Economic
Geology
Vol.91, 1996,pp. 1073-1097
Geology,
TectonicSetting,andOriginof the Paleoproterozoic
Boliden
Au-Cu-AsDeposit,SkellefteDistrict,NorthernSweden
JEANETI'E
BERGMAN
WEIHED,
Instituteof EarthSciences,
Mineralogy-Petrology,
Norbyvtigen
18B,S-75236 Uppsala,
Sweden
ULF BERGSTROM,*
Department
of Geology,
EarthSciences
Centre,Guldhedsgatan
5A,S-41381 GOreborg,
Sweden
KJELL BILLSTROM,
Laboratory
for Isotope
Geology,
Museum
of NaturalHistory,Box50007,S-10405 Stockholm,
Sweden
AND PAR WEIHED
Geological
Surveyof Sweden,
Box670,S-75128 Uppsala,
Sweden
Abstract
The Skellefte
districtin northern
Sweden
comprises
morethan85 pyriticvolcanic-hosted
massive
sulfide
deposits
whichmainly
occur
within,andatthetopof,afelsic-dominated
volcanic
unitoverlain
byasedimentary
sequence.
The Boliden
Au-Cu-As
deposit
wasoneof thefirstdiscovered
in thedistrict,andit hasattracted
a continuous
interestsincethendueto its significant
sizeandhighgoldgrade(avg15 ppm).The Boliden
orecanbe dividedintomassive
ore,with arsenopyriteandpyrite-dominated
lenses,
andveinorewhich
comprises
a quartz-chalcopyrite-sulfosalt-dominated
assemblage,
occurring
inbrecciated
partsofthearsenopyritebodies,
andquartz-tourmaline
veinsmainlyin hostrocksbelowthemassive
ore.Asa rule,thegoldis
foundin deformational
structures
in veinore.Mostgoldis presentas an Au-Ag-Hgalloywithvariable
compositions,
fromAuo.
17Ago.68Hgo.
mto tuo.93Ago.07
(in atomicproportions).
For the lasttwo decades,
the approximately
1.88 Ga massive
sulfideoresin the Skelleftedistricthave
collectively
beeninterpreted
asvolcanic
exhalative
formations
resembling
theMiocene
kuroko
oresofJapan.
However,
thisviewhasrecently
beenchallenged
anda subsurface
replacement
originhasbeenproposed
for
some of the ores in the district.
TheBoliden
oreisnotboundto oneparticular
hostrockbutoccurs
in feldspar
porphyritie
daeite,quartz
porphyry,
andbasalt-andesite.
Textural
observations
suggest
thattheserocksrepresent
intrusions
or lavas.
Geoehemieally,
theyaretypical
eale-alkaline
volcanic
rocks,
enriched
inlargeionlithophile
elements,
depleted
in heavy
rareearthelements,
andwithtroughs
forTh, Nb,Hf, andTi. Theorezone,in itspresent
setting,
isina moreorless
vertical
position
andoblique
tolithologieal
contacts.
Ore-related
hydrothermal
andregional
metamorphic
processes
(loweramphibolite
facies)
havecreated
a complex
alteration
system
around
theore.
Thisformsa symmetric
patternwithaninnerserieite-rieh
zone,locally
containing
abundant
andalusite,
and
anouterehlorite-dominated
zone.Thenatureofthealteration
isconsistent
withleaching
ofelements
anda
silica-alumina-rich
residue--features
whichareoftenfoundin epithermal
environments.
Structural
observations
suggest
thatthreeductilefoliation-forming
events
haveaffected
therocksnearthe
ore.Theseinclude
a regional
Sl foliation,
formedduringisoelinal
folding,
whichwassubsequently
sheared
eausing
formation
of a strongcleavage
Ssandextensive
deformation
of the oreitself.A lateS2cleavage
erenulated earlier fabrics.
The available data and observationsare not consistentwith a volcanic exhalativemodel for the ore and the
following
scenario
is favored.Shallow
intrusions
of daeiteandandesitc
intounlithified
sediments
occurred
around1.87Ga.At thistime,theearliermarineenvironment
hadbeenliftedup to a shallow-marine
or
possibly
subaerial
position.
Shortly
thereafter,
fluids
whichgenerated
themassive
oreatBoliden
werefocused
alonga fault,andarsenopyrite
andpyritelenses
wereprecipitated
in morethanonehostrockdiscordantly
to lithologieal
contacts.
Regional
deformation
withfoldingandshearing,
possibly
at around1.85Ga,ledto
breeeiation
ofpreviously
formed
oresandstretching
oforebodies.
In relation
tothisshearing
event,
Auwas
introduced
and/orremobilized
andconcentrated
in breeeiated
portions
oftheorezone.Thereafter,
oresand
hostrocksreerystallized
duringpeakmetamorphism
at around1.82Ga,anda second
deformation
at around
1.80Ga causederenulation
of earlyfabrics.
Thecrosscutting
natureoftheorewithrespect
tothehostrocks,
thehydrothermal
alteration
pattern
with
strongly
leached
hostrocks,
andtheoreassociation
withearlymassive
sulfides
followed
bygold,ehaleopyrite,
andsulfosalts
in brittlestructures
all indicate
thata modern
analogue
for oreformation
maybe a highsulfidation
epithermal
environment.
Theepigenetie
nature
oftheBoliden
deposit
hassignificant
implications
forexploration
of golddeposits
elsewhere
in theregion.
* Present
address:
Geological
Survey
ofSweden,
EarthSciences
Centre,
Guldhedsgatan
5A,S-41381Gfteborg,
Sweden.
0361-01•8/96/1858/1073-•555.00
1073
1074
BERGMAN •VEIHED ET AL.
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•
(Revsundtype), c. 1.80-1.78 Ga
Post-volcanic
granitoids
ofA-and
I-type
/I•
(Skelleftetype), c. 1.82-1.80 Ga
Post-volcanic
granitoids
ofS-type
• Gabbro
and
diorite
,•-• Synvolcanic
calc-alkaline
granitoids
ofI-type
(J0rn type), c.
1.95-1.85 Ga
-•Conglomerates
and
polymict
(VargforsGroup),
c. sandstones,
1.87-1.85 Ga
..'......'...'..•
ß Group,Vargfors
Mudstone,
black
shales,
sandstone,
and
turbidites,
(Bothnian
Group,
Skellefte
Group),
c. >1.95-1.85
Ga
•
Skellefte
Group
volcanic
rocks,
c.1.89-1.87
Ga
•
Major
massive
sulfide
deposits
•
Major
gold
deposits
•'] Major
faults
and
shear
zones
F•c.1. Simplified
geology
oftheSkellefte
district
withthelocation
oftheBoBden
deposit.
Modified
froInAllenetal.
(1996).Coordinates
in nationalgrid.
Introduction
THE Skellefte district in northern Sweden has been mined
pyriteoreandin quartz-tourmaline
veins
withinaltered
rocks
belowthe massive
ore.The highgoldgradeis unusual
comparedwith othermassive
sulfidedeposits
in the Skellefte
forbaseandprecious
metalssincetheearly1920s.Sincethis districtandmakesthe oretypeespecially
prospective.
Interpretations
of the genesis
of massive
sulfidedeposits
ore hasbeenprovenandpartlymined(Allenet al., 1996). in theSkellefte
districthavegenerally
followed
theconcepts
Theearlyperiodof miningconcentrated
onmassive
sulfide of current times. Until around 1970, the formation of the
deposits
but,duringthelastdecade,
twolodegolddepositsmassive
oreswasattributed
to granitoid
intrusions
andmihaveopened
formining(Fig.1).In addition,
Ni orehasbeen grating
metamorphic
fluidsconcentrating
metals
in structurminedfromthe Lainejaurdeposit(Grip, 1942;Martinsson, allyfavorable
traps(e.g.,Gavelin,
1939,1942,1955;Kautsky,
1987)andLi mineralsfromtheVarutr•isk
pegmatite
deposit 1957).In 1975,RickardandZweifelpublished
a paperon
(Quensel,
1960).Porphyry-type
deposits
(cf.Weihedet al., themassive
ores,comparing
themwiththeMiocene
kuroko
1987; Weihed, 1992; Weihed and Fallick, 1994) have also deposits
ofJapan.
Thiswasthebreakthrough
formoderu,
ore
beendescribed.
The Bolidenminewasthe firstlargemassive genetic
concepts
intheSkellefte
district.
Mostmajordeposits
sulfide
mineintheSkellefte
district.
It wasexceptionally
gold have since then been described as volcanic-hosted massive
exhalative
sea-floor
deposits
(e.g.,Svenson,
1983;Willrich(avg15ppm)andwasEurope's
largest
goldmineanda sulfide
1989;Duckworth,
1991;Nicolson,
principal
producer
of arsenic
untilit closed
in 1967.High d•n, 1986;Trepka-Bloch,
have,however,
reinterpreted
goldgrades
werefoundin arsenopyrite
podswithinmassive1993).Recentinvestigations
time, more than 160 million metric tons (Mt) of basemetal
BOLIDEN DEPOSIT, SKELLEFTE DISTRICT, N. SWEDEN
1075
8,/
84•80
82
.F.
!gur'e3:,!::
/'
OK2
'-!:d•fo'rrr{ation'.:'
80,,,,,,,•
50
•80•
85
•
62
85
80
/
_' •_',_ breccias,
Rhyolitic-dacitic
and sandstones
lavas, /78
Bedding
/•62 S1cleavage
Basalt-andesite•
Ore
Sedimentary
sequence ß.. -
Strike
line
ofbedding
/ 69S2cleavage
Dacitic porphyritic lavas
StrikelineofS1
f ' Faultorshearzone
and domes
•
F1 and/or
F2 fold axis
F•c. 2. Generalized
]napof the geolo• aroundthe Bolidendeposit.
Compiled
usingresults
fromthisstud)'and
information
in Slade(1986)andSmith(1986).A. Bedcling
information
andlocation
ofregional
samples
forRb-Srisotope
analysis.
B. Cleavage
information.
1076
BERGMAN WEIHED ET AL.
I-$OO
200
.
I-•$o
Io
250
500
m
- ,..,oc" o o
o0 o
o øø •_o•
•ø•0o UOc•00
•½
O0o0
0
0o0
o
0
0
o(•o
0
0
o
0
00 o
o
0
0
o,
,o
.-[•:-.-.-•
Quartz porphyry
^[--•--'•
Basalt-andesite
-::• Sedimentary
sequence
•-•
Feldspar-phyric
dacite
•
Rhyolitic
massflow
[TT•T• Pyriteore
/
Arsenopyrite
ore
•[•-• Pyrrhotite
ore
,•'
_•
---'"
Younging direction
,,=--
- J
-500
-250
0
250
Shearzone
Late fault
500
Fla. 3. Detailedgeology
around
theBoliden
deposit.
Interpretation
isbasedondrillcores(represented
bysolidblack
lines),preserved
samples,
andmagnetic
surveys
(Boliden
AB,unpub.data).Coordinates
in localgrid.
someofthesevolcanic-hosted
massive
sulfidedeposits
assub- roundings.
Samples
forchemical
analyses,
mineralogical
studies,andisotopic
workwere alsocollectedduringthe core
sea-floor
replacement
deposits
(Allenet al., 1996).
Since
thefirstdetailed
study
oftheBoliden
minebyOd- logging.
man(1941),manynewhypotheses
concerning
ore genesis
RegionalGeology
havebeenpresented.
Earliermodels
fortheformation
ofthe
oftheSkellefte
district
hasbeensummarized
Bolidendeposit
include,for example,
a replacement
meta- Thegeology
(1986)andWeihedet al.(1992).Theareaconsists
morphic
origin(Odman,
1941)anda synvolcanic
exhalativebyRickard
volcanic
rocks(Skellefte
genesis
(e.g.,Rickard,
1986).Results
fromthepresent
study of ca. 1.89to 1.87Ga subaqueous
showthatit is impossible
to attributeall features
of theBo- Group;Fig. 1) whichare overlainandpartlyintercalated
lidendeposit
to anexhalative
massive
sulfide
deposit
andthat with a ca. 1.87 to 1.85 Ga (cf. Billstr6m and Weihed, 1996)
of younger
sedimentary
rocksandmainlymafic
at leastpartsof the orehaveanepigenetic
origin.Although sequence
rocks(Vargfors
Group).The supracrustal
rocksof
the oreis metamorphosed
anddeformed,
it is stillpossiblevolcanic
to findcluesasto howtheoreformedthrough
detailedstud- the Skelleftedistrictare borderedto the southandeastby
graywackes
(Bothnian
iesof regional
geology
andstructures,
alteration,
orestruc- a vastareaof highlymetamorphosed
Group)whichprobably
spanin depositional
agefrom> 1.95
turesandtextures,
andradiogenic
isotopes.
pers.commun.,
1995)andto the
Sincethe mineclosedin 1967,it is no longeraccessibleto 1.85Ga (T. Lundqvist,
for direct studies. Much information remains, however, in northby subaerial
volcanic
rocks(Arvidsjaur
Group)which
Group(Allenet
theformof verydetailedminemaps,hundreds
of ldlometers aresimilarin ageto rocksof theVargfors
sequence
of drillcore,a largenumberof precisely
locatedhandspeci- al., 1996)of the Skelleftedistrict.The supracrustal
byca.1.95to 1.85Ga,calc-alkaline,
I-typegranitmens,andthin sections.
A portionof thisinformation
has isintruded
beenusedin the presentstudy,togetherwith information oids(J6mtype;cf. Billstr6mandWeihed,1996),by S-type
granitesat ea. 1.82to 1.80 Ga (Skellefte-H•irn6
collected
in the field,to compilethe regional
anddetailed anateetie
Claesson
and Lundqvist,
1995;P. Weihedand
surface
geology
(Figs.2 and3). Morethan2.5 km of drill granites;
unpub.data),andby younger,
postvoleanie,
corehasbeenloggedandcombined
with the informationP.-O.Persson,
roeksat ea.1.80to 1.78Ga (Revsund
mentioned
abovein orderto construct
a verticalsection
(Fig. A- to I-typeintrusive
Sld61d,
1988).
4) approximately
throughthe centerof the ore andits sur- granitoids;
BOLlDENDEPOSIT,SKELLEFTE
DISTRICT,N. SWEDEN
1077
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eellille
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:::::::::::::::::::::::
eeeeeeleeeeeeJeeeeeeeee
::::::::::::::::::::::::
eeeeeeeeeeeee•ee,
•ß
Qua•zporphy•
•
Basalt-andesite
:•
Sedimenta•
unit
•
Feldspar-phyric
dacite
•
Pyrite
ore
I
Arsenopyrite
ore
'..,./r•Pyrrhotite
ore
ß¾ Tourmaline
vein
• ...... • Sericite
alteration
•
•
{
41
II Younging
direction
Bounda• of
chloritezone
•
ß
Andalusite
/
Fault
Handspecimen
Drillhole
FIG.4. Approximately
north-south
vertical
cross
section
through
thecenteroftheBoliden
ore.Drillcores
andpreserved
handspecimens
examined
in thisstudywereusedto construct
thesection.
Longdrillholedenoted
withø wasloggedby
R. Allen.Scalein meters,
coordinates
in localgrid.Seetextfor discussion.
The SkellefteGroupconsists
of various
volcanic,
subvol- sedimentary
rocks,mainlyturbiditicmud- and sandstones
canic,volcaniclastic,
andelastic
faciesandrepresents
theold- of volcanic
debriswithintercalated
blackphyllites,
arelocal
estandlowermost
rocks
in theSkellefte
district.
Themajority equivalents
of the Bothnian
Groupsedimentary
rocks(see
of volcanic-hosted
massive
sulfidedeposits
of thedistrictare below).TheVargfors
Groupalsoincludes
basaltic
lavas,mifoundin the upperpartsof the Skellefte
Group.Rocksare norfelsicignimbrites,
andseveral
typesofconglomerate.
The
mainlyfelsic(morethan50%rhyoliteanddacite),buta con- DOdmanberg
conglomerate
contains
clasts
of theArvidsjaur
tinuous
rangefromrhyolitic
tobasaltic
rocksoccurs,
andthe volcanic
rocks,
theAbborrtj'•irn
conglomerate
contains
mainly
volumeof rhyoliteto basaltvariesbetweendifferentareas clastsfromsynvolcanic
JOrngranitoids,
andthe Menstr•sk
withinthe Skellefte
district(Allenet al., 1996).The major conglomerate
consists
mainlyof volcanic
clasts
withscarce
volcanic
facieswithinthe SkellefteGroupare syneruptivegraniteclasts.
subaqueous
massflows,syneruptive
sills,cryptodomes,
and
The deposition
of rocksof the Vargfors
Groupgenerally
lavaswithassociated
hyaloclastites
(Allenet al., 1996).The marks
a periodof extensive
upliftin theareaasshown
bythe
totalthickness
of the sequence
is morethan3 km.
shallow-water
sedimentary
rocks
containing
erosional
products
Rocksof the Skellefte
Groupare overlain
by bothfine- of synvolcanic
granitoids
aswellasweldedignimbrites.
The
Groupis coevalwith the mainphaseof subaerial
andcoarse-grained
sedimentary
units.Theseunitsaremosfiy Vargfors
in the north (Arvidsjaur
volcanicrocks),where
conformable
and interfinger
with the underlying
volcanic volcanism
rocksof the Skellefte
Groupwithouta majorunconformity.weldedandnonwelded
ignimbrites
with accretionary
lapilli
subaerial
volcanism.
Thisis confirmed
by available
Thesesedimentary
unitstogether
withintercalated
volcanic manifest
rocksare definedasthe Vargfors
Group.The fine-grainedagedeterminations
(cf.Billstr6m
andWeihed,1996).
1078
BERGMAN WEIHED ET AL.
