University of Montana
ScholarWorks at University of Montana
Theses, Dissertations, Professional Papers
Graduate School
1982
Magmatic and hydrothermal history of the
Emigrant Gulch igneous complex Park County
Montana
Richard F. Moore
The University of Montana
Follow this and additional works at: http://scholarworks.umt.edu/etd
Recommended Citation
Moore, Richard F., "Magmatic and hydrothermal history of the Emigrant Gulch igneous complex Park County Montana" (1982).
Theses, Dissertations, Professional Papers. Paper 7434.
This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for
inclusion in Theses, Dissertations, Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more
information, please contact [email protected].
COPYRIGHT ACT OF 1976
Th i s
s is t s
.
is
an
un published
m anuscr ipt
Any f u r t h e r r e p r i n t i n g of
in
it s
which
contents
copyright
m us t
be
BY THE AUTHOR.
M ansfield L ib ra ry
U n i v e r s i t y o f Montana
Date
■/
19 8 2
sub­
approved
MAGMATIC AND HYDROTHERMAL HISTORY
OF THE EMIGRANT GULCH IGNEOUS COMPLEX,
PARK COUNTY, MONTANA
by
Richard F. Moore
B .S ., U niversity of Washington, 1979
Presented in p a rtia l f u lf illm e n t o f the
requirements fo r the degree of
Master of Science
UNIVERSITY OF MONTANA
1982
Approved by:
Chairman, Board o^Examiners
Dean, G/iduate School
/
Date
UMI Number: EP38235
All rights reserved
INFORMATION TO ALL USERS
The quality of this reproduction is dependent upon the quality of the copy submitted.
In the unlikely event that the author did not send a complete manuscript
and there are missing pages, these will be noted. Also, if material had to be removed,
a note will indicate the deletion.
UMT
OisMTttttion Publishing
UMI EP38235
Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author.
Microform Edition © ProQuest LLC.
Alt rights reserved. This work is protected against
unauthorized copying under Title 17, United States Code
ProOuesf
ProQuest LLC.
789 East Eisenhower Parkway
P.O. Box 1346
Ann Arbor, Ml 48106- 1346
Dedicated to Bettye and Fred Moore
fo r years o f understanding.
ABSTRACT
Moore, Richard F . , M.S., Summer, 1982
Geology
Magmatic and Hydrothermal History of the Emigrant Gulch Igneous
Complex, Park County, Montana
D ire c to r:
Donald W. Hyndma
The Emigrant Gulch igneous complex is a composite quartz monzonite
stock which intruded andésite breccias of the Absaroka Volcanics
Supergroup. These rocks have been dated at 53 m illio n years to 45
m illio n years old, and l i e on the northwestern margin of the Beartooth
Mountains, Montana.
Extrusive and in tru s iv e a c t i v i t y a t Emigrant Gulch may be s im ila r
to a c t i v i t y a t other igneous centers in the Absaroka-Gall a t i n province.
In ad dition to Emigrant Gulch, andésite country rocks form vent
complexes at Independence, Montana; Sepulcher Mountain, Montana and
other lo cations.
I t is d i f f i c u l t , however, to conclude th at the
andésites in Emigrant Gulch originated from an actual stratovolcano.
Five d is t in c t periods of quartz monzonite porphyry intrusion
followed emplacement of a rhyodacite stock in Emigrant Gulch. The
d i s t r ib u t io n , shape, and composition of phenocrysts in the quartz
monzonite su ite suggest th at these magmas were derived by tapping
magma from progressively deeper levels of a high-level magma chamber.
An e a rly and a la te in tru s iv e series o f quartz monzonites were
mapped, with each series probably representing d is t in c t periods of
magma chamber f i l l i n g , d i f f e r e n t ia t io n , and draining.
Chemical and pétrographie data also suggest th a t magma d iffe r e n ­
t i a t i o n took place by v o l a t i l e d iffu s io n , a process th a t enriched
s i l i c a and other ' f e l s i c ' elements in the upper part of the chamber.
Magma d i f f e r e n t ia t io n also caused accumulation of an active
hydrothermal phase associated with the la te in tru sive series.
In
ad dition to severe a rg il l i e and s e r i c i t i c a lt e r a t io n , th is hydrothermal phase also triggered brecciation and molybdenum m ineralizatio n
following emplacement o f quartz porphyry. The bulk of th is molyb­
denum m in eralizatio n occurs in the matrix of the A llison breccia,
but quartz-molybdenite stockworks may be present below or adjacent
to the A llis o n breccia along a north-south trend. The molybdenum
deposit at Emigrant Gulch has more ch aracteristics of molybdenum
deposits found in western Canada such as Boss Mountain, than of
Climax-type deposits.
n
ACKNOWLEDGMENTS
I would lik e to sincerely thank Drs. Donald Hyndman, Ian Lange,
and Keith Osterheld o f the U niversity of Montana fo r th e ir c r i t i c a l
review of the ideas presented in the thesis.
The study was o r ig in a lly
made possible by Joseph McAleer, Regional Manager o f Molycorp,
Incorporated, Spokane, Washington, and Charles Lee, of Tucson, Arizona,
who encouraged me to do the thesis on the Emigrant Gulch property.
Steve Castor o f Molycorp, Incorporated, Spokane Washington, and
Robert Leonardson o f Molycorp, Incorporated, Questa, New Mexico, offered
much d ire c tio n and in s ig h t.
Molycorp, Incorporated provided lo g is tic a l
support during the summer o f 1981, and also provided a Union Oil Research
Foundation Grant fo r ad ditional chemistry, two potassium/argon age
dates and thesis expenses fo r the following academic year.
Brian Smith
o f Union Oil Research in Brea, C a lifo r n ia , and R. T. Van Zandt, Vice
President, Union Oil Company o f C a lifo rn ia Foundation, Los Angeles,
C a lifo r n ia approved the grant and offered much guidance.
I would also l i k e to thank S h irley Pettersen who typed the fin a l
d r a f t , and to Laurie Emmart and Ruth Duff who drafted the geologic maps,
cross sections, and many o f the diagrams.
Bruce Davis and Kathy Tureck
assisted in preparing rock samples fo r geochemical analysis and age
dating.
Potassium/Argon age dating was conducted through Brian Smith,
and lab orato ries at the U niversity o f C a lifo rn ia a t San Diego.
Special
thanks go to Susan Bloomfield fo r countless hours o f patience, help,
and encouragement.
11 i
TABLE OF CONTENTS
Page
ABSTRACT........................................................................................................
ACKNOWLEDGMENTS
......................................................................................
ii
iii
LIST OF TABLES AND P L A T E S ..................................................................... v ii
LIST OF F IG U R E S ........................................................................................v i i i
CHAPTER
I.
II.
INTRODUCTION .............................................................................
1
Purpose
...............................................................................
1
Method
...............................................................................
7
REGIONAL GEOLOGICAL OVERVIEW ...........................................
9
...................................................................
9
S t r u c t u r e ...........................................................................
11
Age Data
...........................................................................
18
GEOLOGY AND PETROGRAPHY...................................................
22
....................................
22
......................................................
27
S tratigraphy
III.
Absaroka Volcanics HostRocks
Rhyodacite Porphyry
Quartz Monzonite Suite
.................................................
31
Early In tru s iv e Series ..............................................
41
Early quartz monzonite ..........................................
42
Hornblende porphyry
..............................................
43
Late In tru s iv e Series
..............................................
44
......................................................
45
Quartz porphyry
iv
CHAPTER
Page
Plagioclase porphyry ......................................................
46
Late p o r p h y r y ........................................................................48
Summary............................................................................................ 49
IV.
V.
............................................................
52
Physical Parameters ..............................................................
53
Cooling Model ...........................................................................
55
Reaction Textures ..................................................................
63
CHEMISTRY AND DIFFERENTIATION ...............................................
67
MAGMA CRYSTALLIZATION
C h e m is try ........................................................................................ 67
Major and Minor O x i d e s ....................................................... 68
Trace E l e m e n t s ........................................................................74
V I.
V II.
D iff e r e n tia tio n .......................................................................
79
......................
91
Review of Hydrothermal History...... ....................................
91
HYDROTHERMAL HISTORY AND DIFFERENTIATION
CONCLUSION..........................................................................................98
REFERENCES CITED
..........................................................................................
99
APPENDICES
1.
X-ray analyses o f potassium feldspar and
groundmass minerals ................................................................... 105
2-1.
Mass balance calculations o f CaO contents from
plagioclase phenocrysts in Emigrant Gulch
quartz monzonites
................................................................... 117
2-2.
Mass balance calculations of SiOgcontents
from phenocrysts in Emigrant Gulch
quartz monzonites
................................................................... 120
V
APPENDICES
3.
4-1.
4 -2 .
5-1.
5-2.
Page
Stokes' Law s e ttlin g times fo r plagioclase
phenocrysts in Emigrant Gulch quartz monzonites . . .
121
Summary of petrography o f Emigrant Gulch
quartz monzonites .......................................................................
124
Modal compositions of Emigrant Gulch
quartz monzonites .......................................................................
142
Chemical analyses of Emigrant Gulch
in tru s iv e rocks ...........................................................................
148
Normative compositions o f Emigrant Gulch
in tru s iv e rocks ...........................................................................
152
VI
LIST OF TABLES
TABLE
Page
1.
Summary o f regional fa u ltin g and other
stru ctu ra l features ....................................................................... 12
2.
Summary o f age data pertaining to Emigrant
Gulch r o c k s .......................................................................................... 20
3.
C la s s ific a tio n o f andésite breccias in
Emigrant Gulch ............................................................................... 25
4.
Summary o f in tru s iv e and hydrothermal history
in Emigrant G u lc h ..............................................................................92
LIST OF PLATES
Page
PLATE
la .
Geologic map of Emigrant Gulch .....................
lb .
Geologic map showing location of
breccias in Emigrant Gulch .............................
2a2d.
Geologic cross sections
.................................
vn
LIST OF FIGURES
FIGURE
la .
lb .
2.
3.
Page
Generalized regional geology map of the
Absaroka-Gall a t i n volcanic province ..................................
3
Generalized stru ctu ra l geology o f the
Absaroka-Gall a t i n volcanic province
..............................
5
Location of study
a r e a .........................................................6
Modal compositions and c la s s ific a tio n of
Emigrant Gulch in tru s iv e rocks ...........................................
23
4a4c. Hand specimen and th in section photographs
o f rhyodacite porphyry ............................................................ 30
5a5b. Hand specimen and thin section photographs
of e a rly quartz monzonite ........................................................ 34
6a6d. Hand specimen and thin section photographs
of hornblende p o r p h y r y
34, 36
7a7b. Hand specimen and thin section photographs of
quartz p o rp h y ry
36, 38
8a8b. Hand specimen and th in section photographs
of plagioclase porphyry ............................................................ 38
9a9c.
1Oa10b.
11.
12a.
Hand specimen and thin section photographs
of la t e p o r p h y r y
Hand specimen photographs of the A llison breccia
38, 40
. . 40
Plagioclase-orthoclase binary phase diagram ...................
Temperature -
54
^ diagram fo r synthetic
quartz m onzonite^m elt..................................................................58
vi i i
FIGURE
12b.
Page
Pressure -
g diagram fo r synthetic quartz
monzonite m e l ? ................................................................................ 58
13a.
13b.
Id e a lize d cross section o f quartz monzonite
pluton a f t e r 1,250 years of cooling .............................
60
Id e a lize d cross section of quartz monzonite
pluton a f t e r 12,500 years of cooling .....................
60
. .
14.
Quartz-orthoclase-plagioclase ternary diagram
15.
A n o rth ite -a lb ite -o rth o c la s e ternary diagram
16.
S ilic a v a ria tio n diagrams fo r oxides in Emigrant
Gulch quartz monzonites ......................................................
70
S ilic a v a ria tio n diagrams fo r average contents
of oxides in Emigrant Gulch quartz monzonites
. . .
71
Peacock a lk a li- lim e index and s i l i c a va ria tio n
diagrams fo r average contents o f trace elements
in Emigrant Gulch quartz monzonites .............................
76
Average contents o f trace elements versus
potassium in Emigrant Gulch quartz monzonites
78
17.
18.
19.
. . .
. . . .
. . .
62
65
20.
Element enrichment trends in la te in tru siv e
series rocks and the Bishop T u f f ...........................................83
21.
Element r a tio enrichment trends in the la te in tru siv e
series and the Bishop T u f f ....................................................... 85
22.
Diagram showing stru ctu ra l state of potassium
feldspar from Emigrant Gulch rocks .................................
108
23.
Diagram showing Or content o f potassium
_
feldspar from Emigrant Gulch rocks, based on 201
peak r e f l e c t i o n s .......................................................................... 110
24.
Typical X-ray pattern o f groundmass of
Emigrant Gulch porphyry ......................................................
IX
114
CHAPTER I
INTRODUCTION
The Emigrant Gulch igneous complex is located 45 kilometers south
of Livingston, Montana ( f i g s . l a , l b ) , and consists of m ultiple quartz
monzonite intrusions which cut a rhyodacite stock and Eocene rocks of
the Absaroka-Gall a t i n volcanic province.
The in tru siv e rocks form
irregularly-shap ed stocks and anastomozing dikes on steep talus covered
slopes two kilometers east of Emigrant Peak (F ig . 2 ).
These rocks, and
t h e i r andesitic host rocks represent an in tru s iv e -e x tru s iv e center
s im ila r to other igneous centers through the Absaroka-Gall a t i n province.
Magmatic h isto ry a t Emigrant Gulch is complex, as the rhyodacite
stock was intruded by at lea st fiv e d if f e r e n t quartz monzonite porphyries,
Recurring periods of hydrothermal a lt e r a tio n also accompanied emplacement
o f these rocks.
Several stages of brecciation and su lfid e m ineralization
are associated with hydrothermal a lt e r a tio n .
Purpose
This study describes the igneous and hydrothermal history in d e tail
in an e f f o r t to understand the re la tio n s h ip between m ineralization and
magmatism.
This has not been well understood even though su lfid e and
precious metal deposits have been explored in Emigrant Gulch fo r over
100 years.
The present study delineates in tru s iv e and hydrothermal
events not previously recognized.
1
Figure la .
Generalized geologic map of the AbsarokaGal la t in volcanic province o f northwestern
Wyoming and southcentral Montana. Map
shows location of Emigrant Gulch study
area and other lo c a lit ie s discussed in
te x t. A fter Smedes and Prostka (1972).
iiY o o
' no' oo' '
.STUDY AREA MAP ON FIGURE 2
M IL L CREEK
ARROW PEAK
■INDEPENDENCE
4 5 ' oo'
SEPULCHER
MOUNTAIN
YELLOWSTONE
PARK
r*
4 4 '0 0
0
25
50
KILOMETERS
THOROFARE CREEK GROUP
ABSAROKA
VOLCANIC
SUPERGROUP
SUNLIGHT GROUP
43 00
WASHBURN GROUP
FIGURE 1 a
Figure lb.
Generalized geologic map showing stru ctu ral
features of the Absaroka-Gallatin volcanic
province. CS: Cherry Creek - Squaw Creek
f a u lt ; SP: Spanish Peaks f a u lt ; DC: Deep
Creek fa u lt ; MC: M ill Creek f a u lt ; RC:
Reese Creek f a u lt ; MH: Mol Heron f a u lt ;
G: Gardiner f a u lt ; CL: Cooke City lineament;
R: Rattlesnake f a u lt . A fte r Smedes and
Prostka (1972), and Shaver (1974).
'i i r 00'
110“ 00
FAULTS
REVERSE
: NORMAL
vSTUDY AREA MAP ON FIGURE 2
MC
45*0 0 '
YELLOWSTONE
PARK
15
I
44' oo'.
'50
KILOMETERS
THOROFARE CREEK GROUP
ABSAROKA
VOLCANIC
SUPERGROUP
SUNLIGHT GROUP
o
43 00
WASHBURN GROUP
FIGURE 1 b
/
lllMgW IM Bl
US 93
YELLOWSTONE RIVER
EMIGRANT CREEK
FRIDLEY CREEK
EMIGRANT PEAK
GOLD P R IZ E CREEK
EMIGRANT
GULCH
, STUDY AREA
S I X MIL E
CREEK
KILOMETERS
FIGURE
Figure 2.
2
Map showing location of Emigrant Gulch
study area.
Textural and compositional features in the quartz monzonites are
used to support a model proposed fo r d if f e r e n t ia t io n of these rocks.
This model explains contrasting petrography between d iffe r e n t rocks and
c l a r i f i e s the h isto ry of in tru sio n .
Hydrothermal a c t i v i t y , including
s u lfid e m in eralizatio n can be tie d to processes th a t generated the quartz
monzonite magmas.
Method
A ll major rock types in th is study were id e n t ifie d from mapping of
f iv e square kilometers in upper Emigrant Gulch (1 inch = 400 fe e t or
1 centimeter = 48 meters, Plate l a ) .
D r i l l core from ten diamond d r i l l
holes (Med 1-9,11 ) confirmed crosscutting relationships between rock types,
and revealed numerous lit h o lo g ie va riants.
Plates 2a, 2b, and 2c show
subsurface geology in fe rre d from these holes.
Core from d r i l l holes
Med-9 (below 76 meters), and Amax-1 could not be obtained, although in ­
formation from previous d r i l l
sections.
logs was used to construct geologic cross
The Med holes were d r i l l e d by Duval Corporation between 1972
and 1976, the Amax hole was d r i l l e d by American Metals Climax, Incor­
porated, in 1963.
Thirty-one rock-chip samples were analyzed fo r major, minor, and trace
elements including SiO^, TiOg, AlgO^, FegO^, FeO, MnO, MgO, CaO, Na^O,
KgO, PgOg, Rb, Sr, and F (Appendix 5 - 1 ) .
Bondar-Clegg and Company, L td .,
(Vancouver, B r itis h Columbia) conducted the analyses using atomic absorbtion. X-ray fluorescence, and c o lo rim etric techniques.
covered a l l major rock types.
Sampling
8
Two potassium/argon age dates were intended to bracket a ll in tru siv e
and hydrothermal events in the suite of quartz monzonites.
Determinations
used b i o t i t e mineral separates, and s e r ic it e from a whole-rock sample.
San Diego State U niversity performed the analyses.
Mapping, pétrographie work, and geochemistry produced three main con­
clusions about igneous a c t i v i t y in the complex:
1)
The rhyodacite stock and quartz monzonites intruded an
an desitic vent complex composed mainly o f flow and vent
breccias.
I t is d i f f i c u l t to conclude th a t these vent
breccias originated from a stratovolcano since structural
features normally associated with stratocones are absent
a t Emigrant Gulch.
2)
The most important periods of s u lfid e m ineralizatio n occurred
following emplacement of the rhyodacite and during intrusion
of the quartz monzonites.
brec cia tio n .
Both periods also involved
Most o f the observable molybdenum m in e ra li­
zation is closely associated with quartz monzonite in ­
trusion and occurs as breccia m atrix.
3)
Textural and compositional trends in the quartz monzonites
support d if f e r e n t ia t io n by v o l a t i l e d iffu s io n , with magmas
d i f f e r e n t ia t in g in a small, near surface chamber.
The
nature and timing of hydrothermal a c t i v i t y also agree with
th is scheme.
CHAPTER I I
REGIONAL GEOLOGIC OVERVIEW
Andesitic eruptions in the northwestern part o f the AbsarokaG a lla tin volcanic province produced abundant flows and breccias during
the middle Eocene.
Intrusion o f stocks, dikes, and s i l l s followed the
volcanic a c t i v i t y at Emigrant Gulch and other centers in the province.
D is trib u tio n o f these extrusive centers may have been controlled by
major northwest-trending fa u lts .
However, dike emplacement w ithin the
Emigrant Gulch complex appears to be controlled by j o i n t sets in the
Precambrian basement rather than la rg e r fa u lts .
Stratigraphy
Numerous studies on T e rtia r y volcanic rocks in the Absaroka-Gallatin
province yielded lengthy summaries oF regional stratigraphy and petro­
graphy (Hague and others, 1899; Covering, 1929; Parsons, 1958; Wilson,
1963; Chadwick, 1964, 1969; Smedes and Prostka, 1972; Wedow and others,
1975; E l l i o t and others, 1977).
Smedes and Prostka (1972) revised e a r l i e r volcanic stratigraphy
(Hague and others, 1899) and divided extrusive and reworked andesitic
sediments in to Washburn, Sunlight and Thorofare Creek Groups (F ig . 1).
The older Washburn Group rocks characterize the northwestern part of the
province, with the younger Sunlight and Thorofare Creek rocks found to
the southeast.
The study also divided these groups in to vent and
9
10
a l l u v i a l faciès from the terminology o f Dickinson (1968).
a llu v ia l
The
facies represents the reworked, sedimentary equivalent of the
igneous-vent fa cies .
In Emigrant Gulch, andésites comprise a vent facies of the Washburn
Group volcanics.
In the northern G a lla tin Range, ea rly to middle Eocene
Golmeyer Creek and H y a lite Peak volcanics are contemporaneous with
the Washburn Group rocks (Smedes and Prostka, 1972; Shaver, 1978, Fig. 2 ).
In th is way, volcanic rocks in Emigrant Gulch are referred to as e ith e r
Washburn or Golmeyer Creek volcanics,although younger Sunlight Group rocks
may outcrop on cirque headwalls to the southwest of the map area (Pfau,
1981; E l l i o t and others, 1977).
Individual centers of igneous a c t i v i t y in the province were studied
by Emmons (1908), Covering (1929), Parsons (1939), Krushenski (1962),
Wilson (1963), McMannis and Chadwick (1964), Chadwick (1966), Casella
(1967), Schultz (1968), Ruppel (1969), Rubel (1971), Love (1972), and
Fisher (1972).
Origin of magmas, sty le of emplacement, and d e ta ils of
Plutonic a c t i v i t y are generally unclear.
In the Emigrant Gulch complex, Basler (1965) described Golmeyer
Creek volcanic rocks and s p l i t the younger in tru s iv e rocks into two dacite
groups.
Shaver (1974, 1978) remapped the area in his study of dacites
in the region and proposed a lower crust or upper mantle o rig in fo r the
magmas.
E l l i o t and others (1977) discussed the resource potential fo r
molybdenum and other minerals.
Pfau (1981) fu rth e r divided the su ite of
in tru s iv e quartz monzonites and described m in e ra liza tio n .
The current
n
study revised Pfau's c la s s ific a tio n and c l a r i f i e d the sequence of
magmatic and hydrothermal events.
Structure
Previous work on regional fa u ltin g and stru ctu ral trends allows
speculation about tectonic control over emplacement of Emigrant Gulch
prophyries.
Prominent northeast- and northwest-trending fractures in
the Precambrian basement underlying Emigrant Gulch were probably the
primary control over near surface channeling of magma.
Major reverse
fa u lts peripheral to the study area may have been more important during
magma migration at deeper crustal levels (Fig. l b ) .
Table 1 summarizes the nature and timing of stru ctu ral a c t i v i t y in
the region.
Major northwest-trending reverse fa u lts were apparently active
during the Laramide u p l i f t of the Beartooth block.
is s im ila r to other 'foreland-type'
The Beartooth block
basement blocks such as the Wind River
Mountains, which were u p lift e d in la t e s t Cretaceous to e a rly T e r tia r y time.
McMannis and Chadwick (1964) cited major northwest-trending reverse fa u lts
as probably the most important featu re lo c a liz in g Eocene igneous a c t i v i t y
in the province (F ig . l b ) .
Shaver (1974) reviewed this idea in d e t a i l ,
demonstrating th a t volcanic and in tru s iv e centers are aligned along the
projected trends o f the Cherry Creek-Squaw Creek and Spanish Peaks f a u lt s .
These trends define two su b-p arallel belts of volcanic and in tru s iv e com­
plexes extending to the southeast under the Absaroka plateau.
little
There is
d ir e c t evidence, however, to draw a d ir e c t relatio n sh ip between
Table 1.
Summary o f f a u ltin g and other stru c tu ra l features of the Absaroka G a lla tin volcanic province
Fault or
stru ctu ral
feature
NW-trending
structures
Source
Squaw Creek
Fault
McMannis
and
Chadwi ck
1964
Cook City
1ineament
Foose
and
others
1961
Spanish Peak
1ineament
Gardi ner
fa u lt
Sense of
movement
high-angle
reverse
Timing
p o s t-la te
Cretaceous
to
e a rly Eocene
s tra tig ra p h ie
o ffs e t
<910m to
>610 m near
Independence
s tra tig ra p h ie
o ffs e t ?
p o s t-la te
Cretaceous
to
ve ry -ea rly
Eocene
4120 m dip
si ip
s tra tig ra p h ie
o ffs e t
3050 m
s tra tig ra p h ie
o ffs e t
p o s t-la te
Cretaceous
to
pre-middle
Eocene
>3050 m dip
s lip
thickness of
Mesozoic sedi­
ments from
Beartooth block
120 m dip
s lip
displaced
Pliocene basalt
?
highangle
reverse
Fraser
and
others,
1969
highangle
reverse
Comments
>1370 m
"Laramide" to
McMannis
and
Chadwick,
1964
H a ll, 1961
O ffset
Quaternary
reac tiva tio n
ro
Table 1 (Continued)
Fault or
stru ctu ra l
feature
NW-trending
structures
N-trending
structures
Mi 11 Creek
fa u lt
(W-trending)
Source
Sense of
movement
Timing
O ffset
<13 km?
Comments
Ruppel,
1972
Wilson,
C.W..
