aa rustic, Li'l Abner
Abner setting).
setting).
Continueon
on H-58
H-58 an
an additional
additional
intersection ...excellent vittles!
. Continue
0.55 miles to
to Washington
Washington St.,
St., turn
turn left
left and
and proceed to Munising
Munising Falls
Falls visitors
visitors center
center parking lot
lot
(0.3 miles).
miles).
Munising Falls
Return to H-58 from the parking lot and
and proceed
proceed north
north 3.7
3.7miles
milesto
toMiners
MinersCastle
CastleRoad.
Road. Turn
left. Travel
Travel 4.9
4.9 miles
miles to
to the
the Miners
Miners Castle
Castle parking
parking lot.
Miners Castle
turn east (left).
(left). Travel 5.9 miles to the Bear Trap Inn, veer left to
to
Return to H-58 (4.9 miles) and turn
stay on H-58. End
End of
of pavement
pavement 4.4
4.4 miles
miles from the Bear Trap Inn. Continue
Continueto
to Kingston
Kingston Corner
Comer
junction (11.2
(1 1.2 miles).
Kingston Plain
Travel north on H-58 7.1 miles to the small 2-track road on the left. Proceed
Proceed about
about50
50yards
yardson
on
the road and park. Walk
Walk about
about 55 minutes
minutes due
due west (this
(this walk may be strenuous for some). We
We
will remain at this stop
for
about
½
hour
to
allow
time
to
walk
to
the
bottom
of
the
kettle.
stop
?h
NOTE: At 2.1 miles
miles drop-off
drop-off onto
ontosurface
surfaceB
B (Hughes).
(Hughes).
NOTE: At 6.7 miles
miles steep
steephill
hill marking
markingedge
edge of
of surface
surface C.
C.
Turtle Lake
Return to H-58 and continue north.
north. Travel 8.75 miles to the
the Log
Log Slide
SlideRoad.
Road. Turn north. Travel
Travel
0.75 miles to the
the Log
h g Slide
Slide parking lot.
139
Log Slide
Slide
Return to H-58. Proceed
Proceed east.
east.
NOTE: At 3.7 miles, the Grand
Grand Sable
Sable dunes
dunes are
areon
onthe
theleft
left(north).
(north). At
At the
the intersection
intersection of
of
Newburg and William Hill roads, turn north on William Hill road and travel 0.7 miles to the
Sable Falls parking lot. Walk
Walkto
tothe
theGhost
Ghostforest
forest then
then to
to Sable
Sable Falls.
Falls.
Ghost Forest and
and Sable
Sable falls
Return to parking
parking lot
lot and
and travel
travel to
to Grand
Grand Marais
Marais (east).
(east),
harbor
Grand Marais harbor
Follow M77 to M28,
M28, turn
turn e,
e, follow
follow to
to US41,
US41, N
N to
to Marquette.
Marquette.
Meitwater
Meltwater channels
channels
Observe the outwash channels along M-77
appearance
M-77 as
as we
we proceed
proceed south.
south. The "terrace"-like appearance
of the topography is due
due to M77
M77 crossing
crossing proglacial meltwater
meltwater terraces at right angles.
angles.
Streamfiow was to the east. Note
Streamflow
Note the
the gentle
gentle southward
southward slope
slope of individual
individual terraces, marked by
drained, swampy
swampy areas
areas near
near their
their southern
southernedge.
edge.
poorly drained,
98 miles to Marquette. Sleep!
Sleep!
Grand Marais to M-28
M-28 south
south on
on M-77
M-77
West on M-28
M-28 to
to U.S.
U.S. 41
41
North on U.S. 41 to
to Marquette
Marquette
140
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Stop 8
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Grand Sable
GRAND
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_______
and Pleistocene Geology
Geologyfrom
fromAu
AuTrain
Trainto
toGrand
Grand Marais,
Marais, Michigan
Michigan
Paleozoic and
(FIELD TRIP
#4)
(FIELD
TRIP#4)
by Robert S. Regis and John B.
B. Anderton
Anderton
Northern Michigan University
University
The geological history of the Pictured Rocks National Lakeshore (PRNL)
(PRNL) is limited
limited to
to
intervals of geologic
two intervals
geologic time;
time; the Pleistocene
Pleistocene epoch
epoch when glaciers reworked
reworked and
and mantled
mantled the
the
underlying
underlying bedrock
bedrock with
with aa nearly
nearly continuous
continuous veneer
veneer of drift,
drift, and the
the late
late Proterozoic,
Proterozoic,Cambrian
Cambrian
and early Ordovician periods when sediments deposited in shallow seas became the
the sandstones
sandstones
that form the "pictured rocks" section
section of
of the
the lakeshore.
lakeshore.
Late
the Central Upper
Late ProterozoiclCambrian/Ordovician
Proterozoic/Cambrian/OrdovicianHistory of the
Peninsula
Peninsula including
including PRNL
PRNL
Hamblin (1958), who
who performed
performed a
Most material for this section was compiled from Hamblin
detailed study of the Cambrian rocks outcropping in the PRNL and elsewhere
elsewhere in northern
Michigan.
Michigan. The
The Paleozoic
Paleozoicgeologic
geologic history
history of
of the
the PRNL
PRNL is
is best described
described as
as aa period
period of
of eroding
eroding
highlands
highlands and sedimentary
sedimentary deposition
deposition into
into transgressing
transgressing empeiric
empeiric seas.
seas. The
The Northern
Northern Michigan
Michigan
Highlands
Highlands and the
the Wisconsin
Wisconsin Arch
Arch stood
stood as
as eroding
erodingremnants
remnants of
of former
former highlands
highlandsand
and
mountains, trending
east-west
across
northern
Michigan
and
more
north-south
across
Wisconsin,
trending
east-west
across
northern
Michigan
and
more
north-south
across
Wisconsin.
respectively. Encroaching
respectively.
Encroachingshallow
shallowseas
seas(the
(theSauk
Sauk transgression)
transgression) during
during the late Proterozoic
Proterozoic and
and
throughout
the
Cambrian
were
depositional
journey's
end
for
of
eroded
sediments.
the
journey's
most
the
sediments.
Bedrock
Bedrock is
is best
best exposed
exposed in
in the
the pictured
pictured rock
rock section
section where
where bluffs
bluffs rise
rise up
up to
to 200'
200' above
above
short
Lake Superior and extend seventeen miles from Munising to
to the
the Beaver
Beaver Basin.
Basin. For aa short
distance
distance inland from
from the
the escarpment,
escarpment, usually
usually no more than several
several hundred
hundred yards, bedrock may
may
occasional outcroppings.
outcroppings. Elsewhere
Elsewherein
in the
the National
National Lakeshore
Lakeshore bedrock is
is only
only found
found
be seen in occasional
in the vicinity of the
the Grand
Grand Sable
Sable bank, where
where it forms a low bluff around
around the
the north
north and
and east
east side
side
of Au Sable Point,
Point, and
and in
in the
the gorge
gorge at
at Sable
Sable Falls.
On this field trip, we will observe the rock
rock formations
formations in
in reverse
reverse order.
order. The first rocks
we will see are the youngest
youngest exposed in the
the park (western
(western end) and last rocks we see
see are
are the
oldest (eastern end). The
TheJacobsville
Jacobsvillesandstone,
sandstone,of
of questionable
questionablelate-Proterozoic
late-Proterozoicto
to lower
lower and
and
age, is
is the
the oldest
oldest formation
formation in
in the
the lakeshore
lakeshore (more
(more than 520,000,000
520,000,000years
years old).
old).
middle Cambrian age,
It is a feldspar-rich, quartz
quartz sandstone,
sandstone, deep
deep red in color
color with white mottlings. Although
Although the
the
Jacobsville has a thickness
thickness of
of 1100
1100feet,
feet, only
only the top several
several feet rise above
above lake level,
level, and
and only
only
Jacobsville
Onthis
thistrip,
trip,we
we will
will see
see the
the Jacobsville
Jacobsville at
at Sable
Sable Falls
Falls (Stop
(Stop 10).
10).
in the eastern part of the park. On
The early-to-middle
early-to-middle Cambrian-age
Cambrian-age Munising- formation underlies most of the
the lakeshore,
lakeshore,
mostof
and forms most
of the significant bedrock outcroppings in
in the
the park.
park. It lies unconformably on top
of the Jacobsville
Jacobsville sandstone. The
TheMunising
MunisingFormation
Formation isis divided
divided into
into two
two members,
members, the
the lower
lower
and 5)
5) members
members (500
(500 -Chapel Rock
Rock (Stops
(Stops 10
10and
and 5)
5) and
and the
the upper
upper Miners
Miners Castle
Castle (Stops
(Stops 44 and
520,000,000
520,000,000 years old).
old). Except
Except where
where the friable Miners Castle member is being rapidly
rapidly
undermined
by
wave
action
it
forms
slopes
rather
than
cliffs.
undermined
forms slopes rather
cliffs.
In the western half of the "pictured rocks" the late Cambrian Au Train formation
formation cropscropsout only along
the
top
edge
of
the
cliff
(480
The Au
Au Train
Train is
is aa light
light
along
edge
(480 500,000,000
500,000,000 years old). The
brown or white, hard, dolomitic
dolomitic sandstone
sandstone usually highly
present, it forms
forms a
highly glauconitic.
glauconitic. Where present,
-
142
cap
cap rock
rock on
on the
the weaker
weaker Miners
Miners Castle
Castle member. We
Wewill
will observe
observethe
the"type
"type locality"
locality" of
ofthe
theAu
Au
Train
Trainformation
formationatatAu
AuTrain
TrainFalls
Falls(stop
(stop1)1)on
onthis
thisfield
fieldtrip.
trip.
All
All formations
formationsdip
diptoward
towardthe
thesouth
southaadegree
degreeor
ortwo
twoas
as shown
shownin
in the
thephoto
photohere
here(view
(viewisis
toward
toward the
the east).
east). All
Allthe
theformations
formationsalso
alsoarise
ariseeastward
eastwardso
sothat
thatthe
theAu
Au Train
Trainformation
formationisismissing
missing
east of
of Miners
MinersCastle
castlebut
but
east
the
the Jacobsville,
Jacobsville,which
whichisis
below
below lake
lake level
levelat
atMiners
Miners
Castle,
Castle,carries
carriesabove
abovelake
lake
level. East
Eastof
ofMiners
Miners
level.
beach,
it
is
well
exposed
beach, it is well exposed
beneath
beneath the
the conglomerate
conglomerateof
of
ChapelRock
Rockand
andin
inthe
the
Chapel
gorgeat
atSable
SableFalls.
Falls.
gorge
Thereare
aremany
many
There
waterfalls along
along the
the
waterfalls
escarpment
escarpmentformed
formed mostly
mostly
by the
the Au
Au Train
Trainformation,
formation,
by
but all
all are
arefed
fedby
by small
small
but
streamswith
with small
small
streams
watersheds and
and are
are only
only
watersheds
imposingin
in the
the spring
springand
and
imposing
during wet
wet periods.
periods.
during
MunisingFalls
Falls(stop
(stop4)
4) isis
Munising
view
View along
along the
thePictured
PicturedRocks
Rocksshowing
showing the
the Munisiug
Munisine formation
formation dipping
dipping to
to the
the
the most
most unusual
unusual and
and
the
south
south at
at aalow
lowangle.
angle.
interesting
of the
the falls.
falls. ItIt isis
interesting
- of
an
an extremely
extremelywell-developed
well-developedcap-rock
cap-rockwaterfall
waterfall with
withthe
the Au
Au train
train Formation
Formation forming
forming aa shelf
shelf that
that
overhangs
overhangs the
the Miners
Miners Castle
Castle member
member by
by 25
25 to
to 30
30 feet
feet permitting observers
observers to walk behind
behind the
the
torrent
torrent which
which falls
falls from
fromthe
the center
center of
of the
the overhang
overhang as
as aa concentrated
concentrated stream
stream and then
then descends
descends
another
another 30
30 to
to 40
40 feet
feet in
in aa series
series of cascades. AArecent
recent collapse
collapseof
of aa section
section of this formation
formation onto
onto
the
walkway
prompted
Park
officials
to
close
a
part
of
the
walkway.
Some
well
developed
the walkway
officials
of the walkway. Some
potholes
potholes may
may be seen
seen in the cataract section. Miners
MinersFalls
Falls has
has developed
developed where
where the Miners
Miners River
River
drops
from
the
upper
to
the
broad
lower
valley.
The
fall
is
vertical
and
about
30
feet
high.
drops from the upper to the broad lower valley. The fall is vertical and about 30 feet high.
Chapel
Chapel Falls
Falls isis fed
fed by
by aa tributary
tributary to
to Chapel
Chapel Lake
Lake that courses down the valley slope where there
is no cap rock. The
fall
is
a
long
cascade,
about
The fall is a long cascade, about 90
90 feet
feet high that broadens fan-like from ten feet
at the top
top to
to 30
30 feet
feet at
at the
the bottom.
bottom. Bridalveil
Bridalveil Falls
Falls and Spray
Spray Falls are formed where streams
streams fall,
fall,
about
90
feet,
directly
into
Lake
Superior,
adding
greatly
to
the
beauty
of
the
Pictured
Rocks.