Dressingplant
Central
shaft
shaft
w
E
200
Volcanic
units
II
II
•oom
II
II
. . Andalusite
rock
Tourmaline
II
II
II
II
rock
with
ladder
quartz
veins
II
Pyrite
ore
II
ß
Arsenopyrite
ore
ßPyrrhotite
ore
,,...-, Latefaults
• Quartz_+
tourm.
veins
Position of
vertical section
Approximate plunge
stretching lineation
FIc. 5. Simplified
longitudinal
section
throughtheore.Constructed
fromverticalcross
sections
andmineplans.Scale
in meters,coordinates
in localgrid.
Areas to the south and east of the Skellefte district are, the southandeast.The metamorphic
gradein Bolidenwas
asmentioned
above,dominated
by highlymetamorphosed,
studied
byBerglund
andEkstrOm
(1978),whousedarsenopyturbiditicmud-andsandstones
andintrudedby several
gen- rite andsphalerite
compositions
as a geothermometer
and
erations
of granitoids.
Thisvastareais generally
referredto geobarometer,
respectively.
Theirresults
indicate
anequilibasthe Bothnian
basin(Lundqvist,
1979).The sedimentaryriumtemperature
of about430øCanda pressure
of 5 to 7
rocks are intercalated with marie volcanic rocks with minor
kbars.However, the commonoccurrenceof andalusitein the
felsiceruptive
centers.
Thissuccession
isprobably
morethan areaindicates
thatthispressure
estimate
is probably
some10 km thick(Lundqvist
et at., 1990)andspansin agefrom whathigh (Sharpet al., 1985).No detailedmetamorphic
> 1.95to ca.1.85Ga.Ni deposits
arefoundwithinultramarie studies
havebeenperformed
onthevolcanic
or sedimentary
sillsanddikesintrudingthissequence
immediately
southeastrocksof the area.Sinceall rocksdiscussed
in thispaper
of the Skelleftedistrict.The older, >1.95 Ga, Knaften area, aremetamorphose&
theprefixmeta-will be droppedin the
whichconsists
partlyof marieandfelsicvolcanic
rocks(Was- following
discussion.
str6m,1993),is alsolocatedin thisgeneralregion.Its stratiGeologyof the BolidenDeposit
graphicrelationship
to the surrounding
supracrustal
rocks
remains unclear.
The rocksequence
surrounding
the Bolidendeposit(Fig.
The supracrustal
rocksof the Skelleftedistricthavebeen œ)belongsto the upperpart of the volcanicrocksin the
affected
bytwomajorphases
of foldingandweremetamor- SkellefteGroup,nearthe contactwith the sedimentary
sephosed
duringthe Svecokarelian
orogeny
at ca.1.85to 1.82 quence.Observed
younging
directions
in the volcanicand
Ga.Thefirstphaseof deformation
resulted
in uprightisocli- sedimentary
unitsareconsistently
towardthesoutheast,
with
nalfoldswitheast-to northeast-striking
axialplanesandfold nomajorfoldsobserved
in the area(Fig.9.).The lowermost
axesplungingin average60ø E in the easternpart of the exposed
volcanic
unit is foundin the northernmost
partof
of coherentrhyolitesand
Skellefte
district.A penetrative
grainshapeandgrainalign- the areain Figure9..It consists
mentfoliationdeveloped
parallelto the axialplanesof these voleanielastie
rocks.Fine-grainedlaminatedsedimentary
folds.Thesecond
deformation
produced
gentletoopenfolds rockscapthe rhyolitierocksandareoverlainby a hererogewithsteepnortheast-striking
axialplanesanda locallydevel- nousassemblage
of volcanicbrecciasand sandstones,
and
opedaxialplanarcrenulation
cleavage.
Thesecond
phaseof feldspar
porphyritie
subvolcanic
intrusions
withamainlydaeifoldingwaslargelycoaxial
withthefirstfoldgeneration
in the tie composition.
Thesesurround
the Bolidenore. Someof
eastern
partof the Skellefte
district.Shearing
wasimportant the daeitievolcanic
sandstones
arepumiceous
flowswhereas
duringbothphases
of deformation.
In otherpartsof the othersare morereworkedwith a polymietelastpopulation
Skellefte district,the structuralevolutionwas similar to that
and well-rounded
elasts. Pumiceous flows are found in out-
of the easternarea,although
orientations
of structural
ele- cropsabout400 to 500 m northwest
of the ore andin the
mentsare different (of. Weihed et al., 1992).
minedeclinewestof the ore.Theyare alsofoundin drill
All rocksin the Skelleffe
districthavebeensubject
to re- coreswestand southwest
of the orebody.The pumiceous
gionalmetamorphism
in greenschist
to lower amphibolite flowsarepartlyfeldspar
porphyritic
ashflowswithlithieripup
facies,
andthemetamorphic
gradeincreases
generally
toward clastsnearthebaseandanincreasing
abundance
of centime-
BOLIDENDEPOSIT,SKELLEFTEDISTRICT,N. SWEDEN
1079
TABLE1. MajorandTraceElementCompositions
of LeastAlteredRocks
nearthe BolidenDeposit
i
2
3
4
5
6
7
8
9
SiO2
TiO2
68.7
0.61
55.0
0.73
68.7
0.45
67.3
0.56
72.0
0.41
73.0
0.28
48.4
1.05
48.2
0.85
A12Oa
16.3
18.7
13.6
14.5
14.0
12.7
16.2
18.3
12.1
13.5
10.5
0.20
0.15
20.5
12.4
Fe,2Oa
2.23
9.89
4.13
5.39
3.34
3.68
MnO
0.03
0.12
0.12
0.09
0.02
0.08
0.12
0.17
MgO
1.52
4.22
1.21
2.39
1.89
1.78
9.07
8.48
47.5
0.37
10
7.97
47.4
0.57
10.9
9.71
CaO
2.43
3.21
2.54
4.01
1.61
2.39
6.43
4.04
9.17
Na,20
K,20
P,205
5.50
1.56
0.15
3.40
3.06
0.18
3.86
1.86
0.09
2.25
2.86
0.15
2.31
2.57
0.01
2.80
2.12
0.07
1.03
0.39
0.15
0.48
3.09
0.16
0.23
0.38
0.11
0.95
0.19
0.11
1.85
100.4
1.30
97.9
0.65
100.1
1.55
99.7
1.20
100.1
2.87
99.8
2.37
95.6
L.O.I.
Sum
0.80
99.8
3.2
98.1
3.1
100.4
10.8
Ba
283
501
508
555
595
387
169
868
102
84
Rb
29
40
32
38
43
36
8
47
32
22
247
285
219
65
139
Sr
81
147
143
46
Cr
6
10
10
92
36
2,060
Ni
V
4
5
5
3
9
528
Co
Sc
Zr
Y
12
15
158
12
12
20
179
24
6.0
16.0
136
24
8.0
15.8
151
21
11.0
3.0
10.0
3.7
Nb
Hf
11
3.0
10
5.0
200
27
4.0
11
171
24
84
16
12
5.0
36
48.2
48
11
5
1.0
68
23.2
32
7
5
1.6
2.7
U
1.8
2.1
1.6
3.1
3.0
2.0
0.9
1.2
1.38
3.3
4.4
4.1
3.9
5.3
4.1
1.4
0.5
1.76
22
44
26
53
22
44
23.1
46.0
22.0
50.4
26
52
9.9
22.9
7.6
20
Nd
19
26
21
22
25.1
19.8
13.2
10
271
186
45
27.8
56
14
4
0.78
Th
La
Ce
157
1,460
2.6
7.6
17.4
11
24.5
8.2
12.8
Sm
Eu
4.5
1.2
4.9
1.3
4.5
1.4
4.2
1.1
6.2
1.05
4.9
0.8
3.3
1.09
2.4
0.9
1.9
0.7
3.2
0.9
Gd
Tb
0.6
0.7
0.6
0.5
5.3
0.8
0.6
3.O
0.5
0.3
0.3
3.3
0.4
Dy
4.7
2.9
Er
2.7
1.6
Yb
Lu
1.6
2.6
2.4
2.4
1.3
2.1
2.9
2.9
1.5
0.9
0.94
1.3
0.35
0.39
0.35
0.23
0.13
0.13
0.20
L.O.I. -- lossonignition;
oxides
andL.O.I. in weightpercent,
traceelements
in ppm
Samples:
1 = feldspar
porphyritic
daciteintrusion
(150-mlevel,Softheore);2 = younger,
coarse,
feldspar
porphyritic
daciteintrusion
(Gillervattsbodliden
3 km SWBoliden);
3 -- pumiceous,
feldspar
porphyritic,
dacitic
volcanic
sandstone
(1 kmN of Boliden);
4 = dacitic
volcanic
sandstone
(570-mlevel);5
= rhyolitic
volcanic
sandstone
withquartzandfeldspar
phenocrysts
(volcanic-sedimentary
contact
zone,1 kmSWBoliden);
6 = quartz
porphyry
intrusion
(northern
stock);
7 = basalt(50-mlevel,N of ore);8 = maficsedimentary
rock(410-mlevel,E of ore);9 = sill,ultramafic
composition
(Str/smfors
village,
N of Boliden);
10 = sill,maficcomposition
(BoRden)
ter-sizepumiceclasts
towardthetop.Reworked
breccias
be- sequence
(Fig.2). Theyarecoherent
plagioclase
porphyritic
comeincreasingly
abundant
upwardandgradeintoconglom- rocks
witha veryfinegrained,
almost
glassy
matrix.Themaserate.The pumiceous
flow materialformsthe matrixto a sivetextureandthe unbroken
feldspar
phenocrysts
indicate
polymict
population
ofclasts
including
coherent
feldspar
por- thatthe rockis a lavaor an intrusion.Texturallythe dacites
phyry,angularmaficclasts,biotite-richsedimentary
clasts, maybedividedintotwoseparate
types:anolderfine-grained
andpumice.
However,
insomeconglomerate
beds,thematrix typewitha regional
distribution
anda coarse-grained
dacite
iscomposed
of mudstone.
Several
bedsof conglomerate
oc- whichlocallybrecciates
thefine-grained
type.Manysamples
cur separated
by unitsof brecciaand sandstone.
The con- fromthe regionshowalteredandcorroded
phenocrysts,
but
glomerates
canbe distinguished
by the morepolymict
clast well-preserved
porphyritic
textures
witheuhedral
to subhepopulation,
clastroundness,
clastsizesin the range5 to 50 dralphenocrysts
of albiteor sodicoligoclase
arenotuncomcm,anda muchhigherclast/matrix
ratio.Thevolcanic
sand- mon.Theaverage
phenocryst
contentis 10to 15percent,but
stones
andbreccias
arerecrystallized,
fine-grained,
quartz- intheporphyry
partlyhosting
theBolidenore,thephenocryst
plagioclase-biotite
rocks
withscattered
quartzphenocrysts.
A contentaverages
20 percent.Phenocryst
sizeis normallyin
variable
amountof sericite
andchloriteiscommonly
accom- the range1 to 2 mmbut reaches
5 to 10 mmin the coarsepaniedbytheappearance
of calcite-bearing
fractures.
Com- grained
variety.
Thematrixiscomposed
ofquartz,plagioclase,
monaccessories
are apatiteandsulfides.
andbiotitewithvaryingamounts
of sericite.
Dacitesareexposed
in a beltsoutheast
of the daciticbrecThe contactrelationships
betweenthe coherentfeldspar
ciasandsandstones,
adjacent
to the overlying
sedimentaryporphyritic
dacitesandthe daciticvolcanicsandstones
and
1080
BERGMAN WEIHED ET AL.
a)
Rhyolite
(•, V
75
7O
•Oß
'-'
Comendite/
Trachyte
Rhyodacite/
dacite
Pantellerite
,
65
Andesire
ß O•
I1•ß
03 55
5O
te
ß
basalt-andesite
ß
cgfeldspar-phyric
dacite
Subalkaline
I•• •
O fgfeldspar-phyric
dacite
basalt
45
ß
daciticvolcanicsandstone
V quartzporphyry
40
0,001
' ' ' '''"1
' ' ' '''"1
0.01
i
0.1
i i i i iii I
I
i
i i i i iii
lO
Zr/TiO2
b)
observed
in polymict
breccias
andconglomerates
whichcover
the pumiceous
sandstone.
Thissuggests
rapiderosionand
uneonformities
in theupperpartof thissequence.
The contact relationship
betweenthe daeitiesubvolcanic
intrusions
andthe sedimentary
sequence
to the southeast
is unclear.
In someplaces,the contactbetweendaeiteandsiltstone
is
characterized
bya breeeia
ofdaeite
elasts
orblocks
in a sandy
to siltymatrix.In otherplaces,
blobsandveinsof feldspar
porphyritie
daeiteoccurin a mass-flow
unitwhichis banded
andappears
bakedalongthecontacts.
Thedaeitealsolocally
contains
fragments
of the sedimentary
units.Theseobservationsindicatethatthedaeitelocallyintrudedintounlithified
sediments
givingriseto peperitie
contacts.
Thisimpliesthat
the daeites
aresimilarin ageto thebaseof the sedimentary
sequence.
Thebaseofthesedimentary
sequence
(Fig.2) iscomposed
of quartzor feldsparporphyritic
rhyoliticsandstone.
This
is overlainby brownish,
biotite-andplagioclase-dominated,
andesiticto basalticsilt- and sandstonewhich, in turn, is
overlain
bya unitofgraphiteandsulfide-bearing
mudstones.
The sedimentary
sequence
abovethe graphitemudstones
maybe dividedintotwoprincipalunits;a lowerunitof finegrainedmudstone-siltstone
with a few distinctthick sandstonebedsdeposited
asmajormass
flowsandanupperunitof
FeO* + TiO2
more well-defined turbidites. The mass-flow units can reach a
ß
basalt-andesite
[]
mafic sill
thickness
of up to 50 m. Theyhavecoarse
bases,
commonly
withripupclastsof mud-andsiltstones,
andfineupwardto
a weaklylaminated
top,withan average
grainsizeof i mm.
A characteristic
component
of themassflowsisupto 1 percent,<3-mmbluishquartzcrystals.
Mostofthesource
materialisprobably
volcanic
inoriginandmineralogically
theflows
containmainlyplagioclase,
quartz,andbiotite.The sedimentarysequence
hasbeeninterpreted
asa medialto distalturbiditcsequence
deposited
wellbelowthe wavebase(Slade,
1986; Smith, 1986).
Semimassive,lead-zinc mineralization occursin the sedi-
HFT
mentary
sequence
of mud-andsiltstones
southoftheBoliden
deposit
(Fig.3). Thismineralization
isstratiform
withchalco-
BK
[]
lOO
PK
V quartz porphyry
•-- ... .•.
AI203
"'"
'"•_.• •;
MgO
•-'•,
FIC. 6. Geochemical
discrimination
diagrams.
a. Rockclassification
(Winchester
andFloyd,1977)of leastalteredhostrocksin theBolidenarea.
AB = alkalinebasalt,cg -- coarse-grained,
fg = fine-grained,
TrAn = trachyte-andesite.
b. Classification
of leastalteredbasalt-andesites
andhighMg
sillsfromthe Bolldenarea(Jensen,
1976).BK = basaltic
komatiite,
CA =
ß=,, •,
o
feldspar
porphyritic
dacite
ß basalt-andesite
...
_
10
_:-"•i
/ ::\,••..,,,,
/•
ß
•' '•.•...•.
calc-alkalineandesitc,CB = calc-alkalinebasalt,CD = calc-alkalinedacite,
CR = calc-alkaline
rhyolite,HFT = highFe tholelite,HMT = highMg
tholeiite,
PK = peridotitic
komatiite,
TA = tholeiitic
andesitc,
TD = tholeiiticdacite,TR = tholeiitic
rhyolite.
-
',,, ;
"•--H
--
-
Norm:
•)
MORB
breccias
arenotknownin anydetail.However,theircommon
U KNbLaCeSrNdHfZrSmEuTiGdDy
YErYbLu
texturalandcompositional
properties
suggest
that theyare 0.1 CsRbBaTh
coevalandpossibly
genetically
linkedto eachother.ThedaeFIG. 7. Spidergrams
of the threemostcommonhostrocksto the ore,
itesareinterpreted
asrepeatedsubvolcanic
eryptodome
in- normalized
to midocean
ridgebasalt(MORB).'['hecurvesshowspikedpattrusions into older domes and associated extrusive rocks.
i
Someelasts
ofcoherent
feldspar
porphyritie
daeitehavebeen
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
ternwithtroughsfor Th, Nb, Sr, Hf, andTi, typicalfor the fractionation
patternsin calc-alkaline
volcanics.
BOLIDEN DEPOSIT,SKELLEFTEDISTRICT,N. SWEDEN
1081
pyritestringers
overlying
a chlorite-amphibole-garnet
alter- thanone rocktypeis hostto the ore. Feldsparporphyritic
ation in the mud- and siltstones,and serieite-ehloritealter- daciteandbasalt-andesite
wereidentifiedverycloseto the
orebodyandlocallybetweenore lensesand,in addition,a
ationin thefeldspar
porphyritie
daeite.
Severalnarrowbodiesof massive,
darkgray-green
basalt- numberof handspecimens
of quartzporphyry
werestrongly
withveryfinegrainedarsenopyrite
andpyrite.
andesiteintrudethe dadtiefeldsparporphyries,
volcanic impregnated
sandstones,
andbreeeias
nearthe Bolidenore.Theypartly
Hydrothermal
Alteration
formhostrocksto the ore (Figs.3 and4). A largerbasaltandesite
unitis presentnortheast
of the mine,immediately The hydrothermal
alteration
aroundthe Bolidenore denorthofthecontact
withthesedimentary
rocks.Geochemicalposithaspreviously
been described
in detailby Nilsson
with the Bolidenore forms
analyses
of thebasalt-andesite
showtwocompositionally
dis- (1968). The alterationassociated
zonedpattern,withanandalusite-bearing
astinctgroups(seebelow).However,thesegroups
couldnot a characteristic
be distinguished
maeroseopieally
andthe geochemical
data semblage
belowthemassive
orein thecenter,followed
outwereinsufficient
to separate
the basaltandandesite
on the wardby a sericitezoneanda chloritezone.The alteration
mapandverticalsection
(Figs.3 and4). Nearthe ore,the zone is less than 15 m wide on either side of the ore at
in widthwith depth(Fig. 4). The
basalt-andesite
is finegrained
withsmall(<0.5 ram)plagio- the surfacebut increases
zonecanalsobetracedwestoftheore.In general,
elasephenoerysts
in a matrixof feldspar,
aetinolite,
andbio- alteration
tite,withminorepidoteandcalcite.Thebasalt-andesite
typi- the pervasive
character
of the alteration
associated
withthe
macroscopic
identification
of the precallycontains
up to 1-em-large
quartz-and/orcalcite-filledBolldenoreprohibits
amygdales.