1934
Reid and
others
1975
l e f t la t e r a l
( Precambrian)
high-angle
reverse
Precambri an
"Larami de"
to midEocene
Wilson,
J .T .,
1936
high-angle
reverse to
high-angle
normal
mainly prevolcanic
>1220 m
s tra tig ra p h ie
displacement ?
NW-trending
fractures in
northern
Snowy block
Shaver,
1974
Reid and
others
1975
relax atio n
features?
Precambrian
?
related to Pre­
cambri an deformational events
Mol-Heron
f a u lt
(E. G a lla tin
fa u lt)
Ruppel,
1972
normal
p o st-early
Eocene to preP1iocene?
(mainly postEocene?)
610 m- 910 m
s tra tig ra p h ie
o ffs e t
post-Pliocene
3 m - 10 m
o ffs e t volcanic ro
Shaver,
1974
450m1,100 m
o f fs e t Pre­
cambri an units
s tra tig ra p h i c
displacement
Table 1 (Continued)
Fault or
stru ctu ra l
featu re
Reese Creek
fa u lt
(W. G a lla tin
fa u lt)
Source
Fraser and
others,
1969
Sense o f
movement
normal
Ruppel,
1972
Mammoth
fa u lt
Fraser
and
oth ers,
1969
Ruppel,
1972
Timing
Comments
post-Cretaceous
to
p re -e a rly
Eocene
460 m - 1310 m
s tra tig ra p h ie
o ffs e t with less
than 500' o f
post-Laramide
movement
Quaternary
re a c tiv a tio n
minor
o ffs e t
Quaternary
deposits?
460 m - 1310 m
greater inferred
displacement of
Eocene volcanic
rocks
p o s t-la te Cre­
taceous, possible
l a t e r movement?
460 m - 610 m
s tra tig ra p h ie
o ffs e t
mainly postEocene
>610 m
s tra tig ra p h ie
o ffs e t
mainly postEocene
normal
O ffset
Table 1 (Continued)
NE-trending
structures
F a u lt or
stru ctu ra l
feature
Source
Deep Creek
fa u lt
Horberg,
1940
Sense of
movement
normal
Timing
mainly Larami de
with in te r m itte n t
Cenozoic move­
ment
O ffset
>1520 m
Quaternary
reac tiva tio n
Comments
covered by Suce
Creek-Window
overthrust to the
N; geomorphic
evidence
Offset alluvium
Chadwick,
1969
some Eocene
tiltin g
changing dip
10°) of Eocene
volcanic rocks
Foose and
others,
1961
Paleocene to
Eocene
major u p l i f t of
Beartooth block
Boni ni and
others,
1972
Reid and
others,
1975
5490 m to
6100 m
r ig h t la t e r a l
(Precambrian)
Precambrian
normal
post-Late Cretaceous
to
Paleocene
6 Km
400 m to
830 m
O ffset of Precambrian units
s tra tig ra p h ie o ffs e t
Table 1 (Continued)
Fault or
s tru ctu ra l
feature
Source
Sense of
movement
Timing
O ffset
Quaternary
re a c tiv a tio n
Luccock Park
fa u lt
E-NE-trending
fractures in
northern
Snowy block
Reid and
others,
1975
Reid and
o thers,
1975
r ig h t la t e r a l
(PreCambrian)
Precambrian
normal
post-Laramide ?
extension
and/or
shear ?
Precambrian ?
Comments
o ffs e t Holocene
deposi ts
6 Km
o ffs e t of Pre­
cambrian units
related to Pre­
cambrian deformational events
CT»
17
these fa u lts and igneous a c t i v i t y (see also Basler, 1965; Chadwick,
1968a, 1970; Fraser and others, 1969; Fisher, 1972).
J. T. Wilson (1936) also proposed th at the M ill Creek f a u l t con­
t r o l l e d lo c a liz a tio n of a west-trending strin g o f volcanic vents and
m ineralized in tru sio n s.
Presence o f mappable shear zones and offsets
which predate Eocene igneous a c t i v i t y suggests a s im ila r relation ship
along the Cooke C ity lineament (Parsons, 1958; Foose and others, 1961;
Rubel, 1971).
J. T. Wilson (1936) and E l l i o t and others (1977) claimed
the in tersec tio n of these two f a u l t zones influenced magma lo c a liz a tio n
near M ill Creek, 11 kilometers northeast o f Emigrant Gulch.
In Emigrant Gulch i t is d i f f i c u l t to demonstrate control of porphyry
emplacement by any of these major structures.
In ferred jo in ts and fr a c ­
tures in the Precambrian basement would provide a more s u ita b le , open
system of conduits fo r upper crustal d is tr ib u tio n of magmas in Emigrant
Gulch.
R. Burnham (1982, pers. comm.) mapped prominent northeast-trending
fractures in Archean rocks near Six Mile Creek, 6 kilometers west of
Emigrant Gulch (F ig , 2 ) .
Reid and others (1975) documented both northeast-
and northwest-trending basement fractures north o f M ill Creek.
Dike trends
in Emigrant Gulch closely p a r a lle l these fracture trends as expected i f
the fractures underly the in tru s iv e complex and were the dominant struc­
tu ra l control during emplacement.
Conversely, the Squaw Creek and Spanish Peak fa u lts may have provided
a zone of weakness deep in the crust where magmas collected and migrated
upward.
Minimum v e rtic a l displacement of these fa u lts is 1500 and 3000
meters, resp ectively (Table 2 ).
Seismic data from the Wind River Range
18
support th is idea (Smithson and others, 1979).
The study found reverse
fa u ltin g during Laramide u p l i f t of the Wind River block occurred on
structures th a t extend to at le a s t 24 kilometers depth.
Reverse fa u ltin g
on the Squaw Creek and Spanish Peaks f a u l t may well be analogous.
Age Data
Age data on rocks w ithin or related to rocks in Emigrant Gulch sug­
gest a maximum time in te rv a l of 8 m illio n years between andesitic volcanism and hydrothermal a lt e r a tio n which terminated igneous a c t i v i t y .
These
dates bracket in tru s iv e a c t i v i t y between 53 m illio n years (Washburn time)
and about 45 m illio n years (Sunlight tim e).
Table 2 shows radiometric
age determinations, and ages from normal remnant magnetism (NRM).
Two new potassium/argon age dates from th is study were intended to
bracket a l l
s u ite .
in tru s iv e and hydrothermal events in the quartz monzonite
In the e a r li e s t porphyry dated, however, e ith e r potassium loss
from b io t i t e or in flu x of argon yielded an age which predates the volcanic
host and is c le a r ly unreasonable.
The rock was not v is ib ly a lte re d a l ­
though some minor a lte r a tio n of b i o t i t e to c h lo r ite occurred.
Recent
weathering and groundwater c irc u la tio n probably f a c i l i t a t e d potassium loss,
as potassium was exchanged fo r other cations such as calcium, iro n , and
magnesium.
Reactions such as th is commonly convert b i o t i t e to c h lo r ite
and clay minerals during weathering, as discussed fu rth e r in the section
on chemistry.
Even minute potassium loss from b i o t i t e can s ig n if ic a n t ly
a l t e r the age determination (B. Smith, pers. comm., 1982).
19
The potassium/argon date from the la te s t porphyry suggests hydrothermal a c t i v i t y which produced abundant s e r ic it e ceased p rio r to 45
m illio n years ago.
High atmospheric argon content of this rock decreased
precision of the estimate.
Future age determinations in Emigrant Gulch
should involve fis s io n -tra c k dating in zircons, since this method is
less susceptible to hydrothermal a lt e r a tio n .
20
Table 2.
Age data fo r rocks w ithin or related to rocks of the
Emigrant Gulch complex
Age
Rock Unit
Method
Source
Comments
5 4 + 9 m.y.
la te
porphyry
K/Ar,
whole
rock
This
report
rock extremely a l ­
tered, high atmos­
pheric argon; places
fin a l s e r i c i t i c a l ­
te ra tio n event in
Emigrant Gulch at
>45 m.y.
65 +^ 5 m.y.
(inaccurate)
e a rly
quartz
monzon­
ite
K/Ar,
b i o t it e
This
report
recent K loss l i k e l y
due to a r g i l l i c
weathering; or in flu x
of Ar during hydrother­
mal a lte r a tio n
49.0 +
1.7 m.y.
dacite
stock in
lower M ill
southeast
side of
Arrow
Peak
K/Ar,
b i o t it e
Chadwi ck,
1967
probably related to
rocks of the ea rly in ­
trusive series in
Emigrant Gulch
49.5 +
1.5 m.y,
Big Creek
stock
K/Ar,
b io tite
Obradovich,
1968
(in:
Chadwick,
1969)
K/Ar,
b io ti te
53.5
2.3 m.y
andésite
10 k i l o ­
meters
east of
Emigrant
Peak
la te
Washburn
to e a rly
Sunlight
time
group A
NRM
and pos­
s ib ly
group B
dacites of
Basler (1965)
in Emigrant
Gulch complex
cuts H yalite Peak vo lcanics, no close pétro­
graphie equivalent in
Emigrant Gulch
Chadwick,
1967
H ya lite Peak volcanics
Shearer,
1974, 1978
group A equivalent to
rhyodacite stock,
group B equivalent to
some of the l a t e r quartz
monzonites, th is report
21
Table 2 (Continued)
Age
Rock Unit
la t e - e a r ly
Eocene to
e a r ly middle
Eocene
Fortress
Mountain
member of
Sepulcher
formation
to Mount
Wallace
formation
o f Sun­
li g h t Group
Method
s t r a t ig r a ­
phie and
paléontolo­
gie
Source
Smedes and
Prostka, 1972
Comments
CHAPTER I I I
GEOLOGY AND PETROGRAPHY
The description of Emigrant Gulch geology in th is section es­
tablishes the c r y s t a ll iz a t io n history o f these rocks.
This history
includes hydrothermal events th a t produced su lfid e m in eralizatio n .
A
model fo r generation of the quartz monzonite magmas is also introduced
in l i g h t of pétrographie evidence.
The next section focuses on th is
evidence in more d e ta il to explain processes taking place during magma
c r y s t a ll iz a t io n .
Table 3 and Appendix 4 l i s t pétrographie features of the andésite
country rock and in tru s iv e lith o lo g ie s in Emigrant Gulch by rock type.
The andésites are brecciated flows and vent breccias o f the MiddleEocene Washburn Group.
In tru siv e rocks consist of a rhyodacite stock
intruded by a suite of fiv e quartz monzonite porphyries (Plate l a ) .
C la s s ific a tio n of these rocks used the tr a d itio n a l system shown on
Figure 3 (Nockolds, 1954).
Absaroka Volcanics Host Rocks
In Emigrant Gulch, Basler (1965) mapped mono- and h e te r o lith o lo gic and unbrecciated flows which are probably equivalent to ea rly to
middle-Eocene Washburn Group volcanics (Smedes and Prostka, 1972).
In
the northern G a lla tin Mountains, these rocks are contemporaneous with
Golmeyer Creek volcanics (Smedes and Prostka, 1972; Chadwick, 1969).
22
23
QUARTZ
QUARTZ
D IO R IT E
GRANITE
SYENITE
• OX
QUARTZ
MONZONITE
MONZONITE
GABBRO/
DIORITE
GRANO- '
DIORITE
SYENO, D IO R IT E
POTASSIUM
FELDSPAR
Figure 3
PLAGIOCLASE
FELDSPAR
■
LATE PORPHYRY
A
PLAGIOCLASE PORPHYRY
A
QUARTZ PORPHYRY
O
HORNBLENDE PORPHYRY
#
EARLY QUARTZ MONZONITE
X
RHYODACITE PORPHYRY
MODAL COMPOSITIONS OF EMIGRANT GULCH
ROCKS ( a f t e r NOCKOLDS, 1 9 5 4 ) .
INTRUSIVE
24
Andésites in Emigrant Gulch comprise a vent facies (Dickinson,
1968) and lack s ig n ific a n t pyro clastic m a te ria l.
show l i t t l e a l l u v i a l reworking.
The volcanic rocks
Parsons (1960, 1969) lis t e d c r i t e r i a
f o r c la s s ify in g Absaroka volcanic breccias used fo r Emigrant Gulch an­
désites in th is study (Table 3 ).
Monolithogic breccias most l i k e l y
represent flows broken up by in ternal f r i c t i o n , or possibly vent breccias.
H e te ro lith o lo g ic breccias probably resulted from brecciation of semi­
so lid magma in vents or fractures with subsequent extrusion as breccia
flows or s o l i d i f i c a t i o n w ithin the vents.
Both breccia types grade into
each other and unbrecciated flows.
Other in te rp re ta tio n s of the Emigrant Gulch breccias as lahars or
even pyro cla stic flows (Shaver, 1974; Pfau, 1981) do not f i t the c r i t e r i a
l is t e d by Parsons as well
(Table 3 ).
dication of bedding or so rting .
Massive outcrops also show no in ­
I n i t i a l dips of the flows are d i f f i c u l t
to determine and the source vents hard to recognize.
For these reasons
the o rig in a l d is tr ib u tio n of volcanic vents around Emigrant Gulch remains
uncertain.
Although th is area has been refe rred to as a stratovolcano vent com­
plex (Pfau, 1981), the following observations make this conclusions tenuous
1)
Abundant pyro clastic material i n t e r s t r a t i f i e d with lava
is not present.
2)
Basler (1965) claimed a crude cone sheet and radial dike swarm
e x is t in Emigrant Gulch.
with stratocones.
Such features are commonly associated
The present study found no evidence sup­
porting th is assertio n, and dike o rie n ta tio n appears to be
Table 3. Comparison o f Emigrant Gulch volcanic breccias with flow and vent breccias, lah ars, and ash-flows
____________________________ ________________ ________________ C r i t e r ia from Parsons (1960, 1969)_____________
Character­
is t ic s
Type o f breccia
Emigrant Gulch
volcanic breccias
Typical autobrecciated
lava flows, brecciated
vent facies
Vent or in tru s iv e brec­
c ia s , flow breccias
very near source_______
lahars
ash-flow and
re la te d pyroc la s t ic rocks
monolithologic
h e te ro lith o lo g ic
h e te ro lithologic
h e te r o lith o ­
logic
unsorted
unsorted
crude
grading,
poor sor­
ting
crude grading
of l i t h i c fra g ­
ments, reverse
grading of
pumice fra g ­
ments
n o n s tra tifie d
nonstrati fie d
n o n s tra tifie d
subroundedangular
generally angular,
dense and nonve sicular
sub-roundedangular, porp h y r itic with
dense groundmass,
pumice absent
breccia
type
monolithologic
and hetero­
lit h o lo g ic
sorting
f a i r - poor
bedding
fragments
poor-ex­
c e lle n t
s tra ti f i cation , in ­
dividual beds
commonly 5-30
meters thick
dense, rare
pumice, crude
lin e a tio n
common
nonstrati fie d
unless
welded
ro
cn
Table 3 (Continued)
Character­
is t ic s
Type o f breccia
Emigrant Gulch
volcanic breccias
Typical autobrecciated Vent or in tru s iv e breclava flows, brecciated cias flow breccias
vent facies
________ very near source
groundmass
ap h an itic, dense,
non-vesicular,
no pumice
fin e-g ra in e d fra g ­
mental or mag­
mat i c
fin e-g ra in e d fra g ­
mental or magmatic
c la s tic ,
with sand
and clay
comments
breccia types
gradational
commonly not part
of recognizable
cone
core structure may
or may not be
recognizable
interbedded
with stream
deposi t s ,
gradational
into lava
flows
lahars
ash-flow and
re la te d pyro­
c la s t ic rocks
abundant, recog­
nizable pumice
fragments
r\3
cr*
27
co n tro lled by su b-p arallel basement fractures rather than
volcano-related structures.
3)
No la rg e , central conduit suggestive of a stratovolcanic
vent has been recognized in Emigrant Gulch.
In a d d itio n , few vent complexes elsewhere in the Absaroka-Gallatin vo l­
canic province e x h ib it features which would c le a r ly c la s s ify them as
stratovolcanos.
A notable exception is the Independence complex, 40
kilometers southeast of Emigrant Gulch, where Rubel (1971) id e n t ifie d
cone sheets cuttin g abundant pyroclastic m a te ria l.
Stratovolcanic a c t i v i t y in the province then, may be r e s tric te d to a
few large centers li k e Independence connected by ir r e g u la r , nonpyro clastic vent complexes such as Emigrant Gulch.
Rhyodacite Porphyry
A roughly c ir c u la r stock of rhyodacite porphyry 3-4 kilometers in
diameter intruded the Absaroka volcanics (P late l a ) . The un it extends at
le a s t 76 meters below the c o lla r of Amax d r i l l
hole 1 in upper Emigrant
Gulch (P late l a ) . The rhyodacite porphyry apparently intruded as an ex­
tremely high-level pluton ex h ib itin g very uniform quenching.
Very fin e
s i l i c a laminae up to 1-2 m illim eters wide characterize the rock, and at
f i r s t glance suggest the rock may have been an extrusive flow.
Contorted
and wavy s i l i c a banding also lo c a lly produced textures s im ila r to those
seen in welded t u f f s .
Pfau (1981), however, noted Washburn group roof
pendants on the stock, and X-ray analyses o f potassium feldspar confirmed
28
only the presence of orthoclase and not sanidine (Appendix 1).
This
supports emplacement as a hypabyssal stock rather than a subaerial flow.
The magma contained enough hydrothermal f lu id to promote moderate groundmass d e v i t r i f i c a t i o n and quartz flooding during the la t e s t stages of con­
s o lid a tio n .
Auto-brecciation occurred where f l u i d movement severely
contorted the s i l i c a - r i c h laminae, with zones of massive quartz replacing
and flooding fractured rhyodacite.
Auto-brecciated and flow-banded rhyodacite porphyry commonly grades
into non-laminated v a r ie t ie s .
and in a l l d r i l l
holes.
Clear gradations are seen both in the f ie ld
In te n s ity of flow banding increases in the southern
p art o f the map area, near the contact with volcanic country rocks.
This
suggests th a t hydrothermal a c t i v i t y and m ineralization are localized near
the borders o f the stock.
Elsewhere, flow banding and auto-brecciation
are e r r a t ic and do not c o rre la te with location or depth within the stock.
Color of s i l i c a - r i c h groundmass and flow laminae varies from dark bluepurple to tan.
Peculiar pétrographie features include the predominance of orthoclase
and absence o f quartz in the phenocryst mineralogy.
The orthoclase is
unusually high in potassium (Org^ - Or^^) and shows no exsolution.
Ortho­
clase of th is composition is very uncommon in igneous rocks (Carmichael
and others, 1974).
This may in dicate a comagmatic o rig in with la t e r quartz
monzonites, as the l a t t e r rocks also contain orthoclase which is extremely
ric h in potassium.
The very equigranular, m icro crys tallin e groundmass with
anhedral grain boundaries probably resulted from the d e v it r if i c a t io n of an
o r i g i n a l l y aphanitic groundmass.
Figure 4a.
S ilica-lam in ated rhyodacite
porphyry. Note p a ra lle l a lig n ­
ment o f feldspar phenocrysts and
s i l i c a laminae (0 .5 -1 .5 m illim ete rs )
Hand specimen from d r i l l hole
Med - 10.
Figure 4c.
Photomicrograph of s ilic a -la m in a te d
rhyodacite porphyry (low power,
crossed polars; f i e l d of view is
2.2 X 2.3 m illim e te rs ). Note
p a ra lle l alignment o f orthoclase
phenocrysts (K) and s i l i c a laminae
(S). Groundmass: (G). Pervasive
a r g i l l i c a lt e r a tio n .
Figure 4b.
Auto-brecciated rhyo­
dacite porphyry. S i l i ca­
laminée have become con­
to rte d , and have rotated
pieces of rhyodacite. Hand
specimen from southern ridge
of map area (P late l a ) .
Figure 4a
— I—
1
1
4
I
Figure 4b
4
Cm.
Cm.
Figure 4c
CO
o
31
F a ir ly prevasive a r g i l l i c and minor s e r i c i t i c a lte r a tio n persists
near more intense areas of quartz flooding.
Chalcopyrite and molybdenite
are i r r e g u l a r ly d is trib u te d in these more intensely altered areas, and
base-metal v e in le ts occur in lo calize d patches.
coalesce and fu rth e r brecciate the a lte re d rock.
The base-metal veinlets
Minor potassic a lte r a tio n
in orthoclase ve in le ts up to two m illim eters wide occurs near the forks
o f Emigrant Creek in otherwise fresh rhyodacite, and less commonly in
d r i l l core.
The bulk of th is hydrothermal a c t iv i t y preceded intrusion of
the quartz monzonite su ite (Figs. 4a ,4 b ,4 c ).
Quartz Monzonite Suite
A quartz monzonite suite with fiv e d iffe r e n t rock types intruded the
consolidated rhyodacite stock.
Igneous and hydrothermal history of the
quartz monzinite suite is complex, producing numerous lith o lo g ie variants
and several periods of a lt e r a t io n .
Variations in phenocryst d is tr ib u tio n ,
s iz e , and shape, and rock composition r e f le c t changing magma composition,
pressure, and temperature during c r y s t a ll iz a t io n .
These data point to two
d i s t in c t episodes of d i f f e r e n t ia t io n in the magma chamber, dividing the
su ite in to an e a rly and a la t e in tru s iv e series.
Modal compositions
(Appendix,4 - 2 ) , visual and te x tu ra l differences (Appendix 4 - 1 ) , and cross­
cuttin g relation ship s distinguish the rock types in the s u ite.
Rocks
from the e a rly in tru s iv e series have c h a ra c te ris tic textural and compo­
s it io n a l features which distinguish them from the la t e series.
D iffe re n t
members of the su ite vary in s i l i c a content by less than 3.5 weight per­
cent, and can e a s ily be confused because of visual and compositional
32
s im ila rity .
The e a r l i e r series include emplacement of ea rly quartz
monzonite and hornblende porphyry.
A period o f quiescence was followed
by intrusion of quartz porphyry, plagioclase porphyry, and late
porphyry in the la te in tru s iv e series.
Magmas from both e a rly and la te series may have d iffe r e n tia te d
via v o l a t i l e d iffu s io n o f chemical constituents in a parent magma sim ila r
in composition to ea rly quartz monzonite.
Convection in a r e la t iv e ly
small, near-surface magma chamber presumably enhanced th is process.
Cessation of in tru s iv e a c t i v i t y following emplacement of the ea rly series
allowed time fo r the magma chamber to r e f i l l and r e d if fe r e n tia t e .
Members o f the la te in tru s iv e series represent the s i l i c i c end products
of th is second period of d i f f e r e n t ia t io n .
F ield and d r i l l
core relationships (Plates 1-2) show that younger
members o f the suite generally intruded pe rip h e ra lly to older, s o l i d i ­
fie d plutons.
The older rocks such as ea rly quartz monzonite apparently
formed r i g i d plugs, forcing the younger members to intrude progressively
fa r th e r from the center of the complex.
This explains the paucity of
cross-cutting relationships in Emigrant Gulch.
A few s i l l - l i k e bodies,
dikes and mineralized veins intruded discordant to s i l i c a laminae in the
rhyodacite porphyry.
Petrographically the su ite is characterized by the dominance of
plagioclase phenocrysts with normal zoning, or with no zoning at a l l .
Very minor reverse zoning is r e s tric te d to plagioclase from the ea rly
in tru s iv e series (Fig. 6d).
Orthoclase from every member o f the suite is extremely potassic (Org^
to Orgg) and did not exsolve a lb it e .
Carmichael and others (1974,
Figure 5a.
Early quartz monzonite.
Hand specimen from d r i l l
hole Med - 1.
Figure 5b.
Photomicrograph o f
e a rly quartz monzonite
Oow power, crossed
polars; f i e l d of view
is 3.3 X 2 .3 m illim e te rs )
Note r e l a t i v e l y coarse
groundmass composed o f
mainly orthoclase and
quartz.
Plagioclase: P,
orthoclase: K, quartz:
Q, b i o t i t e : B, G: ground­
mass. Sample from
d r i 11 hole Med - 1.
Figure 6a.
Hornblende porphyry.
Hand speciman from d r i l l
hole Med - 3.
Figure 6b.
Photomicrograph of horn­
blende porphyry (low power,
crossed polars; f i e l d of
view is 3.3 x 2.3 m i l l i ­
meters). Extremely em­
bayed quartz phenocryst
surrounded by fine-g rained
orthoclase. Q: quartz,
B: b i o t i t e , G: groundmass.
Figure 5a
1
4
Cm.
Figure 5b
Figure 6a
I
o
T
r
1
1
Cm.
Figure 6b
Figure 6c.
Photomicrograph o f hornblende
porphyry (low power, crossed polars;
f i e l d of view is 3.3 x 2.3 m illim eters)
Rapakivi rim on orthoclase phenocryst.
Note intergrown plagioclase, quartz,
and orthoclase on rim. Plagioclase
is in optical c o n tin u ity, near ex­
t in c tio n . Orthoclase core: K,
Rapakivi rim: R, groundmass; G.
Figure 6d.
Photomicrograph of horn­
blende porphyry (low power,
crossed polars; f i e l d of
view is 3.3 x 2.3 m i l l i ­
meters). Plagioclase
phenocryst with reverse
zoning. Plagioclase: P,
groundmass: G.
Figure 7a.
Quartz porphyry. Hand speciman from
south of lower Allison Tunnel
(Plate l a ) .
Quartz phenocrysts
ci rcled.
Figure 7b.
Photomicrograph of quartz
porphyry (low power, crossed
polars; f i e l d of view is
3.3 X 2 .3 millimeters).
Note p lagio clase, b i o t i t e ,
and orthoclase, b i o t i t e ,
alte re d to clay minerals
and s e r i c i t e .
Note embayment o f quartz phenocryst.