At
about 90 feet, directly into Lake Superior,
of
At
Sable
Sable Falls,
Falls, the
the stream
stream draining
draining Grand
Grand Sable
Sable Lake
Lake cut into the Chapel Rock and Jacobsville
Jacobsville
formations
formations where
where they
they lie
lie beneath
beneath sand
sand dunes.
dunes. The
Thedeeply
deeplywooded
woodedsetting
settingisisexceptionally
exceptionally
beautiful
beautiful and
and is
is but
but aa short
shortwalk
walk from
from the
the lakeshore
lakeshore and
and the
the parking lot.
The
The Pictured
Pictured Rock
Rock escarpment
escarpmenthas
has been
been carved,
carved, by frost action and wave erosion
erosion of the
the
present
present lake,
lake, as
as well
well as
as higher
higher stages,
stages,into
into aa variety
variety of shore
shore cliff features
features such as: stacks,
stacks, caves,
caves,
sea arches, and promontories. These
Thesefeatures
features have
have been named:
named: Lovers
LoversLeap,
Leap,Rainbow
RainbowCave,
Cave,
Grand
Grand Portal,
Portal, Miners
Miners Castle,
Castle, Chapel
Chapel Rock,
Rock, The
The Battleships,
Battleships, Flower Vase and Indian Drum Cave.
Cave.
Most
Most of
of the
the features
features can
can be
be reached
reached by hiking but only Miners Castle is accessible
accessible by automobile.
The
The best way to appreciate
appreciate the cliffs
cliffs is by boat tour. Increased
Increased accessibility
accessibilityby
by land
land will
will not
not
diminish
diminish the
the advantage
advantageof
of viewing
viewingthe
thecliffs
cliffsfrom
fromthe
thewater.
water.
143
Late Quaternary
QuaternaryHistory
Historyof
of the
the Central
CentralUpper
UpperPeninula
Peninulaincluding
includingPRNL
PRNL
The Great Lakes region experienced
experienced repeated glaciations
glaciations and concommittant
concommittant inundation
inundation
The
the Quaternary.
Quaternary. For
by proglacial lakes throughout the
For the
the purposes
purposes of
of this fieldtrip, the final major
(i.e. 11,800 BP)
BP) marks
marks the
the beginning
beginning of
of
deglaciation of the area during post-Twocreekan times (i.e.
is most
most relevant.
relevant. Between 11,800
the portion of Late Quaternary history that is
11,800and 11,500
11,500 the
Superior
covered the
the Upper Peninsula (Farrand and Drexler 1985).
Superior Ice Lobe nearly completely covered
1985).
By about 11,500
11,500 BP, retreating
retreating glacial ice, followed
followed closely by the high stands
stands of Glacial Lake
Lake
Futyma, 1981).
1981). The retreat of ice continued,
Algonquin, had nearly left the region (Larsen, 1987; Futyrna,
opening up large
portions
of
the
Lake
Superior
basin
in
what
is
large portions the Lake Superior
is known as
as Glacial
Glacial Lake
Lake Duluth
Duluth
(Phases A and B), allowing
allowing discharge
discharge from
from Glacial
Glacial Agassiz during
during Post Algonquin
Algonquin times
times (ca.
(ca.
10,700 BP); however, evidence of this period was largely obliterated by a final readvance of ice
into the Lake Superior
Superior basin. The
Theglacial
glacialreadvance
readvance is
is known
known as
as the
the Marquette
Marquette advance
advance (Drexler
(Drexler
et al. 1983).
1983).
Shoreline features created by these earlier lake stages in the Lake Superior
Superior basin were
Shoreline
largely destroyed during
during the Marquette
Marquette Advance (Hughes, 1978; Drexler, 1981;
1981; Farrand and
and
Drexler, 1985).
1985). By
Byabout
about9800
9800BP,
BP, the
theice
icehad
had begun
begun to
to retreat
retreat again,
again, resulting in a stepwise
stepwise
drop in lake
lake levels
levels which initiated
initiated the
the Post
Post Duluth
Duluth Phase
Phase lakes
lakes (Farrand
(Farrand and
and Drexler
Drexler 1985).
1985).
After about 9,800
BP,
Marquette
Advance
ice
had
retreated
far
enough
to
the
north
to
9,800
Marquette Advance ice
north to allow
allow lake
lake
water from the western part
part of Superior
Superior basin to drain eastward and southward,
southward, eventually
eventually
However, catastrophic
catastrophic discharge
discharge
creating the Main Minong Phase (Farrand and Dexler, 1985). However,
from Lake Agassiz early in the Nipigon Phase is thought to have breached a till barrier at
at the
the east
east
end of the Lake Superior
Superior basin,
basin, causing
causing aa decline
decline in Lake Minong resulting in at least
least five
five or
or six
six
Teller and
and Thorleifson,
Thorleifson, 1983). The
The
post-Minong levels
levels in the Superior
Superior basin (Farrand, 1960; Teller
lowering was finally
finally stopped
stopped as resistant
resistant bedrock at the Sault
Sault outlet
outlet was reached,
reached, culminating
culminatingat
at
the Houghton
Houghton Low around
around 8000
8000 BP,
BP, the
the lowest
lowest lake
lake phase in the Superior
Superiorbasin.
basin. Finally,
Finally, as
as the
the
ice left the south
south shore
shore of
of the
the basin,
basin, Glacial
Glacial Lake
Lake Minong formed
formed along the margins
margins of the
the
retreating glacier reaching
its
maximum
expression
by
about
9500
BP;
however,
shortly
reaching
expression
9500
shortly
afterwards
afterwards the retreating
retreating ice
ice front
front in the eastern
eastern Great Lakes revealed the isostatically depressed
depressed
known as
as the
the Houghton
HoughtonLow.
Low. During this
North Bay outlet, resulting in falling lake levels known
period, lake
lake levels
levels in the
the Superior
Superiorbasin dropped to their
their all
all time
time lowest
lowest elevation,
elevation, which
which may
may
their valley
valley channels.
channels. By about 8000 BP the bedrock
have allowed many rivers to deeply incise their
sill at the Sault
Sault acted
acted to
to halt
halt falling
falling lake
lake levels
levels in
in the
the Lake Superior
Superior basin; however,
however, by then
isostatic
isostatic uplift had
had acted
acted to
to raise
raise the
the elevation
elevation of
of the
the North Bay outlet,
outlet, resulting
resulting in rising
rising lake
lake
levels in the southern
southern basins.
Eventually,
Eventually, the
the rising
rising lake
lake levels
levels reached the elevation
elevation of the Sault
Sault Outlet
Outlet and
and lake
lake levels
levels
in the Superior
Superior basin began to gain in elevation. During
During this
this period,
period, known as
as the
the "Nipissing
"Nipissing
Great Lakes," water in the basins
basins of Lake Superior
Superior and Lakes Huron and Michigan joined
together to create
create one
one huge lake,
lake, which
which was controlled
controlled by the North Bay outlet.
Notwithstanding,
Notwithstanding, the
the North
North Bay
Bay outlet
outlet continued
continued to
to rise
rise and
and eventually
eventually lower
lower drainages
drainages to
to the
the
south became active
active commencing
commencing the three-outlet (Chicago,
(Chicago, Port Huron, and North Bay) phase of
Lake Nipissing (Nipissing
(Nipissing I),
I),which reached its maximum elevation between 4700 and 4000 BP
BP
(Hansel et al. 1985).
1985). During
Duringthe
the Nipissing
Nipissing Maximum,
Maximum, flooding
flooding of many tributary river valleys
embayments along the coastal areas of the Upper Peninsula (Anderton 1993a).
created large embayments
1993a).
The Nipissing II
IIphase, marking a slight drop in levels, was attained about 4000 years BP
as continued
continued isostatic
isostatic uplift
uplift raised the North Bay outlet above the altitude of the two southern
southern
II phase, lake-levels began to
outlets (Hansel et al. 1985,
1985, Larsen 1985b). Following
Following the
the Nipissing II
w
fall as
as downcutting
downcuttingprogressively
progressively lowered
lowered the Port Huron outlet and water volumes fluctuated
fluctuated in
in
the
the basins.
basins. AAshort
shortpeak
peakininlake-levels
lake-levelsatatabout
about3200
3200BP
BPresulted
resulted in
in the
the Algoma
Algoma phase,
phase, but
but by
by
about 2200
2200 BP
BP lake-levels
lake-levels in
in the
the Lake
Lake Michigan-Huron basin had fallen below the elevation
elevation of
of
the
the rebounding
rebounding sill
sill at
at the
theSault,
Sault,separating
separatingthe
the Superior
Superiorbasin
basin from
from the
the lower
lowerGreat
Great Lakes
Lakesand
and
initiating
1960). Modern
Modem lake-levels
lake-levels in
in both the
the Lake Superior
Superior and Lake
initiating the Sault
Sault level (Farrand 1960).
Michigan-Huron
Michigan-Huronbasins
basinswere
werefinally
finallyattained
attainedby
byabout
about2000
2000years
yearsago.
ago.
Larsen
Larsen (1985),
(1985), working
working in the
the southern
southern portions of the Lake Michigan-Huron basins, has
has
also
also proposed an
an alternative
alternativemodel
model of
of Holocene
Holocene lake-level
lake-level fluctuations,
fluctuations, which suggests
suggests aa
complex
record
of
climate-related
changes
that
lasted
between
200
and
300
years
complex record of climate-related changes that lasted between 200 and 300 years each
each and
and
fluctuated
fluctuated with
with an
an amplitude
amplitudeof
of 11to
to 22 meters above the apparent mean lake level. In
In the
the last
last
7000
7000 years at least
least eight
eight episodes
episodes of high water periods separated by intervening low water
periods are
are recognized
recognized (Larsen
(Larsen 1985).
1985).
periods
Stop 1. Au
Au Train
TrainFormation
Formationat
atAu
Au Train
TrainFalls
Falls
At
At Au
Au Train
Train Falls,
Falls, aa 30
30 meter
meter thick
thick section
section of the 100
100 meter thick early Ordovician
Ordovician Au
Train Formation (aka Trempealeau Formation) is exposed.
exposed. The
The formation consists
consists of 2 parts, the
upper and lower members. At
Atthis
thisstop,
stop,we
we will
will examine
examine the
the lower
lower section
section at the lower
lower falls,
falls, and
walk to
to an overview
overview of
of the
the upper
upper section,
section, which is
is exposed about 200
200 meters upstream,
upstream, but is
is
relatively inaccessible.
inaccessible. The
TheAu
Au Train
TrainFormation
Formation lies
lies unconformably
unconformably upon the Munising
Formation,
Formation, and
and is
is representative
representative of
of the
the
continued
continued Sauk
Sauk sea
sea marine
marine
transgression
transgression that
that began
began in
in the
the late
late
Proterozoic. The
environment
of
The environment of
depositionwas
was aa shallow
shallow shelf
shelf
deposition
platform
platform with
with fluctuating
fluctuatingsea
sea levels.
levels.
Paleogeographically,
Paleogeographically,the
theregion
region was
was
equatorial with
with the
the equator
equatortrending
trending
equatorial
nearly north-south
north-south compared
compared to
to
nearly
modern
modem orientation.
orientation.
Note the
the step-like
step-like character
character of
falls (also
(also in
in the
the upper
upperfalls)
falls)
the falls
created by the alternating
alternating layers
layers of
of dolomite,
dolomite, shale,
shale, and
and sandstone
sandstoneof
of varying
varyingdegrees
degrees of
of
resistance. Near
areaaseries
seriesof
ofthin
thinbeds
bedsof
of glauconitic
glauconiticsandstone
sandstone
Nearthe
thebase
baseofofthe
thelower
lowerfalls
fallsare
(hydrous
(hydrous silicate
silicate of iron
iron and potassium)
potassium) and shale. Glauconite
Glauconiteisisdisseminated
disseminatedas
asflakes
flakes
throughout the sandstone.
sandstone.Glauconite
Glauconite comprises
comprises as much as 35% of some beds in the lower
lower
Individual
beds
Individual
beds vary
vary
section
and
imparts
a
speckled
green
to
dark
green
color
to
the
formation.
section and imparts a speckled green to dark green color to the formation.
from pure sandstone
sandstone to pure dolomite (Hamblin, 1958). Dolomitic
Dolomitic beds are
are bluish gray in color.
An intraformational
conglomerate(?)
with
pebbles
of
sandstone
and
dolomitic
intraformational conglomerate(?)
dolomitic sandstone
sandstone are
are
found in a few of the units near the base of
of the section (see
(see cross
cross section
sectionon
onnext
nextpage).
page). At the
base of the lower
lower Au
Au Train,
Train, the
the contact
contact with the underlying Miners Castle member of the
Munising Formation is easy to recognize because the friable sandstone is quite
quite different in
Munising
appearance from the hard, resistant dolomitic sandstone
sandstone of the Au Train. Fossils are rare in the
formations. Following deposition of the Au Train
Au Train but become abundant in later formations.
Formation was
was aa minor
minor regression
regression that
that formed
formed an
an unconformity.
unconformity.