Towardthe alteration
zonesurrounding
the ore cursor rocks.
The centralandalusite
zoneoccurs
immediately
belowthe
(shown
in Fig.4), feldspar
becomes
progressively
alteredto
serieite.
massive
ore (Figs.4 and5). It includes
bandedandalusiterock,massive
andalusite-sericite-quartz
rock,
Twoquartzporphyry
stocks
intrudethedaeitiemassflows sericite-quartz
andintrusions
in the vicinityof the ore (Fig. 2), andsome and bodies of almost monomineralic andalusite. The andalubothasminutegrains
disseminated
alongthemain
narrow(<20 em wide)quartzporphyry
dikesintrudethe siteoccurs
daeiteneartheore(Figs.3 and4).Thequartz
porphyry
stock penetrative
cleavage
andaslargereuhedral
crystals
cutting
thatappears
immediately
northwest
of theoreonthesurface the maincleavage.
Withinthe andalusite
zone,corundum
andpodsof monomineralic
sericite
rock,
geologic
map(Fig.3) plunges
ca.60øSEandcomes
doseto waslocallyobserved
the orein the deeperpartsof the mine(Fig.4). Thisstock rutfie-rich
rock,andapatite-rich
rockhavebeendocumented
is invariably
stronglyalteredevenoutsidethe serieiteand (Odman,
1941;Nilsson,
1968;GripandWirstam,
1970).
the massive
ore (Fig. 4) and
ehloritezonessurrounding
theore,withpervasive
fracturing, The sericitezoneenvelops
sulfideimpregnation,
and a serieitiematrix.The northern locallycontains
minoramounts
of andalusite.
The dominant
stockisbetterpreserved.
The quartzporphyry
contains
< 10 assemblage,
however,
is sericiteandquartz.Apartfromthis
Nilsson(1968)alsodistinpercentmonoerystalline,
bluish,andupto 1-em-large
quartz normaltypeof serieitealteration,
phenoerysts
with embayments
and inclusions
of the fine- guished
an extremely
silieievarietyof the serieiterock,with
grainedmatrix,whichis composed
mainlyof quartz,plagio- up to 80 percentSiO2,whichoccursin the deeperpartof
elase,andserieite_ ehlorite.Anyfeldspar
phenoerysts
that theserieite
zone.Adjacent
totheore,substantial
arsenopyrite
impregnations
arepresent.
originally
mayhavebeenpresentwereentirelyobliteratedandehaleopyrite
by the pervasive
alteration.
The distribution
of thin quartz The outermost
ehloritezoneis almostentirelylackingat
porphyritie
dikesis unclear,sincetheywereonlyobservedthesurface
(Fig.4) butwidens
withdepth.Thiszoneisdomiin a fewhandspecimens.
The dikesare,however,
texturally natedby quartz,serieite,andehlorite,commonly
with imof pyrite.Theboundary
between
theserieite
and
similarto the quartzporphyry
stockat depthto whichthey pregnations
maybe related.
ehloritezonesis commonly
diffuse,withalternating
bandsof
A numberof marieintrusions
are foundin all rocktypes serieite and ehlorite on a scale of tens of centimeters. In
of thearea,including
the massive
ore.Theseareusually1 to contrast,the transitionzone between the ehlorite alteration
2 m wide and, when the orientationof the intrusionscan be zoneandthe relatively
unalteredhostrocksis alwaysquite
tracedin doselyspaceddrill cores,theyare moreor less sharp.
parallelto lithologieal
layering.
Thus,the majoritymaybe
Begionalalteration
typesandtheir influenceon the geodefinedassills.Themarieintrusions
showbrownish,
hybrid- chemicalcomposition
of the rockshavebeendiscussed
by
ized plagioelaseand biotite-richcontacts
towardthe wall Vivallo (1987). He noted that mostvolcanicrocksin the Borocksandare composed
of a metamorphic
assemblage
of liden-L•tngdal
areahaveexperienced
a regionalalteration
aetinolite,
biotite,andplagioelase,
withaccessory
titaniteand whichaffected
mainlythelargeionlithophile
elements.
Pocompositions
are foundmainlyin volcacalcite.In rareexamples,
a lowermetamorphic
grade,ehlo- tassium-enriched
nielastierocks,whereassodium enrichment is most common
rite-dominated
assemblage
ispresent.
Thenorth-south
verticalsection
throughthecenterof the in well-preserved,
feldspar
porphyritie
daeites
nearthe Boore(Fig.4) shows
thatallobserved
lithologieal
contacts
have lidenore.It ispossible
thatearlyhydrothermal
fluidsrichin
a uniformdip of about50ø towardthe south,whereasthe CO2 altered intermediateto marievolcanicrocksand this was
massive
oreitselfissubvertical.
Asa resultof shearing
along followed
byregional
spotty
metamorphic
growth
ofaetinolite.
the orezone,it is notpossible
to tracelithologieal
contactsAetinolite has not been observed within the alteration zone
from the southern side of the ore to the northern side. Desurrounding
theore,suggesting
thateithertheBolidenalterspitethe intensealterationnearthe ore,it is dear thatmore ationzonewasnot geoehemieally
susceptible
to aetinolite
1082
BERGMAN WEIHED ET AL.
a)
growthor theBolidenalteration
waslaterandobliterated
all
tracesoftheactinolite.
Sincetheactinolite
isapeakmetamorphicmineralandtheBoliden
ore,together
withitsalteration
zone,alsocontains
peakmetamorphic
assemblages,
the sec-
Ti/1 O0
'• ßbasalt-andesite
Bollden
ond alternative can be ruled out.
Zr
Geochemistry
Samples
for geochemical
analysis
werecollected
in order
to classifypetrographically
leastalteredrock typessurrounding
the Bolidendeposit,
to discriminate
the rockstectonically,
andtouseimmobileelements
fordetermining
original rocktypein areaswithintensehydrothermal
alteration.
All leastalteredsamples
werecollected
asdoseto the ore
aspossible
but outside
the alteration
zone.Fromthe quartz
porphyry
stockclosetotheBoliden
ore,however,
allsamples
were intenselyaltered.Therefore,the texturallyidentical
stockfartherto the northwassampled
instead.Representativegeochemical
analyses
fortheleastaltered,texturally
wellpreserved
samples
arepresented
in Table1.
Earlierstudies
by Claesson
(1985),Vivallo(1987),Vivallo
andClaesson
(1987),andVivalloandWilld6n(1988)suggest
Y*3
that the volcanic rocks of the Skellefte district have a bimodal
composition,
withvolcanic
rocksofandesitic
anddaciticcomposition
beingrelatively
rare.Thisviewisnotshared
byAllen
et al.(1996)whoarguethatnobimodal
distribution
isevident
in the Skellefte
districtandthatthe oldopinionis merelyan
artefactof sampling
strategy.
The analyzed
rocksin thecurrent investigation
represent
compositions
fromrhyoliteto
basalt.
No attemptwasmadetoestimate
thevolumeofdifferentcompositions
in thearea,butAllenet al. (1996)describe
the Bolidenareaasdominated
by dacite(60%)andrhyolite
(30%)withonlyminorareaswithandesitc
andbasalt.In this
respect,
theareacloseto theBolldenoreisrepresentative
of
theBolidenareaasa whole(Fig.3). Thefeldspar
porphyritic
intrusions
andthe volcanic
sandstones
havemainlydacitic
compositions
andare geochemically
similar,exceptfor the
largeion lithophileelements
whicharemoresusceptible
to
b)
100000
-
Ocean-floor
•
10000
basalt
E
Volcanic-arc
1000
basalt
alteration(Fig. 6a andTable 1). The quartzporphyries
are
O.1
I
10
1 O0
1000
10000
Or (ppm)
c)
600
10 Arc <- 20 -> Ocean-floor
basalt
500
•-•400 []
50
300
rhyolitic
to rhyodacitic
(Fig.6a).Basalt-andesite
samples
plot
in twodistinctgroups(Fig.6a)but cannotbe distinguished
macroscopically.
The maficsillsarehighMg basalts
andmay
be dividedintotwomeancompositional
groups,
asindicated
in Table 1. The first group,whichis morecommonin a
regional
sense,
is ultramafic
in composition
whereas
the secondgroup,whichdominates
in the areaof the oredeposit,
includes
modalplagioclase.
In a Jensen
plot (Jensen,
1976),
bothcompositional
groupsof highMg maficsillsdisplay
a
clearaffinityto komatiites
whereas
the basalt-andesite
sampleshavemorecalc-alkaline
affinities
(Fig.6b).
Spidergrams
for thebasalt-andesite
samples,
the feldspar
porphyritic
dacites,
andthe quartzporphyry
suggest
thatthe
200
FIc. 8. Tectonic
discrimination
diagrams
for thebasalts
andandesites
in theBollden
andLSngdal
(Vivallo,
1987)areas.
a.AfterPearce
andCann
100
(1973). A, B = low K tholeiites;B = ocean-floorbasalts;B, C = calc-alkaline
0
0
5
10
15
Ti/1000 (ppm)
20
25
basalts;
D = within-plates
basalts.
The Lttngdal
basalts
arelowK tholeiite
basalts,
whereas
the BoBden
basalts-andesites
showan affinityto boththe
calc-alkaline
andwithin-plate
fields.b. Ti-CrplotafterPearce(1975).c. TiV plotafterShervais
(1982).The lineof Ti/V = 20 discriminates
between
are and ocean-floor basalts.
BOLIDEN DEPOSIT, SKELLEFTEDISTRICT, N. SWEDEN
rocksare typicallytalc-alkaline
volcanic
rocks,enrichedin a)
largeion lithophileelements,
depletedin heavyREE, and
withtroughs
forTh, Nb, Hf, andTi (Fig.7) whennormalized
relativeto midocean
ridgebasalts
(MORB).
Thebasalt-andesite
at Boliden
hasbeencompared
withthe
stratigraphically
equivalent
maficvolcanic
rocksin Lfmgdal
TiO2*100
ß
farther to the south(Vivallo, 1987) in tectonicdiscrimination
diagrams
(Fig.8). Bothpopulations
of maficvolcanic
rocks
showanarc-related
signature
butthe Bolidensamples
have
a more fractionated,
plagioclase-depleted
charactercomparedto the Lfmgdalsamples.
A system
basedonimmobile
elements
isnecessary
forthe
interpretation
of originalrocktypesin the alteredrockvolumesaround
theBoliden
ore.Byusinglessmobileelements
andplotting
allsamples
in a Zr-TiO2-A120•
diagram
(Fig.9a),
it is commonly
possible
to identifythe originalrocktype,
evenof strongly
alteredrocks.The leastalteredsamples
in
Figure9a definea fractionation
trendandmostrocktypes
are easilydistinguished
in separate
fields.Onlythe mafic
sedimentary
rocksshowa heterogeneous
distribution
from
Zr-poorbasaltic
compositions
(oneexample
isshown
inTable
1) to oneZr-rich,quartz-rich,
andbiotite-poor
samplesituatedbetweenandesires
anddacites.Figure9b showsthe Zr
compositions
ofstrongly
alteredsamples
(representative
analysespresented
in Table2) fromthewallrockadjacent
to the
massive
oreontheupperlevels,
plottedin thesamediagram
astheunaltered
samples.
Chlorite-sericite-pyrite
schist
from
the wall rocksin the east-central
part of the ore zoneand b)
bandedsericite-chlorite-pyrite
schistfromthe westernmost
orezoneplotinthesamefieldastheunaltered
basalt-andesite
samples.
Onesamplefroma massive
sericite-chlorite-pyrite
schist
fromtheeasternmost
partofthedeposit
hasthetypical
Zr-poorbasalticsedimentary
signature.
Mostotheraltered
samples
plotin thedacitefieldorbetweenthebasalt-andesite
andthe dacitefields.Thesesamples
werecollected
in the
upper-central
partof the orezone.Theyareinterpreted
to
be dacites
witha possible
contribution
frommaficsedimentaryrocksof theZr-enriched
quartz-rich-biotite-poor
type.
1083
basalt-andesite
,•7fg
feldspar-phy
dacite
quartz porphyry
dacitic volcanic sandstone
basaltic-andesitic
sediment
AI203-5
TiO2*100
ß
Ore TypesandTheir Distribution
Thetotaltonnage
of the Bolidendeposit
was8.4 Mt and
the average
composition
of theentireorewas15.5g/t Au,
50 g/t Ag, 1.43percentCu, 0.9 percentZn, 0.27percentPb,
6.8 percentAs, and 25 percentS. Other metalsfoundin
minoramounts
in the depositwereSb,Co, Se,Te, W, Mo,
andBi.Duringtheperiodof mining(1925-1967),about128
t of Au, 118,000 t of Cu, and 566,000 t of As were extracted
(GripandWitstam,1970).
Twodifferentoretypescanbedistinguished
intheBoliden
mine: massive ore which constitutes the bulk of the ore and
Zr
AI203-5
consists
of about75 percentmassive
pyritewithlensesand
irregularbodiesof massive
arsenopyrite
(25%)and minor
pyrrhotite(Figs.3 and4), andveinorewhichis a termused FiG.9. Trianglediagrams
for recognition
of originalrocktypein highly
forall orethatoccurs
in structures
crosscutting
themassivealteredsamples.a. Leastalteredrocksin the vicinityof the Bolidenore.
ore andcountryrocks.Thesestructures
includea finevein Thedifferentrocktypesplotinwell-defined
clusters,
except
fortheheterogesediments.
fg = fine-grained.
b. Alteredrocksadjacent
networkcontaining
quartz,sulfides,
andsulfosalts
in brecci- neousbasalt-andesite
oreontheupperlevels,mainlysericite-chlorite-pyrite
schists.
atedarsenopyrite
ore, largerveinswith sulfides,
sulfosalts,to themassive
Fieldsfortheleastaltered
rocks
areadded.
Thisplotwasusedforrecognizing
andquartzin massive
pyriteoreandhostrocks,
andquartz_+ originalrocktypein thecompilation
of the detailmapof the deposit(Fig.
sulfide
_+gold-bearing
veinsin tourmaline
bodies,
commonly3). No samplesfromthe andalusite-bearing
alteration
belowthe ore are
occurring
immediately
belowthe massive
ore.
included
in thisplot.
1084
BERGMAN WEIHED ET AL.
T^BLE2. MajorandTraceElementCompositions
of AlteredRocks
nearandin theBoliden
Deposit
SiO2
TiO•
A12Oa
Fe20•
MnO
MgO
CaO
Na•20
K20
P20•
ia
2
3
4a
66.9
0.75
23.1
0.42
<0.01
0.44
0.13
0.45
2.80
0.10
64.0
0.52
15.2
5.18
0.04
7.89
0.42
0.23
2.20
0.16
59.7
0.79
15.1
7.95
0.16
2.41
6.76
2.87
1.39
0.23
56.4
1.72
28.3
0.09
<0.01
0.40
0.22
1.08
6.81
0.13
5.10
L.O.I.
4.40
2.70 •
Sum
100.2
100.2
100.1
Ba
223
35
34
2
309
27
20
33
543
26
55
150
1
5
< 10
Rb
Sr
Cr
Ni
V
52
Co
13
13
Sc
Zr
12.7
238
8.4
16.6
152
24
10
10
Y
Nb
Hf
3.20
98.4
379
105
60.5
i
5a
46.7
0.94
17.7
11.0
0.08
11.0
1.34
0.32
2.41
0.18
8.40
100.1
84.2
0.21
11.1
0.16
<0.01
0.20
0.08
0.25
2.83
0.04
10.22
99.9
1.40
8
73.6
0.26
12.8
5.01
0.05
2.67
0.20
0.23
2.24
0.09
2.99
14
29
35.7
103
8.1
86
<10
6
41
30.2
95
10
7
4.952
1
15
28.1
195
5.7
4.10
187
25
1
700
< 1
19
54.0
0.50
12.5
9.89
0.20
14.2
1.54
0.12
1.43
0.23
307
59
64
3
1
25.5
109
19
37.6
2.00
50.7
0.09
<0.01
0.26
0.01
0.84
4.66
0.06
100.3
831
20
19
11
3
10
100.1
599
48
16
< 1
213
9a
100.5
253
29
56
33
1
<10
41.4
1.12
16.0
19.4
0.09
8.30
0.56
0.84
1.76
0.22
7a
269
40
36
6
258
9
6
7.1
31
13.2
159
6.9
88
8
8.6
159
34
<10
99.6
17.7
214
6.8
<10
22
25.8
87
10
10
7
4.7
2
5
2
2.1
3
5
2.5
U
3.3
4.1
1.4
2.9
0.9
2.0
2.1
4.2
3.8
Th
2.8
9
18
10
2.3
0.6
3.6
21.2
44
21
4.4
1.0
1.4
19
40
4.0
1.7
3.4
74
150
50
9.2
2.5
2.4
15
29
14
3.6
1.6
1.8
14.5
32
19
4.3
1.5
3.9
23
50
22
4.7
1.1
4.6
98
203
110
22.4
3.1
4.4
14.5
32
16
3.5
1.5
0.6
0.6
0.6
0.5
0.6
0.7
0.8
0.4
La
Ce
Nd
Sm
Eu
Gd
Tb
6.91
<0.5
7.95
Dy
Er
3.98
Yb
Lu
Cu
Zn
Pb
As
19.8
45.5
28.7
6.67
0.95
2.5
0.36
10.7
4.5
<2
2
2.1
0.23
120
98
<2
<2
2.3
0.35
29
87
< 10
<3
2.4
0.34
1.4
3.9
<2
<2
1.5
0.26
58.7
300
73
54
1.6
0.23
269
301
59
130
3.7
0.47
0.6
3.1
<2
3
3.72
<6
31
< 12
2.4
0.39
<0.5
5.4
<2
5
0.6
0.09
328
518
84
240
L.O.I. = lossonignition;
oxides
andL.O.I.in weightpercent,
traceelements
in ppm
Samples:
i = sericite-andalusite-altered
feldspar
porphyritic
dacite,570-mlevel;2 = chlorite-altered
feldspar
porphyritic
dacite,570-mlevel;3 =
actinolite-altered
feldspar
porphyritic
dacite,
Tj'•ilamyrberget
hill;4 = sericite-altered
basalt-andesite,
270-mlevel;5 = chlorite-altered
basalt-andesite,
270m level;6 = chlorite-sericite-altered
basalt-andesite
clostto massive
ore;7 = sericite-altered
quartzporphyry,
570-mlevel;8 = chlorite-altered
quartz
porphyry,
ca.500-mlevel;9 = andalusite-sericite-altered
rock,270-mlevel;10 = sulfosalt-impregnated
chlorite-altered
highMgbasalt
stillcuttingmassive
ore
• Unpublished
analyses
byAnders
Hallberg
2 Includes1.4% CO•
3 Includes S
Massive ores
of compression
andshear.Boththe roundedarsenopyrite
fragments
and
the
morehomogeneous
interiors
of arsenopyMassivearsenopyrite
ore commonly
occursas irregular
arenearlyeverywhere
brecciated
intoangular
bodies,completely
enclosed
in massive
pyritein mostcases riteorebodies
fragments
(Fig.10B)withquartz,minorsulfobutlocallyextending
alsointothecountry
rock(Fig.4). Arse- tosubrounded
pyrrhotite,
nopyritebodies,whichare concentrated
nearthe southern salts,andsulfidemineralssuchas chalcopyrite,
and
sphalerite
constituting
the
matrix.
sideof themassive
ore,areusually
elongate
andplungeca.