Plagioclase: P, B io t it e :
8, Orthoclase: K. Quartz:
Q, groundmass: G.
Figure 6c
Figure 6d
Figure 7a
T
r
1
Cm,
Figure 7b
co
en
Figure 8a.
Plagioclase porphyry. Hand
speciman from southern ridge
o f map area (Plate la ) . Note
abundant, w hite, fine-grained
plagioclase phenocrysts.
Figure 8b.
Photomicrograph of plagio­
clase porphyry (low power,
crossed polars*, f i e l d of
view is 3.3 x 2.3 m i l l i ­
meters). Note abundant
dark-gray plagioclase pheno­
crysts alte re d to lo w -b iré ­
frin g e n t clays. Plagioclase:
P, Quartz: Q.
Figure 9a.
Late porphyry. Orthoclase
phenocryst with rapakivi rim
c ir c le d . Hand speciman from
dike north o f the lower
A llison Tunnel (Plate la ).
Figure 9b.
Photomicrograph of la te
porphyry (low power, crossed
polars; f i e l d of view is 3.3 x
2.3 m illim e te r s ). Extreme
clay and s e r i c i t i c a lt e r a t io n .
Note paucity o f quartz pheno­
crysts . Plagioclase: P,
B io t ite : B.
Figure 8a
1
Cm.
Figure 8b
Figure 9a
m
I
o
Cm.
Figure 9b
CO
00
Figure 9c.
Figure 10a.
Photomicrograph of la te porphyry
(low power, crossed polars; f i e l d
of view is 3.3 x 2.3 m illim e te rs ).
Rapakivi rim on small, remnant
orthoclase core. Note extreme
clay and s e r i c i t i c a lte r a tio n of
plagioclase in rapakivi rim.
K: orthoclase core, R: rapakivi
rim, G: groundmass.
Block of A llison breccia (25 centimeters
across) showing molybdenite occurring
with quartz as breccia matrix (dark
c o lo r ). Lighter-colored fragments
are s e r i c i t i c a l l y a lte re d and
si 1ic i fie d .
Figure 10b.
Smaller fragments of
A llison breccia
s im ila r to sample
shown in Figure 10a.
Figure 9c
Figure 10a
Cm.
Figure 10b
o
41
p. 231) claim orthoclase more potassic than Or^g is rare in g ra n itic
r h y o l i t i c rocks where subsolidus r e c r y s ta lliz a tio n of the orthoclase
has not taken place.
No evidence was seen fo r subsolidus orthoclase
r e c r y s t a ll iz a t io n in any Emigrant Gulch rocks.
Also, T i l l i n g (1968)
found th a t the Rader Creek pluton, a quartz monzonite stock in southcentral Montana, contains orthoclase which is Org^ and higher, and that
no subsolidus r e c r y s t a lliz a t io n has taken place.
In addition, Whitney
(1975a) c le a r ly showed orthoclase phenocryst composition from synthetic
melts quenched from 700° Celsius and two kilobars can reach OrgQ and
higher.
Other general features include resorbed quartz phenocrysts and
rapakivi rims on orthoclase phenocrysts which are common in many of the
rocks.
Within a single rock type, phenocryst d is trib u tio n and groundmass
quenching are commonly ir r e g u la r .
These textures are an in teg ral part of
the proposed c r y s t a lliz a t io n model presented below.
Early In tru s iv e Series
The e a rly in tru s iv e series began with intrusion of ea rly quartz mon­
zonite followed by hornblende porphyry.
a compositionally zoned magma chamber.
These magmas l i k e l y formed in
Both rock types have s im ila r
compositions, in d ic a tin g a lim ite d amount of d if f e r e n t ia t io n kept both
magmas close to the composition of the parent magma.
Textural differences
point to derivation o f each magma a t d if f e r e n t depths in the chamber.
Textural differences presumably developed in response to pressure and
temperature gradients between the upper and lower parts o f the magma chamber
42
V o la t ile d iffu s io n would have produced r e la t iv e ly f e ls ic magma in the
upper portion of the chamber, as s i l i c a , sodium, rubidium, and other
' f e l s i c ' elements migrated roofward.
Early quartz monzonite represents
th is f e l s i c magma whereas hornblende porphyry is more mafic, and probably
formed a t g reater depths.
Mafic magmas generated a t these greater depths
would be expected to be depleted in such elements.
Early quartz monzonite.
Early quartz monzonite forms generally small,
p o rp h y ritic stocks, ir r e g u la r plutons, but ra re ly dikes (P late l a ) .
Poorly
developed jo in t in g in massive outcrops gives the rock a very g ra n itic
appearance (Figs. 5a ,5b ).
less-commonly aphan itic.
Groundmass ranges from fine-grained s e r ia te , to
Phenocryst content in th is u n it is greater than
in any other porphyry in the su ite (Appendix 4 ).
Early quartz monzonite
contains some pegmatite dikes and lenses which appear to be absent from
other members of the s u ite .
Thin sections of e a rly quartz monzonite contain more hornblende and
b i o t i t e than rocks from the l a t e r se rie s.
Abundant mafic minerals in this
rock and in hornblende porphyry distinguish members of the early series
from subsequent porphyries which are d e fic ie n t in hornblende.
Plagioclase
phenocryst composition averages An^^ and quartz phenocrysts show only
minimal resorption.
Quartz and potassium-rich orthoclase dominate the
groundmass, as in a l l members o f the quartz monzonite su ite.
Orthoclase
phenocrysts lack rapakivi rims, but granophyre-like replacements occur on
sutured borders of quartz and orthoclase phenocrysts.
These granophyre-
l i k e borders consist of intergrown quartz and orthoclase, with no apparent
myrmekitic textu res.
Early quartz monzonite has a coarser groundmass than
43
any other rock in the complex, with grains up to two m illim ete rs .
Variants with a fin e r-g ra in e d groundmass are completely gradational with
coarser v a r ie t ie s , as noted near the bottom of d r i l l hole Med-1 (Figs. 5a,
5b).
Feathery b i o t i t e replaced hornblende, probably during post-magmatic
consolidation.
Hornblende porphyry.
Intrusion of hornblende porphyry magma followed
emplacement o f e a rly quartz monzonite.
Since hornblende porphyry is more
mafic than e a rly quartz monzonite, v o l a t i l e diffusion apparently did not
produce renewed compositional zonation in ris in g hornblende porphyry
magma.
For th is reason, hornblende porphyry retains textu ral and compo­
s itio n a l features of magma which may have been generated deeper in the
chamber.
Hornblende porphyry dikes c h a r a c t e r is tic a lly have an ap hanitic, darkgreen groundmass and conspicuous hornblende phenocrysts.
Dikes commonly
outcrop w ithin 250 meters o f the margins of e a rly quartz monzonite plutons
(P la te l a ) . Dikes with darker borders up to ten centimeters wide are
exposed along the A llison Tunnel t r a i l and in the creek bed in upper
Emigrant Gulch.
Compositionally, hornblende porphyry is the most mafic
member of the quartz monzonite su ite (F ig . 6a).
Scarce quartz phenocrysts are ir r e g u la r ly d is trib u te d in hornblende
porphyry.
Quartz phenocrysts ra re ly exceed three percent of the rock and
grains are extremely embayed (F ig . 6b).
Orthoclase phenocrysts are even
less common and e x h ib it rapakivi rims (Fig. 6c).
Calcium content in
plagioclase phenocrysts consistently reaches An^^ or higher, commonly with
minor o s c illa t o r y zoning.
44
The paucity of quartz and orthoclase phenocrysts suggests th at horn­
blende porphyry magma d iffe r e n tia t e d deep in the magma chamber, where
pressures and temperatures were too high fo r these minerals to c r y s t a ll iz e .
R e la tiv e ly high calcium content of plagioclase also suggests c r y s ta lliz a tio n
began a t depth in the chamber a t r e la t iv e ly high temperatures.
The few
quartz and orthoclase phenocrysts probably began c r y s ta lliz in g as magma
rose to higher le v e ls .
Disequilibrium between these minerals and the melt
may have been caused by changing pressure and temperature during ascent of
the magma.
The minerals reacted with the m elt, with quartz resorbed and
orthoclase mantled by rapakivi rims.
These phenocryst textures contrast with
those seen in e a rly quartz monzonite where quartz and orthoclase are more
common and show l i t t l e
in dication of disequilibrium with the melt.
clase is r e l a t i v e l y sodic.
Plagio­
This is expected i f e a rly quartz monzonite magma
was derived a t shallower depth under lower pressures and temperatures.
Late In tru s iv e Series
Following emplacement o f the ea rly in tru s iv e series, the magma chamber
presumably r e f i l l e d with parent magma s im ila r in composition to ea rly
quartz monzonite.
The magma d iffe r e n tia t e d to produce members of the
la te in tru s iv e series.
In tru s iv e histo ry o f the la te series is much more complex than in the
e a rly s e rie s .
Rocks in the la te series are a l l more s i l i c i c than ea rly
quartz monzonite or hornblende porphyry.
This may indicate th a t d i f ­
fe r e n tia tio n proceeded to a greater degree in the la te s e rie s, and that
mafic magmas o rig in a tin g deeper in the chamber underwent renewed d i f ­
fe r e n t ia t io n as they ascended.
Draining o f the chamber may have been
45
discontinuous, allowing time fo r r e d if fe r e n tia t io n of the r is in g , mafic
magmas.
This contrasts with the e a rly s e rie s , where magmas appear to
have intruded in a s in g le , continuous event without r e d if fe r e n tia t io n .
Quartz porphyry.
D iff e r e n tia tio n in the la te in tru siv e series would
presumably have generated a s i l i c a - r i c h , f e ls ic cap atop the magma
chamber.
This magma is represented by quartz porphyry, the i n i t i a l phase
o f the la t e s e rie s .
Quartz porphyry dikes contain d is tin c tiv e quartz
phenocrysts, and a creamy-tan aphanitic groundmass (Fig. 7a).
cross-cut hornblende porphyry dikes in upper Emigrant Gulch.
The dikes
Quartz
porphyry also intrudes the ea rly quartz monzonite stock on the main ridge
in the center of the map area (P late la ) .
Dikes coalesce north of this
main rid g e , forming irregularly-shaped plutons.
Fresh outcrops can be
found to the north, 210 meters from the end o f the
upper d r i l l
road.
Intru sio n o f quartz porphyry dikes closely coincides with a major
period of molybdenum m in e ra liz a tio n , intense a r g il l i e and s e r i c i t i c a l ­
t e r a t io n , and b rec cia tio n .
This a c t i v i t y would be expected as hydro-
thermal flu id s evolved from quartz porphyry magma in higher levels of the
magma chamber.
The magmatic system also generated many quartz-porphyry
variants th a t have an a p l i t i c groundmass.
These variants grade into a
younger rock type with abundant plagioclase phenocrysts.
Severely resorbed quartz phenocrysts in quartz porphyry have sutured
boundaries where replaced by groundmass (F ig. 7b).
Orthoclase borders
also show the same type o f replacement, producing in c ip ie n t granophyric
te x tu re s .
Rapakivi rims replace some borders as w e ll.
Changing pressures
and temperatures in the roof zone o f the chamber might produce these
46
reaction textures by causing disequilibrium between these minerals and
the m elt.
Plagioclase between Ang^ and Angy probably resulted from the
r e l a t i v e l y h ig h - s ilic a content of quartz porphyry magma and lower tem­
peratures in the roof zone.
Thin section study reveals b i o t it e altered to muscovite and c h lo r ite ,
with accompanying r u t i l e .
This a lt e r a tio n is diagnostic of quartz porphyry
and other members of the la t e in tru s iv e series.
Severe a r g il l i e and
s e r i c i t i c a lt e r a t io n makes quartz porphyry e a s ily confused with other a l ­
tered rock types in the quartz monzonite suite (F ig. 7b).
The severe a l ­
te ra tio n in quartz porphyry records intense hydrothermal a c t i v i t y in the
la t e in tru s iv e series.
Quartz porphyry variants and plagioclase porphyry.
Depletion of
v o l a t i l e - r i c h porphyry magma from the top o f the chamber would have allowed
magma o rig in a tin g a t s l i g h t l y deeper levels to migrate upward.
Quartz
porphyry variants and plagioclase porphyry represent such magmas.
Such
rocks occur in r e l a t i v e l y small amounts, commonly adjacent to quartz
porphyry (P late l a ) . The gradational nature of these rock types indicates
a magmatic continuum, with la t e r phases emplaced from magmas o rig in a tin g
progressively deeper in the chamber.
units is also complex.
The nature o f contacts between these
Plagioclase porphyry crosscuts the e a r l i e r phases
near the la s t switchback of the d r i l l road on the southernmost ridge of
the map area (P late l a ) . Good examples of gradational contacts and i n t e r ­
mediate types are found between 120 and 190 meters depth in d r i l l hole
Med-3.
47
Two generations of plagioclase phenocrysts in plagioclase porphyry
record c r y s t a ll iz a t io n at d i f f e r e n t depths.
Most phenocrysts occur as
numerous, fin e -g ra in e d laths which give the rock i t s peculiar texture
(Figs. 8 a ,8 b ).
Larger plagioclase phenocrysts are not as abundant, are
unzoned, and have compositionsreaching An^g.
The la r g e r, calcic phenocrysts
possibly c r y s ta lliz e d a t deeper, hotter levels of the chamber whereas the
smaller laths grew a f t e r the magma rose to the roof zone.
The smaller
laths are probably more sodium-rich than larger phenocrysts, although clay
and s e r i c i te a lt e r a tio n makes estimation of calcium content and degree of
zoning speculative.
Orthoclase phenocrysts have well-developed rapakivi
rims, and quartz phenocrysts are extremely embayed.
A v o l a t i l e phase would be expected to have accumulated in the roof zone
of the magma chamber as v o l a t i l e d iffu s io n enriched the s i l i c a content of
r is in g , mafic magmas.
Resorbed quartz and rapakivi rims are evidence
o f changing pressure and temperature during accumulation o f the v o la t i le
phase.
Presence of an active v o l a t i l e phase during intrusion of quartz
porphyry variants and plagioclase porphyry is also suggested by the
a p l i t i c textu re o f many of these rocks.
Water, flu o r in e , and other con­
s titu e n ts in the v o l a t i l e phase would tend to i n h ib i t nucléation of mineral
grains, producing a sugary, fin e-g rained a p l i t i c groundmass according to
Hyndman (unpub. manusc., 1982, p. 153).
The v o l a t i l e phase also apparently
induced progressively more intense a lte r a tio n in the quartz porphyry
variants and plagioclase porphyry.
48
Hydrothermal a c t i v i t y resulted in the mostpronounced
and molybdenum m in eralizatio n in Emigrant Gulch.
brecciation
The bulk of brecciation
probably took place during plagioclase porphyry in tru sio n , as many of
the brecciated zones contain fragments of quartz porphyry and plagioclase
porphyry (P late l b ) .
A few molybdenite stockworks cut these breccias,
but most m in e ra liza tio n occurs in the matrix of the breccias with intense
s ilic ific a tio n .
Brecciation around the A llison Tunnel i l lu s t r a t e s this
s ty le of m in eralizatio n (Figs. 10a,10b).
Late porphyry.
The fin a l stage of magmatism in the la te in tru sive
series occurred with intrusion of la t e porphyry dikes.
These dikes cross­
cut a l l e a r l i e r phases but r a r e ly intrude the la rg e r plutons in the center
of the map area (P late l a ) . Late porphyry therefore forms the most p e r i­
pheral phase of the quartz monzonite s u ite , emplaced l a t e r a l l y to the
e a r lie r , consolidated plutons.
One-half kilometer north o f the map area on steep c l i f f s , are two
separate stages of la t e porphyry dikes.
The younger la te porphyry dike
cuts an older dike, but rocks from each event are t e x tu r a lly and com­
p o s itio n a lly id e n t ic a l.
These outcrops, li k e other la te porphyry rocks,
commonly appear flaggy, with pronounced j o i n t surfaces p a ra lle l to dike
margins.
Some v a r ie it ie s of la te porphyry resemble quartz porphyry w ith ­
out quartz phenocrysts (Figs.
9a ,9b ).
The dike
the map area il l u s t r a t e s th is
(P late l a ) .
along the main ridge of
R e la tiv e ly c a lc ic plagioclase phenocrysts in la t e porphyry suggest
th a t magma began c r y s t a ll iz in g at deeper, hotter parts of the same magma
chamber which generated quartz porphyry and plagioclase porphyry.
49
Plagioclase co nsistently yielded values of An^Q to An^^ despite pervasive
clay and s e r i c i te a lt e r a t io n .
Other phenocryst textures imply marked disequilibrium with the ris in g
magma.
Uncommon orthoclase phenocrysts almost in variab ly have rapakivi
rims, and some aggregates contain no orthoclase cores, or only small,
anhedral remnants (F ig . 9c ).
Severely embayed quartz phenocrysts com­
monly have sutured quartz overgrowths which are o p tic a lly continuous with
the surrounded c r y s ta l.
Extreme a r g il l i e to s e r i c i t i c a lte r a tio n t y p ifie s la te porphyry and
fresh rock could not be found (F ig. 9b),
S e r ic ite in v a ria b ly replaces
feldspars and b i o t i t e .
Summary
The summary below describes sequential events in the in tru siv e history
of the Emigrant Gulch quartz monzonite s u ite .
The discussion above
describes pétrographie features of the suite and introduces the idea
th a t rock types originated from magmas at d if f e r e n t depths of a magma
chamber.
Pétrographie evidence supporting th is idea is reviewed in the
next section, using th e o re tic a l aspects from a magma c r y s t a lliz a t io n model
by Whitney (1975b).
1)
Parental magma s im ila r in composition to ea rly quartz mon­
zonite f i l l e d a small magma chamber w ith in 6 kilometers of
the surface.
Magma probably represented a high level
apophysis o f a deeper pluton.
V ariations in pressure
between one and two k ilo b a rs , and l i qui dus
temperatures
50
between 900° and 700° Celsius resulted in a zonal d i s t r i ­
bution of phenocrysts in the chamber.
2)
Early quartz monzonite magma d iffe r e n tia t e d a t higher levels
of the chamber, c r y s t a lliz in g unresorbed quartz, and
orthoclase lacking rapakivi rims.
Hornblende porphyry magma
was generated deeper in the chamber.
3)
Meager q u an titie s o f v o l a t i l e flu id s accumulated in the roof zone,
in s u f f ic ie n t to cause pressure fluctuations and phenocryst d is­
equilibrium in e a rly quartz monzonite magma.
little
Consequently,
a lt e r a t io n , m in e ra liz a tio n , brecciation or groundmass
quenching accompanied in tru s io n .
Both ea rly quartz monzonite
and hornblende porphyry magmas were expelled from the chamber
in a s in g le , continuous event.
Scattered quartz and ortho­
clase phenocrysts c r y s ta lliz e d in hornblende porphyry magma
during ascent and reacted with the melt to form resorbed rims
on quartz, and rapakivi rims on orthoclase.
This disequilibrium
was apparently caused by decreasing pressure and temperature,
along with increased water content in the magma.
Emplacement
of e a rly quartz monzonite and hornblende porphyries terminated
the e a rly in tru s iv e series.
4)
The chamber r e f i l l e d with parental magma.
Physical gradients
again produced d i f f e r e n t stable phenocryst assemblages at
d i f f e r e n t depths in the chamber.
Quartz porphyry magma formed
towards the top o f the chamber with a coexisting v o l a t i l e phase.
51
Accumulating v o la tile s in the roof zone caused increases
in pressure which induced resorption o f quartz pheno­
crysts and rapakivi rims on quartz porphyry and
plagioclase porphyry.
5)
V o la t ile degassing provided the driving force fo r quartz
porphyry in tru s io n , trig g e rin g severe a rg il l i e and
s e r i c i t i c a l t e r a t io n , molybdenum deposition, and
brecciation (Figs. 10a,10b).
Plagioclase porphyry
magma replaced quartz porphyry magma atop the chamber,
as in tru sio n continued.
Progressively more intense
v o l a t i l e a c t i v i t y alte re d and brecciated intruding
phases, many o f which were a p lit ic - q u a r t z porphyry
v a ria n ts .
Plagioclase porphyry shows phenocryst-
reaction textures s im ila r to quartz porphyry, l i k e l y as
a r e s u lt of hydrothermal processes in the roof zone.
However abundant, r e l a t i v e l y ca lc ic plagioclase pheno­
c rys ts, and few orthoclase phenocrysts imply th at the
magma began c r y s t a ll iz in g at deeper, hotter lev els .
6)
Emplacement of la t e porphyry terminated the la t e in tru siv e
se rie s.
Phenocryst minerals include ca lc ic plagioclase
with uncommon quartz and orthoclase.
Phenocrysts show
reaction textures analogous to those in hornblende
porphyry, most l i k e l y due to magmatic disequilibrium
during ascent.
V o la t ile a c t i v i t y during la te porphyry
emplacement was s t i l l severe enough to cause pervasive
s e r i c i t i c a lt e r a t io n , but l i t t l e
brecciation or s u lfid e
CHAPTER IV
MAGMA CRYSTALLIZATION
This section presents theoretical aspects of magma c r y s ta lliz a tio n
from a cooling model proposed by Whitney (1975b).
Whitney (1975b) studied
an experimental system using synthetic quartz monzonite melts s im ila r
in composition to Emigrant Gulch porphyries.
According to this model,
pressure and temperature gradients in a r e la t iv e ly small magma chamber
w i l l s t a b il iz e d i f f e r e n t phenocryst assemblages a t d iffe r e n t depths.
This zonation implies a sequential order of phenocryst c r y s ta lliz a tio n
as temperature and pressure decrease roofward in the chamber.
The order
o f phenocryst c r y s t a lliz a t io n in ferred from inclusion relationships in
Emigrant Gulch rocks is compatible with this cooling model.
The o rig in of reaction textures in phenocrysts is also discussed,
with reference to experimental studies and phase diagrams th at explain
diseq u ilib riu m between phenocrysts and the surrounding magma.
Such dis­
equilib rium presumably occurs with rapidly changing pressure, tempera­
tu re , and water content in the magma.
E ither rapid ascent of magma
towards the top of the chamber or buildup and release of v o la tile s in the
roof zone might cause these severe fluctu ations in pressure and tem­
perature.
52
53
Physical Parameters
Emigrant Gulch magmas presumably d iffe r e n tia t e d from a parent magma
which ascended high into the continental crust.
The level to which this
magma rose c r i t i c a l l y determined pressure and temperature gradients,
and the amount o f dissolved water during d if f e r e n t ia t io n .
Phenocrysts
probably began c r y s t a ll iz in g in a magma chamber a t depths no greater
than fiv e to six kilom eters, as most porphyries have a quenched,
aphanitic groundmass.
This depth corresponds to less than two kilobars
l i t h o s t a t i c pressure, with a maximum of four to six weight percent
dissolved w ater, as shown by Hyndman (unpub. manusc., 1982, p. 135-136)
and Brown (1970, p. 356).
At these conditions magma c r y s ta lliz a tio n
would terminate near 700° Celsius, according to experimental work on
synthetic quartz monzonites compositionally s im ila r to Emigrant Gulch
porphyries (Whitney, 1975a).
The presence of both orthoclase and plagioclase phenocrysts in
Emigrant Gulch rocks provides another way o f estimating solidus tem­
perature.
Phase diagrams on Figures 11 and 14 demonstrate th a t members
of the quartz monzonite su ite c r y s ta lliz e d a t sub-sol vus temperatures.
This means th a t feldspars formed as discrete potassic and sodic phases
rath er than a s in g le , homogenous a l k a li feldspar.
Temperatures of the
solvus-solidus boundary on the binary diagram in Figure 11, and the
ternary e u te c tic on Figure 14 agree with the solidus temperature of about
700° Celsius.
54
MELT
T (*C)
LIQUIDUS
EUTECTIC
SOLIDUS
K -r ic h
700 -
FELDSPAR
SOLVUS
PLAGIOCLASE
FELDSPAR
PERTHITE
Qr
PLAGIOCLASE
(AN25)
Fig u re 11.
spars
A p p r o x i m a t e phase r e l a t i o n s h i p s
CRYSTALLIZING AT ABOUT 0 . 5
PLAGIOCLASE SIMILAR
in
KILOBARS (H 2O)
fe ld ­
FOR
IN COMPOSITION TO PLAGIOCLASE IN
Em i g r a n t Gu l c h rocks
(AN2 5 ) .
Two
f eld spa r s^ potassium
RICH FELDSPAR AND PLAGIOCLASE WILL CRYSTALLIZE AT THE
EUTECTIC.
gram
D i a g r a m r e p r e s e n t s s e c t i o n of t e r n a r y d i a ­
ON f i g u r e
14,
PLAGIOCLASE J O I N ,
DRAWN PARALLEL TO THE ORTHOCLASEAND NEAR THE TERNARY MINIMUM.
A f t e r Hy nd ma n
(1982,
unpub.
manusc. ,
p
. 310)
55
Solidus temperature could have been lowered by an estimated,
ad ditional 35° Celsius by the presence of flu o rin e in the magmas
(B a ile y , 1977).
In experimental systems studied by Von Platen and
Winkler (1961), and Von Platen (1965) a granite melt with 0.5 molar
d is­
solved flu o r in e and saturated in water a t two kilobars s o lid if ie d 35°
Celsius lower than the same melt without flu o rin e .
Fluorine concentrations
in Emigrant Gulch rocks are much lower, however, and 35° Celsius li k e l y
represents the maximum temperature depression of the solidus.
Chemically, flu o r in e in h ib its lin kin g of s i l i c a t e chains, with
silica-oxygen bonds broken and s i l i c a - f l u o r i n e bonds formed.