145
eq
a.
a
Cl)
I-
a
a
Cl)
Footage
Footage
Section
section
-
top
Upper
Train
rep of
of ~
m Au
AU
s~
r r i nr7.11.
a n
Sandy
grayi.h..blu. to
to dark
4-10 Inch*.
inches [hick.
thick, some
M d y dolomite,
dtilmnitq, grayi#h-blu.
dark blue,
h h à §b.ds
bed# 4-10
miu interbedded
buff
fin., ttoo mediurn.grained
1
iwft finemedium-grained sandstone
¥andçtounits.
ualte, .everal
¥¥vr horizons contain
contain abundant
• '/.
120
120
Description
Detcription of
of lithology
lithology
,1auconite and
and a
uthl'enic
pyrite
authigenic
pyrite
Iglauconit.
Olagco.ddc
Gl.ueonldc
.
Sand.tone
bed6""thick,
S u d i t o i r bed
thick, buff
buff to
to light gray, medium- to flne-grain.d,
fine-grxincd, well
wellrounded
rounded
well sorted
and well
sorted
Sand.tone
bed, same
above
S
a d i t o n e bad,
#Â¥mas above
110
110
Sandstonebed,
bed, mame
same ,a.iabove
Suidçton
above
Ito
110
Authigenic pyrite filling vu ge
Pure dolomite
I::.:
•
90
90
-
:1
Highly glauconitic
Authigenic pyrite filling wg.
80
80
70
70
-
Sandstoneb ibed
thick,finefine-totomedium-(rained,
medium-grained,buff
bufftotolight
light gny,
gray, well rounded
thick,
rounded
SkndatonÃ
d 8"8"
sad
well
a dw
e l l sorted
sorted
_
Abundant tuthigenic pyrite
-: •
-
60
60
Highly glauconitic
of Uppç
Upper An
Au Train
Train FP41.
1Base
!am ef
dln
interysi
(Covered
:overed interval
-
50.
50
40
40
-
betweenUpper
Upperaud
andLowr
LowerAu
AuTrain
Train fills
Falls
between
-2
and" dolomite
thick,generally
generally
Sandy
dolomit,to
todolomitic
dolomiticsanditone,
sandstone,buff
bufftotobluish-#ray,
bluieh-gray,bçd
beds2"-t"
2"-" thick,
separated
by thin
thin !.me*
lenses of
of silt
silt or
c p r t t e d by
or clay,
clay, bedding
bedding planes
plane* are
ere undulatory,
undulatory, percent of sand
sand
b e
' Ã § r lthroughout
throughout
Ã
the section,
#action, glauconite
gluuconit. extremely
extremelyabundant
abundant mmpeclaily
vane,
the
especially near the base
GlauconitÃ
Glauconit. and clay galls
Crest of
of 1.ow.r
lower Au Train
Train Falls
Crçs
Fall.
30
30
Olauconit. concentrated along several horizons, sandy units ezhitat email-scale
croci bedding, thin shale lense. separate unite
20
20
10
10
00
-
_
_
Clauconite is
is extremely
10-30 percent
percent of
of many
many
extremely abundant
abundant as iitt conslitutee
Gl*ucÈnit
coirtltu1.m between
bçtwç10-30
unit.
thin uaite
Had, weather, dark brown, beds Iron.
thick, several horizons contain
pebbles of .and.tone and dolomitic .sndstone which ,.pparently constitute an
intraformational conglomerate
IBase of Lower Au Train F'41s
Columnar
Columnarsection
sectionofofthe
theAu
AuTrain
TrainFormation
FormationatatAu
AuTrain
Trainfalls
falls(1-tamblin,
[Hamblin, 1958)
1958)
147
147
Stop
Stop 2. Au
Au Train-Whitefish
Train-WhitefishChannel,
Channel, Barrier,
Barrier,and
andBeach
BeachRidge
RidgeComplex
Complex
Train Whitefish
Whitefish Channel
Channel
Au Train
The
The Whitefish-Au
Whitefish-Au Train
Train Channel
Channel isis aa former
formerproglacial
proglacial lake
lake spillway
spillwaythat
that drained
drainedacross
across
the
the central
central Upper
Upper Peninsula.
Peninsula. From
FromLittle
LittleBay
BayDe
DeNoc,
Noc,north
northto
toAu
Au Train
TrainBay
Bay on
on Lake
Lake Superior,
Superior,
kilometers
the
channel
is
over
60
kilometers
(36
miles)
in
length
and
ranges
between
about
2.5
the channel is over 60 kilometers (36 miles) in length and ranges between about 2.5 kilometers
(1.5 miles) to 6.7
6.7 kilometers
kilometers (4
(4 miles)
miles) in width. The
TheWhitefish
WhitefishRiver,
River,aatributary
tributaryto
to Little
LittleBay
Bay De
De
Noc,
Noc, drains
drains the
the southern
southerntwo-thirds
two-thirds of
of the
the Channel.
Channel. The
Thenorthern
northernthird
thirdisisdrained
drainedby
bythe
theAu
Au
Train
Train River,
River, aa tributary
tributary to
to Lake
Lake Superior.
Superior. Most
Mostof
ofthe
thechannel
channelbottom
bottomisisunderlain
underlainby
by thin
thin
deposits of
of gravel
gravel or
or till
till over
over bedrock.
bedrock.
deposits
The
The lowlands
lowlands of
of the
the Au
Au Train-Whitefish
Train-WhitefishChannel
Channel were
were first
first noted
notedby
by Foster
Fosterand
andWhitney
Whitney
(1851),
(1851), but itit was
was Winchell
Winchell (1871)
(1871) who
who interpreted
interpreted the feature as a meltwater spillway. Russell
Russell
(1904),
(1904), however, identified
identified the
the channel
channel sides
sides as
as kame
kame terraces
terraces having
having been
been deposited
deposited by
by aa thin
thin
lobe of ice that had stretched
stretched across the Upper Peninsula. The
The channel
channel has
has since
since been
been studied
studied by
by
a number
(1929), Hough
Hough (1958),
(1958), Drexler
Drexler et
et al.
al. (1983),
(1983), Farrand
Farrand and
and
number of workers
workers including
including Leverett
Leverett (1929),
Drexler (1985),
(1985), and Hughes
Hughes (1990).
(1990). Incidently,
Incidently,Leverett
Leverett (1929)
(1929) indicates
indicates that during
during the 1920s,
1920s,
government
government engineers
engineers considered
considered cutting
cutting aa ship
ship canal
canal through
through the
the Au
Au Train-Whitefish
Train-WhitefishChannel
Channel
and connecting
connecting Lake
Lake Superior
Superior to
to Lake
Lake Michigan, thus avoiding
avoiding the Sault
Sault Locks.
According to Hughes (1990),
(1990), the Au Train-Whitefish Channel reached its
its full expression
expression
According
during
during the Marquette
Marquette Advance
Advance about 10,000
10,000 BP. At
At this
this time,
time, lake
lakelevels
levels in
in the
the Lake
Lake MichiganMichiganHuron basins
as retreating
retreating ice
ice had revealed
revealed the
the
basins were
were extremely
extremely low
low (Chippewa-Stanley
(Chippewa-Stanley Low) as
isostatically
isostatically depressed
depressed North
North Bay
Bay outlet,
outlet, allowing
allowing water
water to nearly drain from the southern
southern basins.
basins.
Before,
Before, during,
during, and,
and, for
for an
an unknown
unknown period
period after
after the
the Marquette
Marquette Advance,
Advance, proglacial
proglacial lake
lake water
water
drained across the central
Upper
Peninsula
to
the
lower
lakes,
creating
the
Au
Train-Whitefish
central Upper Peninsula
lakes, creating
Train-Whitefish
Channel. Hughes
Hughes(1990)
(1990)has
hassuggested
suggestedthe
the Channel
Channel extends
extends under Little Bay De Noc and Green
Bay to a large, Chippewa-level
off the east
east side
side of
of the
the Door
Door Peninsula.
Peninsula. The
Chippewa-level delta submerged off
The
Channel was abandoned
abandoned at
at around
around 9700
9700 BP when retreating
retreating ice allowed
allowed lake water to reach the
the
Sault Outlet
Outlet on the
the east
east end
end of
of the
the Superior
Superior basin.
Barrier
Au Train Barrier
During Nipissing
Nipissing times,
times, lake
lake water
water flooded
flooded some
some 66 kilometers
kilometers (3.6 miles)
miles) into
into the
the
northern end of the
the Au Train-Whitefish
Train-Whitefish Channel
Channel creating
creating Au Train Relict
Relict Bay, which is
is one
one of
of
Nipissing II wave-cut bluffs
the largest Nipissing embayments in the central Upper
Upper Peninsula.
Peninsula. Nipissing
and cliffs, cut
cut in bedrock
bedrock and
and unconsolidated
unconsolidated sediments
sediments at
at 192
192meters,
meters, rim much
much of
of the
the east
east and
and
west sides
of
the
former
embayment;
however,
in
places
along
the
southeast
fringes,
the
shoreline
sides the former embayment;
southeast fringes, the shoreline
is less developed, appearing
by eolian
eolian activity.
activity. In fact, small relict
appearing to have been modified by
along the
the southeast
southeast side of Au
foredunes may be present along
Au Train Lake.
Lake. A massive mid-bay barrier
extends
extends from
from northwest
northwest of
of Paulson
Paulson Lake,
Lake, east
east to
to Joel
Joel Creek,
Creek, forming
forming the
the northern
northern boundary
boundary of
of
modern Au Train Lake. The
Thebarrier,
barrier, having
having an
an elevation
elevation of 186
186 meters on its north side
side and 189
189
its south
south side,
side, reaches
reaches an
an elevation
elevation at
at its
its crest
crest between 195
195 and
and 192
192 meters
meters and
and aa
meters on its
maximum elevation
dune remnant.
remnant. In
elevation of 201 meters on what appears to be a dune
In places,
places, the barrier
severely eroded
eroded by
by the
the Au
Au Train
Train River.
River.
has been severely
The barrier
barrier created
created aa huge
huge lagoon
lagoon between the barrier proper and the southern
southern rim of
of the
the
The
bay, now occupied
occupiedby
byAu
Au Train
TrainLake,
Lake, Paulson
Paulson Lake,
Lake, and surrounding
surrounding wetlands. Based on the
depth of Au Train
Train Lake
Lake (USDA,
(USDA, 1938),
1938),during
during Nipissing II times,
times, the lagoon would have
have had
had aa
maximum
maximum depth
depth of
of at
at least
least7.5
7.5 meters
meters (25
(25feet),
feet), an
an average
average depth
depth of 33 to
to 44 meters
meters (9.8
(9.8 to
to 13
13feet),
feet),
miles). Drainage may have exited from the
and covered about 6 square kilometers (2 square miles).
148
148
Au Train
Lake
$Iapnck Cr.
AU TRAIN-
CisySand Cliffs
Baa n
SPI LLWAY
Cr.
WhIt.fish P.
SCALE
SCALE
kilomstsrs
I
a
,
j
I
0
-I
$
IQ
Ovation In ff11 vs
Little
Bay Do Noc
AuTrain-Wiifeflsh Spiliway
AuTrain-Whitefish
Spillway (Hughes,
(Hughes, 1971)
1971)
149
D
Moraines and channels in the Marquette area and the Eben-Chatham channels (Hughes, 1971)
SUBMARINE VALLEY
IN
IN THE
THE FLOOR
FLOOR
OF GREEN
GREEN BAY
BAY
I
11
SCALE
I.
I.
ItI•
KIlO S•I SI I
SSIk,Ic lIt,SI - S S4—•
151
lagoon on its
its northeast
northeast corner
comer near
near the
the eastern
eastern end
end of
of the
the Au
Au Train
Train Barrier
Barrier or
or at
at some
somemidmidposition breach in the barrier, where the Au Train River runs through today.
today. Nipissing
Nipissing II
IIlevels
levels
may have modified the
the northern
northern edge
edge of the
the barrier, which is
is at an
an elevation
elevation of
of 186
186meters.
meters.
Au Train
TrainBeach
BeachRidge
Ridge Complex
Complex
The
The Au Train
Train Beach
Beach Ridge
Ridge Complex
Complex occupies
occupies the
the area
area between
between the
the barrier
barrier and
and the
the
modern
shoreline
of
Lake
Superior.
The
complex
is
composed
of
some
50
separate
recessional
modem shoreline
Superior. The complex is composed of some 50 separate recessional
ridges which form a compact, arcuate shaped "corrugated plain"
plain' as first
first termed by Bergquist
Bergquist
(1936).
The
rIdges,
which
range
in
elevation
between
186
and
184
meters,
are
currently
being
(1936). The ridges, which range in
186
184
eroded
eroded by Lake
Lake Superior,
Superior, which has
has created
created an irregular
irregular line of actively forming coastal
coastal
foredunes on the northern end of the complex. The
The Au
Au Train
Train River has also
also meandered through
the ridges
ridges resulting
resulting in
in erosion
erosion and
and truncation,
truncation, suggesting
suggesting that
that the
the river's
river's present
present course
course isis
relatively recent. The
Theridges
ridgesare
areparallel
parallel to
toeach
each other,
other, and
and there
there is
is no
no evidence
evidence of cross-cutting
cross-cutting
relationships,
relationships, indicating
indicatingfluctuating
fluctuatinglake
lake levels.
levels.