The arsenopyrite
ore is usually
veryfinegrainedwith an
60ø E (Fig.5).
grainsizeof 0.01to 0.04rnm(adman,1941).It
Withina meterof their mutualcontact,bothpyriteand average
mainlyofrecrystallized
euhedral
tosubhedral
arsenoarsenopyrite
oresarebanded
(adman,1941)and,in these consists
Quartzcommonly
occupies
aninterstitial
posiareas,rounded
arsenopyrite
orefragments
withinpyriteore pyritecrystals.
includechalcopyrite,
pyrrhotite,
are common(Fig. 10A).This is interpretedas durchbe- tionandotherminorphases
wegung(e.g.,Vokes,1969)andattributed
to a combinationsphalerite,
andsulfosalts.
A bimodalgrainsizeis commonly
BOLlDENDEPOSIT,SKELLEFTE
DISTRICT,N. SWEDEN
1085
developed
withextremely
finegrainedinteriorsandcoarser tactbetween
pyriteoreandcountry
rock(0dman,1941).
recrystallized
margins
of arsenopyrite
fragments,
wherearse- Noneof the pyrrhotite
bodieswasintersected
by drill core
nopyritegrainscommonly
form well-developed
rhomboid in theverticalsection
shown
in Figure4, buthandspecimens
crystals
protruding
intothe matrix(Fig.10C).Thiswasalso showsmallrounded
fragments
of countryrockin a matrixof
observed
in anelectron
microprobe
studywherecoarser,
con- pyrrhotite.
Thismineralization
typehasalsobeenfoundin
fromthe Bolidendeposit.
centrically
zonedarsenopyrite
crystals
enclose
a finergrained drill coreat somedistance
spongy
arsenopyrite
aggregate
withgalenaandquartzasminorphases
in interstices
(Fig.11).Thisspongy
arsenopyriteVein ore
is interpreted
asthe oldestpreserved
texturein the deposit.
Theveinoreis mineralogically
verycomplex
with a large
Thezonation
in thearsenopyrite
crystals
iscaused
by a variaof sulfosalt
minerals
andprecious
metalsin addition
tionin Co content.Brokenarsenopyrite
crystals
wererarely number
to the common sulfidesand calcite. In all vein ore, however,
observed
alongthefragment
boundaries
whichindicates
that
constituent.
Theveinore
the bulk of the deformation which caused brecciation oc- quartzis,byfar,themostcommon
in thebrecciated
arsenopyrite
orewhereit
curredpriorto recrystallization.
Somebrecciation,
however, is mostcommon
ofveins.Larger
alsooccurred
at a laterstage,
asseenbyveinscuttingrecrys- formsa matrixin theformof a finenetwork
veinsalsooccurin pyriteore andcountryrocks.The most
tallizedarsenopyrite
andquartz.
constituents
in thenetwork
ofveinsinthearsenopyApartfromthetypeof arsenopyrite
oredescribed
above, common
pyrrhotite,
andsphalerite
0dman(1941)alsodescribed
extremely
finegrained
and rite ore are quartz,chalcopyrite,
(Fig.
10F).
Sulfosalts
are
also
rather
common
(Isaksson,
compact,apatite-banded,
and quartz-banded
arsenopyrite
1973),
especially
in
the
deeper
parts
of
the
mine.
The
highest
ore.Thesetypeshave,however,
notbeenencountered
in our
Cu
and
Au
grades
of
the
deposit
are
closely
correlated
with
study.
brecciated
arsenopyrite
ore.Wherethe veins
The pyriteore constitutes
morethan75 percentof the the strongly
quartz,palisade
texturehasdeveloped
massive
oreanditsdistribution
is shown
in Figures3 and4. containdominantly
perpendicular
to
some
faces
of
arsenopyrite
ore fragments.
Thepyriteoreis dividedintotwoseparate
bodies,
a western
This
preferential
orientation
of
quartz
grains
indicates
anexandaneastern.
Bothareabout350m long.Thewestern
body
component
in the formation
of thebreccia.
isonlyaround15mwideandpinches
outata depthof 130m, tensional
Largerveinsin pyriteore andcountryrockalsocontain
whereas
theeastern
bodyisabout50 mwideandcontinues
to
galena,
sulfosalts,
andgold,apartfromthe othercommon
250 m belowthe surface.It is apparentfromlongitudinal
sulfides
and
quartz.
The sulfomineral
assemblage
hasbeen
sections
of the ore (e.g.,Fig.5) thatpyritebodiesare also
studiedin detailby Isaksson
(1973).Therefore,no attempt
elongate
andplunge60øE.
mineralspecies
in thisstudy,
The pyriteorevariesconsiderably
in grainsize,between wasmadeto identifyindividual
but
the
veins
commonly
contain
boulan
erite
'amesonite
g
..,J
,
0.5 and5 mm,andit is commonly
banded(0dman,1941).
tetrahedrite,
bournonite,
and
gudmundite
(Odman,
1941;
The oreis normally
strongly
recrystallized
andthe electron
1973).Verycomplex
intergrowths
betweenthedifmicroprobe
studyshowed
Co-zoned
pyriteovergrowths
on Isaksson,
werereportedby Isaksson
(1973)and
spongy-textured,
fine-grained
pyritewherepyritecrystals
oc- ferentsulfominerals
also
observed
in
this
study.
Deformation
structures
arecomcurin contact
withquartz.Fragments
ofarsenopyrite
oreare
of the veinore and
common
withinthe pyriteoreandarsenopyrite
occurs
also monlyfoundin the quartzcomponent
a slightundulose
extinction,
deformation
lamelasintimateintergrowths
withpyrite.Interstitial
phases
are theseinclude
development.
chalcopyrite,
sphalerite,
pyrrhotite,
and,in a fewplaces,
ga- lae,andsomesubgrain
oreconsists
of lens-shaped
tourmaline
bodies
lena and sulfosalts.
Roundedinclusions
of countryrock, Tourmaline
of quartzandminorsulfides.
It occurs
mainlyquartzandsericiteschist,
werealsoobserved
within withtensiongashes
orewithinthe sericitic
alteration
the pyriteore.All thesefragments
mayhavebeenincorpo- mainlybeneaththemassive
ratedinto the pyriteore duringdeformation.
In addition, zone. Three main bodies of tourmaline ore were mined with
mylonitic
banding
of pyriteore(Fig.10A)commonly
occurs goldgradesabove10 g/t. One suchbodyis shownin the
cross
section
ofFigure
4.According
to0dman(1941),
alongthe margins
of arsenopyrite
bodiesenclosed
in pyrite vertical
bodies
arestrongly
elongate
inthesameorienore('0dinah,
1941).In thinsections,
thisbanding
isdefined thetourmaline
arsenopyrite
andpyriteorebodies.
Only
by streaked-out
fragments
of fine-grained
arsenopyrite.
An- tationasindividual
a
few
samples
of
this
ore
type
were
studied
since
the
bulk
of
otherobserved
typeof banding
wasdefinedby layersconit fallsoutside
theselected
profile.Morethanonegeneration
tainingsphalerite
interstitial
to pyritegrains.
wasobserved
in thisoretype.A lightgreenishVeinsofsulfosalts
andquartzlocally
cutthepyriteore,and oftourmaline
is cut by fibrousveins
the margins
of theseareusually
strongly
recrystallized
with brown,veryfinegrainedtourmaline
tourmaline,
withthetourmaline
well-developed
pyritecubes
occurring
somedistance
intothe withdarkergreenish-brown
at rightangles
to theveinwalls.Bothtypesarecut
veins(Fig.10D).Inclusions
andembayments
in pyritecubes needles
tensiongashes.
The tourmaline
bodiesare
contain
fine-grained
fragments
of arsenopyrite,
chalcopyrite,by quartz-filled
ona smallerscaleandsmallveinlets
commonly
sphalerite,
andpyrrhotite
(Fig.10E).Theseobservations
indi- alsofractured
sulfosalts,
andgold.In the deeper
catethatthepyriteorerecrystallized
aftertheincorporationcontainquartz,sulfides,
of arsenopyrite
fragments
intothe pyriteore andafterthe partsofthemine,randomly
oriented
tourmaline
needles
also
formation
of veins.Late-stage
deformation
in thepyriteore occur
inthesericite-quartz
schist,
andmostanalyzed
tourmalinesare dravites(Odman,1941).Isaksson
(1973)notedthat
isvisibleasbrittlefracturing
of pyritegrains.
Pyrrhotite
ore(Fig.4) iscommonly
situated
alongthecon- thedeepest
andlargestofthetourmaline
orelodescontained
1086
BERGMANWEIHED ET AL.
q
(2
.
FIC.10. Photographs
oforetextures.
A.Mylonitic
fabric
inarsenopyrite
ore.B.Brecciated,
fine-grained
arsenopyrite
withmatrix
ofchalcopyrite
andpyrrhotite.
Length
ofspecimens
ca.7 cm.C.Photomicrograph
ofbrecciated
arsenopyrite
BOLIDEN DEPOSIT,SKELLEFTEDISTRICT, N. SWEDEN
ST1021
1087
2. Goldwasalsofoundassmall(•10-pm) inclusions
to-
getherwithBi-Seminerals
in fragments
of veryfinegrained
arsenopyrite
orein a sample
of strongly
brecciated
arsenopyrite ore.Thesegoldgrainsoccuralonggrainboundaries
of
recrystallized,
compositionally
zonedarsenopyrite
crystals
andasinclusions
inlarger,recrystallized
arsenopyrite
crystals.
Thesetwogoldassociations
weremostimportant
in termsof
thecontribution
togoldore(cf.M6rtsell,
1931;Odman,
1941;
GripandWirstam,1970).
3. In pyriteore,goldwasfoundin a sulfosalt
+ galena
veincuttingthepyriteore.Thesegoldgrainsare20 to 100
pm in sizeandareirregular
in shape.
Theyaredoselyassociatedwith galena,sulfosalts,
nativeBi, quartz,andstannite.
In thisassociation,
onegoldgrain(about3 pm in size)was
alsofoundenclosed
in a pyritecrystal
surrounded
bysulfosalt
andgalena.
4. In the tourmalineore, goldwas foundin fractures
20
. OkU
800
r20 . OOP E:SE
throughquartzandtourmaline
(Fig. 10H) andasa matrix
to
broken
tourmaline
crystals.
In
bothcases,the goldwas
FIG.11. Backscattered
electron
imageof zonedrecrystallized
arsenopyrite growing
on spongy-textured
fine-grained
arsenopyrite
to the left. The
zonation
iscaused
bya variation
in Cocontent.
Whitespots
in thearsenopyrite aregalena.
associated with the common sulfides and/or sulfosalts.
Analyses
of the composition
of goldshowthatall goldis
presentasan Au-Ag-Hgalloy.Compositions
varyconsiderably(Fig.12),fromAu0.•TAg0.6sHg0J6
to Au0.9aAg0.07
withan
Sb-free
sulfosalts
andappreciable
amounts
ofAg,contrary
to average
of
Au0.s6Ag0.s9Hg0.05
(atomic
proportions).
The
lowest
the other tourmaline bodies.
goldcontent
wasfoundin goldassociated
withsulfosalt
veins
A highconcentration
of gold-bearing
quartzveinswas in pyriteore andthe compositions
of thesegoldgrainsare
termedthe "goldriseore"(Odman,1941)andthiswaslo- distinctly
differentfromthoseof theothergoldassociations.
catedbeneaththe massive
ore in the centralpartsof the Thehighest
goldcontent
wasfoundin a goldgrainassociated
deposit.
Several
narrow,
parallel
quartzveinswithtourmalinewithaurostibite
(AuSb2)
andanothergoldgrainwitha lower
formeda lodewithcoarse
goldandveryhighgrades
(Grip goldcontent
in tourmaline
ore.Goldoccurring
in brecciated
andWitstam,
1970).Theaverage
goldgradein thegoldrise arsenopyrite
ore and in tourmalineore cannotbe distinorewascalculated
to be 50 g/t anda 1-msamplefroma guished
onthebasis
ofgoldcomposition.
In somegoldgrains,
stopeat the245-mlevelcontained
620g/t goldand630g/t a zonationin composition
was
observed
with the margin
silver.
poorerin goldthanthe center.Nysten's
(1986)analyses
on
goldin the Lftngsele
massive
sulfidedeposit,
situated
some
Occurrence
of gold
15 km southof the Bolidendeposit,
alsoshowed
a variable
Asmentioned
previously,
thegoldisexclusively
associated
wheremostgoldgrainscontain
substantial
with vein ore in deformational structures that cross the mas- goldcomposition
amounts
of mercury.
siveoreandlocally
thehostrocks.
A survey
of themodeof
A polished
section
ofveryfinegrained,
unbrecciated
arseoccurrence
of goldwascarriedout usingbothan optical nopyriteorewas
scanned
for goldusingthe electronmicromicroscope
andanelectron
microprobe,
andthecompositionprobe.No goldwasfoundbutsomegalena
wasidentified
as
of the goldwasdetermined
for differentassociations.
Gold
inclusions
interstitial
to theveryfinegrained
arsenopyrite.
A
was encountered in four different environments:
section
of relatively
finegrained
pyriteorewasalsoscanned
1. In strongly
brecciated
arsenopyrite
ore,goldwasfound forgoldbutnonewasfound.Thisisin agreement
withM6rtas4- to 200-pm-large,
rounded
grains
in thequartz-rich
ma- sell(1931)andOdman(1941)whoreported
goldin pyrite
trix.Minorassociated
sulfides
includechalcopyrite,
pyrrho- ore onlywhereassociated
with arsenopyrite,
chalcopyrite,
tite,andsphalerite.
Thegoldgrains
occuralongtheinterface pyrrhotite,sulfosalts,
andcalcite.Otheridentifiedminerals
between
quartzandarsenopyrite
fragments
andasdroplets,were,apartfrompyrite,boulangerite,
galena,tetrahedrite,
or locally
crystals,
insidetheveinquartz(Fig.10G).
nativesilver,gudmundite,
andanunidentified
Se+ Pbphase.
withrhomboids
ofarsenopyrite
protruding
intothematrix.
Reflected
light,slightly
uncrossed
polars.
Length
ofscale
bar
isI mm.D. Photomicrograph
ofwell-developed
pyritecubes
along
edgeofsulfosalt
vein.Reflected
light.Length
ofscale
baris1 mm.E. Photomicrograph
ofinclusions
ofarsenopyrite
(asp)fragments
in recrystallized
pyrite(py).Quartz
(black)
andchalcopyrite
(chp)formmatrix
topyritecubes.
Reflected
light.Length
of scale
baris I mm.F. Photomicrograph
of
veinorecomposed
of chalcopyrite
(chp),pyrrhotite
(po),sphalerite
(sph),andquartz(black)
in brecciated
arsenopyrite
(asp)
ore.Reflected
light.Length
ofscale
baris1 min.G.Photomicrograph
ofdroplets
ofgold(Au)withina quartz
(q)and
sphalerite
(sph)
veininbrecciated
arsenopyrite
(asp)
ore.Reflected
light.Length
ofscale
baris1mm.H. Photomicrograph
of
fractures
through
tourmaline
(tin)filledwithgold(lightgray),
tellurobismuthite,
andanunidentified
Te-Bi-S-Se-Pb
mineral
(bothmedium
gray).ser= sericite.
Transmitted
andreflected
light.Length
of scale
baris 1 mm.
1088
BERGMAN WEIHED ET AL.
thissampleandattempts
to achieve
Pb-Pbevaporation
ages
for thesegrainswere successful
in onlyone case.ConventionalU-Pbanalysis
hasalsobeencarriedoutononetitanitc
Au
fractionobtainedfrom the dacite (Table 3). In addition,com-
monPb, stableisotope,
andNd isotopestudies
havebeen
carried out and will be discussed elsewhere.
!
ß
Au in a
•,Au
Au
in
p!