This de-
polymerizes the chains, decreases magma v is c o s ity , and delays fin a l
c ry s ta lliz a tio n .
Cooling Model
Whitney's experimental cooling model (1975b) i llu s t r a t e s th a t d i f ­
fe re n t phenocrysts w i l l be stable a t d if f e r e n t depths of a magma chamber
due to pressure and temperature gradients alone.
In the experimental
system, orthoclase appears on the liquidus a t lower temperatures than
e ith e r quartz or plagioclase (Figs. 12a,12b).
Orthoclase phenocrysts
would be in equilibrium with magma only in the upper, cooler portions of
the chamber, and any crys tals subjected to higher pressures or tem­
peratures would be unstable.
Late c r y s t a lliz a t io n of potassium feldspar
is apparently common in many g r a n itic magmas with a s im ila r amount of
dissolved water (Hyndman, unpub. manusc., 1982, p. 333-334).
56
Quartz appears a t higher liquidus pressures and temperatures than
orthoclase in both phase diagrams on Figure 12.
Quartz would be expected
then, to p e rs is t to greater depths in the magma chamber than orthoclase.
D is trib u tio n of quartz and orthoclase phenocrysts in Emigrant Gulch
quartz monzonites generally agrees with these observations.
Magmas such
as e a rly quartz monzonite which contain more of these minerals probably
began c r y s t a ll iz in g higher in the chamber.
Magmas postulated as having
formed a t greater depth, such as hornblende porphyry, contain few quartz
or orthoclase phenocrysts.
Composition of plagioclase from rocks from each in tru siv e series rein*
forces the idea th a t l a t e r rocks in each series originated from magmas
a t greater depth.
Plagioclase phenocrysts become progressively more
c a lc ic with time in each s e rie s , with rocks lik e hornblende porphyry
and la t e porphyry having a higher An content.
Figure 13 i llu s t r a t e s the hypothetical d is trib u tio n of phenocryst
minerals in a quartz monzonite magma chamber, based on phase diagrams in
Figure 12.
Both e a rly and la t e in tru s iv e series in the quartz monzonite
s u ite presumably originated in th is way, with each series representing a
d i s t in c t episode of magma chamber f i l l i n g and d if f e r e n t ia t io n .
D iffe re n t
members of each series represent magmas derived a t d iffe r e n t depths.
Intrusion o f these rocks apparently took place before protracted cooling
could cause the magmas to s o lid if y w ithin the chamber i t s e l f .
Figure 12.
Phase relationships fo r synthetic quartz mon­
zonite magma containing zero to six weight percent
dissolved water. Figure 12a: temperature
versus weight percent water at two kilobars.
Figure 12b: pressure versus weight percent d is ­
solved water at 750° Celsius. Quartz s t a b i l i t y
fie ld s are highlighted in both diagrams.
Arrow in Figure 12a shows magma composition
moving out of quartz s t a b i l i t y f i e l d with in ­
creasing amount of dissolved water during
c r y s t a lliz a t io n . Solid arrow in Figure 12b
shows s im ila r phenomenon. Dashed arrow in
Figure 12b shows magma s h iftin g out of quartz
s t a b i l i t y f i e l d due to pressure increase.
Such a pressure increase may be the re s u lt of
increased water content in the magma and the
accompanying v o la t i le phase in the roof zone
of a magma chamber.
Composition of synthetic quartz monzonite:
( a f t e r Whitney, 1975a)
70.34
SiO^:
^^2^3'
CaO :
Na^O :
KgO
:
weight percent
18.00
3.97
4.37
3.32
Normative equivalents:
quartz:
orthoclase:
a lb it e :
anorthite:
23.7
19.8
37.3
19.8
58
1000
PL+AF+L
900
PL+L
PL+AF+ V
. Q+L: • ■ V
800
T CC)
700""
PL+. Q+L'-
•.PL+AF+. 0 + L + V - ' ; PL +AF+ Q+L
600’
PL+. Q+L+V.
PL +AF+ Q+V
PL + A F +
750*C
PL+L
Q+L •
PL +L +V
PL+ Q+L+V /
PL + A F+
Q+L+V
PL +AF+ Q+V
WT% H 0
F
ie l d s
where
PLAGI OCLASE
AL KALI FELDSPAR
QUARTZ
quartz
is
stable
liq u id u s
V:
mineral
VAPOR
FIGURE 12
Figure 13.
Experimentally determined zones o f s t a b i l i t y fo r phenocryst minerals in a
cooling quartz-monzonite pluton. Figure shows hypothetical cross section
of the pluton 1,250 years and 12,500 years a f t e r in tru s io n , with the 700*
Celsius isotherm collapsing inward. Stable phenocryst phases are shown
as a function of pressure and temperature gradients. Composition o f the
synthetic quartz monzonite used in this cooling model is given in Figure 12.
Time = 1 2 ,5 0 0 years
Time = 1,250 years
b.
G
Depth
(km )
p
P
(kb)
(kb)
PkQ
P1 + Q +
Af
1 .5 -
1.5
P+Q+Af
xl's + liquid
FIGURE 13
□
xl's + liquid-rvapor
S
Modified
xl's+vapor
from
Whitney (1 9 7 5 b )
CT»
O
61
and orthoclase appearing la s t .
This sequence, in ferre d from inclusion
re la tio n s h ip s , is the same fo r a l l the quartz monzonites studied
(Appendix 4a).
Sequence o f c r y s t a ll iz a t io n is also in agreement with quantities
o f phenocryst minerals seen in the quartz monzonites.
Plagioclase
dominates phenocryst mineralogy in every member of the s u ite .
In contrast,
orthoclase and quartz phenocrysts are not as abundant (Appendix 4b). This
is expected i f plagioclase was the f i r s t mineral to appear on the liquidus
and was therefo re able to c r y s t a ll iz e in greater qu an tities than other
phenocryst minerals.
Figure 14 il l u s t r a t e s th is sequence using the qu artz-plagioclaseorthoclase tern ary phase diagram.
Each quartz monzonite magma in the
su ite probably began c r y s t a ll iz in g in the plagioclase f i e l d since plagio­
clase is the most abundant phenocryst mineral.
Continued cooling bought
magmas to the plagioclase-quartz c o te c tic , rock types with more quartz
phenocrysts remaining on the co tec tic longer.
Liquidus temperatures
dropped s u f f i c i e n t l y fo r each magma to c r y s t a ll iz e orthoclase at the
eu tec tic ju s t p r io r to groundmass quenching and s o lid if ic a t io n .
Rock
types l i k e hornblende porphyry and la te porphyry remained at the eu tec tic
only long enough to c r y s t a ll iz e a few, scattered orthoclase phenocrysts.
This probably resulted from rapid ris e and emplacement of magma from
deeper lev els of the chamber.
Conversely, e a rly quartz monzonite and
quartz porphyry cooled longer at lower temperatures and contain more
orthoclase than other rocks in the s u ite .
In a d d itio n , estimates of
AN
An/Ab = 3.8
AB
QUARTZ
OR
PRESSURE
EUTECTIC
TEMPURATURE
= 695 "C
POTASSIUM
FELDSPAR
SS
PLAGIOCLASEgg
Or
Ab + An
PERCENT BY WEIGHT
Figure 14 . IsOBARIC PHASE DIAGRAM FOR THE SYSTEM:
K A l S I j O g - N a A l S I j 0 g - C A A L 2 S i ^ O g - S i O 2- H 2O
P ro jected
f r o m H2O o n t o t h e a n h y d r o u s b a se :
O r-(A b + A n )-S i
■
LATE p o r p h y r y
A
PLAGIOCLASE PORPHYRY
A
QUARTZ PORPHYRY
o
HORNBLENDE PORPHYRY
EARLY QUARTZ MONZONITE
AFTER VON PLATEN
(1 9 6 5 )
63
groundmass composition fo r a l l members of the suite (Appendix 4a) plot
close to the e u te c tic on Figure 14.
Reaction Textures
The phase diagram on Figure 12a shows th a t ris in g quartz monzonite
magma could s h i f t outside the quartz s t a b i l i t y f i e l d i f water content
increased to more than 3.5 percent, as temperature dropped below 750*
Celsius.
This s h i f t is depicted by the arrow in Figure 12a.
Such an
increase in water saturation is expected as the magma rises to shallower
le v e ls , and the water becomes less soluble in the melt (Hyndman, unpub.
manusc., 1982, p. 152-153).
This amount of dissolved water is appropriate
fo r quartz monzonite magmas w ithin the postulated pressure and temperature
ranges (Whitney 1975b).
Figure 12b shows a s im ila r s h i f t of the magma outside the quartz mon­
zonite s t a b i l i t y f i e l d as pressure decreases from about 1.6 kilobars
and water content in the melt approaches four percent.
Again, such an
e f f e c t is expected when magma becomes more saturated in water at
shallower depths where pressures are r e l a t i v e l y low.
As magma composition s h ifts out of the quartz s t a b i l i t y f i e l d ,
previously c r y s ta lliz e d quartz would p a r tly redissolve in the magma and
e x h ib it resorbed rims.
Severely resorbed quartz in rocks lik e horn­
blende porphyry and la te porphyry suggests magmas originated deep in the
chamber, then rose and became more water saturated.
Disequilibrium
occurred between quartz phenocrysts and magma, and resulted in embayed
quartz borders (F ig . 6b).
64
I t can also be argued th a t buildup of
water and other v o la tile s in
the roof zone the chamber might cause quartz embayment in magmas in the
roof zone.
The dashed lin e in Figure 12b shows th at pressure increase
from increasing v o l a t i l e water in the system would cause magma com­
position to s h i f t out o f the quartz s t a b i l i t y f i e l d .
Resorbed quartz
in rocks from the la te in tru s iv e series l i k e quartz porphyry then,
probably arose because of an active v o l a t i l e phase (F ig . 7b).
Presence
of th is v o l a t i l e phase was established above, in view of the intense
hydrothermal a lt e r a tio n associated with quartz and plagioclase por­
phyries.
Numerous episodes of v o l a t i l e buildup and release during intrusion of
the la t e in tru s iv e series probably produced repeated periods of quartz
resorption.
Embayed quartz in plagioclase porphyry suggests this magma
rose to the roof zone a f t e r intrusion of quartz porphyry and was sub­
je c te d to the same increasing pressures and water saturated as was the
e a r l i e r magma.
In th is way, embayed quartz in rocks from the la te in ­
tru s iv e series r e fle c ts the recurring buildup and venting of v o la tile s
in the roof zone, concurrent with hydrothermal a lt e r a t io n , brecciation
and s u lfid e m in e ra liz a tio n .
In a s im ila r manner, rapakivi rims on orthoclase may indicate s h iftin g
of melt composition out of the orthoclase s t a b i l i t y f i e l d .
phase diagram on Figure 15 demonstrates th is phenomenon.
The ternary
The phase
boundary drawn between the orthoclase and plagioclase fie ld s s h ifts
towards the lower border of the diagram with decreasing pressure, as
shown by Abbott (1978).
Composition of a magma c r y s t a ll iz in g orthoclase
65
PLAGIOCLASE
CEP
K- RICH
FELDSPAR
CP
M
Figure 15 .
IsOB ARI C LI QU ID US DIAGRAM FOR THE SYSTEM KA. LSljOg-
NAALSlj0g-CAAL2Sl20g-H20PROJECTED
FROM THE H2O APEX OF
THE TETRAHEDRON ONTO THE ANHYDROUS TERNARY BASE O r - A b ~
An at 2,000
FIGURE 64).
BARS
AFTER
TuTTLE AND BoWEN (1958,
THE VERY SMALL NONQUATERNARY F I E L D FOR
LEU CITE + MELT I S
WITH L I Q U I D
PpLuiD-*
IS
IGNORED.
IN D IC A T E D ;
THE SOLID PHASE
IN E Q U IL IB R I U M
MAXIMUM SOLID SOLUTION L I M I T S
ARE SHOWN DIAGRAMMATICALLY BY THE STIPP LED PATERN.
D =
INTERSECTION OF COTECTIC WITH A n ~O r J O IN
CEP = C R I T I C A L ENDPOINT ON THE TWO FELDSPAR + MELT BOUNDARY
CP = C R I T I C A L POINT ON THE TERNARY FELDSPAR SOLVUS
M = THERMAL MINIMUM OF THE QUATERNARY SYSTEM
SS = SO L ID SOLUTION
66
w i l l move in to the plagioclase f i e l d i f i t is close to the phase
boundary p r io r to the drop in pressure.
Abbott (1978) concluded that
th is would induce the growth of plagioclase rims around an unstable
orthoclase c r y s ta l, and produce a rapakivi te xtu re .
The pressure de­
crease required fo r th is process could be a ttrib u te d to sudden venting
of v o la t ile s in the roof zone.
Conversely, lack of e ith e r rapakivi rims
or resorbed quartz in e a rly quartz monzonite suggests only meager amounts
of v o la t i le s accumulated in magmas of the ea rly in tru siv e series.
Con­
sequently, l i t t l e a lt e r a t io n , m in e ra liza tio n , or groundmass quenching is
associated with e a rly quartz monzonite.
F i n a lly , decreasing l i t h o s t a t i c pressure in magmas ris in g from depth
could conceivably cause rapakivi textures on orthoclase phenocrysts.
The
few orthoclase phenocrysts th at did form in hornblende porphyry and la te
porphyry are in v a ria b le mantled by plagioclase, giving fu rth e r evidence
th a t these rocks formed from magmas derived deeper in the chamber.
CHAPTER V
CHEMISTRY AND DIFFERENTIATION
Whitney's cooling model explains variatio ns in phenocryst d i s t r i ­
bution and mineralogy in Emigrant Gulch quartz monzonites.
I f magmas
d i f f e r e n t ia t e d a t d if f e r e n t depths, however, the magma chamber would
have become compositionally zoned in addition to c r y s t a lliz in g d iffe r e n t
phenocryst assemblages a t d i f f e r e n t depths.
f a i l s to explain th is compositional zonation.
The cooling model alone
Felsic magmas would have
been generated higher in the chamber and mafic magmas at greater depth,
as indicated by compositional trends in the ea rly and la te in tru siv e
s e rie s.
This section examines trends fo r major and minor oxides, and trace
elements, and the e ffe c ts o f hydrothermal a lt e r a tio n in the quartz mon­
zo n ite s u ite .
These compositional trends r e f l e c t changing pressure,
temperature, and chemical gradients during magma d i f f e r e n t ia t io n .
Textural
data presented above is coupled with compositional data to propose th a t
magmas d if f e r e n t ia t e d by v o l a t i l e d iffu s io n .
This model, applied to
in tru s iv e rocks in Emigrant Gulch, was adopted from H ild re th 's study of
the Bishop T u f f (1979).
Textural and compositional data also discount
other d i f f e r e n t ia t io n mechanisms such as crystal fra c tio n a tio n .
Chemistry
Whole-rock oxide and trace-element trends fo r rocks in the Emigrant
67
68
Gulch quartz monzonite su ite i l l u s t r a t e :
1)
Early in tru s iv e series rocks are chemically d is tin c t
from the la t e in tru s iv e series rocks.
2)
Compositional differences between members o f the suite
r e f l e c t changing pressure, temperature, and compositional
gradients during magma d if f e r e n t ia t io n .
3)
Compositional differences are also produced by hydrothermal
a lt e r a tio n in the la t e in tru s iv e series.
Rocks in the
e a rly series were not s ig n if ic a n tly a lte re d .
Major-and Minor-oxide Chemistry
Rock-chip samples o f each member in the quartz monzonite suite were
collected from the freshest rocks a v a ila b le but hydrothermal a lte r a tio n
has affec ted rocks from the la t e in tru siv e series.
Figures 16 and 17 p lo t
whole-rock oxides and t h e ir average values against s i l i c a fo r each rock
type.
Figures 16 and 17 show th a t members of the ea rly in tru s iv e series
contain greater q u an titie s of elements th a t are associated with 'mafic'
rocks.
Early quartz monzonite and hornblende porphyry have higher iro n ,
magnesium, calcium, and phosphorus than rocks from the la te in tru siv e
s e rie s .
Conversely, la t e -s e r ie s rocks contain more s i l i c a and potassium,
oxides normally associated with 'f e l s i c *
rocks.
on Figures 18 and 19 confirm th is d is tin c tio n .
Trace-element trends
The rocks from the la te
series represent the m o re -fe ls ic , m o re-d iffe re n tia ted quartz monzonites
in Emigrant Gulch.
Figure 16.
Major and minor oxide contents o f Emigrant Gulch quartz
monzonites plotted against SiO^ contents. Ail
samples for each rock type are p lo tte d .
Corre­
latio n c o e ffic ie n t R, and R also shown fo r each
oxide. R measures degree of dependence of each
oxide with s ilic a :
R=
: perfect correlation
R = 0: no co rrelation
2
R measures the amount of v a r i a b i l i t y in each
oxide trend that is explained from the lin e a r
dependence with SiOg:
2
R
= 1 : 100 percent of the v a r i a b i l i t y
2
can be explained.
R = 0: 0 percent of the v a r i a b i l i t y can
be explained.
Figure 17.
Average major and minor oxide contents of
Emigrant Gulch quartz monzonites plotted
against average SiOg contents. R and R^
fo r each oxide trend are also shown (see
Figure 16 for explanation).
KEY:
B LATE PORPHYRY
A PLAGIOCLASE PORPHYRY
A
QUARTZ PORPHYRY
O HORNBLENDE
e
PORPHYRY
EARLY QUARTZ MONZONITE
70
0 6
-
0 4
-
R
R '
-.260
.068
.130
.017
FegO^
-.894
.799
-
4
-.893
.798
-
2
026
.001
------- TiOc
024
AI2O3
-
4
-
18
-
16
-
14
-
FeO
008-
MnO
o*3§: -
004-
_2__
30
MgO
20
-.791
625
-.811
658
-689
.475
.279
.078
h10
504030-
CaO
8 ■
■O--O . B I
B * - - B_
* a*"#
0
20-
10-
b
-5 0
-4 0
NOgO
-3 0
-
50-
KpO
...................
40-
20
30-
-0.4
P2O5
■-
0
.
A -0 2
..........
59 0
62 4
65 8
SiOp
Figure 15
69 2
735
-.613
.375
71
R.
K
06
TiO:
0 4
-.027
.001
.025
.001
P 82o^03 - .778
.606
- .845
.714
.418
.175
-.8 00
.640
- .691
.478
-.6 6 3
.440
.614
.377
0 2
A l,0
2^ 3
-
18
-
16
-
14
2%>
FeO
*
--------------------- " - A ----------------------------- A
MnO
0080 .0 4 -3 0
MgO
-20
-A
—
—
—
—
^
A t
1.0
-
50403 .0 -
CaO
20-
1.0
NOgO
-
5.0
-
40
30
h20
5040
KgO
-
30
P2,0
^5
b66 5
672
67 8
SiO,
Figure 17
68 5
-
0.2
-
0.4
694
-7 8 6
.618
72
R e la tiv e ly high c o rre la tio n c o e ffic ie n ts fo r f e r r i c io rn , ferrous
ir o n , magnesium, and calcium indicate close association with s i l i c a .
Poorer c o rrela tio n s fo r titan iu m , aluminum, potassium, and sodium stem
p a r t ly from a n a ly tic a l precision but more so from a rg il l i e and s e r i c i t i c
a lt e r a t io n in the la t e in tru s iv e series.
The apparent decrease of sodium
with increasing s i l i c a fo r example, is uncommon in unaltered g r a n itic
rocks (Hyndman, unpub. manusc., 1982, p. 9 6 ).
Sodium may be leached
from s e r i c i t i c a l l y a lte re d rocks through hydrogen metasomatism and cation
exchange according to Meyer and Hemley (1968, Table 6 . 1 ) , and Hemley
and Jones (1964).
Younger quartz monzonites in Emigrant Gulch underwent
the most extensive a lt e r a tio n of th is type and consequently retained the
le a s t sodium.
Analogous cation exchange can convert potassium feldspar to
potassium clays and micas during intense a r g il l i e and s e r i c i t i c a lte r a tio n
(Hemley and Jones, 1964).
These reactions lib e r a te potassium ions which
may be flushed in aqueous flu id s from zones of severe a lt e r a t io n .
This
accounts fo r anomalously low potassium in plagioclase porphyry, and
results in poor c o rre la tio n of th is oxide with s i l i c a (Figs. 16 ,17 ).
Although aluminum is generally
immobile or s l i g h t ly depleted in
s e r i c i t i c and p o ta s s ic a lly a lte re d zones (Hemley and Jones, 1964, Table 1 ) ,
Meyer and others (1968) documented aluminum content increase in s e r i c i t i ­
c a lly a lte re d quartz monzonite at Butte, Montana.
This could explain the
higher aluminum content o f a lte re d plagioclase porphyry and la te porphyry
compared to less alte re d rocks in the e a rly s e rie s.
Scatter of titanium
values fo r porphyries on Figures 16 and 17 may also re s u lt from
73
a l t e r a t i o n , but supporting data are lacking.
In ad dition, hydrogen
metasomatism commonly depletes calcium and magnesium in more severely
a lte re d rocks.
Good correlatio ns of these oxides with s i l i c a , as well
as lower i n i t i a l values are l i k e l y a product of a r g il l i e and s e r i c i t i c
a lt e r a t io n in the la t e in tru siv e series.
Figure 17 shows averages fo r oxides in the quartz monzonite suite
and demonstrates th a t mafic rocks from the e a rly in tru sive series are
chemically d i s t in c t from rocks in the f e l s i c , la te series.
A combination
of o rig in a l magma composition, and a lt e r a tio n contribute to th is d is ­
t in c t io n .
Despite the above argument th a t a lt e r a tio n modifies calcium
trends, only differences in o rig in a l mineralogy between e a rly and la te
in tru s iv e series rocks are required to explain observed calcium v a r i ­
a b ility .
Calculated calcium contents o f rocks from both the e a rly and
la t e in tru s iv e s e rie s , based on percent plagioclase phenocrysts, closely
approximates whole-rock values fo r each respective series (Appendix 2 ) .
Likewise, plagioclase phenocryst composition and content appear to con­
tr o l calcium v a r i a b i l i t y between members w ithin each series, such as
e a rly quartz monzonite and hornblende porphyry.
Conversely average values
o f sodium no longer r e f l e c t differences in o rig in a l magma composition.
Calcium p lotted against sodium in Figure 18
illu s t r a t e s compositional
dependence of Emigrant Gulch quartz monzonites on both magma chemistry
and hydrothermal a lt e r a t io n .
Emigrant Gulch quartz monzonites can also be distinguished by modal
and normative mineral content (Figs. 3 ,14; Appendix 5 - 2 ) .
Normative
q u a r tz -a lb ite -o rth o c la s e estimates are plotted in Figure 14, and closely
74
p a r a lle l modal values shown on Figure 3.
Both figures show th at f e l s i c ,
la t e in tru s iv e rocks p lo t closer to the ternary minimum than ea rly series
rocks.
Trace-element Chemistry
The trace-element content of rocks from the quartz monzonite suite
fu rth e r characterizes the e a rly series as being more mafic than the la te
series.
Members of the e a rly series generally have more strontium than
rocks from the more f e l s i c , la t e series (Figs. 1 8 ,19 ).
In ea rly quartz
monzonite and hornblende porphyries, cation exchange between the melt
and phenocrysts probably enriched hornblende and plagioclase in strontium.
S im ila r reasoning explains lower rubidium/strontium ratio s in the
e a rly series than in la t e quartz monzonites th at have fewer calcic plagio­
clase phenocrysts, and lack abundant hornblende.
In the la te in tru siv e
s e rie s , rubidium probably substitutes fo r potassium in b i o t i t e and ortho­
clase.
Figure 19 supports t h is , since rubidium content increases with
increasing potassium in younger, more siliceous rocks that contain more
modal and normative orthoclase.
more a v a ila b le s it e s .
More rubidium exchange is possible with
Rubidium and rubidium/strontium plotted against
potassium on Figure 19 reveal s im ila r trends.
The preceding discussion describes trace-element variations as a
function of i n i t i a l magma composition.
Effects of hydrothermal a l ­
te ra tio n in plagioclase porphyry however, produced poorer correlations
fo r ru b id iu m -s ilic a , strontium-potassium, and rubidium-strontium.
Low
and e r r a t i c potassium values fo r the la te series rocks which were subjected to
Figure 18.
Peacock a lk a li- lim e index (Na^O + K^O
versus CaO, a f t e r Peacock, 1931); Na^O
versus CaO; and Rb, Sr and Rb/Sr versus
SiOg fo r Emigrant Gui ch quartz monzonites.
Average oxide and element contents fo r each
rock type are plotted. R and R^ values are
also shown for each oxide and element trend
(see Figure 15 fo r explanation).
76
R
ALKAy
KgO
-
r
:
.614
.377
-.691
.478
.710
504
.286
072
- .741
.549
.687
.472
CaO
NOgO
550
56 3
-40
NOgO
-1 6 6 0
- 1455
Rb
h 125.0
354
273 H
- f 822
h 593
R b /S r
.364
67.2
665
67.8
685
SiOa
Figure 18
KEY:
■
LATE PORPHYRY
A
PLAGIOCLASE
PORPHYRY
▲ QUARTZ PORPHYRY
O
HORNBLENDE
PORPHYRY
e
EARLY QUARTZ MONZONITE
69.4
Figure 19.
Rb versus Sr; and Rb, Sr and Rb/Sr
versus K^O for Emigrant Gulch quartz
monzonites. Average oxide and element
contents fo r each rock type are plotted
(see Figure 16 fo r key). R and R^
values are also shown fo r each oxide
and element trend (see Figure 16 fo r
explanation).