Grand Island/Powell
Stop 3. Grand
Island/PowellPoint
Point Scenic
Scenic Overlook
moraine. The
This stop
stop is on the crest of a Marquette Advance moraine.
The overlook
overlook affords
affords aa view
view
of Grand Island,
Island, Grand island
Island Harbor,
Harbor, Murray Bay, and Sand Point. Sand Point, a large cuspate
spit, is visible in the east channel of Grand Island Harbor.
Harbor. The
The spit,
spit, which ranges in elevation
elevation
between 185
185 and 183
183 meters, appears
appears to
to have formed in post-Nipissing times,
times, and
and is
is still
still being
being
actively modified by wave-action
wave-action from
from Lake
Lake Superior.
Superior. It is composed of a number of lesser
bathers that
deve]opment of
of the
the spit. A
sandy barriers
that appear
appear to
to have
have created smaller
smaller lagoons during the development
A
massive Nipissing
I,
wave-cut
bluff
at
192
meters
is
present
along
the
southeast
side
of
the
spit,
192
southeast side the spit,
Nipissing I,
which extend
extend to the
the northeast
northeast from
marking the start of the modem cliffs of the Pictured Rocks, which
Sand Point.
Point.
On Grand Island,
Island, Nipissing
Nipissing I wave-cut bluffs developed in bedrock at 192
192 meters
meters rim
rim
much of the southern portions of island. Wave-cut
Wave-cut caves,
caves, shoreline
shorelinestacks
stacks and
and arches,
arches, and pocket
pocket
shoreline along
beaches are present along the Nipissing I shoreline
along the west side of Murray Bay. WaveWavecut bluffs at 192
192 meters
meters rim portions
portions of the
the Island's
Island's western high ground, known locally as
as the
the
Thumb.
Thumb. AAwell-developed
well-developedsystem
systemof
ofstream
streamgullies
gullieshas
has dissected
dissected the
the eastern
eastern hillsides
hillsides of
of the
the
main lobe of the island, as well as much of the hillsides on
on the
the Thumb.
Thumb. These
These narrow,
narrow, deep
deep
incised valleys end abruptly
at
the
Nipissing
I
shoreline,
indicating
that
they
formed
in
preabruptly
shoreline,
Nipissing, Houghton Low times
Nipissing,
times when stream
stream base levels were extremely
extremely low.
Cuspate
Cuspate barriers
barriers grading
grading from the Nipissing I bluffs to about 186
186 meters are
are also
also visible
visible
on Grand Island. The
Cuspate Bather,
Barrier,which
which isis located
located
Themost
most obvious
obvious of
of these is the Duck
DuckLake
Lake Cuspate
on the east side of the main lobe of the island. ItIt isis composed
composed of
of a number
number of large ridges
ridges that
trend east to west on the south side, and north to south on the east
east side of
of the
the barrier.
barrier. Duck
Duck Lake,
Lake,
a small inland lake,
lake, is
is aa lagoon
lagoon remnant
remnant associated
associated with the
the barrier. The
The Muskrat
Muskrat Point
Point Cuspate
Cuspate
Bather,
Barrier, formed
formedoff
off the
the southwestern
southwestern end
end of
of the
the Thumb, also contains a lagoon remnant, now a
small swampy pond. Wave
Waveaction
actionfrom
fromLake
Lake Superior
Superiorhas
has eroded the southeastern
southeastern tip of the
the
Duck Lake Cuspate
Cuspate Barrier, the
the southern
southern limb
limb of the
the Muskrat
Muskrat Point
Point Cuspate
Cuspate Barrier,
Barrier, and
and the
the
southeastern
southeastern tip
tip of
of the
the Williams
WilliamsLanding
Landing Spit,
Spit, exposing
exposing imbricated
imbricated gravel
gravel and
and cobble
cobble beds
beds in
in
both cases. These
Theseeroded
erodedareas
areasshow
showup
upas
ashigh,
high, sandy
sandy bluffs
bluffs along
along the modern shoreline.
The Grand Island Beach Ridge Complex, a complex of at least 25 beach ridges ranging
ranging in
tombolo between the Thumb
Thumb and
and the
the main
main part
part
elevation between 186
186 and 183.5
183.5 meters,
meters, forms a tombolo
of the island. 1i
Inplaces
placeson
onthe
thesouthern
southernedge
edgeof
of the
the complex,
complex, beach
beach ridges are submerged by
152
1 52
Stops 3 & 4
Stop 5
153
Lake Superior,
Superior, while
while the
the northern
northern edge
edge is
is currently
currently being eroded
eroded by the modern
modem lake,
lake, creating
creating
small coastal foredunes
foredunes along
along Trout
Trout Bay.
of Grand Island
Island presents
presents an
an enigma.
enigma. Some
A steep cliff visible high on the west side of
Some
workers (Don
and
Eschman,
1971)
have
suggested
it
is
a
wave-cut
bluff
from
Lake
Algonquin.
(Dorr and Eschman, 1971) have suggested it is a
However, such
such aa feature
feature would
would have
have been
been destroyed
destroyed during
during the
the Marquette
Marquette Advance.
Advance.
Alternatively,
the
escarpment
may
be
a
remnant
of
an
ice
marginal
stream
channel
Alternatively,
escarpment may
stream channel created
created during
during
the retreat of Marquette
Marquette Advance
Advance ice
ice or
or itit may
may have been created
created by headward
headward erosion
erosion and
and retreat
retreat
of a caprock on the
the highest
highest portions
portions of
of Grand
Grand Island.
Island.
Stop 4. Munising
Munising Falls
Falls
At Munising falls, the Au Train Formation (see
(see stop
stop 11for
for a more complete
complete description)
description)
meters thick. Conformably
forms a resistant
resistant caprock
caprock of
of dolomitic
dolomitic sandstone
sandstoneabout
about 55 meters
Conformably below is the
weaker Miners Castle member of the Munising Formation, which has
has been
been undercut
undercut to
to form
form a
steep high-walled valley. Munising
MunisingCreek
Creek plunges
plunges 10
10meters
meters over
over the
the falls
falls in
in its
its short
short journey to
to
Lake Superior. At
plunge pools.
pools. They were
At the
the base
base of
of the
the falls
falls are
are several potholes and plunge
abraded into the sandstone
sandstone by
by hard cutting
cutting tools
tools carried from
from the overlying
overlying caprock
caprock and
and from
from
glacial drift. Visitors
Visitorsused
usedtotowalk
walkbehind
behindthe
thefalls,
falls,underneath
underneath the
the overhanging
overhanging caprock,
caprock, but
but aa
recent collapse
collapse of aa section
section onto
onto the
the walkway
walkway prompted
prompted park officials
officials to
to close
close that
that part
part from
from the
the
general
general public.
public.
The Munising
Munising formation
formation represents
represents aa transgression
transgression of
of the
the shallow
shallow Sauk
Sauk sea
sea onto
ontothe
themidmidcontinent region in Cambrian
Cambrian time. The
Theupper
upper Miners
MinersCastle
Castle Member
Member isis aa light
light gray
gray to
to white
white
colored, poorly
poorly sorted,
sorted, cross-bedded sandstone
sandstone that comprises
comprises the
the upper
upper 30
30 meters
meters of
of the
the
Munising Formation. In the lower part of the member, thin
thin lenses
lenses of
of blue
blue shale
shale are
are common,
common, but
but
become scarce
scarce in the
the upper
upper portions.
portions. Quartz
Quartz grains
grains dominate
dominate the
the lithology
lithology (>95%)
(>95%) of
of the
the Miners
Miners
Castle member,
member, with feldspar comprising most of the remainder. Most
Most quartz
quartz grains
grains exhibit
exhibit
straight extinction and are likely of igneous origin.
origin. ItIt is
is not
not uncommon to find
find 1-3
1-3 cm
cm thick
thick beds
beds
of shale alternating with 10cm
10 cmthick
thickbeds
bedsof
of coarse
coarsesandstone
sandstoneand
and even
even conglomerate!
conglomerate! Especially
Especially
in the lower parts of the Miners Castle member. Higher
Higher up in the member,
member, fewer beds of shale
and conglomerate
conglomerate are
are found,
found, and
and beds
beds consist
consist mostly
mostly of well-sorted,
well-sorted, cross-bedded sandstones
sandstones
with a greater degree of mineralogical and textural
textural maturity.
maturity. Some
Some individual
individual beds are
are quartz
quartz
arenites, but in sum the formation
formation is poorly sorted. Quartz
Quartzisis also
also the
the dominant
dominant cementing
cementing agent,
agent,
but with only
only slight
slight secondary
secondary enlargement of the detrital
detrital grains,
grains, it is porous
porous and
and friable.
friable.
Authigenic
Authigenic pyrite
pyrite is
is common
common enough
enough in
in some
some of the middle
middle beds that it is
is megascopically
megascopically
discernable. Discussion
Discussionof
ofthe
theMiners
MinersCastle
Castlemember
member continues
continuesat
at the
the next
next stop.
stop.
Stop
Stop 5.
5. Miners
Miners Castle
This stop
stop examines
examines the
the type locality
locality of the Miners Castle member of the Munising
Munising
formation (see description in stop 4). Sedimentary
structures
such
as
mud
cracks
(mostly
Sedimentary structures
as
(mostly in the
shale beds), ripple marks
marks and cross
cross bedding
bedding (as seen at this stop) suggest an alternating
alternating
fluvialllacustrine environment of deposition. Significantly,
fluvial/lacustrine
Significantly,the ripple marks and cross bedding in
the Miners Castle
Castle member
member indicate
indicate aa transport
transport direction
direction from the east-northeast,
east-northeast, as
as opposed
opposed to
to
the northwesterly transport direction
direction measured in the older Chapel Rock member (see
(see figure
figure
illustrating cross-bedding directions). The
The abrupt
abrupt change
change in
in transport
transport directions
directions in the two
members provides
provides further
further evidence
evidence of an unconformity that separates
separates them. The change
change in
transport direction also
also suggests
suggests that
that the
the Wisconsin Arch and the
the Northern Michigan
Michigan highlands,
highlands,
1544
15
Miners Castle
Castle Member
Member of the Munising Formation
Au Train
Train Formation
Formation
dolomitic, yellow-brown, coarse-and
coarse-and medium
medium grained with much
148 'Sandstone, very
very dolomitic,
148
' -152'
152' Sandstone,
very coarse and little fine,
fine, scattered
scattered very
veryfine
fineand
andfine
finequartz
quartzpebbles.
pebbles. Some
irregularly bedded.
bedded. Some large
zones conglomeritic. Massive
Massive bedded and irregularly
sandstone
sandstone pebbles.
pebbles.
Munising
Formation, Miners
Munising Formation.
Miners Castle
Castle Member
Member
144' - 148'
148'
Sandstone,
144'
Sandstone, light
light yellow gray, medium grained
grained with much fine
fine and
and aa little
little fine,
fine,
even bedded and thin-medium
thin-medium beds.
Another bed as above
above
140' - 144'
haccessible
122' - 140'
Inaccessible
Light yellow-gray sandstone, coarse and very coarse grained with a trace of
121'- 120'
granules. Cross-bedded
Cross-beddedin
inmedium-thin
medium-thin beds.
beds.
Light yellow-gray sandstone medium to fine grained with traces
traces of granules.
granules.
115'- 121'
bedding. Poorly sorted with much fine
Medium - thick bedded with cross bedding.
material in lower foot
foot of bed.
112' - 115'
coarse to
to medium
medium grains.
grains. Trace
Light yellow gray sandstone, coarse
Trace of granules.
Massive, cross
cross bedded.
111' -112'
Light yellow gray sandstone, poorly sorted with sizes ranging from silt
silt to
to
granules. Abundant
Abundant limonite
limonite staining. Appears
Appears to
to be
be burrowed. Narrow
Narrow ledge.
ledge.
108' - 111'
Light brown gray sandstone. Medium to coarse grained
grained with much
much fines.
fines.
Massive. Trace
Traceof
of granules.
granules.
Light yellow gray sandstone. Poorly
Poorly sorted
sorted with grains
grains ranging from silt
silt to
to
106' - 108'
granules. Abundant
Abundantburrows
burrowsbest
bestseen
seenfrom
from overhangs.
overhangs.
103' - 106'
Light yellow brown sandstone. Medium
Medium to
to coarse
coarse grained
grained with
with few
few granules.
granules.
Massive bedded.
Massive
bedded.
Same as above
above except
except more silty and abundant
abundant burrows.
99' - 103'
95' - 99'
gray sandstone.
sandstone. Silty
Light yellow gray to gray
Silty medium to fine grained with thin beds
of red, blue and green shale. Poorly
Poorly sorted.
sorted.
40' - 95'
gray sandstone.
sandstone. Poorly sorted to well
well sorted beds.
beds. The
Light yellow gray
The upper
upper part,
part,
of medium to fine grained
beginning at the top of Miners Castle pinnacle, consists of
wed
wee1sorted
sortedbeds
beds of
of sandstone,
sandstone,interbedded
interbedded with
with poorly
poorly sorted
sorted beds
beds of
of sandstone
sandstone
containing abundant silt and thin beds of silty shale, and conglomeritic beds with
silty shale pebbles. The
The upper
upper 66 feet contains burrowed beds. At
At 10-12
10-12feet
feet
below the top, are
prominent
poorly
sorted,
shaly,
conglomeritic
and
cross
are prominent
conglomeritic and cross bedded
bedded
sandstones.
sandstones.