.• ++
.'•
in
s
If+gal -•
•
Au in qz
Auintn+qz+sulf
v
¾
'•
+ AuinL•ngsel
e
•,
¾
v
v
v
v
v
Ag
\
Hg
FIo. 12. Ternaryplotshowing
the composition
of gold(atompercent)
in Boliden,
analyzed
in 48 spots
withawavelength
dispersive
Cameca
microprobe.Opensquares
andopencirclesrepresent
inferredposition
of data
pointswhichwereanalyzed
on a differentmicroprobe.
Dataon goldfrom
theLfingdal
deposit
fromNysten(1986).asp= arsenopyrite,
gal= galena,
py = pyrite,qz = quartz,sulf= sulfosalt,
tn = tourmaline.
Analytical
procedures
andisotopic
results
Rb-Srisotope
datahavebeenobtained
forninewhole-rock
samples
usingstandard
ionexchange
techniques
for separatingRb andSr (Table4). The complexity
of the Bolidenore
zone,witha varietyof lithologies
affected
bybothhydrothermalandmetamorphic
processes,
wasappreciated
in theearly
stageof thestudy.In orderto overcome
someof thesecomplexities,
thestudywasdesigned
to includea largearea(Fig.
2A) surrounding
the ore zoneproper.The samples
chosen
forisotopic
analyses
wereselected
froma largersetofwholerocksamples
(analyzed
formajorandtraceelementcomposition)fromrelatively
unaltered
rocks,
located
0.3to 3 kmaway
from the Bolidenore zone.The samplesincluderhyolite,
andesitc,
dacite,andquartzporphyry.
In addition,
a coarsegrained
maficdikesituated
withintheorezonewasincluded.
Despitethe approach
usedfor selecting
samples,
it was
foundthat the Rb-Srisotopedatascatterconsiderably
in
an Rb-Srisochron
diagram(opensquares,
Fig. 13a).The
magnitude
of the observed
scattersuggests
open-system
behaviorof the Rb-Srisotopesystemsincethe time of rock
crystallization.
Whencompared
witha reference
linehaving
a slopecorresponding
to an ageof 1870Ma (a reasonable
estimate
for the ageof the magmatic
rocksconsidered;
see
The presence
of invisiblegold,i.e., goldthat cannotbe above)andwith an initialS7Sr/S•Sr
ratioof 0.703 (cf. Wilson
observed
by opticalor electronmicroscopy,
hasbeenre- et al., 1987),the datascatterin an unpredictable
manner,
portedfrommanysulfideores(cf.HealyandPetruk,1990). plottingbothdistinctly
aboveandbelowthe reference
line
Thisgoldmayoccurin solidsolution
or assubmicron
inclu- (Fig. 13a).Onlytwo samples
(K7 and K8, Fig. 13a)have
sions
in sulfide
minerals,
mostcommonly
pyriteandarsenopy- remained
unaffected
bypostcrystallization
processes
or,alterrite(Cabriet al.,1989;CookandChryssoulis,
1990).It isstill natively,the net effectof suchpostulated
processes
hasreunknown
if the Bolidensulfides
containanyinvisible
gold. sultedin isotopic
ratiosthatappearto be undisturbed.
It is
clear
that
most
initial
Sr
isotope
ratios
(filled
squares,
Fig.
Rb-SrandU-Pb IsotopeSystematics
13a, as calculated from available Rb and Sr concentration
In orderto setconstraints
on the agesof oresandhost data(ICP-MS)andusinga common
ageof 1870Ma for all
rocks,
minerals
androckssuitable
fordatingbyvarious
radio- samples
investigated)
are anomalously
lowanddo nothave
genicisotopemethods
werecollected.
Rb-Srdatafromleast anygeologic
significance.
alteredwholerocksare reported
here,alongwithconven- Zirconandtitanitcfractions
weredigested
in anHF-HNOa
tional U-Pb data from one monazite and one titanitc fraction
mixturefollowing
the procedure
of Krogh(1973),whereas
fromthe alteration
zonessurrounding
the massive
ore.A monazite was dissolved in concentrated sulfuric acid. The
largedrillcoresample
wasalsocollected
fromthe feldspar chemical
preparation
ofPbandU andsubsequent
mass
specporphyritic
dacite,whichformspartof thehostrockto the trometricanalyses
followedessentially
the procedures
deore,in orderto datethe rockby the U-Pbzirconmethod. scribedby Johansson
et al. (1995).Blanklevelsduringthe
However,onlya few zircongrainscouldbe extracted
from treatmentof titaniresare generally
lessthan40 pg for Pb
T/tBLE3. U-PbIsotopeDatafor Mineralsfromthe BolidenOre
Concentration
Radiogenic
Pb
Atomic
(at %)
ratios
(ppm)
Sample
no.
Weight
(mg)
U
545 monazite
797 titanitc
3.29
2.04
90032 titanitc
0.494
85.5
96.0
137.2
Pb
235.6
187.5
22.87
Apparentages
(Ma)
2ø•Pb/•ø4pb
(measured)2ø•PbZø7pbaøspbaø•Pbœzasu
Zø7pb/235U
aø7pb/•øapb
ZøePb/•3SU
aø7pb/•SU2ø7pb/Zø•Pb
173
19.22
118
21.4
86.7
2.4
8.6
76.2
4.7
0.6251
0.1274
9.7594
1.7305
0.1132
0.0985
3130
773
2412
1019
1852
1597
86.5
9.2
4.3
0.1078
1.5898
0.1069
660
966
1747
BOLIDENDEPOSIT,SKELLEFTE
DISTRICT,N. SWEDEN
TABLE 4.
1089
Rb-Sr Whole-Rock Data from the Boliden Area
age,S•,formed
during
folding
waslaterdeformed
byshearing
in a subvertical,
east-striking
shearzonealongthe Boliden
Sample
Rb
Sr
S7Rb/ S78r/86Srs7Sr/S6Srorezone.Thissheafing
caused
thedevelopment
of a strong
no.
Petrology(ppm) (ppm) S6Sr (measured)(initial)
cleavage,
Ss,in thealteredrocksneartheore,andextensive
K1
Rhyolite
50
102
1.424
0.7493
0.7108 deformationof the ore itself. Both the S• and Ssfoliations
K2
K3
Dacite
Dacite
46
26
104
195
1.280
0.386
0.7103
0.7043
0.6757
0.6939
K4
Andesitc
43
414
0.301
0.7310
0.7229
K5
Quartz
36
65
1.606
0.7336
43
79
1.582
0.7540
1.339
0.7380
porphyry
K6
Quartz
porphyry
K7
Quartz
30
65
K8
porphyry
Bhyolite
30
82
dh 24,
Marie vein
25
1
1.061
72.3
0.7329
0.7048
werelatercrenulated
bya locallydeveloped,
steep,north-to
northeast-stfiking
crenulation
cleavage,
S2.
The firstfoliation,S1,is steepandstrikesnortheast,
but
0.6901
neartheshearzonealongtheoreS1swings
toa moreeasterly
0.7112 direction
(Fig.2B).In theblackschists
southoftheBoliden
deposit,
S•iswelldeveloped
witha pyrrhotite
impregnation
0.7017
along
thecleavage
whichisdefined
bythealignment
ofmicas.
In
the
volcanic
rocks,
the
S•
is
also
penetrative
but
less
intense
0.7042
<0.6
113.85m
a)
K6
[] present-daycomposition
whereasthe monaziteblankwason the orderof 100 pg.
0.75
K1
ß calculated initial composition
Regression
analysis
(York,1969)andisotopic
ratios(Ludwig,
1:3
K7
1991) are at the 2•r level.
K8
Themonazite
fromthealteration
zone
occurs
asyellow, o.73
K5
transparent
grains
witha highlustreandhasdistinct
cleavage
planes.
Whenplottedon a concordia
diagram,
it wasfound 87sr
that the monazitesampleshowsa stronglyreversedis- 86Sr
cordancy.
Thismaybe aneffectof thedifficulty
of getting 0.71
the samplecompletely
into solution
to obtaina correctU
concentration.
Thecalculated
2ø7Pb/•ø6Pb
agewas,however,
preciseat 1852+ 3.5 Ma (Fig. 13b)andthe significance
of
U-Pbagesisdiscussed
furtherbelow.
0.69
The titanires from the dacite rock and the alteration zone
aredarkreddishbrown.Theyexhibitcracksandinclusions
andoccurasirregular
fragments
of largercrystal
aggregates.
Bothtitanitc
fractions
arecharacterized
byveryhighcommon 0.67
Pb contents
andU-Pbdataare,in bothcases,
significantly
I
I
I
0.4
•
i
0 8
I
i
1.2
i6
I
1.
2.0
S7Rb
discordant.For reasonablecommonlead corrections(Anders
Hallberg,
pets.commun.,
1995),theresulting
Pb-Pbagesare
1747_+50and1597+_75Maforthedacite
andalterationb)
zonesamples,
respectively.
Theseagesare inferredto be
minimumages,notingthe highdiscordancy,
andtheyonly
confirmthatthe crystallization
of the titanires
is dueto an
EarlyProterozoic
event.
0.4
Mineral ages refer to
Onelightbrown
zircon
crystal
fromthefeldspar
porphy- o.3
rifledacite(90032),whichoccurred
asa brokenfragment
of
a prismatic
crystal,
wasdatedbythe=øzPb/•ø6Pb
evaporation206pb
technique
(of.Kober,1986).Unfortunately,
onlyonesetof 238u
datacouldbe acquired
fromthe grainat a temperature
of
ca.1,480øC.
Therefore
it wasnotpossible
toconfirm
thatthe
monazite-
2000 Ma
2ø7pb/2ø•Pb
ages 1852+3.5
Ma•
o.2
calculated
ageis a plateauage.Thus,theageof 1869+_15
Ma (l•r) mustbe lookeduponasa minimumage.However,
theindividual
agescalculated
foratotalof30scans
areconsistent,andthehigh•ø6pb/•ø4pb
ratios
(generally
above
10,000) 0.1
suggest
that the analyzed
Pb wasreleased
from an undisturbeddomainwithinthe crystal.
The preferredinterpretationis thattheobtained
Pb-Pbageis a goodapproximation
of thetruecrystallization
ageofthezirconanditshostdacite
0
[] measured
ratios
• inferred
concordancy
•
•
1800
.,-•.,'"
.,;"
:::c
zircon-'dacite
rock
(evaporation
age
1Z.;:O•;•?
1869+
Ma)
Oy.•_.[•.,,.-,'
titanite
7alteration.(•597•75.
800
ß',')'
,.;?;
l•l--titanite
- dacite
rock
(1747+50
Ma)
2
rock.
2o7pb
4
6
235u
Structural
Evolution
Fig. 13. a. Rb-Srdiagram
showing
whole-rock
dataforrocksfromthe
Threeductile,foliation-forming
eventshaveaffectedthe Bolidenarea.b. Concordia
diagram
forminerals
fromtheBoliden
ore.Note
agesare Pb/2Pbages.
rocksin thevicinityof theBoliden
deposit:
a regional
eleav- thatindicated
207
06
1090
BERGMAN WEIHED ET AL.
a)
N
b)
N
easternpart of the mine.However,younging
directions
in
thesedimentary
sequence
areconsistently
towardthesouth
and considerable core loss in some sections indicates that the
contact
wasoffsetbysheafing.
Theshearzonealsolocalized
straintothealteredrocks
hosting
theBoliden
oreandcaused
intense deformation of the massive ore. The ore structures
are desefibed in more detail below.
The strongcleavage,
S•,alongthe shearzonein sefieiteand ehlofite-altered
.f
',
ß Poles to bedding(n=70)
C)
wall rocks to the ore is conformable to
theorebodies
(0dman,1941).Thiscleavage
isdefined
bythe
orientation
of ehloriteandmicaandthe shapeof veryfine
feldspar
andquartzgrains.
TheSsshearfabrichasobliterated
ß Polesto contacts(n=27)
r• Polesto Sscleavage(n=265)
* Fold axes and lineations(n=15)
N
almostall tracesof the St foliation. However, the St fabric
wasobserved
in lessdeformedshearpodsbetweenSsin a
handspecimen
of sefieiteschist.
A stretching
linearion,
Ls,
relatedto the sheafing
plunges
about55øE, andindividual
arsenopyrite
orebodies
andquartz-tourmaline
orebodies
are
alsostretched
alongL• (Fig.5). Combining
thisplungewith
thesinistral
strikeseparation
of thecontact
between
volcanic
andsedimentary
units,anobliquemovement
withthenorthem sidedisplaced
westandup relativeto thesouthern
side
can be inferred. The St and S• foliationsare macro- and
+
+
+
microscopically
identicalin appearance,
and both have
formedpriortopeakmetamorphism
andthesecond
deformation. It is, therefore,probablethat the sheafing
occurred
duringor shortly
afterthefirst,mainregional
cleavage-forming event.
ß Polesto S• cleavage(n=25)
+ Foldaxes and lineations(n=4)
A secondregionaldeformation
produced
gentleto open
foldswhichhaveaxialsurfaces
strikingnorthto north-northeastwithalocally
developed
axialplanarerenulation
cleavage,
S2.Thisisparticularly
welldeveloped
in thealteration
zone
surrounding
the
ore
due
to
the
presence
of
miea-fieh
rocks
surfaceseastand southeastof Bolidenmine (from Slade,1986, and Smith,
anda strongpreexisting
cleavage.
On a regionalscale,the
1986).b. Polesto lithological
contacts
andshearfoliation(S•)withintheore
cleavage
dips60øto 70ø ESE. A erenulation
zone(dataextracted
fromminemaps).
c. Polesto S•cleavage
andlineations S2erenulation
lineation,
L2,plunges
about50øESE andisverywelldevelwestandnorthof mine(thisstudy).
opedin the vieinit)/ofthe ore.In a stereographic
projection
(Fig. 14b)polesto the shearlabfiemeasured
in the mine
than in the black schists. Lack of markers in the volcanic
(fromminemaps)definea poorlyconstrained
partialgreat
unitsprecludes
recognition
of foldsrelatedto the firstdefor- circlewith an F2 foldaxisplunging
about55ø E. A stereomation.In the sedimentary
unitseastof the Bolidenmine, graphic
projection
(Fig.14c)ofpolesto Stcleavage
measured
however,
Slade(1986)reports
a fewisoclinal
minorfoldswith in the field,mainlynorthandwestof the mine,showsa
thepenetrative
Stfoliation
asaxialplanarcleavage.
Foldaxes similarpartialgreatcirclewithanF2 foldaxisplunging
54ø
to thesefoldsplungeabout60ø E. Comparison
withother toward 070 ø.
areasin the Skellefte
districtsuggests
thatthe firstfoliation Late faultscut the ore as shownin mine maps(Fig. 3).
formedasanaxialplanarcleavage
to tightor isoclinal
folds. Most of these faults strike north-northeast with a minor set
Stretching
lineations,
Lt, relatedtothefirstdeformation
have of northwest-stfiking
faults.Displacements
areonthe order
the samegeneralorientationas the minorfold axes.The of tensof meters.North-northeast-stfiking
faultsgenerally
intense
foldingin thesedimentary
unitssoutheast
of theBo- showa sinistral
strikeseparation,
whereas
northwest-stfiking
lidenmine(Fig.2A)isinterpreted
assoft-sediment
deforma- faultsshowa dextralstrikeseparation.
One late faultwas
tion(Slade,1986;Smith,1986)anda stereographic
plot(Fig. identified
infeldspar
porphyritic
dacite
in adrillcoreimmedi14a)of polesto bedding
givesa verycomplex
picture.
atelyeastoftheore.It ischaracterized
bybrittletextures
and
Theshearzonethrough
theorezone(Fig.2) isvisibleon the development
of pseudotachylite,
indicating
morebrittle
groundmagnetic
maps(Boliden
AB,unpub.data)asa gener- conditions
duringthisdeformation
thanfor earlierdeformaallyeast-striking,
lowmagnetic
lineament.
Theshearzonehas tions.Thisfaultingis interpreted
asthelastdeformation
afdisplaced
thecontact
between
thevolcanic
andsedimentaryfectingthe area.
FIG. 14. Lowerhemisphere
equalareaprojections.
a. Polesto bedding
sequences
eastof the minein a sinistralfashion.This offset
Oresin relationto tieformation
andevidence
of
haspreviously
beenshown
asa foldandOdman(1941)inter- remobilization
pretedthe structure
asa dragfold.A numberof drillcores
east of the ore zone indeed show that the contact between
Banding
in pyriteandarsenopyrite
oresis inferredfrom
volcanic
andsedimentary
rocksswings
to the northin the drill coreobservations
andfromdescriptions
in Odman
BOLIDENDEPOSIT,SKELLEFTE
DISTRICT,N. SWEDEN
1091
isa common
metamorphic
mineralin intermedi(1941)to be parallelto the shearfoliation,Ss,andindividual Actinolite
orebodies
areelongate
parallelto thestrongstretching
direc- ate to marie volcanic rocks and mud- and siltstones surthe Bolidendeposit.
Locally,actinolite
is soabuntionin the shearzone.Tourmaline
bodiesarealsogenerally rounding
conformable
to the main Ssfoliation(•)dman,1941)in the dantthat it obscures
the originallithology.
The actinolite
ore zone and are stretched in the same orientation as the
occursboth as a disseminated
spottyoverprint,in larger
massive sulfide orebodies. This indicates that the tourmaline
masses
togetherwith feldsparand quartz,and as fracture
albite-andcalcite-bearing
halos.The
bodieswere eraplaced
prior to shearing.
Duringshearing fillingswith bleached
commonly
attacks
plagioclase
phenocrysts
and,at
alongthe orezone,tensiongashes
formedperpendicular
to actinolite
the wallsof tourmaline
bodies(Odman,1941).Fracturinghigherconcentrations,
alsoinvades
the matrix.Epidotemay
alsooccurredon a smallerscalewith the development
of a appearin somesamples.