78
. 166.0
-153 9
-1 443
346
' 125.0
Rb
166 0
153.9
1-1440
-134 6
-1 2 5 0
Rb
354 -J
306
268
229 {
191 -
Sr
Rb/Sr
-.822
-.687
-.580
-.472
364
cr
4.15
390
KgO
Figure 19.
KEY:
■
LATE PORPHYRY
A
PLAGIOCLASE PORPHYRY
▲ QUARTZ PORPHYRY
O
HORNBLENDE
PORPHYRY
*
EARLY QUARTZ MONZONITE
450
R
R.
.3 4 5
.119
.88 6
.786
- .526
.2 7 7
.7 8 7
.619
79
a r g il l i e and s e r i c i t i c a lte r a tio n is expected however, as explained
above.
Plagioclase porphyry is an extreme example o f t h is , containing
very l i t t l e potassium and rubidium.
Both elements are mobile and
reac tive during intense a r g il l i e and s e r i c i t i c a lt e r a t io n , and are
e a s ily leached.
These considerations also explain the unusual position
of plagioclase porphyry on the rubidium-potassium, and rubidium/
strontium-potassium plots (F ig . 19).
D iff e r e n tia tio n
The discussion o f chemistry shows th a t two d is tin c t series of magmas
were generated in Emigrant Gulch.
I t was proposed in the sections above
th a t the e a rly in tru s iv e series represents the f i r s t episode of magmachamber f i l l i n g , d i f f e r e n t i a t i o n , and draining.
I t was argued th at
te x tu ra l differences seen in rocks from the ea rly series arose from
magmas generated a t d if f e r e n t depths in the chamber.
Compositional
v a ria tio n s between the lower and upper parts of the chamber would ex­
p la in differences in chemistry w ithin rocks of the ea rly in tru s iv e series.
S im ila r reasoning explains chemical and te xtu ra l trends in rocks from
the la te in tru s iv e series.
The trends point to a second period of magma-
chamber f i l l i n g , d i f f e r e n t i a t i o n , and draining.
Unlike the ea rly series
however, d i f f e r e n t ia t io n produced magmas ric h e r in s i l i c a and other
f e l s i c constituents.
D if f e r e n t ia t io n in the quartz monzonite suite must take into account
compositional trends in both the e a rly and la t e series.
These trends
imply two periods o f compositional zonation a ffe c tin g the d if f e r e n t ia t in g
80
magmas.
A process involving upward tra n s fe r of v o l a t i l e constituents
in the magma chamber offers the most reasonable explanation of th is com­
po sitional zonation.
This process is known as v o l a t i l e d iffu s io n .
Shaw
Cl974) described th e o re tic a l aspects of v o la t i le d iffu s io n , whereas
H ild re th (1979, 1981) and Luddington (1979) applied these ideas to d i f ­
fe r e n tia tio n in natural magmatic systems.
The ra tio n a le fo r applying
th is d i f f e r e n t ia t io n process to natural systems including the Emigrant
Gulch complex must consider:
1)
Presence o f a well-documented hydrothermal phase coexisting
with convecting magma.
2)
Chemical enrichment trends between magmas formed in the roof
zone of the magma chamber versus those formed a t greater
depth.
3)
F a ilu re o f other d i f f e r e n t ia t io n schemes such as crystal
fra c tio n a tio n to explain te x tu ra l and chemical features.
In natural systems, the Bishop T u ff has yielded the most convincing
evidence fo r d i f f e r e n t ia t io n by v o l a t i l e d iffu s io n (H ild re th , 1979).
Oxide and trace-element contents of magmas derived from higher in the
chamber compared with those in magmas derived at depth
proved to be
c r i t i c a l in supporting v o l a t i l e d iffu s io n in th is r h y o lit ic system.
Chemistry
and rock textures also discounted crystal fr a c tio n a tio n , magma imm i s c i b i l i t y , and assim ilation as other means of d i f f e r e n t ia t io n .
H ild reth
(1981) expanded th is theory to suggest d if f e r e n t ia t io n proceeds through
v o l a t i l e d iffu s io n in volcanic rocks less s i l i c i c than the Bishop T u ff.
Using the chemistry o f several d a c itic and rhyodacitic systems, H ild reth
81
concluded th a t oxide enrichment trends were s im ila r to those in the Bishop
T u ff and would be promoted by v o la t i le d iffu s io n .
Oxide and trace-element trends o f rocks in the la te in tru siv e series
in Emigrant Gulch p a r a lle l element-enrichment trends in the Bishop Tu ff
(Fig. 20).
These trends, as in the Bishop T u ff and other volcanic rocks,
argue fo r v o l a t i l e-phase tra n s fe r of s ilic o n , rubidium, and other ' f e l s i c '
elements in to r e l a t i v e l y f e l s i c magmas in the roof zone of the magma
chamber.
Calcium, phosphorus, strontium, and other elements are less
mobile and tend to remain behind in the v o la tile -p o o r mafic magmas.
The
mafic magmas generally form deeper in the magma chamber, erupting or in ­
truding only a f t e r f e l s i c material vents from higher in the chamber
(Smith, 1979).
Although the Emigrant Gulch quartz monzonites display oxide and traceelement trends very s im ila r to the Bishop T u ff, some discrepencies appear.
S im ila r trends support d i f f e r e n t ia t io n via v o l a t i l e d iffu s io n in the
quartz monzonite s u ite , whereas diverging trends i l l u s t r a t e problems of
recognizing v o l a t i l e d iffu s io n in complex, in tru s iv e systems:
1)
Quartz monzonite rock textures and chemistry point to two
d i s t in c t periods o f magma-chamber f i l l i n g , d i f f e r e n t ia t io n ,
and draining.
These periods correspond to the early and
la t e in tru s iv e series in Emigrant Gulch.
Trends shown on
Figures 20 and 21 compare d i f f e r e n t ia t io n in the la te
in tru s iv e se rie s, to trends in the Bishop T u ff.
The Bishop
T u ff and other volcanic rocks presumably d iffe r e n tia t e d and
erupted in a single continuous episode.
Figure 20.
Enrichment factors fo r 13 selected elements in the la te in tru s iv e series
compared to those in the Bishop T u ff. For the la te in tru s iv e se rie s,
enrichment factors are element concentrations in quartz porphyry divided
by concentrations in unerupted parent magma s im ila r in composition to
e a rly quartz monzonite. For the Bishop T u ff, enrichment factors are
element concentrations in e a rlie s t-e ru p te d rocks divided by concen­
tra tio n s in samples erupted la t e r in the sequence. In each case, rocks
emplaced or erupted ea rly in each sequence represent magmas generated
in the top of the magma chambers, whereas l a t e r rocks were presumably
derived at greater depth,
(modified from H ild re th , 1979).
Mn
Na
I
En r i c h m e n t f a c t o r s
f or
la te / early
phases
f or
selected
elements
BISHOP TUFF ROCKS
EMIGRANT GULCH ROCKS
Figure 20.
taa
n J o j
Figure 21.
Enrichment factors fo r four selected element
ratios in the la te in tru s iv e series compared
to those in the Bishop T u ff.
For the la te
intrusive se rie s, enrichment factors are the
ratios of element concentrations in quartz
porphyry divided by those in unerupted parent
magma, presumed to be s im ila r in composition
to early quartz monzonite. For example,
Mg/Fe increased from 0.41 to 0.66 between
emplacement of quartz porphyry (0.41) and
and s o lid if ic a tio n of the hypothetical parent
magma at depth ( 0 . 66 ).
For the Bishop T u ff, enrichment factors are
the ratio s o f element concentrations in the
ea rliest-eru p ted rocks divided by concen­
trations in samples erupted la t e r in the
sequence. For example, K/Rb increased
from about 235 to 450 during progressive
tapping of the chamber.
(modified from H ild re th ,
1979).
85
.9
K /R b
.8
.7
.6
.5
F e /M n
S r/R b
Mg / F e
.3
.2
.1
E n r i c h m e n t f a c t o r s for e a r l y / l a t e
ELEMENTAL RATIOS
EMIGRANT GULCH ROCKS
Figure 21.
BISHOP TUFF ROCKS
86
2)
Contrasting magnitudes in enrichment factors in Figures 20
and 21 possibly r e f l e c t differences in parental magma
composition between r h y o l i t i c and quartz monzonite systems.
3)
Hydrothermal a lt e r a tio n d isto rte d o rig in a l compositional
differences of magmas in Emigrant Gulch.
This apparently
did not occur in the Bishop T u ff.
The following discussion examines these points in d e t a i l .
Depletion of the magma chamber generating the quartz monzonites
probably took place in sequential pulses.
During derivation of the early
s e rie s , presumably only lim ite d v o l a t i l e d iffu s io n occurred.
This f i r s t
pulse o f magmatic a c t i v i t y culminated in intrusion of e a rly quartz mon­
zonite and hornblende porphyry.
Both magmas remained compositional ly
s im ila r to the hypothesized parent magma.
In th is way the e a rly in ­
tru s ive series is compositionally representative o f the parental magma
r e f i l l i n g the chamber and generating members of the la te in tru s iv e se rie s.
Chemical trends on Figures 20 and 21 for the la te series consider
quartz porphyry as the e a r l i e s t , most f e l s i c phase, and unerupted parent
magma as the younger, mafic counterpart.
Therefore, rocks s im ila r to ea rly
quartz monzonite should be found a t depth in Emigrant Gulch, beneath
members of the la t e in tru s iv e series.
Smith (1979) claimed th a t these
r e l a t i v e l y mafic magmas forming deeper in the chamber commonly lack suf­
f i c i e n t v o la tile s to drive eruption or emplacement to higher le v e ls .
Even in the la t e s e rie s , magma chamber draining occurred in several
stages, in contrast to the Bishop T u ff.
Between each stage of intrusion
87
v o l a t i l e accumulation in the roof zone probably provided the driving
force fo r the next in tru s iv e pulse.
This was discussed fo r quartz
porphyry and plagioclase porphyry magmas, supported with evidence from
phenocryst te xtu re s.
In a d d itio n , v o la t i le diffusion probably enriched
plagioclase porphyry in s i l i c a and other elements (Figs. 3 ,1 4 ).
This
suggests younger, r e l a t i v e l y mafic magmas underwent renewed d iffe r e n tia t io n
and s i l i c a enrichment as they in te r m it te n tly pushed roofward.
Were i t
not fo r the episodic nature of porphyry in tru s io n , renewed d iffe r e n ­
t i a t i o n would not have taken place.
These rocks then, re ta in composi­
tio n a l and te x tu ra l features of v o l a t i l e d iffu s io n at d if f e r e n t levels of
the same magma chamber.
The Bishop T u ff and other volcanic rocks do not
display these compositional and te xtu ra l i r r e g u l a r it ie s because eruption
occurred in s in g le , continuous events, not episodic pulses.
Figures 20 and 21 stress th a t directions of most element enrichment
trends are the same fo r both the Bishop T u ff and Emigrant Gulch quartz
monzonites.
D iffe rin g magnitudes of these trends probably represent compo­
s itio n a l differences in parent magma generating the r h y o lit ic and quartz
monzonitic systems.
Members o f the quartz monzonite su ite d i f f e r by no
more than 1- 2.5 weight percent s i l i c a , whereas comparable phases of the
Bishop T u ff vary by 2 .0 - 2 .3 weight percent s i l i c a .
Less magnesium, iro n ,
phosphorus, and titanium enrichment in Emigrant Gulch rocks may be a
function of less s i l i c a enrichment.
Also rubidium/strontium is less en­
riched in the quartz monzonites although potassium/rubidium enrichment is
greater than in the Bishop T u ff.
p re ta tio n .
Again, a lt e r a tio n a ffe c ts th is i n t e r ­
88
More d ra s tic a lt e r a tio n probably diminished sodium and flu o rin e
in the la t e in tru s iv e series and produced enrichment trends opposite
to those of the Bishop T u ff.
These trends th e o r e tic a lly would be in the
same d ire c tio n had quartz porphyry retained more o rig in al sodium.
F i n a lly , H ild re th (1979) stressed the importance of convection in
the magma chamber to enhance v o l a t i l e-phase tra n s fe r of chemical com­
ponents.
Rates of chemical d iffu s io n in a s t a tic chamber, without mass
tra n s fe r o f magma are too slow to produce enrichment factors seen in
s i l i c i c rocks derived from roof-zone magmas.
Convection of Emigrant Gulch magmas cannot be d ir e c t ly demonstrated,
although convection in g r a n itic magmas is l i k e l y with large thermal
gradients and r e l a t i v e l y low vis co sity (Norton, 1978; Norton and Knight,
1977; Shaw, 1974, 1965).
In ferred temperature and visco sity ranges in
Emigrant Gulch magmas f i t these c r i t e r i a .
Textural and chemical features of the Emigrant Gulch quartz monzonites
also argue against fra c tio n a l c r y s t a lliz a t io n :
1)
Reverse zoning in plagioclase is uncommon, and r e s tric te d
to narrow o s c illa to r y zones in the ea rly in tru s iv e series
(F ig. 6b).
2)
Phenocryst mineralogy alone does not account fo r differences
in s i l i c a between rock types:
s i l i c a content o f a ll pheno-
crysts in quartz porphyry is 16.8 percent; to ta l s i l i c a
from phenocrysts in e a rly quartz monzonite is 25.5 percent.
This amounts to 8.8 percent difference in s i l i c a between these
rock types, where only 1.5 percent is a c tu a lly observed
89
(Appendix 2- 2 ).
S ilic a variatio ns must therefore depend
on groundmass composition.
3)
Crystal s e t t lin g rates give 10-100 meters maximum
s e t t lin g in 100,000 years, using Stoke's law, and
v is c o s ity of g r a n itic magma with four percent dissolved
water (Appendix 3 ).
The calculated cooling time of
small quartz monzonite plutons such as the one depicted in
Figure 13 (Whitney, 1975b) would be only about 10,000
years.
This cooling time would re s u lt in less than 10
meters of s e t t lin g (Appendix 3).
Less than 10 meters of crystal s e ttlin g is also consistent with the
plagioclase zoning seen in Emigrant Gulch rocks.
I f crystal s e ttlin g had
occurred to any appreciable ex ten t, plagioclase would be expected to show
prominent reverse zoning, with sodic cores and calcic rims.
Crystals would
sink to deeper levels o f the chamber where hotter temperatures would cause
c a lc ic plagioclase rims to form, as opposed to formation of sodic cores at
higher, cooler le v e ls .
Such zoning is not seen in any Emigrant Gulch rocks
Minor reverse zoning could, however, have occurred as plagioclase
phenocrysts s e ttle d less than 10 meters in to s li g h t ly more-mafic magma.
In th is v e r t ic a l distance, magma would presumably become more s i l i c i c
with time, as v o l a t i l e d iffu s io n progressed.
O s c illa to ry zoning would
re s u lt from minor c a lc ic reversals amidst an overall trend towards
more-sodic rims with time.
In any case, crystal fra c tio n f a i l s to re­
solve v a ria tio n in s i l i c a content between members of the quartz monzonite
s u ite .
90
Evidence also is not av aila b le to support d iff e r e n t ia t io n through
liq u id im m is c ib ility .
Textures and outcrop features suggesting liq u id
im m is c ib ility in a lk a lin e la c c o lith s are t o t a l l y absent in Emigrant
Gulch rocks.
Quartz monzonite compositions display no s ig n ific a n t gaps
in s i l i c a in the e n tir e s u ite , nor globules of one rock type in another.
Such features might otherwise suggest im m is c ib ility (Hyndman, unpub.
manusc., 1982, p. 397).
F in a lly , im m is c ib ility in experimental a lk a lin e
systems occurs only with upwards o f one mole percent titanium and three
mole percent phosphorus (Freestone, 1978).
Both these values g rea tly
exceed observed q u an titie s in Emigrant Gulch quartz monzonites.
Textural data argues against assim ilation as an important d iffe r e n ­
t i a t i o n mechanism.
Inclusions in quartz monzonites are generally recog­
nizable Precambrian basement rocks and are in variab ly more mafic than
the surrounding quartz monzonite.
Inclusions commonly have fuzzy borders,
but the composition o f adjacent in tru s iv e rocks does not appear sig ­
n i f i c a n t l y changed by the assimilated m a te ria l.
Large-scale assim ilation
deep in the crust or upper mantle conceivably generated the orig inal
p rim itiv e magma which produced the Emigrant Gulch complex, and perhaps
volcanic rocks in the region.
Massive assim ilation well below the level
o f emplacement would have been extremely complex, and supportive data are
scant.
In a l l lik lih o o d . Emigrant Gulch quartz monzonites d iffe r e n tia t e d
by v o l a t i l e d iffu s io n at shallow depth.
CHAPTER VI
HYDROTHERMAL HISTORY AND MINERALIZATION
The following discussion summarizes the histo ry of m in e ra liz a tio n ,
a lt e r a t i o n , and brecciation in Emigrant Gulch.
These events were d is­
cussed in the previous section, as hydrothermal a c t i v i t y accompanied
in tru sio n of rhyodacite porphyry, and the quartz monzonite s u ite .
Compositional c r i t e r i a given by Westra and others (1981) c la s s ifie s
the Emigrant Gulch complex as a c a lc -a lk a lin e -ty p e molybdenum deposit.
These c r i t e r i a show the Emigrant Gulch system is more closely related to
molybdenum deposits such as Boss Mountain, B ritis h Columbia than to
Climax-type deposits.
Review of Hydrothermal History
Hydrothermal a c t i v i t y at Emigrant Gulch
is summarized in Table 5.
Brecciation which is s p a t ia lly confined to rhyodacite porphyry and pre­
sumably predates quartz monzonite emplacement includes:
1)
auto-brecciation caused by quartz flooding, with contorted
or broken laminae,
2)
a matrix-supported, p y r it e - r ic h explosion breccia with
rhyodacite rock fragments in d r i l l hole Med-11,
3)
brecciation from a dense network of coalescing base metals u lfid e ve in le ts in a breccia known
as the
Basic Metals
breccia (Pfau, 1981) and lo c a l i t i e s
to the
east.
91
Table4
Igneous/Structural
A c tiv ity _______
M in e ra liz a tio n
A lte ra tio n & Brecciation
Hypabyssal intrusion of
rhyodacite porphyry.
Gradational into v a r ie tie s
with s i l i c a laminae.
Minor c h a lc o p y rite -p y rite
molybdenum with quartz
flooding in s i l i c a laminae
Later quartz s u lfid e stockworks (minor) cut s i l i c a
laminae and auto-breccia.
Possibly galena-sphaleritechalcopyrite with la t e s t
brecciation event.
Auto-brecciation in rhyodacite
from intense quartz flooding
in s i l i c a laminae.
Very lo c a lize d p y r it e - r ic h ex­
plosion breccia with numerous
rhyodacite fragments. Possible
la te brecciation due to coalescing
base-metal s u lfid e v e in le ts .
Moderate a r g il l i e and s e r i c i t i c
a lt e r a tio n with quartz flooding
in au to-breccia, and lo c a lly in
s ilic a -la m in a te d zones.
Q u a r t z -s e r ic itic a lt e r a tio n in
each of the l a t e r brecciation
events.
Minor potassic a lt e r a t io n .
Intrusion of small, stock­
li k e bodies o f ea rly
quartz monzonite.
Mafic border phase, pegma­
t i t e and a p lit e containing
molybdenite rosettes.
Very minor ch alcopyritemolybdeni te -p y r ite -q u a r tz
v e in le ts . Chalcopyrite
a f t e r b io t i t e common.
Extremely lim ite d , intense
s e r i c i t i c a lt e r a tio n near in ­
trusion boundaries.
Widespread a r g il l i e weathering
with preservation of primary
b io t i t e .
lO
Table 4 (Continued)
Igneous/Structural
A c t iv it y _______
M in e ra liza tio n
A lte ra tio n & Brecciation
Hornblende porphyry dikes
with m afic, c h ille d border
phase
Very minor s u lfid e veining.
Chalcopyrite a f t e r b io t i t e
common.
Generally minor a lt e r a t io n ,
but with lo c a lly severe a r g il l i e
to s e r i c i t i c bleaching.
Probably from l a t e r event.
Quartz porphyry dikes and
ir r e g u la r in tru s iv e masses
Quartz porphyry variants
grade in to plagioclase
porphyry.
Main period of brecciation
(A lliso n breccia)
Generally minor p y r it e molybdeni te-chalcopyri te
veining + quartz through
intru sio n of quartz
porphyry and plagioclaseporphyry.
Some disseminated chal­
copyrite a f t e r b i o t i t e in
porphyry.
Main period of molybdenitepyri te m ineralizatio n
associated with quartzgangue m atrix in brecciated
zones.
Extreme a r g i l l i e and s e r i c i t i c
a lt e r a tio n with few fresh out­
crops.
Moderate-extreme a r g il l i e and
s e r i c i t i c a lt e r a tio n in quartzporphyry va ria n ts .
Main period of brecciation
(A llis o n breccia) producing northsouth trending strin g o f 8
breccias.
Occurs p r io r to la t e s t periods of
plagioclase porphyry in tru sio n .
Late porphyry dikes intrude
p e rip h e ra lly to e a r l i e r
porphyry intru sio ns.
Minor, r i g h t - l a t e r a l s trik es lip fa u ltin g .
Disseminated chalcopyrite.
Pervasive, g re e n -s e ric ite
a lt e r a t io n .
VO
CO
94
Minor molybdenite, chalcopyrite, and p y rite are associated with
the auto-breccia.
Increased molybdenum m ineralizatio n at depth is
considered u n lik e ly , because areas o f higher molybdenite concentrations
at the surface and re la te d to the auto-breccia are quite local and patchy.
Areas o f more intense auto-brecciation e x h ib it s i l i c i f i c a t i o n and moderate
to pervasive a r g il l i e to s e r i c i t i c a lt e r a t io n .
S ulfide m ineralization
does not always coincide with the areas o f more intense a lt e r a t io n .
Later brecciation in rhyodacite porphyry consists o f coalescing
q u a r tz -s u lfid e v e in le ts which become dense enough to rotate fragments
of host rock and entrain them fo r several centimeters.
The Basic Metals
breccia, ju s t northwest of the northwest corner of the map area (Plate lb )
and several other occurrences to the southeast record th is event.
Spatial
r e s t r ic t io n of these base-metal stockworks to the rhyodacite porphyry
strongly suggests th is hydrothermal event occurred p rio r to intrusion
of the quartz monzonite s u ite.
possibly occurred at depth.
Concurrent copper-molybdenum m ineralizatio n
Base metal m in eralizatio n is also associated
with q u a r tz -s e r ic ite a lt e r a t io n .
Extremely lim ite d potassic a lt e r a tio n occurs in orthoclase ve in le ts
near the confluence of the east and south forks of Emigrant Creek
(P late l b ) . This style of a lt e r a tio n was not observed in other in tru s iv e
rocks in Emigrant Gulch and probably took place before emplacement of
the quartz monzonite s u ite .
Varying degrees of hydrothermal a c t i v i t y accompanied both e a rly and
la t e in tru s iv e series in the quartz monzonite s u ite .
The only brecciation
95
observed in e a rly quartz monzonite was a small patch on the western
boundary o f the large stock, 100 meters north of the lower road (P late l a ) ,
Chalcopyrite and molybdenite v e in le ts cut this brecciated patch, but
m in e ra liz a tio n and hydrothermal a c t i v i t y may have resulted from a la t e r
period of hydrothermal a c t i v i t y related to the la te In tru s iv e series.
Disseminated chalcopyrite a f t e r b io t i t e Is common In the e a rly In tru siv e
s e rie s .
A lte ra tio n In the e a rly quartz monzonite generally results from
a r g il 11c weathering rath er than a hypogene source, and decreases In In ­
te n s ity with depth.
The main period o f brecciation and m ineralizatio n In Emigrant Gulch
occurred a f t e r the bulk of quartz porphyry emplacement but before fin a l
In tru sio n of plagioclase porphyry.
plained above.
Relationships In these rocks are ex­
Seven separate brecciated areas containing quartz porphyry
fragments were noted (P late l b ) .
The A llison breccia contains the l a r ­
gest amount o f v is ib le molybdenum.
These breccias form a north-trending
zone centered on the A llis o n breccia.
This trend possibly resulted from
major structures at depth as Pfau (1981) noted s ig n ific a n t shearing In
d r i l l hole Med-11.
However only a few minor, surface fa u lts can be
documented, and lack of both demonstrable o ffs e t In porphyry dikes and
abundant shearing In rhyodacite porphyry argues against existence of
major f a u l t zones In the study area.
Common sllckensldes on porphyry
dike margins record minor movement expected during consolidation and
cooling.
No pattern of m in eralizatio n d ir e c t ly Indicates fa u lt-c o n ­
t r o l l e d d is tr ib u tio n of hydrothermal flu id s or m in e ra liz a tio n .
96
Emplacement of a l l the breccias along the north-south trend probably
did not occur simultaneously.
Depth of emplacement and dip of the zone
of brecciation are unknown, although d r i l l hole Med-1 bottomed below or
adjacent to the A llis o n breccia without in tersecting brecciated rock.
The A llis o n breccia and breccias to the north could pass downward into
molybdenite stockworks which did not behave as explosively.
Supporting
th is is a section of d r i l l core in Med-1 between 305 and 341 meters depth
which displays w el 1-developed quartz-molybdenite stockworks with core
assays up to 0.44 percent molybdenum.
This stockwork may represent a
deeper or peripheral phase of the A llison breccia.
In any case, the
A llison breccia is located near the in tru s iv e and mineralized center of
the Emigrant Gulch complex, and in the most altered part of the system.