20' - 40'
Gray sandstone. Poorly
Poorlysorted,
sorted,medium
medium grained
grained with
with abundant
abundant thin
thin blue
blue shale
shale
partings.
Some
conglomerate.
Cross
bedded.
Some
conglomerate.
Cross
bedded.
10' - 20'
Light yellow gray sandstone, coarse
coarse grained,
grained, well
well sorted.
sorted. Contains
Contains clay pebbles.
pebbles.
Formation, Chapel
Munising Formation,
Chapel Rock Member
Light yellow gray sandstone. Medium
0'
0' -- 10'
10'
Medium to
to fine
fine grained
grained with
with aa trace
trace of
of coarse,
coarse, wellwellsorted sandstaone. Cross
Cross bedded.
bedded.
Lake Level
183 m (602')
Level == 183
(602')
155
+
Inland—0Prairie du
du Chlen
Chien inland
PJI
members arise to
to the
the east
east so that
that JacobsvllIe
All members
Jacobsville
is
above lake
Is above
lakelevel
levelat
atGrand
GrandMarais
Maraisand
andthe
theAu
AuTrain
Train
fornialion
formationIsismissing
missingfrom
fromThe
the top
top of
ofcliffs,
cliffs.
—
7
a
—— ——
-
-
Unconformity
- -I- —- ——
Light brown, hard, dolomilic
sandstone. Giauconillc.
Au Train Formation
Resistant
Resistant
A
Upper 5-12
5-12 meters
meters
Upper
Moderate
Moderateto
tovery
vewdoiomitlc
doiommcwttti
with
O L O gronJies
~
on0
pebDie5.
qUarlz
granuies
and pebbles.
Burrowed
br the
ELnowea "for
tnebirdC
Diros"
i
Soft
riable ,,poorly
pooW sorted
sorted
Soft, ffriable
silty
silly, shaM
shov, sandstone, often
often
cross-bedded.
cross-bedded,
Light yellow-gray
color
Light
yellow-gray color
imparted
importedby
bylimonite.
llmontte.(In
(In
varying
varying amounts
amounts
Miners Castle Member
C
0
About 3Dm to lake
0
U-
0)
C
Cl,
C
0'
A
———
_Uo9rmjy_
——
Pink. light
Pink,
Ughtbuff
buffto
to brown,
brown, medium
medium
omoquommcsandstone.
sandstone.
grainedorthoquartzWc
grained
Thin
blue shale
shale beds
beds 2-3
2-3cm
Thin blue
cmthick.
thick.
Mudcracks
Mudcrocksininshale
shalebeds.
beds.
""bdsôh"é'(°h€ri Uórtz
quartzite, and chert w/minor amounts
of siate, BiF, basalt and granite)
Chapel Rock Memberô a
—————a——
c
b
a
Jacobsville
Jacobsville Formation
Formation
I
-
—a———
a
I
Nigular unconformity
Nkosic
to
Arkosic quartz
quartz sandstone.
sandstone. Red
Redto
red-brown
abundant mottles
red-brownwith abundant
mottles
and
where
on0sireaks,
s ~ ~ o < sespecially
es~ecioirv
.
wnere
permeable.
Th cmessvaries
vanesgrecztts
greatly,
Denneooie.Thickness
from > 1m to 500 m,
from>lmtoSOOm.
____________________________________________________________________________________
Ga rat Zircon
looter,
5.çtton
Dcecflptioaotltthotogy
ntis
0
I
DeIsmilic ttnd.tae. light brown, Very resistant
3ssso1 kU TRAIN formation tencanlormity
•
•
ISO
''.•.'
Top of MUNISINC t.rmatlea Miner's C..tj. nwmb.r
teStIsa. .Mte, m.dlwn-grasn.4, friable, ma.!,. badtsg (I' tot thick),
fonti vertical di/l.
—
Details of ..dimtntsry strvcttr.s ob•c,and hyw.ath.riag; ataytadte appnrt
be
cr*tt.b.dd*d
120 Sandstone
wMt. mSwm.praintd. ..U
coned,
masal,. bedding (I' tel thick).
email cavee and arch... friable
N'&marc'aa
tat;. tcrsdeaa along bedding pta.
Smelt—scale crass bedding
to,.
11.4 stains ales; certain hastens
.... b6.
i" bed. very resistant. contains abtaadaat n.atris
Sandstone. pr.tn,iaaatly
poorly sorted
ttt. is gray. tniaor streak.
.1 red, blee and grssn,
5... of Mint's Castle piradle
55
Coarse .and.sene conc.atrat.d along cantata bedding p1....
Abundant day pail.;. deposited parallel to the cro.*
bedding
cbann*I. and Ian... shine cflgtOrnhrstO
iO
Small ceagiarat. lens..
So.
Coarse eand and conglomerate I.....
S
Slut, grita, red, and yells'. colorado. foflo.tn bedding ,a.'...
S.ndst.. gray to grayteh'.bl.a., m.tum,grain.d, pearly .on.d amall_.caIa
crosa bedding 4".,i" thick ..parat.d by thin blue hal. I.n.e. l14"-t" thick
'4
3°
(approalmaisly 2030 percent .hal*), torn, coarse and fin. coagloanarat.
/
20
''i
L'&3c
10 ,
&
¶cat.r t*v.l
of
bk.
r
/
y
I,
(
1
pa.'
of Itt.,'0 tail. m..r unonntor
Top of Chapel Rock rna,.bor
5aad.tots, pink, coors.-grainsd, nil sorted, rounded, abundant clay pellet.
Mud cracks 4"-S' In tonnetar dontloped in beds of bIn. shale ines than I' thick.
Clay pellet.
sandseont light brown to buff well carted
large •cale cross bedding It thick
j
-
Superior
Columnar
Columnarsection
sectionof
ofthe
theMiners
Miners Castle
Castle member
memberof
ofthe
theMunising
MunisingFormation
Formation
at
atMiners
MinersCastle
Castle(after
(afterHamblin,
Hamblin,1958)
1958)
157
157
Paleogeography
member of
Paleogeographyduring
duringdeposition
depositionof
of the
the Miners
Miners Castle member
of the
the
Munising
Munising Formation
Formation(after
(afterHamblin,
Hamblin, 1958)
1958)
£IPLSNAtION
bSSJ
V
tsuSe
ndI.*
d—
- — •_N_ •m
Caib — — fl4 —*•
lö?
I—
I*aj
•f — t* un—*.
_*._ cni .4
*
t
tS3a,
La
-
is
0
iG
it
sai—. t—
'0
•0S.iii
Cross bedding
bedding directions
MinersCastle
Castlemember
member of the
the
directions in the Miners
Munising
(after Hamblin,
Homblin, 1958)
Munising FormatHon
Formatiion (after
1958)
158
158
which
which provided
providedthe
thesource
sourcefor
forsediments
sedimentsduring
duringthe
the Jacobsville
Jacobsvilleand
and Chapel
Chapel Rock
Rock deposition,
deposition,had
had
eroded
erodedenough
enoughthat
thatthe
thetransgressing
transgressingSauk
SaukSea
Seasubmerged
submerged most
most of
of the
the Upper
Upper Peninsula
peninsula and
and
Wisconsin.
Wisconsin.
Stop6.
6. Kingston
Kingston Plain
Plain
Stop
Stop
Stop66 isison
onthe
theKingston
KingstonPlain,
Plain,an
an extensive
extensiveoutwash
outwashplain
plain that
that isislocated
locatedbetween
betweenthe
the
towns
towns of
of Munising
Munising and
and Grand
Grand Marais.
Marais. The
ThePlain
Plainpresents
presentsaaunique,
unique,prairie-like
prairie-likesetting
setting compared
compared
to
to much
much of
of the
the more
more heavily
heavily forested
forested areas of the Upper Peninsula. The
The multitude
multitude of
of pine
pine stumps
stumps
preserved
presemed on
on the
the Kingston
Kingston Plain
Plain serve
serveas
as testimony
testimony to
to the
the intensity
intensity of
of historic
historic clear-cut
clear-cut logging.
logging.
A
(Leverett,1911,
1911,
A number
numberof
of workers
workershave
havestudied
studiedthe
the geology
geology of
of the
the Kingston
Kingston Plain
Plain (Leverett,
1929;
1929;Bergquist,
Bergquist, 1936;
1936;Martin,
Martin, 1957;
1957;Hughes,
Hughes, 1968;
1968;Drexler,
Drexler, 1981,
1981, Drexler
Drexler et
et al.,
al., 1981;
1981;Blewett
Blewett
and
and Rieck,
Rieck,1987;
1987;Blewett,
Blewett,1994),
1994),which
which has
has resulted
resulted in
in aa wide
wide range
range of
of theories
theories concerning
concerningits
its
formation
formationhistory.
history.Largely,
Largely,the
thecontroversy
controversyhas
hasfocused
focusedon
onthe
theinterpretation
interpretationof
ofthe
theMunising
Munising
Moraine,
which
the
Kingston
Plain
is
part
of
(Blewett,
1994).
According
to
Hughes
Moraine, which the Kingston Plain is part (Blewett, 1994). According to Hughes (1968
(1968and
and
unpublished
unpublishedwork),
work),and
andlater
laterby
by Blewett
Blewett (1994),
(19941,whom
whom has
has done
done the
the most
most current
currentQuaternary
Quaternary
geology
geology research
research in
in the
the area,
area,the
the Plain
Plain initially
initially formed
formed when
when Marquette
Marquette Advance
Advance ice
ice probably
probably
reached
reached an
an equilibrium
equilibriumand
andsouthward
southwardflowing
flowingmeltwater
meltwater streams
streamsbuilt
built the
the Upper
Upper Kingston
Kingston
Outwash
I). Ice
Outwash Plain
Plain surface
surface at
at about
about 10,000
10,000 BP (Phase 0.
Ice retreated
retreated somewhat
somewhat from this position,
opening
opening up drainage
drainage to
to the
the east
east between
between the
the ice
ice front
front and higher ground to the south,
south, creating
creating aa
channel
M-77. The
channel at
at 283
283 m
m that
that currently
currently parallels state highway M-77.
The upper
upper portions
portions of
of the
the
Kingston
Kingston Lake
Lake Kettle
KettleChain
Chainformed
formedatatthis
thistime.
time.
Next
Next (Phase
(PhaseII),
lI), the
the ice
ice margin
margin retreated to a new position along
along the Lower Kingston
Kingston
Plain,
opening
even
lower
drainage
outlets
from
the
west.
Consequently,
water
flowing
to the
Plain, opening even lower
west. Consequently, water
east
east between
between the
the ice
ice front
front and
and the
the now
now abandoned
abandoned Upper
Upper Kingston ice-contact
ice-contact slope,
slope, formed
formed aa
broad,
broad, kame
kame terrace
terraceatatabout
about285
285m,
m,which
whichslopes
slopestotothe
theeast.
east.
Marginal
Marginal retreat
retreat of
of the
the ice
ice from
from highlands
highlands south
south of Grand
Grand Marais
Marais allowed
allowed still
still lower
lower
outlets
outlets to
to open
open (Phase
(PhaseIII),
I Q ,which
which resulted
resulted in
in the
the incision
incision of a channel
channel at 250-253 m, into the
the
Lower
Lower Kingston Plain
Plain surface.
surface. The
TheHurricane
Humcane River
River currently
currently follows this channel. Also
Alsoatatthis
this
time,
time, some
some meitwater
meltwaterspilled
spilledsouthward
southward from
from the
the Lower
Lower Kingston
Kingston ice-contact
ice-contact slope
slope south
south of
of
Beaver
Beaver Lake,
Lake, forming
forming the
the Long
Long Lake Channel. The
Thelower
lowerportions
portionsof
ofthe
theKingston
KingstonLake
LakeKettle
Kettle
Chain,
Chain, including
includingKingston
KingstonLake
Lakebasin
basinmust
musthave
haveformed
formedduring
duringthis
thisphase.
phase.
IV,and
and55 of
of Blewett, 1994),
1994), the ice withdrew to a position
position several
several
Eventually(Phase
(PhaseIV,
Eventually
kilometers
kilometers north
north of
of the
the Lower
Lower Kingston
Kingston ice-contact
ice-contact slope,
slope, allowing
allowing meltwater
meltwater drainage
drainage across
across aa
broad
terrace
heading
at
230
m,
known
as
the
Beaver
Basin
surface.
Further
retreat
of
ice
broad terrace heading at 230 m, known as the Beaver
Further retreat of ice
allowed
allowed even
even lower
lower channels
channelsat
at 225
225 and
and 215
215 m
m to
to be
be incised
incised into
into the
the Beaver Basin surface.
surface.