On an outcropscale,the actinolite
areflattenedin the firstpenetrative
cleavage
(S•).In
fine networkof quartz_+sulfide+_gold-bearing
veinlets. masses
however,
actinolite
commonly
occurs
asradiatInternalremobilization
(Gilliganand Marshall,1987) oc- thinsections,
curredin response
to theshearing
whichcaused
thedevelop- ing aggregates
crossing
the firstcleavage.
Thiscanbe exmentofbanding
in pyriteore,thebreeeiation
ofarsenopyriteplained
bya regional
alteration,
priortothefirstdeformation,
a suitable
rockcomposition
whichallowed
the
ore,andthe durehbewegung
strnetures
(Vokes,1969;Mar- thatproduced
shallandGilligan,1989).The alteredmariedikesthatcutthe growthof actinolite
duringlatermetamorphism.
The local
oreandhostrockin various
directions
wereemplaeed
prior occurrence
ofepidoteaftercalcite-actinolite
in marievolcanic
to theshearing
sincetheyhavebeendeformed.
In thearseno- rocksindicatesloweramphibolite
facieswhichmaycorrepyriteore,the dikesarerelatively
undeformed,
whereas,
in spondto peakmetamorphic
conditions.
of S1andS•areubiquitous
in metamorphic
anthepyriteore,thedikesareboudinaged,
indicating
a substan- Inclusions
by the sectial amountof deformation.
All dikeshavebeenaffectedby dalusiteandbiotite.Thesehavebeencrenulated
both the first and the second foliations. It can be inferred
ondregional
deformation.
In a fewsections,
latebiotitehas
parallelto the secondcleavage
S2.Thus,it
that the hostrockand the pyriteore behavedin a more beenobserved
thatthemetamorphism
peakedeitherlate
ductilemannerthanthearsenopyrite
oreandthemariedikes canbe concluded
duringthe firstdeformation
andthe shearing.
Deformation duringor afterboththefirstdeformation
andtheshearing.
strnetures,
whichare foundmainlyin the quartz-rich
parts Retrogression
is commonly
observed
in mostpeakmetaof the vein ore, are attributed to the second deformation.
morphicmineral assemblages.
Andalusiteis commonly
Piercement
cusps
andpiercement
veins(Gilligan
andMar- strongly
retrogressed
to sericite
alongcrystal
rims,withonly
shall,1987)are structures
commonly
developed
alongthe the centersof crystals
preserved,
andbiotiteandactinolite
interface
between
pyriteoreandhostrock.Piercement
cusps are commonly
retrogressed
to ohiorite.In somecases,
the
assemblage
in marievolcanic
rockshas
are subparallel
to the S2erenulation
cleavage
andtheycut actinolite-dominated
theS•cleavage.
According
to Odman(1941),thepiercementchanged
to a ohiorite-dominated
retrograde
assemblage
corcusps
in pyriteoreconsist
mainlyof fine-to medium-grainedresponding
to low-pressure,
uppergreenschist
faciesmetaehaleopyrite
andpyrrhotite,
oftenwithehaleopyrite,
quartz, morphism.
and gold concentrated
at the pointsof the cusps.The
Discussion
piercement
cuspsalsocommonly
containinclusions
of wall
rockandarsenopyrite
ore,eventhougharsenopyrite
ore is The following
parameters
arecriticalfor establishing
a genotpresentin thevicinityof thepiercement
cusps,
implying neticmodelfortheBoliden
deposit:
therelative
andabsolute
substantial
transportation
of material.
Piercement
cusps
and agesof oreandhostrocks,the structural
settingof the ore,
veinsareusually
interpreted
asevidence
ofexternal
remobili- i.e., isthe ore conformable(stratiform)or discordantto litholzation(Gilliganand Marshall,1987) and, in Boliden,the ogies,
thecomposition
oftheore,thegeometry
andcomposipiercement
cuspsandveinsare interpreted
to haveformed tionof the alteration
zone,andthe petrogenetic
aspects
of
of tectonicsetting.A summaryof
duringor afterthe second
phaseof deformation,
associatedhostrocksas indicators
is presented
in Table5.
withthelate,morebrittlefaulting.
Somedeformation
ofmas- thesecharacteristics
siveandveinoresoccurred
alsoat thisstageasindicated
by
agesof oreandhostrocks
undulose
extinction
andsubgrain
formation
in partsof the ttelativeandabsolute
vein quartz.
Time constraints
canbe placedon the formationof the
Bolidenore if the Bolidenareais compared
to otherareas
Metamorphism
in the Skellefte district. The massive ore in Boliden shows
No detailedstudiesof metamorphism
havebeen per- structures
relatedto bothshearing
andthe two phasesof
formedon the rocksof the Bolidenarea.However,during regionaldeformation
andmust,therefore,havebeenemtheconstruction
ofthevertical
section
andplanmapa large placed
priortodeformation.
Theveinore,incontrast,
formed
number of thin sectionswere studiedand, therefore, some in response
to shearing
alongthe ore zoneduringthe later
conclusions
canbe drawnaboutthe metamorphic
evolution part of the firstregionaldeformation.
Fromstudiesin the
andits relationship
to deformation.
Alongthe dominant
S• centralpartof the Skellefte
district(ef.Weihedet al.,1992),
cleavageand the shearfoliation,S•, ohiorite,muscovite,it is clearthatthe firstregional
deformation
occurred
after
quartz,andlocallybiotitedominate.
Phases
thathavegrown the intrusion
of the Siktrlisk
granitoid
southof the Skellefte
aftertheformation
of thesetwofoliations
arebiotite,garnet, districtatca.1.85Ga(WeihedandVaasjoki,
1993)butbefore
andalusite,
andsometimes
tourmaline,
allofwhichcommonlythe intrusionof Revsund
granitesat ca. 1.80 Ga (Ski01d,
containinclusions
of minerals
definingthe earlierfoliations. 1988).Thisconstrains
theageof themassive
orein Boliden
1092
BERGMAN WEIHED ET AL.
TABLE5. CriticalCharacteristics
of the BolidenDeposit
Tectonicsetting
Host rocks
Destructive
platemargin;
volcanic
are;possibly
developed
onoldercontinental
crust(?)
Feldspar
porphyritie
daeiteintrusion
(ca.1.87Ga);
andesitc-basalt
sills;quartzporphyry
stock;
LowK,
eale-alkaline;
shallow-marine
to subaerial;
waning
volcanic
activity
Alteration
system Inner serieitezone with andalusitebelow ore (ca. 1.85
Ga); outer ehlorite zone
2aSU/2ø6pb
and2aSU/2ø7pb
ratiosdifferby a common
factor
(7.617)fromthe concordant
pointcorresponding
to the PbPb ageof 1852 _+3.5 Ma. This is exactlywhatwouldbe
expected
if the isotopicsystematics
of the monazitewere
reallyconcordant
at 1852Ma, but the laboratory
treatment
resulted in an incorrect estimate of the U concentration. An
ageof 1852__+
3.5 Ma for the monaziteis, thus,indicated
bothfromthe independent
Pb-Pbageandthe manipulated
U-Pb ages.The monazite
occurstogetherwithandalusite
in
Orevertical;alteration
zonewidenswithdepth;ore +
a sericite-altered rock which indicates that the monazite
alteration
obliqueto lithologieal
contacts
Ore minerals
PyTite,
arsenopyrite,
pyrrhotite,
ehaleopyrite,
spalerite, formedin response
to the alteration
processes.
Therefore,
sulfosalts,
galena,
Au
the ageof the monazite
is interpreted
to be an estimate
of
Mineral events
Massive
oreof pyriteandarsenopyrite;
ehaleopyrite
+
the
age
of
mineralization.
Although
errors
in
age
determinaAu sulfosalts
overprinting
massive
ore
Structures and
tionsaresignificant,
it follows
fromthearguments
abovethat
Massive
oredeposited
priorto deformation;
D• and
metamorphism shearing
affected
alteration
system
andmassive
ore thereis a timedifference
betweenthe crystallization
of the
andgaveriseto brittlestructures
nowhosting
gold daciterock(zirconminimumage,1869_+15 Ma) andthe
ore;peakmetamorhism
postdates
shearing;
lateD2
deposition
of ore(monazite
age,1852_+3.5 Ma). Sincethe
deformation
andf•aulting
caused
slight
Geometry
remobilization
to be older than 1.80 Ga. However, both massiveand vein
1852Ma ageofthemonazite
islikelyto reflectanore-forming
stage,
thisexcludes
a volcanic
exhalative
modelsincesuchan
originwouldimplya mineralization
agecloserto 1.89to 1.87
Ga (agesof hostrocksto exhalative
massive
sulfidedeposits
elsewherein the district;Billstr0m and Weihed, 1996).
oresmusthavebeenemplaced
priorto peakmetamorphism
of thedeposit
sinceall oreminerals
arerecrystallized.
PeakmetamorphismCriticalcharacteristics
is generally
relatedin timeto the intrusion
of lateorogenic Thereis nowgeneralagreement
thatthe Skellefte
district
Harn0-Skellefte
granites
atca.1.81to 1.82Ga (Claesson
and formedat a Paleoproterozoic
destructive
platemargin,alLundqvist,
1990;Billstr0mandWeihed,1996),although
the thougharguments
for bothan island-arc
environment
(cf.
marginsetting(Allen
metamorphism
in theSkellefte
districthasneverbeendated. Weihedet al., 1992)anda continental
Therefore, the massiveand vein ores in Boliden must have et al.,1996)havebeenpresented.
In the Bolidenarea,both
formed some time before the intrusion of Harn0-Skellefte
thelower,mainlyrhyolitic-rhyodacitic
volcanic
rocksandthe
granites
at ca.1.81to 1.82Ga.TheRb-Srwhole-rock
isotope upperdacitesare low K, calc-alkaline
rockstypicalof the
of volcanic-arc
magmatism
(Table5). The
dataobtained
in thisstudyclearlyreflecta disturbed
Rb-Sr felsiccomponent
composition
of the hostrocksin the Bolidenarea
isotope
system
andcannotbe usedto setageconstraints
on chemical
the crustal evolution in the area.
thussupports
the generally
accepted
tectonic
settingof the
To settighterconstraints
on the timingof ore formation, Skellefte district.
intense
alteration
surrounds
theBollden
ore,sufzircons
fromthe feldspar
porphyritic
dacite,partlyforming Although
hostrocktotheBolidenore,weredated.ThePb-Pbevapora- ficient information remains in most cases for the host rocks
Thesearemainlycoherent
volcanic
rocks
tion age of 1869 _ 15 Ma for one singlezirconcrystalis to be determined.
asshallow
intrusions
intoslightly
oldervolcanic
interpreted
to represent
themaximum
ageofthemineraliza-interpreted
tion.A compilation
of newandrecentlypublished
agedata andsedimentary
rocks.The alteration
zonesurrounding
the
(Billstr0m
andWeihed,1996)indicates
thatmagmatic
activity Boliden ore consists of a central sericite zone which is domiin the Skelleftedistrictwasepisodic.
The mainbuildupof natedby quartzandsericite
withandalusite
in the corebethevolcanic
pileoccurred
duringthe 1880to 1890Ma inter- neath the massiveore, and an outer chlorite zone which is
valbutepisodic
magmatic
activity
continued
duringthe1865 dominated
byquartz,chlorite,
andpyrrhotite.
Bothalteration
to 1875 and 1855 to 1860 Ma intervals. The dacite at Boliden
zoneswidenwithdepthandboththeoreandthealteration
to all lithological
contacts
observed
in
isinterpreted
to be a subvolcanic
intrusion
andthe evapora- zoneare discordant
andsedimentary
rockssurrounding
theore(Figs.
tionageof the singlezirconcrystalfallsin the 1865to 1875 thevolcanic
Ma interval,consistent
withthehypothesis
thatthedaciteis 3 and 4, Table 5).
Thereis a closetexturalrelationship
betweenmineralizaintrusive
intobedrockthatrangesup to 1890Ma.
alteration,
whichisseenasa strong
arsenopyThe strongreversediscordancy
of the monazite
fromthe tionandsericite
in sericite-altered
quartzporphyry
anddacalteration
zonesurrounding
theBoliden
orepresents
a prob- riteimpregnation
in ore
lem.Asmentioned
above,
thisbehavior
mayresultfromprob- itc. Sericiteneedlesare locallyfoundas inclusions
Maficdikeswhichcutthe massive
pyriteandarselemsofsample
dissolution
andthefactthatthespiking
proce- minerals.
alteration
and
duresaredifferentfor U andPb.The U spikewasaddedto nopyriteoresare not affectedby pervasive
the total sample,whereasthe Pb spikewasaddedsubse- these dikes intruded before the onset of deformation. This
quently
to theisotopic
dilutionaliquot.
Asa result,thecalcu- indicates that the massive mineralization and sericite alterlatedU contentcouldhavebeenoverestimated
if complete ationare relatedin timeandthatbothformedpriorto any
Theclosespatial
relationship
between
thechloequilibrium
between
spikeandsample
wasnotachieved.
Ac-' deformation.
alteration,
withinterfingering
ona centimeter
cordingly,the U-Pb ratioswould signifya reversedis- riteandsericite
between
unaltered
rockandthe
cordancy.
Thisexplanation
is supported
by the factthatthe scale,andthesharpcontact
BOLlDENDEPOSIT,SKELLEFTE
DISTRICT,N. SWEDEN
1093
maschlorite
zonesuggest
thatboththesericite
andchlorite
alter- tiform. The modelsfor nonstratiform,volcanic-hosted,
herebecause
ationsbelongto thesamealteration
system.
Thesymmetricalsivesulfideores(Large,1992)arenotfavored
distribution of both the chlorite and sericite alteration zones the Boliden
deposit
lacksa recognizable
stringer
zoneand
metalcontent
withhighAsandminorBi, Sb,
aroundthe ore alsoindicates
that the two alterationtypes hasanunusual
alteration
belongto the samehydrothermal
event.In addition,
no evi- Se,Te, Mo, andCo. In addition,the symmetric
theBollden
ore,withthebulkofthealterdenceof olderalteration
systems
wasfoundin thisstudy. zonesurrounding
TheBoliden
deposit
iscomposed
of massive
pyriteandarse- ationlocatedbeneaththeore(Figs.4 and5) anda proximal
nopyrite
ore,whichis overprinted
by brittlestructures
con- sericite zone and a distal chlorite zone, contrastswith most
volcanic-hosted
massive
sulfide
deposits
whichtypraining
mainly
quartz,
chalcopyrite,
sulfosalts,
andgold.
Thevery described
chlorite
zoneanda distalsericite
zone
highgoldgrade(avg15.5g/t)andthelargevolume
of massiveicallyhavea proximal
arsenopyrite
isunusual
compared
to mostothermassive
sulfide (Franklin et al., 1981; Morton and Franklin, 1987).
A genetic
linkbetween
the massive
Boliden
oreandthe
deposits
in theSkellefte
district
(Weihed
et al.,1992;Allenet
al.,1996).Twootherdeposits
showsomesimilarities
withthe Pb-Zn mineralization in the mud- and siltstonessouth of the
sincesedimentary
units
Boliden
deposit
in goldgrades,
sulfide
assemblage,
andalter- Bolidendepositcanbe excluded,
ationassociations.
ThesearetheMg.
ngfallberget
andHolmtj'arnform
only
asmall
component
ofthehost
rocks
tothe•)reand
no roomto foldtectonically
and/or
deposits
(of.Weihedet al.,1992).Holmtj'arn
isa smalldeposit thereis geometrically
sequence
withoreintothepresent
posiin thenorth-central
partof the Skellefte
district(Fig.1),with sheara sedimentary
deposit.
Although
a shearzoneindeedis
highgrades
ofgoldin massive
pyriteandpyrite-sphalerite
(Nic- tionoftheBoliden
presentalongthe Bolidenorezone,the deformation
is not
olson,
1993),andlocally,
withmassive
arsenopyrite-chalcopyrite
ore. Nitolson(1993)linkedthe occurrence
of Asto the Cu-rich intense,and within 10 to 15 m from the ore, the host rocks
lacka cleavage.
The offsetof the contact
between
baseoftheHolmtj'•norebuttherelation
between
themassivevirtually
unitsandthefeldspar
porphyritic
daciteeastof
pyriteoreandthe massive
arsenopyrite-chalcopyrite
lenses
is sedimentary
complex.
A brecciated
arsenopyrite-chalcopyrite
oreisthemost the ore is also rather limited and indicates movement on the
the two
gold-rich
type(Willd•n,1986)andthisis similar
to thebrecci- orderof onlya few hundredmeters,In addition,
in metalcontentandelement
atedarsenopyrite
oreinBollden.
TheHolmtj'am
orehasapoorly oretypesdifferconsiderably
is domdeveloped
stringer
mineralization
anda widespread
sericite
al- distribution;the mineralizationin mud- andsiltstones
the Bollden
oreis dominated
teration
whichis overprinted
by chlorite
alteration
anda later inatedbyZn andPb,whereas
sericite-pyrite
alteration
adjacent
to theore.Latequartzveins by Cu, As,andAu.
We conclude,
in agreement
with radiometric
data,that
withsulfosalts
andgalena
arelinkedto themassive
sulfide
ore
exhalative
(Nicolson,
1993).Holmtj'am
isinterpreted
byNicolson
(1993) the Bolidenoredid notformby synsedimentary
to be an exhalative
volcanogenic
massive
sulfidedeposit--a processes
on the seafloor.Therefore,
otherdepositional
viewwhichischallenged
byAllenet al. (1996)whosuggest
a mechanisms must be considered.
replacement
origin.The Mlngfallberget
deposit
is situated
ca.
model
for theBoliden
deposit
2 kmnorthof Boliden
andischaracterized
byandalusite
alter- A newgenetic
Two
alternative
models
for
the
formation
of the Boliden
ationof rhyolitic
intrusions
andvolcaniclastic
rocks(Gripand
canbeenvisaged.
Thefirstmodelinvokes
thedeposiC}dman,
1942).