Later hydrothermal a c t i v i t y produced l i t t l e s u lfid e m ineralization
but la t e porphyry was pervasively altered to s e r i c i t e , with some a r g i l i c
a l t e r a t io n .
Molybdenum m in eralizatio n at Emigrant Gulch can be described as a
c a lc -a lk a lin e -ty p e deposit in terms of the c la s s ific a tio n system given by
Westra and others (1981).
This system uses the chemistry o f the in tru s iv e
rocks associated with m in eralizatio n to characterize the deposit.
The
c a lc -a lk a lin e nature of the quartz monzonite su ite (Fig. 14) contrasts
with intrusions in Climax-type molybdenum deposits, which commonly con­
ta in more sodium and potassium.
The Emigrant Gulch rocks also contain
lower q u a n titie s of flu o r in e and rubidium than are normally associated
with Climax-type deposits.
The morphology of the Emigrant Gulch deposit,
though s t i l l poorly defined, does not appear to form a cupola-shaped body
97
such as at Climax, Colorado (White and others, 1981).
Although both
these systems orig inated from sp atia lly-ass o ciate d porphyry dikes, the
Emigrant Gulch deposit is ir r e g u la r , with much of the observable m in e ra li­
zation in the matrix o f the A llison breccia.
As explained above, more
d r i l l i n g is needed to define the sp atial and temporal relation ship of
th is brecciation with molybdenite stockworks found at depth.
The
occurrence o f molybdenum in the matrix of the A llison breccia, and the
c a lc -a lk a lin e composition of in tru s iv e rocks makes Emigrant Gulch sim ila r
to deposits such as Boss Mountain, B r itis h Columbia.
c a lc -a lk a lin e molybdenum deposits in the
Many of the other
Canadian C o rd ille ra also lack
regular stockworks, with m in eralizatio n occurring as tabular sheets be­
tween coalescing dike swarms.
The geometry o f stockwork m ineralization
a t Emigrant Gulch may prove to be s im ila r to these deposits such as
K its a u lt , Bell Molybdenum, and B ritis h Columbia Molybdenum (Soregaroli
and Brown, 1976).
CHAPTER V II
CONCLUSION
Textural and chemical data from Emigrant Gulch rocks were pre­
sented in th is study to propose a model fo r the c r y s t a lliz a t io n and
d i f f e r e n t ia t io n of these quartz monzonite magmas.
No single model can
explain a l l the complex, in te r re la te d features of this system, or any
other porphyry system.
The study showed however, th at a cooling model
by Whitney (1975b) can be combined with d if f e r e n t ia t io n by v o la t i le
d iffu s io n to explain variatio ns in rock texture and composition in the
quartz monzonite s u ite .
In ad d itio n , these models account fo r the
timing o f hydrothermal a lt e r a t io n , b rec cia tio n , and m ineralizatio n in
the la te in tru s iv e series.
This approach was used to study the Emigrant Gulch rocks in order
to c l a r i f y the o rig in o f the porphyry system and i t s relation ship to
m in e ra liz a tio n .
These ideas may be applicable to other mineralized
quartz monzonite systems where s im ila r rock textures and compositional
trends are seen.
98
REFERENCES CITED
Abbott, R .N ., 1978, P e r ite c tic relationships in the system An-Ab-Or-QzHgO: Canadian M in eralo gist, v. 16, p. 245-256.
B ailey, J .C ., 1977, Fluorine in g r a n itic rocks and melts:
Chem. Geol. , v. 19, p. 1-42.
A review:
Bambauer, H ., Corbett, M ., Eberhard, E . , and Viswanathan, K ., 1967,
Diagrams fo r the determination of plagioclase using X-ray
powder methods: Schweizer, Mineralog. and Petrog. M i t t . , v. 47,
p. 333-349.
Basler, A .L ., 1965, Geology o f the Emigrant Creek in tru siv e complex.
Park County, Montana: unpub. M.S. th esis , Montana State College,
52 p.
Bonini, W.E., K e lly , J r . , W.N., and Hughes, D.W., 1972, Gravity studies
o f the Crazy Mountains and the west flank of the Beartooth
Mountains, Montana: In_: Montana Geol. Soc., 21st Ann. Geol.
Conf., p. 119-127.
Brown, G.C., 1970, A comment on the ro le of water in the p a r tia l fusion
of crustal rocks: Earth and Planet Sci. L e tte rs , v. 9, p. 355-358.
Chadwick, R .A ., 1970, Belts o f eruptive centers in the AbsarokaG a lla tin volcanic province, Wyoming-Montana: Geol. Soc. Amer.
B u l l . , V . 81, p. 267-274.
__________ , 1969, The northern G a lla tin Range, Montana: northwestern
part o f the Absaroka-Gall a t i n volcanic f i e l d : Univ. of Wyoming
Contr. to G eol., v. 8 , p. 150-166.
, 1968a, Structural and chemical relationships in the AbsarokaGal l a t i n volcanic province, Wyoming-Montana (a b s .): Geol. Soc.
Am. Prog. Ann. M tg., Mexico C ity , p. 50-51.
, 1967, Two Eocene volcanic episodes, G a lla tin Range, Montana
(a b s .): Geol. Soc. Am., Prog. 20th Ann. Mtg., Rocky Mtn. Section,
p. 27-28.
, 1966, Volcanic vent complex a t Point of Rocks, G a lla tin
Range, Montana (a b s .): Geol. Soc. Am., Prog. 20th Ann. Mtg., Rocky
Mtn. Section, p. 27-28,
99
100
Chadwick, R .A ., 1964, Volcanic rocks of the G a lla tin Range, southwestern
Montana (a b s .): Geol. Soc. Am. Spec. Paper 76, p. 267-268.
Carmichael, I . S . , Turner, F . J . , and Verhoogen, J . , 1974, Igneous
Petrology: McGraw-Hill Book Co., New York, N .Y ., 739 p.
C asella, C ., 1967, Sylvan Pass in tru s iv e center, Absaroka Mountains,
Wyoming (a b s .): Geol. Soc. Am., Prog. 20th Ann. Mtg., Rocky
Mtn. Section, p. 27.
Dickinson, W.R., 1968, Problems of s tra tig ra p h ie nomenclature in F i j i
(a b s .): New Zealand Journ. Geol. Geophys., v. 10, p. 1181-1182,
1967.
E l l i o t , J . E . , G a s k ill, D .L ., Raymond, W.H., Peterson, D .L ., Stotelmayer,
R .B ., Johnson, F . L ., Lindsey, D .S ., Ridenour, J . , and Schmauch, S.W.,
1977, Mineral resources of the north Absaroka study area. Park and
Sweetgrass Counties, Montana: U.S. Geol. Surv., Bur. of Mines,
open f i l e report 77-700.
Emmons, W., 1908, Geology of the Haystack Stock, Cowles, Park County,
Montana: Jour. G eo l., v. 16, p. 193-229.
Ernst, W.G., 1976, Petrologic Phase E q u ilib r ia :
Company, San Francisco, 333 p.
W.H. Freeman and
Fisher, F .S ., 1972, T e r tia r y m ineralizatio n and hydrothermal a lte r a tio n
in the stinkingwater mining region. Park County, Wyoming, U.S.
Geol. Surv. B u l l . , no. 1332c, 32 p.
Foose, R.M., Wise, D .V ., and G arbarini, G .S ., 1961, Structural geology of
the Beartooth Mountains, Montana and Wyoming: Geol. Soc, Am.
B u l l . , V . 72, p. 1143-1172.
Fraser, G.D., Walthrop, H .A ., and Hyden, H .J ., 1969, Geology of the
Gardiner area. Park County, Montana: U.S. Geol. Surv. B ull. 1277,
118 p.
Freestone, I . C . , 1978, Liquid im m is c ib ility in a l k a l i - r i c h magmas:
Chemical Geology, v. 23, p. 115-123.
Hague, H ., Iddings, J . P . , and Weed, W.H., 1899, Geology o f Yellowstone
National Park: Part I I , Descriptive geology, petrography, and
paleontology: U.S. Geol. Surv. Mon. 32, p. 1-439.
H a ll, W.B., 1961, Geology of part of the upper G a lla tin V alley o f south­
western Montana: unpub. Ph.D. d is s e rta tio n , Univ. of Wyoming,
Laramie, 239 p.
101
Hemley, J . J . , and Jones, W.R., 1964, Chemical aspects of hydrothermal
a lt e r a t io n : Econ. Geol., v. 59, p. 538-569.
H ild r e th , E.W., 1979, The Bishop Tu ff: Evidence fo r the origin of
compositional zonation in magma chambers: In^' Chapin, G.E.,
and Elston, W.E., eds., Ash-flow Tuffs: Geol. Soc. Am. Spec.
Paper 180, p. 43-75.
, 1980, Gradients in s i l i c i c magma chambers: implications
fo r lith o s p h e ric magmatism: Jour. Geophys. Res., v. 86 ,
p. 10153-10192.
Horberg, L . , 1940, Geomorphic problems and g la c ia l geology of the Yellow­
stone V a lle y , Park County, Montana: Jour. Geol., v. 48, p. 275-303.
Hyndman, D.W., 1982, in press. Petrology of Igneous and Metamorphic
Rocks, Second Edition: McGraw-Hill Book Co., New York, N.Y.
Krushenski, R .D ., 1962, Volcanic features of the Hurricane Mesa area.
Park County, Wyoming (abs.): Geol. Soc. Am. Spec. Paper 68 , p. 214.
Covering, T . S ., 1929, The New World or Cook City mining d i s t r i c t . Park
County, Montana: U.S. Geol. Surv. B ull. 811, p. 1-87.
Luddington, S ., Sharp, W.N., McKowan, D ., et a l , 1979, The Redskin
Granite; a Proterozoic example o f thermogravitational diffusion?
(a b s .): Geol. Soc. Am., Abst. Prog., v. 11 (7), p. 469.
McMannis, W .J., and Chadwick, R.A., 1964, Geology of the Garnet Mountain
Quadrangle, G a lla tin County, Montana: Mont. Bureau Mines and
Geol. B u ll. 43, 47 p.
Meyer, C . , and Hemley, J . J . , 1967, Wall rock a lt e r a tio n , Jn_: Barnes,
H .L ., e d .. Geochemistry o f hydrothermal ore deposits: New York,
H o lt, Rinehart and Winston, In c ., p. 166-235.
, Shea, E .P ., Goddard, J r . , C .C ., et a l , 1968, Ore deposits
a t Butte, Montana, p t. 10, ch. 65, p. 1373-1416, Ijn: Ore Deposits
o f the United S tates, 1933-1967, the Graton-Sales Volume ( I I ) ,
Ridge, J . D . , ed.: A .I. M . E . , New York, 1880 p.
Nockolds, S .R ., and A lle n , R ., 1954, Average chemical composition of
some igneous rocks: Geol. Soc. Amer. B ull. 65, p. 1007-1032.
Norton, D . , 1978, Source lin e s , source-regions, and pathlines fo r flu id s
in hydrothermal systems related to cooling plutons: Econ. Geol.,
V . 73, p. 21-28.
102
Norton, D ., and Knight, R ., 1977, Transport phenomena in hydrothermal
systems; cooling plutons: Amer. Jour. S c i . , v. 2 7 7(8 ), p. 937-981.
Parsons, W.H., 1969, C r i t e r ia fo r the recognition of volcanic breccias:
Review in igneous and metamorphic petrology - A volume in honor of
A ire Poldervart: Geol. Soc. Amer. Mem. 115, p. 263-304.
, 1960, Origin of T e r tia r y volcanic breccias, Wyoming:
In te r n a t. Geol. Congress, XXI session, Pt. X I I I , Copenhagen,
Denmark, p. 139-146.
, 1958, O rig in , age, and tectonic relationships o f the volcanic
rocks in the Absaroka Yellowstone-Beartooth region, Wyoming-Montana:
B illin g s Geol. Soc. Guidebook, 9th Ann. Field Conf., p. 36-43.
, 1939, Volcanic centers of the Sunlight area. Park County,
Wyoming: Jour. Geol., v. 47, p. 1-26.
Peacock, M.A., 1931, C la s s ific a tio n of igneous rock series:
V . 39, p . 54-67.
Jour. Geol.,
Pfau, M ., 1981, Geology o f the Emigrant Gulch Prospect, copper-molybdenum
complex. Park County, Montana: unpub. M.S. th esis, Univ. of Idaho,
89 p.
P h ilp o tts , A .R ., 1976, S ilic a t e liq u id im m is c ib ility ; i t s probable extent
and petrogenetic sig nifican ce: Am. Jour. S c i . , v. 276, p. 1147-1177.
Reid, R.R., McMannis, W .J., and Palmquist, J .C ., 1975, Precambrian geology
of the north Snowy block, Beartooth Mountains, Montana: Geol. Soc.
Am. Spec. Paper 157, 135 p.
Rubel, D .N ., 1971, Independence volcano: a major Eocene eruptive center,
northern Absaroka volcanic province: Geol. Soc. Am. B u l l . , v. 82,
p. 2473-2494.
, 1972, Geology o f the p r e -T e r tia ry rocks in the northern part
of Yellowstone National Park, Wyoming: U.S. Geol. Surv. Prof.
Paper 729-A, 66 p.
Schultz, C .H ., 1968, Mount Washburn Volcano; a major Eocene volcanic
vent (a b s .): Geol. Soc. Am., Prog. 21st Ann. Mtg., Rocky Mtn.
Section, p. 73.
Shaver, K.C., 1978, Chronology of dacite intrusion in the northwest end
of the Absaroka-Gall a t i n volcanic province, Montana: Northwest
Geology, v. 7 ( 4 ) , p. 1409-1424.
103
Shaver, K.C., 1974, Dacites of the northern G a lla tin and western
Beartooth Ranges, Montana: unpub. M.S. th esis, Montana State
U n iv e rs ity , 109 p.
Shaw, H .R ., 1974, Diffusion of HpO in g r a n itic liq u id s: Part 1, Ex­
perimental data; Part I I , Mass tra n s fe r in magma chambers; I n :
Hoffman, A.W., and others, eds., Geochemical Transport and Kinetics:
Carnegie In s t it u t io n o f Washington Publication 634, p. 139-170.
_, 1965, Comments on v is c o s ity , crystal s e t t l i n g , and convection
in g r a n itic magmas: Am. Jour. S c i . , v. 263, p. 120-152.
Smedes, H.W., and Prostka, H .J ., 1972, S tratig ra p h ie framework of the
Absaroka Volcanic Supergroup in the Yellowstone National Park Region:
U.S. Geol. Surv. Prof. Paper 729-C, 33 p.
Smith, R .L ., 1979, Ash-flow magmatism, In^: Chapin, C .E ., and Elston, W.E.,
e d s., Ash-flow Tuffs: Geol. Soc. Am. Spec. Paper 180, p. 5-27.
Smithson, S .B ., Brewer, J .A ., Kaufman, S. O liv e r, J .E ., and Jurich, C.A.,
1979, Structure o f the Laramide Wind River u p l i f t , Wyoming, from
Cocorp deep r e fle c tio n data and from g ra v ity data: Jour. Geophys.
Res., V . 84 { B I D , p. 5955-5972.
S oregaro li, A.E. and Brown, A .S ., 1976, Characteristics of Canadian
C o rd ille ra molybdenum deposits, p. 417-431, h }i Porphyry Deposits
of the Canadian C o r d ille r a , A volume dedicated to Charles S. Ney,
C .I.M . special volume 15: H a rp e ll's Press Cooperative, Ste.
Anne de Bellevue, 510 p.
T i l l i n g , R . I . , 1968, Zonal d is tr ib u tio n and variatio ns in stru ctu ral state
of a lk a lin e feldspars w ithin the Rader Creek pluton, Boulder b a th o lith
Montana: Jour. P e t r o l . , v. 9, p. 331-357.
Troger, C.G., 1979, Optical Determination of Rock-forming Minerals,
Part 1, Determinative Tables: E. Schweizerbartsche
Verlag buchhandlung, S tu t tg a r t , 188 p.
T u t t le , O .F ., and Bowen, N .L ., 1958, Origin of granite in lig h t of ex p eri­
mental studies: Geol. Soc. Amer. Mem. 74, p. 1-153.
Von Platen, H ., 1965, C r i s t a llis a t io n granitScher Schmelzen:
M ineral. P e tr o g r ., v. 11, p. 334-381.
B e itr.
, and Winkler, H .G .F ., 1961, K r i s t a llis a t io n eines Obsidians
bei Anwesenheit von H«0, NH_, MCI, HF, unter 200Atm.: Druck
Fortschr. M in e r a l., v. 39, p. 355.
104
Wedow, J r . , H ., G a s k ill, D .L ., Banister, D ., e t a l , 1975, Mineral
resources of the Absaroka P rim itive area and v i c i n i t y . Park and
Sweetwatergrass Counties, Montana: U.S. Geol. Surv. B ull.
13911-B, 115 p.
Westra, G. and Keith, S .B ., 1981, C la s s ific a tio n and genesis of stockwork
molybdenum deposits: Econ. Geol., v. 76, p. 844-873.
White, W.H., Bookstrom, A . A . , Kamil 1, R .J ., Ganster, M.W., Smith, R .P .,
Ranta, D .E ., and S tein ing er, R.C., 1981, Character and orig in of
Climax-type molybdenum deposits, p. 270-316, In.: Skinner, B .J .,
e d .. Economic Geology, S e v e n ty -fifth Anniversary Volume 1905-1980,
The Econ. Geol. Pub. Co., El Paso, I X , 964 p.
Whitney, J . A . , 1975a, The Effects of pressure, temperature, and X H^O on
phase assemblages in four synthetic rock compositions: Jour.
Geol. , V. 83, p. 1-31.
, 1975b, Vapor generation in a quartz monzonite magma: a
synthetic model with application to porphyry copper deposits:
Econ. G eol., v. 70, p. 346-358.
Wilson, J r . , C.W., 1934, Geology of the th ru st f a u lt near Gardiner,
Montana:
Jour. G eol., v. 42, p. 649-663.
Wilson, J . T . ,
1936, Geology o f the M ill C re e k -S tilIw a te r area, Montana:
unpub. Ph.D. d is s e rta tio n , Princeton U niv ., Princeton, New Jersey.
Wilson, W.H.,
1963, Correlation of volcanic units in the southern Abasaroka
Mountains, northwest Wyoming: Wyoming Univ. Contr. to G eol., v. 2,
p. 13-20.
__________ , 1964a, The Kirwin mineralized area, Park County, Wyoming:
Wyoming Geol. Surv. Prelim. Report no. 2, 12 p.
Wold, R.O., 1972, Composition and stru ctu ral state va ria tio n of potassium
and plagioclase feldspars from the Phillipsburg B ath olith, Montana:
unpub. M.S. th esis, Univ. o f Montana, 95 p.
Wright, T . L . , and Stewart, D .B ., 1968a, X-ray and optical study of a lk a li
fe ld sp ar, I . Determination o f composition and structural state from
refin ed u n it - c e ll parameters and 2V: Amer. M in e ra l., v. 53,
p. 38-87.
, 1968b, X-ray and op tical study of a lk a li feldsp ar, I I .
An
X-ray method fo r determining the composition and stru ctu ra l state
from measurement of 2 theta values fo r three r e fle c tio n s : Amer.
M in e r a l., v. 53, p. 88-104.
APPENDIX 1
106
X-ray analysis o f potassium feldspar and groundmass minerals.
A)
Potassium Feldspar
1)
Method
Degree o f t r i c l in i c it y and Or content o f phenocrysts from
rhyodacite porphyry and the quartz monzonite suite were determined
using the methods o f Wright (1968).
Wright and Stewart (1968)
showed changes in u n it c e ll parameters fo r a l l potassium feldspar
s o lid -s o lu tio n series a f fe c t the positions of 060, 294, and 291
r e fle c tio n peaks.
29
ranges of these peaks for CuKa radiation
are shown fo r maximum m icrocline, orthoclase, and high sanidine
series on Figure 22.
Duplicate runs fo r each sample were made to
check v a r i a b i l i t y in the diffracto m ete r.
Late porphyry samples were
extremely a lte re d to hydrothermal clays and s e r i c i t e , and were un­
su ita b le fo r analysis.
Position of 291 peak re fle c tio n s de­
termined Or content o f each sample.
2)
Results
Potassium feldspar from a l l samples approached the structural
state of orthoclase.
Twenty values fo r 060 peaks f e l l between
4.66° and 41.72° and fo r 794 peaks values f e l l between 50.65° and
5 0 .72 °.
Calculated 29 values fo r 701 peaks generally agreed with
observed 7 oi peak positions with 0 . 1° , in dicating normal monoclinic
symmetry and non-anomalous ce ll dimensions.
Appearance o f 131 re­
fle c tio n s near 29 = 29.8° also revealed the dominant monoclinic
symmetry of each sample (see Wold, 1970, Fig. 7 ).
Potassium content o f orthoclase from a ll rock types covered a
range o f Org^ to Or^g, with most between Or^g and Or^g (F ig. 23).
Figure 22.
Determination of structural state
of potassium feldspars from
Emigrant Gulch in tru s iv e rocks.
Position of 201 peak r e fle c tio n
graphed against position of 060
and 204 peak positions.
KEY:
■ LATE PORPHYRY
A PLAGIOCLASE PORPHYRY
A
QUARTZ PORPHYRY
O HORNBLENDE
•
PORPHYRY
EARLY QUARTZ MONZONITE
108
21.10
2 1 .0 5
21.00
2 0 .9 5
2 0 .9 0
201
90
95
100
We i g h t Pe r c e n t Or
Figure 22.
(C uK« r a d i a t i o n )
Figure 23.
Potassium content of orthoclase
based on position o f the 201
peak r e fle c tio n .
(See Figure 22 fo r key.)
LOW ALB I TE
2 e 201
21.4
4 1 .9 0
MAXIMUM MICROCLINE
21.2
4 1 .8 0
21.0
MAXIMUM
/
MICROCLINE
4 1 .7 0
2
»
060
IGH
ORTHOCLASE
SAN I
I TE
lES
41 .6 0
HIGH SA N ID IN E
41 .5 0
50.50
50.70
2 e %4
Figure 23.
50.90
(CulQ
R ad ia tio n )
Ill
A ll samples contained only one homogenous, potassic feldspar,
without any c ry p to p e rth itic a lb it e component.
Documentation of
orthoclase composition changes in the quartz monzonite suite is
tenuous, due to v a r i a b i l i t y in recorded peak values.
B)
Groundmass Minerals
1)
Method
X-ray analyses o f thin slabs from rhyodacite porphyry and each
member o f the quartz monzonite suite allowed estimate of quartz and
feldspar content o f the groundmass in each sample.
Slabs one to
fiv e m illim ete rs th ick were scanned from 45° to 7° 29 to look fo r
va ria tio n s in c h a ra c te ris tic quartz, plagioclase, and orthoclase
peak r e fle c tio n patterns.
Peaks commonly overlapped and the d is ­
tr ib u tio n of some diagnostic re fle c tio n s was in fe rre d .
Bambauer
and others (1967) described the method fo r plagioclase determination
from 29^21 “
^®241 " ^®241’ ^sed in th is study.
Standard
curves c o rre la tin g in te n s ity o f plagioclase 201 , orthoclase 201 , and
quartz peaks with varying proportions of these minerals provided an
estimate of the amount of quartz and feldspar present in the ground­
mass (J. Wehrenberg, unpub. data, 1982).
B io t it e , hornblende, and
accessory minerals were assumed to be less than fiv e percent o f the
groundmass, with quartz and feldspars normalized to 100 percent.
2)
Results
Samples run from rhyodacite and a l l quartz monzonites produced
recognizable quartz, a lb ite -o lig o c la s e , and orthoclase peak re fle c tio n s
112
Peaks diagnostic o f plagioclase composition when resolvable,
co n sistently yielded values of An^ to An^^
no distinguishable trends.
a l l samples, with
Overall appearance of plagioclase
peaks was also intermediate between a lb it e and oligoclase (An-jy)
(Bambauer and others. Fig. 1, 1967).
Figure 24 shows a typical
X-ray pattern with diagnostic re fle c tio n s labeled.
The degree of hydrothermal a lt e r a tio n in the groundmass seen
in th in section does not co rre la te well to calculated quan tities
of plagioclase in the groundmass, as determined from X-ray in ­
t e n s it ie s .
Accuracy of the method is uncertain and r e l i a b i l i t y depends
p rim a rily on four facto rs:
1)
Accuracy o f the standard curves
2)
Assumption th a t a s u ffic ie n t number of grains are
randomly oriented to y ie ld representative data
3}
Key peaks are resolvable, with no superposition
4)
Phenocryst minerals contribute no s ig n ific a n t re fle c tio n s
V a r i a b i l i t y in determined groundmass mineral q u an titie s and discrepencies
with other modal data r e s u lt from these fa c to rs , although resolution of
these problems requires fu rth e r work.
Figure 24.
Typical X-ray pattern fo r groundmass of early
quartz monzonite. Diagnostic peaks labeled.
20 angle also shown.
Q = quartz
K = orthoclase
P
= plagioclase
p ISl
Plîl
zo
Figure 24.
APPENDIX 2
116
Appendix 2 shows mass-balance calculations comparing:
1)
Calculated weight percent calcium from plagioclase phenocrysts in d i f f e r e n t rock types with to ta l observed calcium
in each u n it from whole-rock chemistry.
2)
Calculated weight percent s i l i c a from plagioclase, ortho­
clase, and quartz phenocrysts in d if f e r e n t rock types with
to ta l observed s i l i c a in each u n it from whole-rock chemistry.