Finally,
the
glacier
retreated
to
a
position
just
south
of
Portal
Point,
but
eventually
Finally,
glacier
eventually further
retreat
retreat allowed
allowedlake
lake water
water in
in the
the western
western Superior
Superiorbasin
basin to
to flow
flow eastward,
eastward, abandoning
abandoning the
the Au
Au
Train-Whitefish
Channel,
into
the
eastern
basin,
forming
an
early
version
of
Lake
Minong.
Train-Whitefish Channel, into the eastern basin, forming an early
of Lake
Continual
Continual retreat
retreat allowed
allowed Lake
Lake Minong
Minong to expand,
expand, eventually
eventually filling the entire Superior
Superior basin.
159
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The Kingston Outwash Plain and meltwater channels near Grand Marais, Michigan
Ti,
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Mosaic
Mosaic of
of digital
digital elevation
elevationmodels
models(DEM)
(DEM)processed
processedtotoshow
showrelief
relief(illumination
(illuminationfrom
fromSE)
SE)
; .—.I
0
/
I,I)
Stop 7.7.Turtle
Lake
(actually
an unnamed
pond,
soso
called
stop
lbrtle
Lake
(actually
an unnamed
pond,
calledononthis
thistrip
tripbecause
becauseon
on previous
previous visits itit
was observed
observed that many
many turtles
turtles make
make itit their
their home)
home) occupies
occupies aa deep
deep kettle
kettle in
in the
the Kingston
KingstonOutwash
Outwash
Plain. The kettle
kettle is
is the
the deepest
deepest (though
(though not
not the
the largest)
largest) in
in aa south-southeast/north-northwest
south-southeasthorth-northwesttrending
trending
chain of kettles that (presumably) were formed as ice stagnated in aa now-buried
now-buried bedrock
bedrock valley.
valley. A
from bedrock
bedrock and a minimum
minimum
seismic study conducted by Regis in the 1980's revealed no reflection from
sediment thickness of 60 meters. The kettle is several hundred meters in diameter
diameter and
and is
is at
at least
least 40
40
meters deep.
deep. Because
Because the
the kettle
kettle chain
chain extends
extends across
across (cuts)
(cuts) each
each of
of the
the outwash
outwash and
and drainage
drainagesurfaces,
surfaces,
the stagnant
Marais) ice was
was building
building
stagnant ice must have been in place from
&om the time the Marquette (Grand Marais)
surface, and was buried by sediments
sediments as the main ice
ice mass
mass retreated.
retreated.
the upper Kingston outwash surface,
00 m
m hike
hike through
through the
the woods,
woods, and
and ifif descending
descending into
into the
the kettle,
kettle, aa strenuous
strenuous
This stop requires a 2-3
2-300
return hike.
164
164
Stop 8. Log
Log Slide
Slide
From the
to the
the east.
east. Two
the Log Slide, the Grand Sable complex is visible to
Two geomorphic
geomorphic
features,
in this
this view:
view: The
features, the "Banks"
"Banks" and the "Dunes" are prominent in
The Grand Sable
Sable Banks
consist
of
a
steep,
north-facing
exposure
of
glaciofluvial
sediments
ranging
in
thickness
consist of a steep, north-facing exposure of glaciofluvial sediments ranging in thickness from
from 30
30
to
to 100
100 meters,
meters, rising
rising directly
directly above
above the
the shoreline
shoreline of Lake Superior.
Superior. To
Tothe
thesouth
southaway
awayfrom
fromLake
Lake
Superior,
Superior, the Banks form
form aa high
high terrace
terrace surface, which is overlain by the Grand Sable Dunes, a
dune
dune field
field "perched"
"perched"up
up to
to 100
100meters
meters above
above the
the lake
lake and
and covering
covering about
about 10
10square
square kilometers.
kilometers.
The
The deposit
deposit that
that underlies
underlies the
the dune
duneplateau,
plateau, exposed
exposed in
in the
the Grand
Grand Sable
Sable Banks,
Banks, has
has been
been
variously
variously interpreted
interpreted as
as aa crevasse
crevasse filling
filling or
or kame
kame terrace
terrace that incorporated
incorporated heterogeneous
heterogeneous
glaciofluvial
glaciofluvialdebris
debris transported
transported by
by rivers
rivers flowing
flowing west to east
east along
along an ice marginal channel
channel
(Drexler,
(Drexler, 1981;
1981;John D.
D. Hughes,
Hughes, personal communication). Within
Within the
the central
central portion
portion of
of the
the
Banks,
Banks, stratigraphy
stratigraphy of the
the lower
lower third
third of
of the
the section
section is obscured by colluvial and aeolian
aeolian sand;
sand; at
at
the
the east and
and west ends
ends of the banks, the section is better exposed. Upper
Upper portions
portions of
of the
the banks
banks
are
several thin
thin gravel
gravel beds.
beds. thterbedded
are dominated
dominated by coarse,
coarse, cross-bedded
cross-bedded sand and include several
Interbedded
silt
silt and
and sand
sand comprise
comprise the
the middle
middle third of the deposit. The
Thelower
lower third
third is
is dominated
dominated by
by fine
fine
textured
textured material,
material, primarily
primarily silt.
silt. AAsurface
surfacegravel
gravel layer
layer is
is presently partially exposed in a
"stripped
"stripped plain" in the central plateau. This
This layer
layer is
is often iron-stained and contributes
contributesto
toaalag
lag
that
that periodically
periodically armors
armors the
the bluff
bluff crest
crest as
as the
the slope
slope retreats
retreats southward
southward (Farrell and Hughes,
Hughes,
1985).
1985).
The
The Grand
Grand Sable
SableDunes
Dunescontain
contain evidence
evidence of
of buried
buried soils,
soils, which
which record
record episodes
episodes of
of
geomorphic
geomorphic change
change consisting
consistingof
of soil
soil profile
profile development
development followed
followed by soil profile
profile burial
burial
(Anderton
(Anderton and Loope,
Loope, 1995).
1995). In
Inmost
mostcases,
cases,an
an entire
entire episode
episode is
is preserved
presewed as a buried soil profile
or
or organic
organic accumulation,
accumulation,capped
capped by
by relatively
relatively finer sediments,
sediments, such as silt and very fine to fine
sand,
which
is
capped
by
medium
to
coarse
eolian
cross-bedded
sand. The contact between the
sand, which is capped
cross-bedded sand.
upper
upper soil
soil horizon,
horizon, usually
usually an
an 0/A
OIAhorizon,
horizon,isisoften
oftenvery
verysharp,
sharp,suggesting
suggestingrelatively
relativelyrapid
rapidburial.
burial.
Such
Such sedimentary
sedimentaryand
andpedological
pedologicalevidence
evidencestrongly
stronglysuggests
suggeststhat
that periods
periodsof
ofstability
stabilityare
are
represented
represented by
by forest
forest vegetation
vegetation colonizing
colonizing landscape
landscape surfaces
surfaces within the Dunes, while
while periods
periods of
of
instability
instabilityare
are characterized
characterizedby
by the
the burial
burial of trees and associated soil profiles and organic
organic
accumulations.
accumulations.
A
A soil
soil catena,
catena, consisting
consistingof
of aa sequence
sequenceof
of buried Spodosols,
Spodosols, may be found
found outcropping
outcropping in
in
various
various places
places throughout
throughoutthe
the Dunes,
Dunes, especially
especially at
at the Log Slide
Slide locality. The
Thecatena,
catena,referred
referredto
to
as
as the
the Sable
Sable Creek
Creek Soil
Soil(Anderton
(Anderton and
and Loope,
Loope, 1995),
1995),is
is traceable
traceable across
across much of the
the Bluff top
top
and
and outcrops
outcrops sporadically
sporadicallywithin
within the
the Dunes
Dunes reflecting
reflecting the preservation
presemation of a past soilscape
soilscape which
began forming
forming immediately
immediatelyafter
after deglaciation
deglaciation about
about 10,000
10,000 years ago, but was isolated from
further
further surface
surface weathering
weatheringunder
under eolian
eolian sediments
sediments beginning about 5000
5000 years ago. The
Theburial
burial
event
event seems
seems to
to have
have been
been initiated
initiatedby
by the
the massive
massive destabilization
destabilization of the
the Bank Front
Front during
during the
the
high
lake
levels
of
the
Nipissing
Great
Lakes.
high lake levels of the Nipissing Great Lakes.
While
While there
there may
may have
have been
been some
some eolian
eolian activity
activity immediately following deglaciation,
deglaciation, the
evidence
tends
to
support
the
hypotheses
of
Farrell
and
Hughes
(1985)
who
suggest
evidence tends to support the hypotheses of
(1985)
suggest that the
Grand
Grand Sable
Sable Dunes
Dunes are
are no
no older
older than 5500
5500 BP. This
Thisisisin
in contrast
contrast to
to Marsh
Marsh and
and Marsh
Marsh (1990)
(1990)
who
who argue
arguethat
that the
the Grand
GrandSable
SableDunes
Dunes began
began forming
forming as
as lake
lake levels
levels rose from
from the pre-Nipissing
pre-Nipissing
lows
lows sometime
sometimearound
around 9000
9000 BP.
BP. Marsh
Marshand
andMarsh
Marsh(1990)
(1990)maintain
maintain that
that sand
sand nourishment
nourishment to
to the
the
Dunes
until around
around 3500
3500 BP.
BP. However,
Dunes increased
increased from
from 9000
9000 BP
BP to
to 5500
5500 BP and remained high until
However,
the
the presence
presenceof
of the
theSable
SableCreek
CreekSoil,
Soil,which
which isis developed
developed into
into glaciofluvial
glaciofluvialsediments,
sediments,suggests
suggests
that
that the
the Bluff
Bluff top
topwas
wasaastable,
stable,forested
forestedenvironment
environment from
from deglaciation
deglaciationuntil about
about 5000
5000BP
BP when
when
eolian
eoliansand
sandbegan
began to
to cover
coverthe
the upper
upper surface.
surface. Rising
Risingpost-Houghton
post-Houghtonlevels
levelsapparently
apparentlyhad
hadlittle
little
165
165
a.
t
0
0
2
0)
0)
0
D
C
0
0
KN
0'
a.
166
effect on the Banks until well into
into the
the mid-Holocene,
mid-Holocene, suggesting that wave action did not begin
to modify the bluff front until lake levels were relatively high during the Nipissing Great Lakes.
Stop 9. Ghost Forest
Subsequent,
Subsequent, post-Nipissing
post-Nipissing fluctuations
fluctuations in lake level also
also seem to have influenced
influenced the
the
Bank front, causing
changes
in
the
sediment
available
for
eolian
transport
on
the
upper
surfaces
causing
upper surfaces
of the Grand Sables,
Sables, resulting in a discontinuous
discontinuous occurrence
occurrence of buried soils
soils and organic
organic layers
layers of
various ages within the Dunes. In
In addition
addition to
to the
the major
major burial event
event that covered the Sable Creek
Soil at least as early
early as around 4500 BP, radiocarbon dates from the Ghost Forest profile also
more recent
recent burial
burial at
at about
about 100
100years
yearsago.
ago. Also a
record a burial event at about 3500 BP and a more
radiocarbon date from other localities
localities in the Dunes suggest that eolian activity also took place at
about 1500
1500BP,
BP, and
and at
at about
about 700
700years
years ago.
ago.
While, these
these data
data may not be conclusive, an analysis of radiocarbon dates from the Grand
Sable Dunes strengthens
strengthens interpretations. Beginning
Beginning with the investigations
investigations of Bach (1978),
(1978),
researchers working in the Grand Sable
Dunes
have
been
collecting
and
radiocarbon
dating
Sable
collecting
the study
study area.
area. In general, the dates seem to
preserved wood, bulk soil, and charcoal from the
roughly delineate
delineate major times of increased eolian activity, documenting burial events within the
Grand Sable
Sable Dunes, and likely
likely recording fluctuations in the lake levels of the Lake Superior
Superior
basin.
Based on the available
available data,
data, episodes of increased eolian activity occurred at about
about 5300,
5300,
2100, 1500,
500, and
4600, and 3500
3500 RCYBP,
RCYBP, and
and may also
also have occurred at about 2500,
2500,2100,
1500, 1000,
1000,500,
and
RCYBP. The
100 RCYBP.
The first
first three
three episodes
episodesseem
seem to
to coincide
coincide well
well with
with rising
rising Nipissing,
Nipissing, Nipissing
NipissingI,I,
and Nipissing II
II phases, which are recognized throughout the southern Great Lakes (Larsen,
1985). The
other,
The other,later
laterepisodes
episodesalso
alsocompare
compare favorably
favorably to
to previous research
research which
which indicates
indicates
fluctuating
fluctuating lake levels
levels in
in the
the Lake
Lake Michigan basin during the last 3000 years (Larsen, 1985).
1985).
Larsen (1985)
(1985) has suggested
suggested that Holocene lake level fluctuations are linked to climatic
climatic changes
changes
that affected the
the temperature
temperature and moisture
moisture patterns influencing the water balance of the lakes.
lakes.
Holocene climate
of proxy
proxy indicators suggest
climate changes
changes in the
the Great
Great Lakes, as revealed by a varity of
alternating
alternating episodes
episodes of cool,
cool, moist
moist and
and warm, dry conditions (Swain, 1978;
1978; Bernabo,
Bemabo, 1981).
1981).