Apartfromandalusite,
thealteration
zonecon- deposit
sists
alsoofcorundum
andsericite
withaccessory
rutile,tourma- tionof massive
pyritewhichwaslateroverprinted
by a vein
of arsenopyrite,
sulfosalt,
andgoldassociated
witha
line,andfuchsite.
Characteristic
minerals
oftheM•gfallberget system
sericite
(andchlorite?)
alteration.
In thesecond
model,
deposit
areBitellurides
withsome
pyrite,tetrahedrite,
andgold. strong
It cannot,at thistime,be conclusively
established
if the theentiredeposit
formedin anepithermal
environment
with
system
involving
strongleaching
of host
massive
oreandthe gold-bearing
veinorerepresent
oneor a zonedalteration
residue
in thecenterof the
two episodes
of ore formation.
If two systems
are present, rockleavinga silicaaluminum
zone.The ore wasthendeposited
in two stages
however,
thetimedifference
is probably
verysmallasindi- alteration
sulfides
overprinted
bygold,copper,
andsulfosalts)
catedbyPbisotope
results
(Anders
Hallberg,
pers.commun., (massive
1995).In thisstudy,nogoldwasfoundin themassive
pyrite in thecentralpartof thealteration
zone.In bothcases,
the
priorto the deformation
andmetamorore,norhasit previously
beenreported.
Mostgoldwasfound orewasemplaced
oreminerals
to theirpresent
in the quartz-rich,
i.e., mostbrecciated,
arsenopyrite
ore phismwhichlaterremobilized
(M0rtsell,1931),whereas
lowgoldgrades
havebeenreported distribution.
fromthe extremely
finegrainedunbrecciated
arsenopyrite Thefirstmodelisbased
ontheinterpretation
thatpodsof
ore.Thismayindicatethatthe goldwasremobilized
mainly massive
arsenopyrite
mayhaveconstituted
morecoherent
fromthearsenopyrite
oreduringshearing
alongtheorezone veinlikearsenopyrite
bodiespriorto deformation.
In some
sections
of the Bolidendeposit,
the arsenopyrite
whichled to intensebrecciation
of the brittlearsenopyritehorizontal
oreandveiningof the moreductilepyriteore.
podsforminclusion
trailsthrough
pyriteoreandhostrocks
Observations
againstan exhalative
originfor theBoliden
deposit
which
arestrongly
reminiscent
offolded
boudinage.
Thearse-
nopyrite-sulfosalt-gold
assemblage
is not uncommonin
smaller
vein-type
deposits,
e.g.,Jales,
northern
Portugal
(CoThe observations that the Boliden ore is situated in shallow teloNeivaandNeiva,1990),Salsigne,
France(Le Guenet
volcanic
intrusions,
that it is discordant
to lithological
con- al.,1992;Leseuyer
et al.,1993),Loddiswell,
UnitedKingdom
tacts,andthat an agedifferenceis indicatedbetweenhost (Stanley
et al.,1990),BaccuLocci,Italy(Bakos
et al.,1990),
rock and alteration zone demonstrate that the ore is not stra- andSanta
Comba-Fervenza,
Spain(Castroviejo,
1990).These
1094
BERGMAN WEIHED ET AL.
veindeposits
alsohavean alteration
stylewhichis similarto whichareobserved
in modemhigh-sulfidation
deposits,
have,
to andalusite.
The seficite
thealteration
foundin Bolidenwitha proximal
intensesefi- at Boliden,beenmetamorphosed
citealteration.
Mostof the deposits
areinterpreted
to have andchlofitezonesatBoliden
correspond
totheargillic-seficitic
beenintroduced
duringdeformation
andmetamorphism,
and andpropylitic
zones,respectively,
in modemdeposits.
The
theycommonly
occurin sedimentary
sequences.
In general, crosscutting
relationship
between
thealteration
system
includarsenopyfite-fieh
deposits
arefoundin areas
wherelargevol- ing ore andthe hostrocksandthe common
occurrence
of
umesofsedimentary
rocks
arepresent.
Young
massive
sulfide tourmaline
areotherfeatures
theBolidendeposit
hasin comdeposits
withabundant
arsenopyrite
arecommonly
foundin monwithmodemhigh-sulfidation
deposits.
back-arebasinsdeveloped
on oldercontinental
crust,e.g.,
The largeamountof arsenopyrite
in the Bolidendeposit,
theOkinawa
trough(HerzigandHannington,
1995).In com- however,
differsfrommodemhigh-sulfidation
deposits
where
parison,the Bolidendepositis indeedsituatedcloseto a enargite-luzonite
assemblages
areubiquitous.
Thismakes
Bosedimentary
sequence
anda numberof smallarsenopyrite-liden,in thisrespect,
moresimilarto low-sulfidation
epithergoldmineralizations
havebeendesefibed,
mainlyin thesedi- mal deposits,
wherearsenopyrite
is ubiquitous
(Whiteand
mentaryunitsof the area(e.g.,Bergman,1992).However, Hedenquist,
1995).Thepresence
of arsenopyrite
in Boliden
most of these smallerarsenopyfite-gold
mineralizationsmaybeexplained
bya slightly
morereduced
fluid,whichwas
formedlate duringor after the secondphaseof regional highin H2Sbut contained
sufficient
HC1 to producethe
deformation
(Bergman,1992),whereasthe arsenopyfite
in aciditynecessary
for leaching
thehostrocks.
Boliden
musthavebeenintroduced
priortoanydeformation. Anotherdifference
between
theBolidendeposit
andmodTherefore,
in thisstudythesecond
modelis favoredsinceit ernhigh-sulfidation
epithermal
deposits
isthata timedifferbest fits our observations.
enceis indicatedbetweenmassive
sulfidedeposition
and
Manyeharaetefisties
of theBolidenoreandalteration
zone gold-bearing
structures
in Boliden.The massive
pyfiteand
aresimilarto modem,high-sulfidation,
epithermal
deposits,arsenopyfite
weredeposited
priorto deformation
andintrue.g., Lepanto,Philippines
(Arfibaset al., 1995),E1 Indio, sionof mariedikes,whereas
gold-bearing
structures
formed
Chile(Jannas
et al., 1990),andthe Nansatsu
distfietin Japan in response
tosheafing
probably
lateduring
thefirstdeforma(Hedenquist
et al.,1994).Thesedeposits
commonly
occurin tion.In modemhigh-sulfidation
deposits
nosuchtimediffertheeireum-Paeifie
regionbuthavealsobeenidentified
in the ence is indicated.
Mediterranean
regionand centralEurope(Arribas,1995). High-sulfidation
epithermaldeposits
are interpretedto
Thehigh-sulfidation
epithermal
deposits
areeharaetefized
by formin a near-surface
environment.
Allenet al. (1996)argue
earlyenargite-luzonite-pyfite
overprinted
byAu andsulfides onthebasisof volcanic
faciesinterpretations
thatthe topof
liketennantite-tetrahedfite,
ehaleopyrite,
andtellufides
(Arfi- thevolcanic
pilein theBolidenareamusthavebeensituated
bas,1995).Typicalalteration
zoningconsists
of aninnerzone abovethe wavebase(shallowwater or subaerial)at the time
of vuggyto massive
silica,whichcontains
mostof the ore, ofdeposition
ofthelowermost
sedimentary
units.In addition,
surrounded
by advanced
argilliealteration
with alunite,ka- fluidinclusion
dataonquartzfromveinorein thebrecciated
olinitc,pyrophyllite,
anddiaspore.
Thisis in turn followed arsenopyrite
indicatea significant
meteoriccontribution
to
outwardby argillicalteration
with quartz,kaolinite,
sericite, thefluidwhich
deposited
the
quartz
and
sulfides
(•berg,
andsmectite,
andin theoutermost
partbya haloofpropylitic
an epithermal,
high-sulfidation
origin
alteration with chlorite, illitc, smectite, and carbonate (Arri- 1995).Thissupports
for
the
Boliden
deposit.
bas,1995;WhiteandHedenquist,
1995).Theadvanced
argilEpithermal
deposits
haverecently
beenidentified
alsoin
lic alteration
isinterpreted
to formbyleaching
fromlowpH
old,
deformed
gold
deposits,
e.g.,
the
Ordovician
Chetwynd
fluidswhichformfromabsorption
of magmatic
volatiles
by
in Newfoundland
(McKenzie,
1986),thePameteoric water near the surface (Heald et al., 1987). The Au-Cu-deposit
En•tsen
Au deposit
in centralSweden
(Hallleachedrockthenformsa permeable
channel
for laterore leoproterozoic
berg,
1993,
1994),
several
deposits
in
the
Archcan
of
Western
fluids,withthezonedalteration
system
indicating
neutralizaAustralia(Rifle, 1990), and the CambrianCarolinaslatebelt
In theBolidendeposit,
themassive
arsenopyrite
andpyrite of the easternUnited States(Rifle, 1990). One reasonthat
tion of fluids outward (Stevenand Ratt•, 1960).
in olderterranes
haverarelybeenclassified
asepioreis overprinted
by a latergold-quartz-eopper-fieh
system deposits
maybe thatthepreservation
potential
of
associated
withsulfosalts.
Thissequence
of mineraldepositionthermaldeposits
orshallow-water
deposits
islow,andthatlatermetaand the common occurrenceof Bi-, Se-, Mo-, Zn-, Pb-, and subaerial
anddeformation
haveobliterated
manyfeatures
Sn-beafing
minerals
in the Bolidendeposit
is similarto that morphism
theoriginof suchdeposits.
described
for modemhigh-sulfidation
deposits
(el. Arfibas, criticalforinterpreting
Archcangolddeposits
in CanadaandAustralia
are com1995;WhiteandHedenquist,
1995).The mineralogy
of the
to bemesothermal
andto haveformedin
centralpartof the alteration
zonearoundthe Bolidenore, monlyinterpreted
to sheafing
relatedto accretionary
tectonicprowithmainly
serieite
andquartz
withpodsandimpregnations
of response
(KerfichandCassidy,
1994).In all cases,
the goldis
andalusite
+ corundum,
indicates
thattherocks
werestrongly cesses
to be emplaced
duringor aftermetamorphism
leached,leavinga silicaaluminumresidue.This strongly interpreted
et al.,1990;Guhaet al.,1991;Toufigny
et al.,
leached
zoneissimilarto theadvanced
argilliealteration
zone (e.g.,Marquis
deposits
alsolackthelargeamountof
observed
in modemhigh-sulfidation
deposits
(Healdet al., 1993).TheArchcan
presentin Boliden.The Bolidendepositis
1987;Arfibas,1995)andin porphyry-type
deposits
in young arsenopyrite
fromtheseArchcan
deposits
in thattheore
geologic
terranes
(MeMillanandPanteleyev,
1986).Thesilica clearlydifferent
gold)wasemplaced
priorto metamorphism.
andaluminosilieates
likekaolinitc,
pyrophyllite,
anddiaspore, (including
BOLlDEN DEPOSIT,SKELLEFTEDISTRICT,N. SWEDEN
1095
Conclusions
We are alsoindebtedto manyothergeologists
workingin
theareafor stimulating
discussions
throughout
thecourse
of
Themainepisode
ofmainlyfelsicvolcanism
in theBoliden thisstudy.RodneyAllenis especially
thanked
for
teaching
area occurred between 1.89 and 1.88 Ga at a time when the
us a newwayof lookingat ancientvolcanic
rocksandfor
area was submarine and below the wave base. This was the
givingus access
to hismatedhal
froma volcanological
study
mainstageof formationof volcanic-hosted
massive
sulfide
in thearea.ThreeEconomic
Geology
reviewers
arethanked
deposits
(Lftngdal,
Lftngsele,
Kankberg,
etc.),someof which forusefulcomments
whichgreatly
improved
themanuscript.
haverecently
beeninterpreted
assubsea-floor
replacement
This
study
is
part
of
a
national
ore
geology
research
program
deposits
(Allenet al.,1996).Theareawassubsequently
up- (PIM) financedby Swedish
National
Board
for
Industrial
liftedduringandafterthewaningstages
of volcanism
at ca.
Development
(NUTEK),the Swedish
Mining
1.88 to 1.87 Ga, and intrusionsof dacite and andesitelavas andTechnical
Industry
Research
Organisation
(MITU),
and
the
Geological
anddomes,with a minimumageof ca. 1.87Ga, wereeraof Sweden
(SGU).Financial
support
fromtheseorgaplacedintothelower,unlithified
partof a succession
of sedi- Survey
nizations
is
gratefully
acknowledged.
ments.According
to Allenet al. (1996)theareawasshallow
marineandpossibly
evensubaerial
atsomestageduringsediREFERENCES
mentdeposition.
inthegoldlodeoresatBoliden;
a keyto
Fluidswerefocused
throughthe rocks,possibly
alongan •berg,A.,1995,Fitfidevolution
in theBoliden
deposit:
Swedish
National
BoardforIndusearlyfault,at ca.1.85Ga.Thesefluidsinitiallyleached
the metallogenesis
andTechnical
Development
Reportof NationalOre Geology
Rewallrocksandformeda zonedalteration
assemblage
which trial
search
Programme
Project93-0135P,12 p.
cutobliquely
across
lithological
contacts.
Oreminerals
were Allen,
R.A.,
Weihed,
P.,andSvenson,
S.-]t.,1996,
Setting
ofZn-Cu-Au-Ag
deposited
in thecenterofthealteration
zonein twoepisodes massive
sulfide
deposits
in theevolution
andfacies
architecture
ofthe1.9
characterized
by contrasting
mineralassociations;
an early Ga marinevolcanicarc, Skelleftedistrict,Sweden:ECONOMICGEOLOGY,
pydhte,
arsenopyrite,
and pyrrhotiteassociation
was over- v. 91, p. 1022-1053.
A.,Jr.,1995,Characteristics
ofhigh-sulfidation
epithermal
deposits,
printedby chalcopyrite,
sulfosalts,
andgold.Regional
defor- Arribas,
andtheirrelationto magmatic
fluid:Mineralogical
Association
of Canada
mationwithfoldinganddevelopment
of a penetrative
cleav- ShortCourse,v. 23, p. 419-454.
age affectedthe rockssometime between1.85 and 1.82 Arribas,A., Jr., Hedenquist,
J.W.,Itaya,T., Okada,T., Concepci6n,
R.A.,
formation
of adjacent
porGa. Duringthe latestageof thisdeformation,
a shearzone andGarcia,J.S.,Jr.,1995,Contemporaneous
over300 ka in northernLuzon,
developed
along
theorezone,causing
a stretching
oforebod- phyryandepithermalCu-Audeposits
Philippines:
Geology,
v. 23,p. 337-340.
iesandthe formation
of durchbewegung
structures
which Bakos,
F., Carcangiu,
G., Fadda,S.,Mazzella,
A., andValera,R., 1990,The
resulted
in a mixingof pydhte
andarsenopydhte
oresandbrec- goldmineralization
of BaccuLocci(Sardinia,
Italy);origin,evolution
and
ciationof the arsenopyfite
ore.Remobilization
(or introduc- concentration
processes:
Terra Nova,v. 2, p. 232-237.
S.,andEkstrOm,
T.K., 1978,Arsenopyrite
andsphalerite
asT-P
tion) of mainlychalcopydhte,
sulfosalts,
and goldoccurred Berglund,
in sulfideoresfromnorthern
Sweden:
Mineralium
Deposita,
duringsheafing.
Thereafter,
alloretypesrecrystallized
during indicators
v. 15,p. 175-187.
peakmetamorphism
at ca.1.83to 1.81Ga.A second
defor- Bergman,
J., 1992,Structural
geology
of Grundfors,
a quartzveinrelated
mationcaused
crenulation
of earlyfabricssometimeafter golddeposit
in theSkellefte
district,
northern
Sweden:
Geologiska
FOre1.80 Ga.
ningens
i Stockhohn
F6rhandlingar,
v. 114,p. 227-234.
K., andWeihed,P., 1996,Ageandprovenance
of hostrocksand
Thepresentstudyindicates
thatthe Bolidendeposit,
and Billstr6m,
Skelleftedistrict,northernSweden:
ECOpossibly
othersimilardeposits
(Mftngfallberget,
Holmtj•im), oresin the Paleoproterozoic
NOMICGEOLOC¾,
v. 91, p. 1054-1072.
earlierclassified
astypicalexhalative
volcanic-hosted
massiveCabri,L.J.,Chryssoulis,
S.,deVilliers,J.P.R.,Laflamme,
J.H.G.,andBuseck,
sulfidedeposits,
arenotstratiform
andhavea differentalter- P.R.,1989,Thenatureof"invisible"
goldinarsenopyrite:
Canadian
Minerationstyleanddifferentmetalcontents
compared
to other alogist,
v. 27, p. 353-362.
R., 1990,Goldoresrelatedto shearzones,
WestSantaCombamassive
sulfidedeposits
in thearea.The Boliden
oresystem Castroviejo,
study:Mineralium
is similarto modemhigh-sulfidation
deposits,
whereearly Fervenzaarea(Galicia,NW Spain):A mineralogical
Deposita,
v. 25, p. S42-S52.
leaching
isfollowed
bydeposition
of massive
pydhte
andCu- Claesson,
L.-Jt.,1985,
Thegeochemistry
ofEarlyProterozoic
metavolcanic
Asoreandthentypically
followed
bylaterAuore.Thetiming rockshostingmassive
sulfidedeposits
in the Skellefte
district,
northern
of mineralization
mighthavebeenassociated
witha period Sweden:Geological
Society
of LondonJournal,
v. 142,p. 899-909.
of shallow-water or subaerial conditions in an otherwise sub-
Claesson,
S.,andLundqvist,
T., 1990,Svecofennian
granites
in theBothnian
mergedarea.In the Bolidenarea,this situationoccurred basin,centralSweden[abs.]:Geonytt,v. 17.1,p. 36.
1995,Origins
andagesof Proterozoic
granitoids
in theBothnian
basin,
some30 to 40 m.y.afterthe mainperiodof deposition
of
centralSweden;
isotopic
andgeochemical
constraints:
Lithos,v. 36, p.
exhalative
volcanic-hosted
massive
sulfidedeposits.