Rock types were chosen from both e a rly and la te in tru siv e series of the
quartz monzonite s u ite .
Conclusions:
1)
The calcium content o f plagioclase phenocrysts alone generally
accounts fo r the to ta l observed quantity o f calcium in each
//
member o f the s u ite .
In the e a rly in tru s iv e s e rie s, the small
amount o f calcic-hornblende phenocrysts probably account fo r
the remaining
calcium in each rock.
In the la te in tru s iv e
s e rie s , some calcium leached during hydrothermal a lte r a tio n
may have decreased the calcium content below calculated
values in each rock (Appendix 2 - 1 ).
2)
S i lic a contained in phenocryst minerals alone does not account
fo r differences in s i l i c a content between rock types.
These
differences must therefore consider groundmass s i l i c a in
addition to phenocryst s i l i c a (Appendix 2 - 2 ).
APPENDIX 2-1
General formula fo r ca lc u la tin g weight percent CaO in plagioclase phenocrysts:
cc = cubic centimeter
a)
cc plagioclase phenocrysts^ 1,OQcc rock
1.00 cc rock
mass o f 1.00 cc rock
modal % plagioclase
phenocrysts
rock density
mass o f ICC plagioclase ^ 1 mole plagioclase , ^oles plagio-
1.00 cc plagioclase
plagioclase density
a t given An content
clase phenocrysts
1 gram rock
grams plagioclase
molecular weight
of plagioclase
b)
moles plagioclase
phenocrysts______
1 gram rock
X
moles Ca
1 mole plagioclase
An content of
plagioclase
1 mole CaO
1 mole Ca
grams CaO
1 mole CaO
molecular
weight of Cao
= 56.08
=
grams CaO
1 gram rock
APPENDIX 2-1 (Continued)
Rock type
Modal % plagioclase phenocrysts
rock density
plagioclase density
molecular
weight
Wt %
CaO
Early quartz
monzonite
28
(A n,*)
2.651
2.660
265.99
1.42
Hornblende
porphyry
20
(An^^)
2.653
2.674
267.67
1.44
(a t 23% phenocrysts
1.65)
Quartz
porphyry
19
(Angg)
2.691
2.663
266.39
1.03
Late
porphyty
18
(An^a)
2.645
2.674
268.69
1.29
00
APPENDIX 2 -1 , (Continued)
Rock Type
Early
quartz
monzonite
hornblende
porphyry
Calculated wt % CaO
from plagioclase phenocrysts
Average
observed wt %*
1.42
1.57
1.44
(1.65)
1.79
quartz
porphyry
1.03
.91
la te
porphyry
1.29
1.21
*See Figure 17.
VO
120
Appendix 2-2
General formula fo r ca lcu latin g SiO^ content from plagioclase, orthoclase,
and quartz phenocrysts analogous to formula shown 2- 1 .
Rock type
Model % p lag io clase phenocrysts
e a rly
quartz
monzonite
Model % orthoModel of % Quartz
clase phenocrysts phenocrysts
Wt %
SiO«
28
5
5
25.62
hornblende
porphyry
20 (An24)
1
3
13.65
quartz
porphyry
19 (An26)
2
4
16.79
la t e
porphyry
18 (An^^)
1
2
13.56
average observed ASiOp
between rock types*
Calculated ASiOg from phenocrysts:
e a rly quartz monzonite = 11.97 wt %
- hornblende porphyry
1.40 wt %
quartz porphyry
- la te porphyry
3.23 wt %
1.10 wt %
8.8 wt %
-1 .5 0 wt %
=
e a rly quartz monzonite =
- quartz porphyry
*See Figure 17.
APPENDIX 3
122
Calculated S to k e 's Law s e ttlin g times fo r plagioclase phenocrysts in
Emigrant Gulch magmas.
Total s e t t lin g distance (d) =
2 gC^ (a p) T^
g = 980 centimeters*second
(Shaw, 1965)
I = to ta l s e ttlin g times
C = crystal growth ra te (centimeters* year"^)
o
Ap = c r y s t a l- liq u id density contrast (grams*centimeters' )
= v is c o s ity (poises)
For Emigrant Gulch quartz monzonites:
_
o
1 ) assume average rock density o f 2.66 grams*centimeter'
(Appendix 2 - 1 ) .
_o
2) Assume average magma density o f 2.37 grams*centimeter'
(Hyndman, unpub. manusc., 1982, p. 139).
3) Assume average plagioclase density o f 2.67 grams*centimeter
-3
(Appendix 2 - 1 ) .
4) Ap fo r plagioclase liq u id :
2.67 - ( 0 .9 ) ( 0 .6 6 ) =
0.276 grams-centimeter'^
5) Assume reasonable vis co sity range of log^ = 7.8 - 9.8 poises
fo r quartz monzonite magma at 1-5 percent dissolved water
(Carmichael, 1974, Figure 4-6 ; Shaw, 1965).
For d = 100 meters to ta l s e t t lin g distance:
C (centimeterS'Vear'T)
l o g n ( p o is e s )
(years)
lO'G
7 .8
10®
10'7
8.8
10®
lO'G
9 .8
10^
123
For d = 10 meters to ta l s e t t lin g distance:
C (centimeters-vear~
lO'S
log
(poises)
T (years 1
7.8
10 ^
8.8
10 4
lO'S
8.7
2 X 10 4
lO'G
8.8
10 ^
3 X lO'S
Conclusions:
1) Total s e ttlin g distances of 100 meters give s e ttlin g times
of 10
5
- 10
7
years.
These times are unreasonably long
according to th eo retical cooling times given by Whitney
(1975b) fo r a small quartz monzonite pluton.
2) Total s e t t lin g distances of 1 0 meters give s e ttlin g times
o f 10 ^ - 10 ^ years.
These times agree with those lis te d
by Whitney (1975b) in the th eo retical model.
APPENDIX 4
Appendix 4 - l a .
Mineral
Size
piagioclase
1-4mm,
2 -3mm
avg.
1-7mm,
1 -2mm
avg.
few
> 6mm
(An
C r y s t a l 1i ni t y
Di agnosti c
Features
I n c l usions
A lteration
Mi n e r a l s
Shape
Angularity
Distribution
subhedralanhedral
equantelongate,
and
irregular
subr oundedsubangul ar
f a ir ly hiatial
and i n g r o u n d ­
mass
glomeroporphyrit i c , commonly
w i t h p ot a ss i um
f e l d s p a r and
bio tite ; alb i t e twinning
common, poor
pericline,
Carlsbad-synnuesis t y p etwins; l i t t l e
z o n i n g ; mi n or undulatory extin c­
tion
apatite,
piagiocl ase
subhedral,
s m a l 1 er
g r a i ns
a nh e dr a l
equantelongate
subangular,
some
rounded
b or de r s
generally
h ia tia l,
and i n
ground­
mass
2V:35°-45°
Ca r l s b a d t w i n ­
ni ng common
very minorapati t e ,
pi a g i o moder a t e
clase,
low-6 c l a y s ,
biotite?
s e ric i te
as p r i ­
mary phases ;
also poly­
crystal lin e q u a r t z a g­
gregates
f i l l i n g hol es
and as r e ­
pl a c e me nt s
2 4 - 28 /
potas­
sium
felds­
par
Rhyodaci te porphyry and r e l a t e d brecci as
*An d e t e r m i n a t i o n by b i s e c t r i x method
o f Troger (1979)
mo d e r a t e pervasive
low -brief
clays,
mi n o r s e r i c ite , epidote, car­
bonat e
Comments
commonly a l i g n e d sub­
p a r a l l e l to quartzr i c h f l o w l ami n a e
wher e not b r e c c i a t e d ;
some ri mmi ng and
c u t t i n g potassium
feldspar
siightly-moderately
embayed; some i r ­
r e g u l a r l y rimmed by
p l a g ; some ri ms r e ­
p l a c e d by m i c r o c r y s t a l i n e , a nh e dr a l
groundmass ( q u a r t z r i c h ) ; most abundant
ph e n o c r y s t phase;
broken and w i t h und ul os e e x t i n c t i o n i n
a ut o b r e c c i a ; c u t by
s i l i c a - r i c h flow
banding; p o l y c r y s t a l l i n e
quartz-segregations
surroundi ng g rai ns
ro
in
Appendix 4 j a . ( Conti nued)
Mineral
Size
quartz
no p h e n o c r y s t s o bs e r v e d
biotite
l -5mm,
1mm
avg. ?
gr ound­
mass
C r y s t a l 1 i ni t y
Shape
Angularity
Distribution
Di a g n o s t i c
Features
I nc l u s i o ns
A1 t e r a t i o n
Minerals
anh edral, equigranu1a r t o s e r i a t e p o l y c r y s t a l l i n e grains
i n g r oundmas s , l i t h i c
fragments, f e l d s p a r border replacements,
f l o w l a mi n a e (<_2mm),
and o t h e r s e g r e g a t i o n s
i n ground­
mass, and
probably
a l s o as
a recrystal i z e d phase
subhedr al
subequantelongate,
irregular
clots
subangul ar
h ia tia l,
and o r i g i n ­
a l l y in
groundmass
generally
anhedral ,
some sub­
hedral
grai n
boundaries
equant
subequant,
few e l o n ­
gate
grains
subrounded
r e g u l a r to
locally
irregular
and
seriate
Comments
f r a g me n t s
q u a r t z , po­
tassi um
felds­
par g e n e r a l l y
identi fiab l e ;
plagioclase,
biotite id e n ti­
f i a b l e i n some
sampl es
pervasive
apatite,
serici t e ,
l ow- bi r e f .
c l a y s , chlori t e ,
opaquei r o n oxides
sphere?,
quartz?
commonly s ur r o un de d by
o r p a r t i a l l y ri mmi ng
glomer opor phyri t i c
feldspars
pervasive
1ow- bi r e f .
clays in
some
sampl es
repl acement along i r r e g u l a r grainborders
indicative of d e v i t r i ­
f i c a t i o n f rom o r i g i n a l
gl a s s ? ; c o a r s e r s e g ­
r e g a t i o n s and v e i n l e t s
o f q u a r t z - r i c h ma­
t e r i a l gradational into
f i n e r groundmass,
may r e p l a c e and cut
potassi um- fe ld sp ar
p h e n o c r y s t s ; p y r i t e com­
monly a s s o c i a t e d w i t h
q u a r t z - r i c h zones
ro
CTi
Appendix 4-1a-
Mi n e r a l
Si ze
lithic
f ragments
1-lOmm?
C r y s t a l ! i n ity
Shape
equantirregular
( Co n t i n ue d ’
Angularity
Di s t r i b u t i o n
subangul ar
seriate
Diagnosti c
Features
groundmass
of frag­
ments i d e n ­
t i c a l to
groundmass
o f nonbrecciated
samples
Inclusions
all p ri­
mary m i n e r a l
phases
described
above
A1 t e r a t i o n
Minerals
Comments
b r e c c i a m a t r i x mag­
n e t i c and i d e n t i c a l
t o groundmass o f
l i t h i c fragments
and n o n - b r e c c i a t e d
sampl es
ro
Appendix 4 - l b . E a r l y q ua r t z monzoni t_e_
Mi neral
plagieclase
Size
Cr y s t a l 1 i ni t y
l - 6 mni,
avg.
subhedraleuhedral
3 mm
Shape
subequantelongate
Angularity
subr ounde ds ubangul ar
(*"21-30)
potass i urn
felds­
par
comp?
Distribution
s e r i a t e , and
i n g r ound-
mass
1-5mm,
2 mm
av g. ,
few
megacrysts
> 1 2 mm
subhedra1 anhedral ,
few e u­
hedral
me g a c r y s t s
equantelongate.
commonly
irregular
subangul ar
s e r i a t e , or
h i a t i a l and
< 1 . 5mm, and
in ground­
mass
Di ag n os t i c
Features.
Comments
I nc l u s i on s
Alteration
mi n o r normal
to s l i g h t l y
o s c i l l a tory
zoning; a l b ite twinn i n g common
wi t h mi nor
p e r i cl 1 n e ,
Carl sbad
twinning;
g lomer opor­
phyri t i c
pi a g i o c l a s e ,
altered
bioti t e ,
apati te ,
quartz?
v e r y mi n o r
t o moder a t e
low -brief,
c l a y s and
mi n o r s e r i ­
ci t e ;
e pi d o t e
mi n or embayn:ent;
sut ured subangul ar
b o r d e r s f rom r e ­
p l a c e me n t by g r o u n d ­
mass common; some
g r a i n s grown t o ­
geth er w i th b i o t i te ;
normal A n - r a n g e i n
b o r d e r phase
Ca r l s b a d
t w i n n i n g , com­
monly em­
b ayed; 2V:
35°-40°
plagioclase.
b io tite.
apati te
very
low-bi
clays
s e r i ci
no p h e n o c r y s t s i n
b o r d e r phase; abundant
r e p l a c e m e n t o f ri ms
by i n d i v i d u a l , a n ­
h e d r a l - m i c r o c r y s t a l 1 i ne
groundmass g r a i n s ;
ri m repl acement prod uc i ng i n c i p i e n t
granophyric t e x t u r e ;
repl acen'. ent p r o d u c i n g
i n c i p i e n t granophyric
t e x t u r e ; repl acement
by l a r g e r b i o t i te
and p l a g i o c l a s e
crystals
(-)
mi n or
ref.
and
te
ro
00
Appendix 4 - l b .
Mi neral
Si ze
C r y s t a l 1i n i t y
Shape
quartz
1-5mm,
1 . 5mm
avg.
s ub he d r a l anhedral
e qua nt elongate,
commonly
sub­
e quant o r
i rregular
l - 3 mm,
subhedra1
subequant elongate
b iotite
1 mm
avg.
Angularity
subr oundedangular
subangul ar angular
(Continued
Distribution
Di ag n os t i c
Fea tures
I nc l u s i on s
Alteration
Comments
generally
h i a t i a l 1 - 2 mm,
and i n
groundmass
o n l y mi n o r embayment ; some
separate grains
in optical
continui ty
v e r y few
piagioc l a s e and
p o t a s s i urn
feldspar
inclusions
common s u t u r e d - s u b a n g u l a r ri ms f r om
r e p l a c e m e n t by potassium-feldspar
r i c h g r oundmas s ;
repl acement p r o ­
ducing i n c i p i e n t
granophyric t e x t u r e ;
few p h e n o c r y s t s i n
b o r d e r phase
h i a t i a l , ^ 1 . 5 mm,
and i n g r o un d ­
mass
p leochr oi sm;
X'= 1 i ght-yellow
t an
z ‘ =brown
apatite,
generally
opaquemi n o r t o
iron oxides, s e r i c i t e .
mi nor
l ow- bi r e f .
r u t ile , all
clays,
f rom a l t e r ­
apatite,
ation
opaqueiron o xid es,
r u t i l e , ch­
lorite,
1 imonite,
e pi dot e?
some s l i g h t l y - b e n t
g r a i n s ; commonly grown
t o g e t h e r wi t h p i a g i o c la s e ; la rg e r grains
may r e p l a c e p ot a s s i u m
f e l d s p a r , and p l a g i o ­
c l a s e ? ; abundant
apatite after b io tite
ro
CO
Appendix 4 - l b .
(ConLinueJ)
Mi ne r a l
Si ze
Crystal 1ini ty
Shape
Angularity
Distribution
Di a g n os t i c
Features
h or n ­
bl ende
1 - 3mm,
1 mm
avg.
subhedrala nh e dr a l
equantirregular
s ub a n g u l a r
h iatial
fragments
acces­
s or y
min­
eral s
goundmass
apatite,
ru tile ,
anhedral,
mi n or sub­
hedral
g r a i nb oundar i es
pyrite,
chaic o p y r it e , opaque-iron o xid es,
e qua nt subequant
rounded
generally
somewhat
seriate
lim nite,
epidote?,
I nc l u s i on s
Alteration
Comments
f i n e - grained
bioti te,
and r e l a t e d
repl acement
minerals
a fte r bio­
t i t e ; per­
vasive
remnant s w i t h no
o rig in a l grains
preserved
v e r y mi n or
low-bi r e f .
c l a y s from
feldspars ;
v e r y mi nor
bio tite a l­
teration
abunda nt q u a r t z feldspar re place­
ment ; a l l above
m i n e r a l phases e x ­
c e p t h or nb l e n de
generally present;
i n c i p i e n t granop h y ritic texture
zircon
very-fine
micro-crystal ■
l i n e to nearphenocryst
si ze
CO
o
Appendix 4 - l c .
Mineral
Size
plagio­
clase
2
(^ " 28-36
po­
t a s s i urn
felds­
par
1 - 6 mm
- 3mm
avg.
few
pheno­
crysts
^ 1 0 mm
1 -5mm
1 mm
avg. ;
few
mega­
crysts
> 1 1 mm
C r y s t a l 1i n i t y
Shape
Angularity
Hornblende porphyry
Distribution
Di agnost i c
Features
I nc l u s i on s
Alteration
sub he d r a l
euhedral
subequantelongate,
mi nor
quant
grains
subroundedrounded,
some sub­
angular
boundaries
generally
h i a t i a l , and
i n ground­
mass
normal zo ni ng
( 4 An m a x i ­
mum); common
a l b it e , Carls­
bad t w i n n i n g ,
p e r i c l i ne
twinning less
distinct;
r a p a k i v i ri ms
on pot a ss i um
feldspar;
glomeropor­
phyri t i c
p lag ioclase, minor-pera l b i t e , b i o - v a s i v e l owt i t e , quartz? b i r e f . c l a y s ,
as p r i m a r y
mi nor - modi n c l u s i o ns
erate s e r i­
ci t e ; mi n o r
carbonate
subhed
mi n or eui
hedral
and an
hedral
grains
subequant elongate;
some
e qu a nt
grains
rounded,
some
sub angul ar
b oundar i es
generally
hiatial
(< 1 mm
and
i n ground
mass
2V:35°-40° ( - ) ;
r a p a k i v i ri ms
of plagioclase
b io tite,
quartz,
pi a g i o c l a s e
a p a t i t e as
primary i n ­
clusions ;
compi ex
quartzplagioclase
replacements
v e r y mi n or
l ow- bi r e f .
cl ays
Comments
sutured-subangular
b o r d e r s commonly r e ­
p l a c e d by a n h e d r a l m i c r y c r y s t a l 1 i ne
groundmass ( q u a r t z r i c h ) ; mi n o r r i m ­
ming by p h e n o c r y s t s iz e , anhedralpot a s s i u m f e l d s p a r ;
some r a p a k i v i - t y p e
aggregates lacking
potassium-feldspar
cores; gr anophyricgroundmass common
near g r a i n borders
some samples c o n t a i n
no p h e n o c r y s t s o r '
mega cry sts , v ery i r ­
regular d is trib u tio n
of 1arger grains ;
r a p a k i v i cor e s v e r y
i r r e g u l a r and r a g ­
ged; mi n or s u t u r e d s u b a n g u l a r b or de r s
f rom r e p l a c e m e n t by
anhedral-mi croc r y s t a l l i n e ground­
mass
CO
Appendix 4 - l c .
(Conti nued
Diagnosti c
Feat ur es
Mineral
Si ze
quartz
l-4inm,
1 mm
avg.
subhedralanhedral
equantelongate,
generally
i rregular
rounded,
come sub­
angular
boundaries
h i a t i a l , few
phenocrysts
> 1 . 5 mm, and
i n gr o un d ­
mass
v e r y embayed
apatite,
v e r y mi n o r
plagioclase
and b i o t i t e
(primary?);
some d e ­
finitely
after
quartz
1-
s ubhedr al
subsequenti rregular
subangular,
angular
terminations
common
s e r i ci t e ;
and i n
gr o un d ­
mass
p l e o c h r o i sm;
X '= 1 i ghty e l low t a n
z '=1 ightr e d d i s h brown
t o d a r k brown
apati t e ,
ru ti1e
f rom a l ­
teration
bio­
t i te
3mm,
2 mm
a v g.
C r y s t a l 1i n i t y
Shape
Angulari ty
D i s t r i b u t i on
Inclusions
Alteration
Comments
sutured-subangular
r i ms r e p l a c e d by a n ­
h e d r a l -mi c r o c r y s t a l l i n e groundmass
(plagioclase-rich)
on o t h e r w i s e rounded
g r a i n s ; b i o t i t e and
plagioclase f i l l
embayed hol e s
generally
mi n o r c h ­
lorite,
apati t e ,
opaquei ron
oxides,
r u t i l e ; very
mi n o r s e r i ­
c i t e ; po­
t a s s i umfeldspar
q ua rt z?
some morepervasive
alteration
f i n e g r a i n e d and
i r r e g u l a r when r e ­
p l a c i n g hornblende
OJ
ro
Appendix 4 _ i ç .
Mineral
hornb l e n de
Size
l-3min
2 mm
avg
accessory
minerals:
ground­
mass
Crystal 1i n i ty
Shape
subhedra 1anhedral
subequantelongate;
i rregular
clots
apatite,
ru tile,
anhedral
w i t h sub­
hedral
b oundar i es
common
sphene,
Angularity
carbonate,
subequant
with elon­
gate
plagio­
clase
common
s ub a n g u l a r
( Conti nued'
Distribution
seriate
opaque-iron oxides,
roundeds ubangular
Di agnost i c
Features
fragments,
a l s o commonly
p r i s ma t i c
1 imonite,
r e g u l a r and
equigranul a r to
commonly
s e r i t i te
wi t h
larger
pi a g i o ­
c l a s e and
quartz
I nc l u s i on s
plagioclase,
p o t a s s i urn
feldspar,
quartz
via a l t e r ­
ation
Alteration
pervasivefeathery
b i o t i t e and
related a l ­
teration
p roduct s
Comments
remnant s w i t h no
o r i g i n a l grains
preserved
pyrite
f i ne-microc r y s t a l 1 i ne
to nearphenocryst
size
a p p a r e n t l y abunda nt
replacement; a l l
above phases e x c e p t
hornblende p r e s e n t ;
q u a r t z - p o t a s s i urn
feldspar in t e r ­
growt hs common, w i t h
granophyric t e x t u r e
CO
CO
Appendix 4 - I d.
Quart z porphyry
Di ag n os t i c
Features
Alteration
Mi n er a l s
Mi neral
Si ze
Cr ys t a l 1i n i t y
Shape
Angularity
Distribution
plagioclase
2
- 5mm,
4mm
avg.
subhedral,
l e s s commonl y e u­
hedral
subequantelongate
subr oundeds ub a n g u l a r
s e r i c i t e , and
i n groundmass
a l b i t e , peri
d i n e , Carls­
bad t w i n n i n g ,
a l b i t e domi ­
n a n t ; gl omer oporphyritic;
mi n o r z o n i ng
biotite,
v e r y mi n or
quartz,
a pa ti te
s ub he d r a l '
anhedral
subequant,
subroundedmi nor e l on- s u b a n g u l a r
g a t e mega­
crysts
s e r i c i t e , b ut
few me g a c r y s t s
> 5mm, and i n
groundmass
Carl sbad t w i n ­
ning; 2V :3 5 °4 0° ( - )
b io tite,
mi n o r s e r i c i t e
p l a g i o c l a s e , and l o w - b i r e f .
mi n o r
clays; carq u a r t z , a pbonat e
a t i t e ; some
may f i l l
embayments
sutured, ir r e g u l a r
boundaries r e ­
p l a c e d by g r o u n d ­
mass common, some
embayment ; some
i r r e g u l a r , discontinuous-plagioclase
r i m s ; me g a c r y s t s
i r r e g u l a r l y di stributed.
sub he d r a l
subequantelongate,
s e r i a t e , but
few phenos
< 1 mm, and i n
groundmass
extremely
embayed
b io tite,
plagioclase,
most f i l ­
l i n g hol e s
and embay­
ments ,
some p r i ­
mary b i o ­
tite ,
apatite
e x t r e m e l y embayed
w ith separate grains
commonly i n o p t i c a l
c o n t i n u i t y ; mi n or
s u t u r i n g o f subangular-grai n
b o r d e r s , and r e ­
pl acement by
groundmass
potassi urn
felds­
par
comp?
quartz
3-
1 - 1 2 mm
2-
3mm
avg
2 -5mm
2-3nim
avg.
roundedsubangul ar
I n c l u s i ons
mo d er a t e lowbi r e f . c l a y s
and mo d er a t e s evere s e r i ­
c i t e ; mi n o r
carbonate
Comments
most b i o t i t e ,
a p a t i t e , inclusions
probably p ri m a r y ,
others po ssib ly
f i l l i n g h ol e s and
embayments; mi n o r
sut ured boundaries
r e p l a c e d by gr o un d ­
mass .
OJ
-PS*
Appendix 4 - I d. (Conti nued)
Mineral
Size
Crys ta l 1i n i ty
bio­
t i te
subhedr al
h or n ­
b l e n de
subhedrala nh e dr a l
accessories:
g r ound­
mass
apatite,
ru tile,
an h e d r a l
v;i t h
mi nor
s ubhe dr a l
g r a i nb ou nd a r i e s
Shape
Angularity
equantelongate,
generally
subequant
subangularsubrounded
Distribution
s e r i a t e , and
i n ground­
mass
equant to
i rregular
clots
zircon,
pyrite,
e qua nt s ubequant
opaque-iron oxides,
rounded
1
Di agnos t i c
Feat ures
p i e o c h r o i sm:
X'= 1 ightp i n k i s h tan
z ' = or a n g e brown; r u t i l e
repl acement s
i n hexagonal
f orm
Inclusions
ru tile ,
apati te ,
opaque
i rono x id e , he­
matite re ­
p l a c e me n t s ;
possibly
primary
piagioclase
Alteration
M i ne r al s
r u t ile , chlor­
i t e , apatite
o pa que , i r o n
o x i d e s , he­
matite, s e r i­
c i t e , mi n or
lov^-bi r e f .
c la ys , per­
vasive
Comments
commonly pseudomor phi c a f t e r h o r n ­
blende; primary
grains moderately
embayed w i t h some
i rregular-ragged
edges.
irre g u la r clots
of f i n e - g r a i n e d
b i o t i t e and
alteration
minerals
b i o t i t e and r e ­
lated b i o t i t e ­
al t e r a t i o n
products, pervasive
common r e p l a c e ­
ment o f f e l d s ­
par s by q u a r t z ,
and v i c e - v e r s a ;
i n c i p i e n t granop h y ritic texture
alteration
uncommon
imonite
abundant a p a t i t e ;
a l l above m i n e r a l s
present.