Comparison of burial
burial events
events within
within Grand
Grand Sable
Sable system
system with lake level and climate
climate data
data from
from the
the
Great
In general,
general,
Great Lakes
Lakes region for
for the
the last
last 3000
3000 years indicate a general correlation of events. In
of cool, wet climatic
increased eolian activity in the dunes appears to correlate with periods of
conditions,
conditions, while
while periods
periods of
of relative
relative stability
stability and soil development
development are
are associated
associated with times
times of
of
of shore-zone
warm, dry climatic conditions. These
These findings
findings do not fit Larsen's (1985) model of
changes during rising and falling lake levels. In
the
Larsen
model,
increased
precipitation
In the Larsen model, increased precipitation and
and
lowered temperature result in high lake levels that cause tributary stream aggradation, marsh
establishment, and soil profile development in
in eolian
eolian sands.
sands. Decreased precipitation and raised
establishment,
temperatures cause
temperatures
cause low lake levels that result in tributary stream erosion, marsh extinction, and
soil profiles becoming buried by eolian sands. While
While Larsen's model seems to work for most
coastal areas, perched dunes,
dunes, such as the Grand Sable
Sable system, appear to be reacting differently to
climatically-induced
climatically-induced changes
changes in
in lake
lake level.
level.
been a nearly
nearly constant
constant rain
rain of
of eolian
eolian sediments
sediments with
with
In general, there seems to have been
relatively short-lived periods of soil development and forest colonization within the Dunes since
about 3000 BP. The
Thedistinct,
distinct,buried
buried soil
soil profiles
profiles of different ages within the Dunes argues for
the Bank
Bank front.
front. Otherwise,
the fact that only relatively high lake levels are influencing the
Otherwise,ifif the
the
of stability, but rather a
Bank front was continually being undercut, there would be no evidence of
157
homogeneous deposit of eolian sands. Thus,
Thus,the
the evidence
evidence of
of geomorphic
geomorphic changes
changes from
from the
the
Grand Sable Dunes seems
shore of
seems to
to be
be an
an indicator
indicator of lake
lake level changes
changes along
along the
the south
southshore
of Lake
Lake
Superior. Th
In sum, the buried soil profiles reflect fluctuations
fluctuations in the amount
amount of sediment
sediment available
available
Hughes, 1985). The
for eolian transport within the Dunes (Farrell and Hughes,
The availability
availability of sediment
sediment
Bank face
face (Marsh
(Marshand
andMarsh,
Marsh,1987)
1987).. High
depends on the condition of the Bank
High lake levels result in
increased wave action, which undercuts the bank toe slope,
slope, causing mass
mass wasting,
wasting, destabilizing
destabilizing
for wind
wind deflation.
deflation, If enough sediment is available
the Bank face and presenting a fresh surface for
on the
the high
high terraces.
terraces. Low lake
for a long enough period of time the result is dune development on
lake
levels allow the Bank face to stabilize
stabilize and
and perhaps
perhaps become
become vegetated,
vegetated, resulting
resulting in
in aa reduction
reduction or
or
near elimination of available sediment. Thus,
Thus, dune
dune building
building
on the
the high
high
terraces is retarded,
retarded,
- on
- terraces
allowing stabilization
stabilization and surface
surface weathering to begin, as well as forest vegetation
vegetation to
to colonize
colonize
the fresh landscape surfaces. Thus,
Thus, soil
soil profiles
profiles formed
formed in
in the
the Grand
Grand Sable
Sable Dunes during
during periods
periods
basin. These
of low lake levels in the Lake Superior basin.
These soil
soil profiles
profiles and in many cases the forests
themselves were, however, buried by eolian sediments,
sediments, eliminating further soil development
development and
and
forest growth, the result
of
high
lake
levels
destabilizing
the
Bank
front.
result
lake
destabilizing
Stop
Stop 10.
10. Sable Falls
At Sable
Sable Falls,
Falls, the
the Jacobsville
Jacobsville Sandstone
Sandstone forms
forms the
the base of the
the section
section exposed
exposed in
in the
the
gorge. The
The Jacobsville
Jacobsvillesandstone
sandstoneisisaa redbed
redbed deposit
deposit of
of questionable
questionable late Proterozoic-early
Proterozoic-early
Cambrian age named by Lane and Seaman (1907) after the Keweenaw Peninsula town of
Jacobsville. There,
There,the
the"Portage
"PortageSandstone"
Sandstone"that
thatwas
was used
used as
as ornamental
ornamental and
and construction
construction stone
stone
was quarried. ItIt isisexposed
exposedall
all along
alongthe
the southern
southern shoreline
shoreline of Lake Superior
Superior from the
Keweenaw Peninsula to Munising. However,
However, from
from Munising
Munising to Beaver Bay, it is below lake
level, spare for a few occurrences slightly above
above water.
water. At Sable Falls and south of Grand
Marais, good exposures are found. Thickness
Thicknessof
of the
the Jacobsville
Jacobsville appears
appears to be extremely
extremely variable
variable
and largely controlled by the underlying Precambrian topography, upon
upon which
which the Jacobsville
unconformably for the most part. For example,
rests unconformably
example, Hamblin (1958)
(1958) cites an instance
instance of aa 22
wells, one of which drilled
loot of
of Jacobsville Sandstone, and the other well drilled
drilled through 11100'
through only 46'
46 of
and the wells are only
of the
the same
same rock
rock before
before reaching Precambrian quartzite, and
10 miles apart (T47N,
(T47N, R1E).
RlE).
The Jacobsville can be easily recognized
recognized by
by its
its striking
striking red
redto
to red-brown
red-browncolor.
color. Much of
the Jacobsville is mottled with white reduction splotches and streaks. The
The color
color change
change is
is abrupt
abrupt
and distinct. Most
of
the
mottles
are
related
to
variations
in
permeability
of
the
beds.
That
is,
Most of the mottles are
permeability of the beds.
is.
where the beds have the greatest permeability, leaching and white coloration is the greatest.
greatest.
Most fine grained beds and shale lenses in the Jacobsville are
are not
not leached
leached at
at all.
all. Leaching most
joints, and
and cross-bedding.
cross-bedding. Spherical
often ffollows
o l l o k bedding planes, fractures such as joints,
Spherical spots
spotiofof
leaching appear to be controlled
controlled by a dark grain or pebble at occurs at the center of the spots.
The grains are probably of some
some lithology that have a composition sufficient to produce a
reducing
reducing environment.
environment.
Texturally,
Texturally, the
the Jacobsville
Jacobsville Formation
Formation ranges from conglomerate
conglomerate to a fine grained
grained
siltstone. The
by Hamblin
Hamblin (1958).
(1958). The
Theformation
formationhas
has 44 different
different facies, as described by
conglomerate
conglomerate facies
facies occurs
occurs mostly at the base of the formation,
formation, especially
especially where it lies
unconformably
unconformably on Precambrian rocks (Presque Isle near Marquette, for example), but may also
be found
found scattered
scatteredthroughout
throughoutthe
theyounger
younger beds.
beds. Thickness of these beds can be as great as 5
meters. The
Themost
mostcommon
commonof
of the
the44 facies
facies(laterally
(laterally and vertically) is a red to reddish brown,
medium-grained,
medium-grained, lenticular
lenticular bedded sandstone. In
In some
some places,
places, lenticular
lenticular bedded sandstones
sandstones are
are
168
over
meters in thickness. Trough
over 100
100 meters
Troughcross
cross bedding
bedding and
and ripple marks are
are common in this
this facies.
facies.
bedding in
in the
the formation
formation accentuate
accentuatethe
thestructures.
structures. The
Selective leaching along cross bedding
occurrence
environment of
occurrence of ripple marks, mud cracks
cracks and clay pebbles clearly indicate a fluvial environment
deposition. AAmassive
deposition.
massivefacies
faciesof
of the
theJacobsville
Jacobsville is
is characterized
characterized by persistent, thick beds of
relatively structureless
structureless sandstone,
sandstone, many over 3 meters in thickness. At Victoria Falls, a measured
measured
section of 200 meters of Jacobsville
Jacobsville consists
consists almost
almost entirely
entirely of 3 meter thick beds of sandstone.
sandstone.
Some laterally persistent oscillation ripple marks are found in the beds and support
support a lacustrine
lacustrine
environment of
of deposition.
deposition. Finally, Hamblin
Hamblin recognized
recognizedaared
redsiltstone
siltstonefacies.
fades. Though not
not
common (except
(except at Laughing Fish Point and Agate Falls), beds of siltstone
siltstone and shale occur
occur
interfingered with other facies
and
is
interpreted
as
a
quiet-water
lacustrine
environment
facies
environment of
deposition.
deposition.
Paleogeographically, the Jacobsville Sandstone represents a continental red bed sandstone
sandstone
from the
the Northern
NorthernMichigan
Michiganhighlands.
highlands. As shown in the
deposited by streams flowing northward from
paleocurrent (cross
(cross bedding and ripple marks) map produced by Hamblin (1958) there is very
little deviation in trend (of indicators)
indicators) from the mean.
An unconformity, in some
some places a disconformity, but as on Grand Island, an angular
unconformity, separates
separates the Jacobsville
Jacobsville from the overlying Chapel Rock Member of the
Munising Formation. At
At Grand
Grand Island
Island the
the Jacobsville
Jacobsville dips a few degrees northward, and the
Chapel Rock member of the Munising
Munising Formation dips a few degrees southward, but at this stop,
stop,
bedding appears
appears to
to be
be parallel
parallel or
or nearly
nearly parallel
parallel at
at the
the contact.
contact.
The lower two to fifteen
foot
section
of
the
Chapel
Rock member is a basal conglomerate,
fifteen foot
conglomerate,
formation
90% vein quartz, quartzite
quartzite and chert with lesser amounts of slate, basalt, granite, iron formation
and sandstone. The
Theupper
upper 49
49to
to 60
60feet
feet isis pink
pink to
to light bluff to brown, medium grained
orthoquartzitic sandstone
blue-clay and
and silty
siltyshale
shalebeds.
beds. The beds in the top 20
orthoquartzitic
sandstone with several thin, blue-clay
very dolomitic
dolomitic and
andfrequently
frequently"burrowed."
"burrowed." Limonite imparts a light
or 40 feet are moderately or very
yellow-gray color to the member
member and occasionally
occasionally red to some
some thin beds.
beds.
Northern Michigan
The (lower) Chapel Rock member represents a period when the Northern
Highlands (to the south)
were
eroding
and
shedding
their
sediments
into
a shallow empeiric sea
south)
to the north (see
page). The
(see the figures on the next page).
The Chapel
Chapel Rock member is composed
composed almost
entirely of quartz (mostly
of
igneous
origin;
Hamblin,
1958)
with
minor
amounts
of feldspar.
(mostly
minor
of tourmaline, and abundant zircon
Heavy minerals in the member include well-rounded grains of
(20-50% of the total heavies, both altered and unaltered), magnetite, hematite, and ilmenite.
Silica is the predominant cement in most places and occurs as secondary overgrowths of varying
degrees. Farther
Farthereast,
east,near
nearTahquamenon
TahquamenonFalls,
Falls, porosity
porosity in
in the
the Chapel
Chapel Rock
Rock member
member isis reduced
reduced
to almost zero due to the silica cement. Texturally,
Texturally, the Chapel Rock member is well-sorted,
medium-grained sandstone. Most
Most grains
grainsare
are 1/4
114to 1/2
112 mm diameter. Color
Colorgradually
graduallyranges
ranges
sedimentary
from white to buff to red. Large
Large scale
scale trough cross bedding is the most abundant sedimentary
The size
size of these troughs range from I1 to
to nearly
nearly 200
200
structure in the Chapel Rock member.
member. The
meters in width (mean about
about 10
10 meters). Megascopically,
Megascopically,they
they are
are useful
useful for
for differentiating
differentiatingthe
the
Miners
Chapel Rock Member
Member from the underlying Jacobsville Formation and the overlying Miners
Castle member. At
At Miners
MinersCastle,
Castle, they
they are
are abundant.
abundant. They
Theyrepresent
represent deep
deep erosional
erosional channels
channels
that were cut into the older units. Near
Near the
the base
base of the Chapel Rock member, pebbles become
abundant in the cross-beds
that
fill
the
troughs.
Hamblin
cross-beds
Hamblin(1958)
(1958)suggests
suggeststhat
thatthey
theyare
areindicative
indicative
along aa cuspate
cuspate shore.
shore. Ripples marks are less abundant and
of marine deposition in embayments along
are both current-formed and oscillatory. Where
Wherecurrent-formed,
current-formed,they
they indicate
indicate aa mean
mean transport
transport
direction to the northwest with little
little standard
standard deviation about that mean (see figure illustrating
illustrating
cross-bedding directions
directions in the
the U.P.). This
Thisindicates
indicatesaafairly
fairlysteep
steepgradient
gradientwith
with little
little
cross-bedding
169
11
UPLI6*TION
S
V
•t•*d
O$Iø *1
— bs#' an a4 —
biSS d.ttli.
4 A'SM I.l..
s4
I
474 t1
isa— ii—.— ii
I———
iii
ii. *i• tsis.