115-140.
Ourconclusion
thattheBolidenAu-fichoreis epigeneticCook,N.J.,andChryssoulis,
S.L.,1990,Concentrations
of invisible
goldin
CanadianMineralogist,
v. 28, p. 1-16.
andepithermalin nature,unrelatedto exhalative
volcanic- the commonsulfides:
hostedmassive
sulfidedeposits
in the region,hassignificantCoteloNeiva,J.M.,andNeiva,A.M.R.,1990,ThegoldareaofJales(northern
Portugal):
TerraNova,v. 2, p. 243-254.
implications
for exploration
of Au deposits
elsewhere
in the
region.
Acknowledgments
Thanksare dueto BolidenAB for theirsupportof this
project.Withouttheir supplyof mapsandsamplematerial
thisinvestigation
wouldnothavebeenpossible
to carryout.
Duckworth,
R.C.,1991,Thegeology
anddepositional
environment
of the
EarlyProterozoic
massive
sulfide-bearing
sequence,
Renstr6m,
northern
Sweden:Unpublished
Ph.D. thesis,Universityof Wales,Collegeof
Cardiff,209p.
Franklin,J.M.,Lydon,J.W.,andSangster,
D.F., 1981,Volcanic-associated
massive
sulfidedeposits:
ECONOMIC
GEOLOGY
75THANNIVERSARY
VOLUME,p. 485-627.
Gavelin,S.,1939,Geology
andoresof the Mal•tn'•etdistrict,
V•isterbotten,
1096
BERGMAN WEIHED ET AL.
Sweden:
Sveriges
Geologiska
Unders6kning,
SeriesC, v. 424,424p.
Th isotope
data,version1.20:U.S.Geological
SurveyOpen-FileReport
-1942,Relations
betweenoredeposition
andstructure
in the Skellefte 88-542.
Lundqvist,
T., 1979,The Precambrian
of Sweden:
Sveriges
Geologiska
Undistrict:
Sveriges
Geologiska
Unders/3kning,
SeriesC, v. 443,15p.
ders6kning,
SeriesC, v. 768,87 p.
--1955, Beskrivning
till berggrundskarta
6vetV'fisterbottens
l•in.1. UrT., Gee, D.C., Kumpulainen,
R., Karis,L., and Kresten,P.,
bergsomr•det
inomV•isterbottens
l•in. Summary:
The pre-Caledonian Lundqvist,
tillberggrundskartan
6verV•isternorrlands
l•in:Sveriges
rocksof the V•isterbotten
county,southern
Swedish
Lappland:
Sveriges 1990,Beskrivning
Geologiska
Undersokning,
SeriesBa,v. 31, 429p.
Geologiska
Unders0kning,
SeriesCa,v. 37,p. 88-99.
Gilligan,L.B.,andMarshall,
B., 1987,Texturalevidence
for remobilizationMarquis,P., Hubert,C., Brown,A.C.,andRigg,D.M., 1990,Overprinting
of early,redistflbuted
Fe andPb-Znmineralization
bylate-stage
Au-Agin metamorphic
environments:
Ore Geology
Reviews,
v. 2, p. 205-230.
Cudeposition
at theDumagami
mine,Bousquet
district,
Abitibi,Quebec:
Grip, E., 1942,Nickelf/3rekomsten
i Lainijaur:Geologiska
F/3reningens
i
Canadian
Journalof EarthSciences,
v. 27, p. 1651-1671.
Stockholm
F/3rhandlingar,
v. 64,p. 273-276.
Marshall,B., and Gilligan, L.B., 1989, Durchbewegung
structure,
Grip,E., and0dman,O., 1942,Thetelluride-bearing
andalusite-sericite
piercement
cusps,
and
piercement
veins
in
massive
sulfide
deposits:
Forrocksof M•ngfallberget
at Bollden,
N. Sweden:
Sveflges
Geologiska
Unmationandinterpretation:
ECONOMIC
GEOLOGY,
v. 84, p. 2311--2319.
ders0kning,
SeriesC, v. 447,21 p.
E., 1987, Lainijaurintrusionens
geokemi:Licentiatuppsats
Grip,E.,andWirstam,
•t.,1970,
TheBoliden
sulfide
deposit:
Sveriges
Geo- Martinsson,
1987:05L,
Sweden,
Lulefi
University,
80
p.
logiska
Unders/Skning,
SeriesC, v. 651,68 p.
McKenzie,C.B.,1986,Geology
andmineralization
of theChetwynd
deposit,
Guha,J.,Lu,H., andDubS,B.,1991,Fluidcharacteristics
ofveinandaltered
southwestern
Newfoundland,
in Macdonald,
A.J.,ed.,Gold'86:Willowwallrockin Archean
mesothermal
golddeposits:
ECONOMIC
GEOLOGY,
v.
dale,Ontario,KonsultInternational,
p. 137-148.
86, p. 667--684.
McMillan,W.J.,andPanteleyev,
A., 1986,Porphyry
copperdeposits:
GeosciHallberg,A., 1993,The genesis
of the Endsengolddeposit:
Unpublished enceCanadaReprintSeries,v. 3, p. 45-48.
Ph.D.thesis,
Uppsala
University.
Morton,R.L., andFranklin,J.M., 1987,Two-foldclassification
of Archean
--1994, The Enfisengolddeposit,centralSweden.1. A Paleoproterozoic volcanic-associated
massive
sulfide
deposits:
ECONOMIC
GEOLOGY,
v. 82,
highsulphidation
epithermal
goldmineralization:
Mineralium
Deposita, p. 1057--1063.
v. 29, p. 150-162.
M0rtsell,S.,1931,Gedigetguldi Boliden-malmen:
Geologiska
FOreningens
Heald,P., Foley,N.K., andHayba,D.O., 1987,Comparative
anatomy
of
i Stockholm
F6rhandlingar,
v. 53,p. 394-414(English
summary).
volcanic-hosted
epithermaldeposits:
Acid-sulfate
and adularia-sericiteNicolson,
D., 1993,The paleoenvironmental
settingandAu genesis
of the
types:ECONOMIC
GEOLOGY,
v. 82, p. 1--26.
EarlyProterozoic
Holmtj'firn
volcanogenic
massive
sulfidedeposit,
SkelHealy,R.E.,andPetruk,W., 1990,Petrology
of Au-Ag-Hg
alloyand"invisileftedistrict,northernSweden:
Unpublished
Ph.D.thesis,University
of
ble"goldin theTroutLakemassive
sulfidedeposit,
Flin Flon,Manitoba: Wales,Collegeof Cardiff.
Canadian
Mineralogist,
v. 28, p. 189-206.
Nilsson,
C.A.,1968,Wall rockalteration
at the BoBden
deposit,Sweden:
ECONOMIC
GEOLOGY,
V. 63, p. 472--494.
Hedenquist,
J.W.,Matsuhisa,
Y., Izawa,E., White,N.C., Giggenbach,
W.F.,
andAoki,M., 1994,Geology,
geochemistry,
andoriginof high-sulfidationNysten,P., 1986,Goldin the volcanogenic
mercury-rich
sulfidedeposit
Lfingsele,
Skellefte
ore district,northernSweden:
MineraliumDeposita,
Cu-Aumineralization
in the Nansatsu
district,
Japan:
ECONOMIC
GEOLv. 21, p. 116-120.
OGY,v. 89, p. 1--30.
andoresof the Boliden
deposit,
Sweden:
Herzig,P.M., andHannington,
M.D., 1995,Polymetaflic
massive
sulfides
at 0dman,O.H., 1941,Geology
Sveriges
Geologiska
Unders6kning,
SeriesC, v. 438,190p.
the modernseafloor,
A review:Ore Geology
Reviews,
v. 10,p. 95-115.
usedto investigate
pasttectonic
Isaksson,
I., 1973,Vismut-antimonrika
mineraliseringar
i Bolidemnalmen:Pearce,J.A.,1975,Basaltgeochemistry
environments
on Cyprus:
Tectonophysics,
v. 25, p. 41-67.
Unpublished
Fil. Lic.thesis,Sweden,
Stockholm
University.
rocks
Jannas,
R.R., Beane,R.E., Ahler,B.A.,and Brosnahan,
D.R., 1990,Gold Pearce,J.A.,andCann,J.R.,1973,Tectonicsettingof basicvolcanic
determined
usingtraceelementanalyses:
EarthandPlanetary
Science
andcoppermineralization
at theE1Indiodeposit,
Chile:Journal
of GeoLetters,v. 19, p. 290-300.
chemicalExploration,
v. 36, p. 233-266.
P., 1960,TheVarutr'fisk
pegmatite:
International
Geological
ConJensen,
L.S.,1976,A newcationplotforclassifying
subalkalic
volcanic
rocks: Quensel,
gress,Norden,1960,Guideto Excursions
A27 andC22.
OntarioDivisionof MinesMiscellaneous
Paper,v. 66, p. 1-22.
Geologiska
Unders6Johansson,
•t.,Gee,D.C.,Bj/3rklund,
L.,andWitt-Nilsson,
P.,1995,
Isotope Rickard,D., ed., 1986,The Skelleftefield:Sveriges
kning,SeriesCa,v. 62, 54 p.
studies
ofgranitoids
fromtheBangenhuk
Formations,
NyFriesland
CaledRickard, D.T., and Zweifel, H., 1975, Genesisof Precambriansulfideores,
onides,Svalbard:
Geological
Magazine,
v. 132,p. 303-320.
GEOLOGY,
v. 70, p. 255--274.
Kautsky,
G., 1957,Ein beitragzurstratigraphie
unddemBaudesSkellefte- Skelleftedistrict,Sweden:ECONOMIC
Rifle,
G.T.,
1990,
A
comparison
of
alteration
assemblages
associated
with
feldes,Nordschweden:
Sveriges
Geologiska
Unders/3kning,
SeriesC, v.
Archaean
golddeposits
in WesternAustralia
andPalaeozoic
golddeposits
543, 65 p.
in thesoutheast
UnitedStates:
Canadian
Journal
of EarthSciences,
v. 27,
Kerrich,R., andCassidy,
K.F., 1994,Temporalrelationships
of lodegold
p. 1560-1576.
mineralization
to accretion,
magmatism,
metamorphism
and deforma- Sharp,Z.D., Essene,E.J.,andKelly,W.C., 1985,A re-examination
of the
tion--Archean
to present:
A review:Ore Geology
Reviews,
v. 9, p. 263arsenopyrite
geothermometer:
Pressure
considerations
andapplications
to
310.
naturalassemblages:
Canadian
Mineralogist,
v. 23, p. 517-534.
Kober,B.,1986,Whole-grain
evaporation
for2ø7pb/2ø6pb
ageinvestigations
Shervais,
j.w., 1982,Ti-V plotsandthepetrogenesis
of modernandophioon singlezirconsusinga double-filament
thermalion source:
Contribuliticlavas:EarthandPlanetary
Science
Letters,v. 59, p. 101-118.
tionsto Mineralogy
andPetrology,
v. 93, p. 482-490.
Ski01d,
T., 1988,Implications
of newU-Pbzirconchronology
to EarlyProKrogh,T.E., 1973,A low-contamination
method
forhydrothermal
decompo- terozoiccrustalaccretionin northern Sweden:PrecambrianResearch,v.
sitionof zirconand extraction
of U-Pb for isotopicagedetermination: 38, p. 147-164.
Geochimica
et Cosmochimica
Acta,v. 37, p. 485-494.
Slade,G., 1986,Thecharacter
andevolution
ofthevolcanic
andsedimentary
Large,R.R.,1992,Australian
volcanic-hosted
massive
sulfidedeposits:
Fearockshosting
the massive
sulphide
mineralization
in Bollden,northern
tures,styles,andgeneticmodels:
ECONOMIC
GEOLOGY,
v. 87, p. 471Sweden:
Unpublished
B.Sc.thesis,Aberystwyth,
University
Collegeof
510.
Wales,50 p.
Le Guen,M.• Lescuyer,
J.L.,andMarcoux,
E., 1992,Lead-isotope
evidence Smith,R.J.,1986,Proterozoic
sedimentation
andtectonics
of the Boliden
fora Hercynian
originof theSalsigne
golddeposit
(Southern
MassifCenarea,N. Sweden:
Unpublished
B.Sc.thesis,
Aberystwyth,
University
Coltral,France):Mineralium
Deposita,
v. 27, p. 129-136.
legeof Wales,57 p.
Lescuyer,
J.L., Bouchot,V., Cassards,
D., Feybesse,
J.L., Marcoux,E., Stanley,c.J., Halls,C., Cam, G.S.,and James,j., 1990,Gold-antimony
Moine,B., Piantone,
P., Tegyey,M., andTollon,F., 1993,Le gisement mineralization
at Loddiswell,
Devon,UK: Terra Nova,v. 2, p. 224-231.
aurifbrede Salsigne
(Aude,France):Une concentration
syntectoniqueSteven,
T.A.,andRatt6,J.C.,1960,Geology
andoredeposits
of the Sumtardivaflsque
dansless•diments
d•tfltiquesetcarbonates
dela Montagne- mitvilledistrict,SanJuanMountains,
Colorado:
U.S.Geological
Survey
Noire:Chronique
de la Recherche
Mini•re,v. 512,88 p.
Professional
Paper343,70 p.
Ludwig,K.R., 1991,PBDAT--a computer
programfor processing
Pb-U- Svenson,
S.-•.,1983,
N•sliden.
A volcanogenic
massive
sulphide
deposit
in
BOLIDEN DEPOSIT,SKELLEFTEDISTRICT,N. SWEDEN
1097
the Skelleftedistrict,northernSweden:
Sveriges
Geologiska
Unders0-
Tallbergporphyry-type
deposit,
northernSweden:
Mineraliunn
Deposita,
v. 29, 128-138.
kning,SeriesC, v. 790,81 p.
Tourigny,
G., Doucet,D., andBourget,
A., 1993,Geology
of the Bousquet Weihed,P.,andVaasjoki,
M., 1993,Regional
innplications
ofanagedetermi2 mine:An example
of a deformed,
gold-bearing,
polymetallic
sulfide nationof a gneissose
granitoidsouthof the Skelleftedistrict,northern
deposit:
ECONOMIC
GEOLOGY,
v. 88, p. 1578-1597.
Sweden:
Geologiska
F6reningens
i Stockholnn
F6rhandlingar,
v. 115,p.
Trepka-Bloch,
C., 1989,Volcanogenie
andtectonic
features
oftheRakkejaur 189-191.
sulfide
deposit,
Skellefte
district,
Sweden:
Mineraliunn
Deposita,
v. 24,p. Weihed,
P.,Bergman,
J.,andBergstr6nn,
U., 1992,Metallogeny
andtectonic
279-288.
evolution of the Early Proterozoic Skellefte district, northern Sweden:
Vivallo,W., 1987,EarlyProterozoic
binnodal
volcanism,
hydrothermal
activPrecannbrian
Researcl•,
v.58,p.143-167.
ity,andmassive
sulfidedeposition
in the Boliden-L3ngdal
area,Skellefte Weihed,
P.A.,Isaksson,
I., andSvenson,
S.-•.,1987,
TheTallberg
porphyry
district,Sweden:
ECONOMIC
GEOLOGY,
v. 82,p. 440-456.
copperdepositin northernSweden:
A prelinninary
report:Geologiska
Vivallo,
W.,andClaesson,
L.-•.,1987,
Intra-arc
rifting
andmassive
sulphide F6reningens
i Stockholnn
F6rhandlingar,
v. 109,p. 47-53.
nnineralization
in anearlyProterozoic
volcanic
arc,Skellefte
district,
north- White,N.C.,andHedenquist,
J.W.,1995,Epithernnal
golddeposits:
Styles,
ern Sweden:
Geological
Society
of LondonSpecial
Publication,
v. 33, p.
characteristics
andexploration:
Society
of Econonnic
Geologists
Newslet69-79.
ter, v. 21, p. 8-13.
Vivallo,
W., andWilld•n,M., 1988,Geology
andgeochennistry
of anEarly Willd•n,M., 1986,Geology
of thewestern
partof the Skellefte
fieldand
Proterozoic
volcanic
arcsequence
at Kristineberg,
Skelleftedistrict,Swethe Kristineberg
andI-Iorntr•isk
sulphide
deposits:
Sveriges
Geologiska
den:Geologiska
F6reningens
i Stockholm
F6rhandlingar,
v. 110,p. 1-12.
Unders6kning,
SeriesCa,v. 62, p. 46-52.
Vokes,F.M., 1969,A reviewof the metannorphisnn
of sulphide
deposits: Wilson,
M.R.,Claesson,
L.-,•.,Sehlstedt,
S.,Snnellie,
J.A.T.,
Aftalion,
M.,
Earth-Science
Reviews,
v. 5, p. 99-143.
Hamilton,
P.J.,andFallick,A.E.,1987,J6rn:AnearlyProterozoic
intrusive
Wasstr6m,
A., 1993,TheKnaftengranitoids
ofV•isterbotten
County,northcomplex
in a volcanic
arcenvironment:
Precannbrian
Research,
v. 36, p.
201-225.
em Sweden:
Sveriges
Geologiska
Unders6kning,
SeriesC, v. 823,p. 6064.
Winchester,
J.A.,andFloyd,P.A.,1977,Geoehennical
discrinnination
of difWeihed,P., 1992,Lithogeochennistry,
alteration
andnnineralization
zoning
ferentmagmaseriesandtheir differentiation
products
usinginnmobile
intheTallberg
porphyry
typedeposit:
Journal
ofGeochennical
Exploration, elements:
Chennical
Geology,
v. 20, p. 325-343.
v. 42, p. 301-325.
York,D., 1969,Leastsquares
fittingof a straight
linewithcorrelated
errors:
Weihed,
P.,andFallick,1994,A stable
isotope
study
ofthePalaeoproterozoic EarthandPlanetary
Science
Letters,v. 5, p. 320-324.