OJ
on
Appendix 4 - 1 e.
P l ag i o c l a s e porphyry
Mineral
Size
Crys ta l 1i n i ty
Shape
Angularity
Distribution
Di agnosti c
Features
piagio­
clase
1 - 1 1 mm,
2 - 3 mm
subhedrala nh e dr a l
subequantelongate,
commonly
1rregular
subroundeds ub a n g u l a r
very s e r i a t e
( < 1 1 mm) t o
very h i a t i a l
w i t h few
g r a i n s > 2 inm,
and i n
groundmass
a lb ite , pcric lin e , Carls­
bad t w i n n i n g ;
v e r y - mi nor
z o n i n g ; glomer oporphyritic; a l ­
teration style
extremely
h iatial ,
l a r g e r mega­
crysts ra re ,
and i n g r ound­
mass
2 V ; 3 5 ° - 4 0 “ ;lCarl sbad
t wins
(Anjo)
avg.
po­
t a s s i urn
felds­
par
l - 2 mm,
subhedralfew
a nh e dr a l
mega­
crysts
( < l 6 n;m)
s ubr oundedgenerally
subangul ar
s ub e qu a nt ;
l a r g e r mega­
crysts
elongate
I nc l u s i on s
b io tite,
plagio­
clase,
quartz
as p r i ­
mary i n ­
clusions
Alteration
Comments
moderatep e r v a s i v e lowbi r e f . c l a y s
and s e r i c i t e ;
polycrystall i n e quartz?
some i n t e r g r o w n w i t h
o r p a r t l y rimmed by
b i o t i t e ; rimmed i r ­
r e g u l a r l y by po­
tassi um f e l d s p a r ;
mi n o r r a p a k i v i type aggregates
l a c k i n g potassium
f e l d s p a r c or e s ;
coarser q u a r t z - r i c h
groundmass a g ­
g r e g a t e s commonly
a d j a c e n t t o g l o me r o phyric p la g i o c l a s e
v e r y mi nor
low-bi re f .
clays
p o s s i b l y no pheno­
c r y s t s ; commonly em­
bayed and r e p l a c e d
by p l a g i o c l a s e ;
intergrown w ith
gloneroporphyri t i c
p l a g i o c l a s e ; some
rapakivi-type
p l a g i o c l a s e ri ms
w
cn
Appendix 4 - 1 e .
Mineral
quartz
Size
1 - 3 mm,
Crystal 1i n i t y
Shape
Angularity
Distribution
s ubhe dr a l
subequantelongate
subr oundedsubangul ar
h i a t i a l , and
i n gr o un d ­
mass
s ubhedr al
s ubequant
2 mm
a vg;
few
pheno­
crysts
( 8 mm)
biotite
1 - 1 0 mm,
2 -3mm
avg.
(Conti nued)
rounded
seriate
Di agnost i c
Feat ures
I nc l u s i on s
p l e o c h r o i sm
( whe r e p r e ­
served ) ;
x ' = l i g h t t an
2 ‘ =reddish
brown;
al­
teration
style
apatite,
r u t i 1e
f rom a l ­
teration
generally
f a i r l y em­
b ayed; i r ­
regular
overgrowths
in optical
continui ty
wi th pheno­
cryst
pr i ma r y
plagio­
clase
and b i o ­
tite,
very
mi n or
Alteration
general l y p er­
vasive s e r i ­
ci t e , r u t i l e ,
chlorite,
apatite,
o p a q u e - i ron
o x i d e ; some
low-bi r e f .
clays
Comments
some t y p e o f
rutile-chlo rite
a l t e r a t i o n as seen
i n cream p o r p h y r y
q u a rt z - ric h over­
growt hs around ri ms
with i r r e g u l a r ex­
te ns i on i n t o ground­
mass; p l a g i o c l a s e ,
b i o t i t e , p ot a ss i u m
f e l d s p a r commonly
inc lu de d in rim
overgrowths; also
rimmed by l a r g e r
f e ld s p a r grains
OJ
Appendix 4 - l e .
Mineral
Size
hornblende
1 - 3 mm
g r oundmass
Crystal 1i n i ty
Shape
Angularity
( Conti nued)
Distribution
subhedrala nhe dr a l
subequantirregular
subangul ar
hiatial
anhedral,
some
s ubhedr al
boundaries
e qua nt elongate
grains
roundedsubangul ar
commonly
seriate
wi th
larger fe ld s ­
p a r and
quartz grains
Oi agnosti c
Features
I nc l u s i on s
Alteration
Comments
feathery
b i o t i te
and r e ­
lated a l ­
teration
products
fragments
abundant
b io ti te
in
quartz
and
feldspars
generally equigranular
w ith coarser segre ­
ga t i o n s ; contains
a l 1 above m i n e r a l s
except hornb le nde ;
some q u a r t z - f e l d s par r epl acement
textures present,
p a r t i c u l a r l y on
p ot a s s i u m f e l d s p a r
megacrysts.
CO
CO
Appendix 4 - 1 f .
Mineral
Size
Crystal 1i n i ty
Lat e porphyry
Shape
Angularity
Distribution
Diagnosti c
Feat ur es
Inclusions
Alteration
Mi ne r al s
1 - 5mm,
l - 2 mm
)
avg.;
^ ^ 33-35
few
pheno­
crysts
< 1 0 mm
subhedral,
some an­
h e d r a l and
euhderal
grains
equantelongate
s ubr ounde dsubangul ar
s e r i a t e , and
i n gr o un dmass
a l b i t e , peri
d i n e , and
peri d i n e twin­
n i n g where not
o bs c ur e d by a l ­
teration ;
l i t t l e o r no
z o n i n g ; some
g l o me r o p o r p h y r i t i c texture
p l a g i o c l a s e , mo d e r a t e t o
b io tite
generally per­
v a s i v e l o wb i r e f . clays
and s e r i c i t e
1 - 5mm,
subhedralanhedral
subequantelongate
subr oundedsubangul ar
la rg e r grains
( 5 mm) as remnamt-rapakivi
cores; o t h e r ­
wi se h i a t i a l
( < 2 mm), and
i n groundmass
some C a r l s b a d
twinning;
2V;35°-45°(-)
quartz,
plagio­
c l a s e as
primary
in­
clusions
plagioclase
potas­
sium
felds­
par
1 - 2 mm
avg.
o n l y mi n o r l owb i r e f . cl ays
on r a p a k i v i
c o r e s ; pos­
s i b l y mod­
erate s e r i­
c i t e and l owb i r e f . cl ays
i n phenocrysts
Comments
r a p a k i v i - t y p e ri ms
on p o t a s s i u m f e l d s ­
p a r c r y s t a l s , and
s i m i l a r aggregates
w i t h o u t r e mn a n t potassium f e l d s p a r
c o r e s ; severe a l ­
t e r a t i o n common;
q u a r t z , b i o t i t e , and
potassium f e l d s p a r
as i n c l u s i o n i n
rapakivi texture
d i f f i c u l t to d i s ­
t i n g u i s h when not
p r e s e n t as remnant
cor es i n r a p a k i v i
aggregates wi th
p l a g i o c l a s e ; some
g r a i n s a t t a c h e d to
p l a g i o c l a s e borders
CO
CO
Appendix 4 - 1 f .
(Conti nued)
Diagnosti c
D i s t r i b u t i o n ________ F e a t u r e s
Mineral
Si ze
Crystal 1i n i t y
Shape
Angularity
quartz
l - 2 mm,
1 mm
avg.
subhedrala nh e dr a l
equantirregular
rounded
hiatial
very
embayed
l - 2 mm,
s ubhedr al
subequantelongate
subangul ar
h i a t i a l , and
i n gr o un dmass
altered
remnant s
b io tite
1 mm
avg.
I n c l u s i o ns
Alteration
Mi n er a l s
Comments
irregular d i s t r i ­
b u t i o n ; commonly
1 % phenocryst s ;
mi n o r s u t u r i n g o f
b o r d e r s and r e ­
p l a c e me n t by m i c r o ­
c r y s t a l l i n e , an­
h e d r a l groundmass
g r a i n s ; s e v e r e embayment ; i n c i p i e n t
ri m overgrowths
o f q u a r t z i n op­
tica l continuity
w i t h sur r ounded
g r a i n common
obscur ed
by
alteration
pervasive
sericite,
apatite,
o p a q u e - i ron
oxide, lim o n i t e ; mi n o r
chlorite,
ru tile
abundant l i m o n i t e
commonly s t a i n i n g
g r ai n s a d j a c e n t to
b i o t i t e ; some
p r i s m a t i c g rai ns
possibly other
hor nb l e n de
Appendix 4 - 1 f . (Conti nued)
Mineral
Shape
Crystal U n i t y
accessory m i n e r a l s ;
groundmass
apatite,
generally
anhedrals ubhedr al
grains
Shape
ru tile,
Angularity
zircon,
s ube qua nt
w i t h some
elongate
grains
Distribution
opaque-iron o x i d e s ,
s ubr ounde ds ub angul ar
grains
Diagnostic
Feat ur es
I n c l usi ons
Alteration
Minerals
Comments
limonite
f a ir ly h iatial
grains with
few c o a r s e r
quartz-rich
segregations
i rregular
grain
boundari es ;
replacement
bet ween
q u a r t z and
feldspars?
mi n o r - m o d e r a t e
a l t e r a t i o n of
biotite
and f e l d s ­
par s
p l a g i o c l a s e in
some samples r e c o g ­
n i z a b l e , b ut o t h e r
sampl es w i t h l i t t l e
a l te r a t i on may be
pla g ioc la s e deficient; all
o t h e r above m i n e r a l s
present
Appendix 4 - 2a
Rhyodacite Porphyry
visual estimate
hand sample
range
average
visual estimate
th in section
range
average
X-ray
% of
groundmass
in te rp re te d
in te rp re te d
phenocryst
groundmass
average_______ average
phenocrysts
plagioclase
10-18
orthoclase
quartz
<1
1-3
2
25
2
22
8-14
10
47
10
40
13
0- 2?
<1
«1
28
24
«1
b io tite
1-3
2
1-3
84
90-78
2
hornblende
groundmass
88-22
Samples
86
86
49,231,
437
49
*modal qu artz-plagioclase-orthoclase recalculated to 100%:
Q=
28
Ab = 26
Or = 46
normative q u a rtz -a lb ite -o rth o c la s e recalculated to 100:
Q=
28
Ab = 32
Or = 41
*plag ioclase recalculated as a lb it e
ro
Appendix 4-2b
Early Quartz Monzonite
visual estimate
hand sample
range
average
visual estimate
thin section
range
average
po int,
count
th in section
X-ray
% of
groundmass
in te rp re te d
phenocryst
average
in te rp re te d
groundmass
average
phenocrysts
plagioclase
20-30
15-45
26
32,30
6
28
3
28
orthoclase
2-5
3
24,5
56
5
29
32,8
38
6
20
quartz
3-7
4
2-8
5
b io tite
5-12
7
3-7
5
<1
1-5
3
60
72-55
6
12,11
hornblende
groundmass
Samples
<1
0-80
RJ,211,
279,281 >
508
58
3
52
0-46
Med-1-1094,RJ
*Modal quartz-plagioclase-orthoclase recalculated to 100:
Q = 29
Ab = 34
Or = 37
normative q u a rtz -a lb ite -o rth o c la s e recalculated to 100%:
q = 28
Ab = 42
Or = 29
^plagioclase recalculated as a lb it e
1
1094 is nonporphyritic, values in dicate to ta l mineral amounts; not calculated in average
RJ contains 25% groundmass feldspars and 21% groundmass quartz
-fs*
oo
Appendix 4-2c
Hornblende Porphyry
visual estimate
hand sample
range
average
visual estimate
th in section
range
average
point
count
thin section
X-ray
% of
groundmass
in te rp re te d
phenocryst
average
in te rp re te d
groundmass
average
phenocrysts
plagioclase
11-29
12-25
orthoclase
quartz
23
17
10
20
<1
<1
56
1
38
1-6
2
6
34
3
23
3-7
5
5
65
66
15
1
1-5
b io tite
5
3-6
hornblende
groundmass
Samples
1-6
84-64
80
83-51
49,50,135,
310,303,344,
443,496.531
50
68
310
*modal qu artz-plagioclase-orthoclase recalculated to 100
Q = 28
Ab = 29
Or = 42
normative q u a rtz -a lb ite -o rth o c la s e recalculated to 100%:
Q = 29
Ab = 43
Or = 28
4:»
*plagioclase recalculated as a lb it e
Appendix 4-2d
Quartz Porphyry
visual estimate
hand sample
range
average
visual estimate
th in section
range
average
point
count
th in section
X-ray
% of
groundmass
in te rp re te d in te rp re te d
phenocryst
groundmass
average_______ average
phenocrysts
plagioclase
10-35
2-5
4
b io tite
3-7
Samples
85-53
8
19
<2-5
<2
47
2
32
3-7
5
45
4
32
2-5
3
<1-6
1
4
4
hornblende
groundmass
19
19
orthoclase
quartz
12-26
73
80-51
70
70
GC,AT,212-213,
262,311,511,521
212-213
*modal qu artz-plagioclase-orthoclase recalculated to 100%
Q = 38
Ab = 26
Or = 36
normative q u a rtz -a lb ite -o rth o c la s e recalculated to 100%:
Q = 37
Ab = 31
Or = 31
^plagioclase recalculated as a lb it e
-Pi
Ol
Appendix 4-2e
Plagioclase Porphyry
visual estimate
hand sample
range
average
visual estimate
th in section
range
average
point
count
th in section
X-ray
% of
groundmass
in te rp re te d
in te rp re te d
phenocryst
groundmass
average_______average
phenocrysts
plagioclase
14-25
orthoclase
2-6
3
1
quartz
1-4
2
4-5
20
14-17
16
29
11
21
8
<1
2
31
3
22
4
7
58
4
41
b io tite
<1
NA
1 ?
1-4
hornblende
groundmass
Samples
83-65
75
80-73
78
165,128,395,
422,514
71
62
165
*modal qu artz-plagioclase-orthoclase recalculated to 100%
Q =45
Ab = 30
Or = 25
normative q u a rtz -a lb ite -o rth o c la s e recalculated to 100%:
Q = 43
Ab = 30
Or = 27
*plag ioclase recalculated as a lb ite
cr>
Appendix 4 - 2 f
Late Porphyry
visual estimate
hand sample
range
average
visual estimate
th in section
range
average
point
count
th in section
estimated
% of
groundmass
in te rp re te d
phenocryst
averages
in te rp re te d
groundmass
averages
phenocrysts
plagioclase
10-15
12-17
14
21
<1
<1
<1
45
<1
33
2
2
50
2
37
79
70
18
13
potassium
feldspar
1-3
quartz
b i o t it e
5-7
3-6
hornblende
groundmass
Samples
75-80
77
83-73
AT,219,308,
351,557
AT
74
AT
*modal quartz-plagioclase-orthoclase recalculated to 100%
Q = 41
Ab = 23
Or = 36
normative q u a rtz -a lb ite -o rth o c la s e recalculated to 100%:
Q = 41
Ab = 27
Or = 32
*plagioclase recalculated as a lb it e
APPENDIX 5
149
APPENDIX 5-1
Whole-rock chemical analyses of Emigrant Gulch in tru siv e rocks.
Analyses conducted by Bondar-Clegg and Company, L td ., Vancouver, B ritis h
Columbia.
A ll values in weight percent.
e or Element
Method of Analysis
SiOg
atomic absorption
A I 2O3
atomic absorption
atomic absorption
MgO
atomic absorption
CaO
atomic absorption
NagO
atomic absorption
KgO
atomic absorption
MnO
atomic absorption
TiOg
colorim etric
atomic absorption
Mo
atomic absorption
F
sp e c ific ion
Rb
X-ray fluorescence
Sr
X-ray fluorescence
*FeO calculated by taking FeO/Fe^O. = 1.19 fo r many quartz monzonites
(Hyndman, unpub. manusc., 1982, p. 325).
Oxi des
Rock t y p e
and samples
Rh y o d a c i t e
P o r p hy r y
W231
437B
E a r l y Quartz
Monzoni t e
GC17
212
Above 279
8 0 0 ' Pa s t
Jet
Road J e t
H or nbl e nde
P o r p hy r y
58
135
303
310
349
Quartz
Porphyry
55
211
212-2138
232
266
311
376
SiOg
APPENDIX 5- 1
FeO
A I 2 O3
MgO
CaO
El ement s
NagO
K 0
MnO
TiOg
2
71.00
69.00
14.50
15.80
61.00
15.00
14.60
14.70
2.67
1.98
^2°5
Mo
1.40
1.84
0.10
0.30
5.60
5.60
0.03
0.04
0.45
0.30
5
1.20
3.30
4.00
0.07
0.35
0.12
2
3.30
1.50
1.20
3.55
0/90
1.55
4.40
3.90
3.80
4.00
4.00
4.00
0.09
0.03
0.04
0.06
0.55
0.25
0.26
0.19
0.18
10
2.02
3.13
2.32
2.38
15.00
14.70
1. 93
1 . 75
2.27
2.05
1.75
1.50
1.60
2.25
4.30
4.20
3.90
3.90
0.02
0.35
0.35
0.20
0.04
2.44
2.86
2.48
1 . 70
2.44
1.75
2.00
1.85
1. 25
1.60
2.25
2.12
2.86
2.10
68.00
15.50
15.30
15.40
1 4. 80
14.40
2.05
0.90
4.30
4.30
4.20
4.10
3.40
3.60
4.10
3.80
3.50
4.10
70.00
71 . 00
70.05
69.00
67.50
69.50
68.50
15.00
15.00
14.80
15. 00
14.40
14.60
14.80
1.24
1.38
1.38
1 . 38
1. 84
1. 46
1.62
1.62
1 . 62
2.16
1.30
1 . 73
0.30
0.30
0.85
3.20
4.40
4.20
5.30
4.10
4.70
4.40
4.40
68.00
68.00
68.00
67.50
64.00
66.50
67.50
77.50
1.20
1.56
1.10
1.47
0.20
0.65
0.60
0.25
1 .20
2.00
2.05
1.55
0.70
0.95
0.30
0.80
1.25
1.45
0.95
2.10
2.30
4.10
3.00
3.20
4.00
0.03
0.01
0.02
0.03
0.04
0.10
0.10
0.10
0.08
0.10
0.06
0.06
0.60
0.45
0.35
0.40
0.30
0.40
0.35
0.50
0.60
0.15
0.25
0.30
F
Rb
330
570
220
245
Sr
50
130
120
2
770
400
700
140
155
410
330
345
7
18
770
600
125
97
345
395
3
9
520
540
140
0.18
120
0.20
8
1200
0.24
0.17
2
2200
5
1200
365
335
385
340
305
0.11
2
0.13
0.15
0.15
0.16
0.06
0.13
2
2
440
280
300
270
5
1000
0.18
0.21
1
4
11
15
660
310
140
190
170
155
175
185
145
160
200
145
150
160
175
310
230
225
285
cn
o
APPENDIX 5-1
Rock t y p e
and samples
PIagioclase
Porphyry
128
Above
165
SiOg
Al^Os
F*203
( Conti nued)
FeO
MgO
CaO
Na^O
KgO
MnO
TiO^
P2 O5
Mo
F
Rb
Sr
68.50
15.30
1.29
1. 51
0.70
0.25
2.90
3.80
0.01
0.55
0.13
5
1100
125
210
73.50
16. 10
0.37
0.43
0.10
0.05
0.30
3.70
0.00
0.30
0.10
4
210
91
59
67.00
67.00
69.50
68.50
69.50
1 6 . 10
1 4. 80
15. 10
16.20
15. 20
1.89
2.21
1.94
1.40
1.73
1.78
0.30
0.55
0.45
0.35
0.35
0.70
2.45
0.45
1.40
1.05
3.00
1.80
2.40
3.00
2.70
4.20
4.30
5.60
3.30
4.40
0.05
0.27
0.13
0.06
0.14
0.45
1.66
0.18
0.04
0.17
4
3
115
185
0.20
9
0.16
2
490
630
770
440
490
290
140
105
250
170
Late
Porphyry
37
175
219
313
351B
1.20
1.47
1 . 52
0.10
0.55
0.35
0.35
2
200
115
170
152
Appendix 5-2
Normative mineral content of Emigrant Gulch in tru siv e rocks,
recalcu lated from oxides in Appendix 5-1.
weight percent.
Q = quartz
OR = orthoclase
AB = a l b i t e
AN = an o rth ite
HY = hypersthene
MT = magnetite
IL = ilm en ite
AP = a p a tite
C = corrundum
DI = diopside
All values in normative
APPENDIX 5- 2
Rock t yp e
and Samples
Normati ve M i n e r a l s
OR
AB
AN
29.37
20.78
33.09
33.09
27.92
33.85
8.48
25.57
24. 91
23.64
23.64
23.64
20.82
HY
MT
IL
AP
1.03
5.17
1. 14
2.54
1.74
2.26
0.85
0.57
0.16
0.28
37.23
33.00
32.16
9.37
3.22
6.51
8.42
5. 51
5.35
3.87
2.87
2.93
1.14
1.04
0.47
0.60
0.44
0.42
—
5.25
2.67
1. 73
——
23.05
35.54
9.74
5.45
2.54
0.66
0.46
1 .27
17.17
20.92
21 . 63
21.28
24.23
22.46
20.12
25.41
24.23
6.91
5.19
5. 71
7.86
5.47
4.14
3.54
3. 07
2.46
3.54
2.67
2.54
0.49
0.42
0.46
0.56
0.46
0.39
0.94
2.04
20.68
9.79
4.78
8.62
8.60
7.13
6.58
1.14
0.85
21.21
36.39
36.39
35.54
34.69
34.69
28.77
Q
Rhyodacite
Po r p hy r y
W231
437B
E a r l y Quartz
M on z on i t e
GC17
212
Above 279
800' past
Jet
Hor nbl ende
Po r p hy r y
58
135
303
310
344
349
27.48
0.66
0.76
0.57
0.57
•
C
DI
2.63
1.26
—
-
——
——
1.22
- -
1.11
——
0.69
1. 96
——
——
CO
APPENDIX 5- 2
Rock t ype s
and Samples
Normat ive M i n e r a l s
OR
AB
3 2 . 37
39. 94
34.80
27. 42
27.78
29.96
25.63
26.00
24.82
31.32
24.23
27.78
26.00
26.00
Plagioclase
P o r p hy r y
128
Above 165
35.50
57.44
Late
P o r p hy r y
37
175
219
313
351B
31.19
32.96
32.53
34.78
33.86
Quartz
Po r p hy r y
55
211
2 1 2 - 2 1 3B
232
266
311
376
(Conti nued)
AN
HY
27.08
17.77
19.46
34.69
25.39
27.08
33.85
2.75
3.86
0.51
2.99
5.16
6.15
3.85
1 . 93
2.19
3.31
1.49
4.00
2.67
22.46
21. 87
24.54
2.54
0.39
0.25
24.82
2 5 . 41
33.09
19.50
26.00
25.39
15.23
20 . 31
25.39
22.85
2.30
11.89
1. 12
5.64
4.16
IL
AP
1.80
0.76
2. 00
2.00
2. 0 0
0.66
2.67
1 .59
2.13
0.95
1.14
0.28
0.47
0.57
0.25
0.30
0.35
0.35
0.37
0.37
0.30
2.04
0.01
1.87
0.52
1.04
0.57
0.30
0.29
6.27
11.60
2.59
3.90
2.04
2.37
2.57
2.74
2 . 41
1.74
2.13
2.20
0.85
0.19
1.04
0.42
0.09
0.39
0.46
0.37
5.78
2.83
4.68
5. 63
4. 47
2.20
2.56
MT
0. 6 6
0. 66
DI
3.96
5.58
5.09
2.72
2.49
2J^
cn
.pi
155
P late la .
Geologic map of the Emigrant Gulch study area. Also
shown are diamond d r i l l hole locations and trend of
cross sections A -A ', B-B', and C-C shown on plates
2a. 2b, and 2c.
(1" = 4 0 0 ', 1 cm = 48 m)
Plate lb .
Map o f brecciated areas in Emigrant Gulch. For
c l a r i t y , geologic units are not shaded.
(1" = 4 0 0 ',
1 cm = 48 m)
Plates2a, 2b, and 2c. Geologic cross sections A -A ', B-B', and
C -C .
Cross sections are interpreted from surface
mapping data, and subsurface d r i l l - c o r e data.
Projections of diamond d r i l l holes also shown. Many
of the d r i l l holes are projected in to the plane of
the section and are a c tu a lly located up to 120 meters
away from the section lin e .
For th is reason, the
top o f holes lik e Med-1 are located below the ground
surface in the plane of the cross section (Plate 2a).
Other holes lik e Med-6 begin above the ground
surface in the plane o f the cross section (Plate 2c).
(1 cm = 48 m),no v e rtic a l exaggeration