0
G••t_ — .9
0
•?
to
Cross bedding
bedding directions
directions in
in the
the Jacobsville
Jacobsville Sandstone
Sandstone (after
(after Hamblin,
Hamblin, 1958)
1958)
Paleogeographyduring
during deposition
depositionof
of the
theJccobsville
JacobsvilleSandstone
Sandstone(after
(afterHamblin,
Hamblin, 1958)
19581
Paleogeography
170
170
1
Cflt*N*?IO
V
—
•Iadad
— ..d at
d.(i.IIa
I
tetIa .1 n. - g*.•
•.n k..—.' - • ••—'
I—
4, t%St fr.at
If SpI —a.
Tnqt.....q P•fIi
t
—
— Mtt
•
ft !a.f •—
If
so
I-.
Is
ma'..
Cross
Crossbedding
beddingdirections
directionsininthe
theChapel
ChapelRock
Rockmember
member
ofofthe
theMunising
MunisingFormation
Formation(affer
(afterRamblin,
Hamblin,1958)
1958)
Paleogeography
Paleogeographyof
ofNorthern
NorthernMichigan
Michiganduring
during deposition
deposition of the
the Chapel
ChapelRock
Rockmember
member
958)
ofthe
theMunising
MunisingFormation
Formation(after
(afterHamblin,
Hamblin, 1958)
of
171
171
meandering.
meandering.InInthe
theupper
upperpart
partofofthe
theChapel
ChapelRock
Rockmember,
member,mudcracks
mudcracksare
arefound
foundininthe
thefinefinetextured
cm in
in diameter
diameterand
and the
theintervening
intervening
textured sandstone
sandstoneand
andthe
theshale
shalebeds.
beds. They
Theyare
areabout
about55cm
cracks
cracksare
arefilled
filled with
with sand.
sand. Paleogeographically
Paleogeographicallyand
andenvironmentally,
environmentally,they
theyindicate
indicatedeposition
depositionin
in
aa shallow
shallowwater,
water, subaerial
subaerialenvironment
environmentwith
with the
the sea
sea regressing
regressing northward. These
Thesebeds
bedsgrade
grade
directly
into
the
Miners
Castle
Member
and
suggest
a
minor
regression
of
the
Chapel
Rock
directly into the Miners Castle Member and suggest a minor regression of the Chapel Rocksea.
sea.
Compact
Compact till
till isis found
found overlying
overlying the
the bedrock in this valley. The
Thetill
tillisisreddish
reddishin
incolor,
color,and
and
texturally
ranges
from
clay
to
cobbles.
It
is
likely
Marquette
age
(about
10,000
YBP)
and
was
texturally ranges from clay to cobbles. It is likely Marquette age (about 10,000 YBP) and was
deposited
deposited as
as the
the Munising
Munisingmoraine
moraineand
and Kingston
Kingston outwash
outwash plain were being deposited (Phase I1of
of
thi previous
previousdiagrams).
diagrams).
the
Grand Marais
Marais Harbor
Harbor
Stop 11. Grand
Grand
Grand Marais
Marais Harbor
Harborisisthe
theonly
only Harbor
Harbor of
of Refuge
Refuge between
between Marquette
Marquetteand
and Sault
SaultSt.
St.
Marie.
Native Americans.
Americans. Later,
Marie. Originally,
Originally,the
theprotected
protectedbay
bay was
was aa summer
summer camping place for Native
French
voyageurs
used
the
bay
as
stopping
place
to
either
prepare
for,
or
recuperate
French voyageurs used the bay as stopping place to either prepare for, or recuperate from,
from, the
the
dangerous
dangerouspaddle
paddle along
alongthe
thecliffs
cliffsof
of the
the Pictured
Pictured Rocks
Rocksduring
during their
their travels
travelsalong
alongthe
thesouth
southshore
shore
of
of Lake
Lake Superior.
Superior. The
Thetown
townitself
itselfwas
wasestablished
establishedininthe
the1860's
1860'sby
by the
the fishing
fishing and
and lumber
lumber
industries
industriesas
as aa boom
boom town.
town. However,
However,by
by1910,
1910,the
thelumber
lumberindustry
industrydeclined
declinedand
and Grand
Grand Marais
Marais
became
became mainly
mainly a commercial
commercial fishing
fishing port. Eventually,
Eventually,this
thisindustry
i n d u s e also
also declined
declined and
and the
the
community
community became
became all
all but aa ghost
ghost town. Today,
Today,Grand
Grand Marais
Maraisis
is seeing
seeing aa renaissance
renaissance as
as aa
summer and
and winter
winterrecreation
recreation town.
town.
summer
Recent,
Recent, soon-to-be-published
soon-to-be-publishedresearch
research at
at the
the Drexler-Carter
Drexler-Carter(D-C)
(D-C)site
site(Anderton,
(Anderton,in
in
progress),
fill section
section exposed
exposed along
along the
the front
front of
of the
the eastern
eastern portion
portion of
of the
theGrand
Grand
progress), aa paleo-valley
paleo-valleyfill
Sable
Banks,
suggests
that
prior
to
the
Nipissing
transgression,
the
Grand
Marais
area
may
Sable Banks, suggests that prior to the Nipissing transgression, the Grand Marais area may have
have
consisted
consisted of
of extensive
extensive lagoon.
lagoon. Evidence
Evidencefor
forthe
thelagoon
lagoonconsists
consistsof
of extremely
extremelythick.
thick. Compressed
Compressed
peat
peat deposits,
deposits,dated
datedatatbetween
betweenapproximately
approximately7000-6000
7000-6000BP,
BP, which
which have
have been
been found
foundatatthe
theD-C
D-C
site,
site, on
on the
the floor
floor of
of Grand
Grand Marais
Marais Harbor
Harbor by
by divers,
divers, and
and during
during drilling
drilling of wells
wells to
to the
the east
east of
of the
the
Harbor.
Harbor. The
Thepre-Nipissing
pre-Nipissinglagoon
lagoonwas
waslikely
likelyprotected
protectedbehind
behindananextremely
extremelylarge
largebay
baybarrier,
barrier,
which
which may have
have extended
extended from
from as
as far
far west
west as
as the
the ancestral
ancestral Grand Sable
Sable Banks
Banks or
or Au
Au Sable
Sable Point.
Point.
The
The high
high ground
groundand
andassociated
associateddune
duneforms,
forms,which
which begin
begin at
at Lonesome
LonesomePoint
Point and
andextend
extendto
tothe
the
east for
for several
several kilometers,
kilometers, may
may be
be remnants
remnants of
of this
this older
older barrier,
bamer, which was
was apparently
apparently breached
breached
during rising Nipissing levels. Much
Muchfurther
furtherwork
work isis needed
needed to
to verify
verify these
these interpretations.
interpretations.
The
The Nipissing
Nipissing transgression
transgressioninundated
inundated the
the Grand
Grand Marais
Marais area,
area, flooding
flooding the
the mouth
mouth of
of the
the
Sucker
Sucker River
River and
and extending
extending inland
inland for
for several
several kilometers
kilometers south
south of
of the
the high
high ground
ground that
that begins
begins at
at
Lonesome
Point.
Steep,
Nipissing-age,
wave-cut
bluffs
at
an
elevation
of
about
195
m
(640
ft)
Lonesome
Steep, Nipissing-age, wave-cut bluffs at an elevation of about I95 m (640 ft)
rim the
the southern
southern edges
edges of the town and harbor. Much
Much of
of the
the town
town appears
appears to
to have
have been
been built
built on
on aa
Nipissing wave-cut
wave-cut platform.
platform.
Nipissing
Recently,
Recently, Grand
Grand Marais
Marais Harbor
Harbor has
has been
been the
the focus
focus of a great
great deal of concern by local
local
residents
West Bay.
Bay. From
residents in regard to the condition of West
Fromthe
thetime
time of
of the
the earliest
earliest written
written records
and
and maps
maps of
of Grand
Grand Marais,
Marais,the
the bay
bay was
was originally
originally portrayed as relatively
relatively deep water lagoon,
protected
protected behind
behind aa low,
low,sandy
sandybaymouth
baymouth barrier
barrier that
that extended
extended from
from the
the west
west side
side of
of the
the current
current
town,
town, east
east to
to Lonsome
Lonsome Point.
Point. AAsmall
smallchannel,
channel,which
whichallowed
allowedwater
waterto
to pass
pass from
from the
the bay
bay into
into
the
to the harbor.
harbor. This
the waters
waters of
of lake
lake Superior,
Superior, provided
provided the only navigable entrance to
This natural
natural
channel,
channel, adequate
adequate for
for several
several centuries
centuries for
for Native Americans
Americans and Voyageurs in canoes, was
was
prone to sand
sand bar development,
development, which
which interfered
interfered with commercial shipping
shipping associated
associated with
with the
the
lumber
800ts. Consequently, various harbor
lumber and
and fishing
fishing industries
industries that
that began
began in
in the
the late
late 11800's.
protection
protection structures,
structures, including
includingeventually
eventually aa large
large breakwater, were built beginning
beginning as
as early
early as
as
172
17 2
L
Stop 1
— jt/ioravel Pita
-
et; Grav4f+t I
-. -
'a $k
—
3__'_
RM868
-
—
r -.
3
87O
•1
lIlT H
V-- -.
—
Eastward-slopingmeliwater
meltwaterdischarge
dischargechannels
channels
southofofGrand
Grand
Marais
Easiword-sloping
south
Morals
17 3
1883
1883 to stop sand bars from
from blocking
blocking the
the entrance
entrance to the bay (USCOE,
(USCOE, 1980).
1980). ,
Over
Over the
the years,
years, the
the harbor
harbor protection
protection structures
structureshave
have effectively
effectively blocked
blocked coastallycoastallytransported
transported sand,
sand, that
that would
would have
have otherwise
otherwisebeen
been conveyed
conveyed by
by littoral
littoral drift
drift processes,
processes,from
from
replenishing the eastern portion of the original bamer.
Consequently,this
this part
part of
of the
the barrier
barrier has
has
replenishing
barrier. Consequently,
been removed
removed by
by wave
wave erosion,
erosion, exposing
exposing nearly
nearly the
the entire
entire east
east end
end of the
the bay
bay to
to Lake
Lake Superior.
Superior.
On the west side
side of
of the
the structures,
structures,sand
sand eroded
erodedfrom
from the
the Grand
Grand Sable
Sable Banks
Banks and
and transported
transportedto
to
the
the east
east has
has been
been deposited.
deposited. Since
Since 1961
1961the
the beach
beach in this area has been growing at a rate of
38,000 cubic m per year (USCOE, 1980).
According
(1990),the
the beach
beach has
has widened
widened by
by
1980). Accordingto
to Marsh
Marsh (1990),
2500-3000
2500-3000 m,
m, and,
and,ifif left
leftunchecked,
unchecked,will
willeventually
eventuallywiden
widen to
to the
thepoint
pointwhere
wherethe
thebeach
beach
effectively cuts off wave-contact
wave-contact with the Grand Sable Banks. ifIf this
thishappens,
happens,the
thesand
sandsupply
supplyto
to
effectively
the
the Grand
Grand Sable
SableDunes,
Dunes,which
which isisultimately
ultimately tied
tied to
to lake
lakelevels
levelsand
andbluff
bluff recession,
recession,will
willbe
be
artificially reduce
reduce dune
dune nourishment
nourishment rates. In
Ineffect,
effect,the
theDunes
Dunesmay
may cease
cease to
to exist.
exist.
artificially
Stop
stop 12
12
Stop
Stop 12
12is
is not
not really
really aa stop,
stop, but
but an
an overview
ovewiew of
of channels
channels as
as we
we leave
leave Grand
Grand Marais
Marais
southward
southward on
on M-77.
M-77. Eastward
Eastwarddischarging
dischargingstreams
streamscut
cutchannels
channelsatatsuccessively
successivelylower
lower elevations
elevations
as the
the Marquette
Marquette (Grand
(Grand Marais)
Marais) ice
ice retreated
retreated into the Lake Superior basin.
At least
least 88channels
channels
as
basin. At
are
are recognized
recognized from
from 207
207 meters
meters to 271 meters. Leverett
Leverett(1929)
(1929)and
andothers
othersmistakenly
mistakenlyinterpreted
interpreted
these
these surfaces
surfaces as
as Lake
Lake Algonquin wave-cut platforms. Hughes
Hughes (1968),
(19681,through
through use
use of
of aneroid
aneroid
barometers and
and topographic
topographicmaps,
maps,determined
determined they
they slope
slopeeastward
eastward and
and asserted
assertedthat
that they
theyare
are
barometers
glacial
glacial spillways.
spillways. Further,
Further,Hughes
Hughesrecognized
recognizedthe
thefact
factthat,
that, in
in profile,
profile, the
the surfaces
surfacesactually
actually slope
slope
southward
southward toward
toward the
the next
next escarpment
escarpment(characterized
(characterized by
by poorly
poorly drained
drained swampy
swampyregions
regions at
at the
the
south
theywere
werewave-cut
wave-cutterraces,
terraces,the
the slope
slope in
in profile
profile would
would be
be in
in the
the
south edge
edge of
of the
the surfaces).
surfaces). ifIfthey
opposite
direction.
opposite direction.
174
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