1005
Canadian Mineralogist
Vol. 30, pp. 1N5-1022 (1992)
OFWALES
AT SALTCHUCKPRINCE
PETROGENESIS
OFTHEPd.RICHINTRUSION
BODY
ULTRAMAFIC
ALASKAN-TVPE
EARLV
PALEOZOIC
ISLAND:AN
ROBERTA.LONEY
IJ.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, U.S.A.
GLEN R. HIMMELBERG
Missouri6521I , U.S.A.
ttniversityof Missouri,Columbia,
SuneyandDepanment
of Geology,
U.S.Geological
ABSTRACT
The early PaleozoicSalt Chuckintrusionhaspetrographicandchemicalcharacteristicsthal aresimilar to thoseof Cretaceous
Alaskan-typeultramafic-maficbodies.The Salt Chuck intnrsion is markedlydiscordantto the structureof the early Paleozoic
andsequences
a ratherindistinctcontactaureolea few meterswide.Fieldrelationships
DesconFormation,in whichithasproduced
andgabbro,arerelatedby fractionalcrystallization.
clinopyroxenite
ofcumulusmineralsindicatethatthetwo rocktypes,magnetite
and
of clinopyroxene
component
a resultofthe highproportionofthe esseneite
All analyzedsamplesarefeldspathoid-normative,
rangesfrom 0.937
magma.The Mg# of the modalclinopyroxene
from a silica-undersaturated
not an indicationof crystallization
andmineral
of crystallization,
sequence
Mineralassemblages,
with a normaltrendoffractionalcrystallization.
to 0.831,consistent
(- 2 kbar)with oxidationconditionsnearthoseof theNNO
thatthe intrusioncrystallizedunderlow pressures
chemistrysuggest
parental magma.The dominanceof Pd over ft [average
buffer, from a hydrous, silica-saturated,orthopyroxene-normative
of Pt overPd [averagePt/(ft+Pd)=
sharplywith the generaldominance
Pt/(ft+Pd)= 0.061in the SaltChuckintrusioncontrasts
in the SaltChuck
bodies.Therearealsogenerallyhigherlevelsof PGEandsulfideconcentrations
0.591in otherAlaskan-type
moreCu- andPd-richthanmagmaticsulfidesin otherultramafic
deposit.Sulfidesin the Salt Chuckintrusionaresubstantially
in whichthe
rocks.The SaltChuckdepositwasprobablyformedby a two-stageprocess:( I ) a stageof magmaticcrystallization
in a disseminated
mannerin cumulusdeposits,possiblylargelyin the gabbro,and (2) a later
sulfidesand PGE accumulated
in veinsandfracture-fillings,
andconcentrated
magmatic-hydrothermal
stageduringwhichthesulfidesandPGEwereremobitized
- gabbrocontact.In thismodel,
but controlledby thecomplexmagnetiteclinopyroxenite
largelyin thi magnetiteclinopyroxenite
the sourceof the sulfidesandPGE wasthe magmathat producedthe Salt Chuckintrusion.
early Paleozoic,Salt
elements,sulfides,arc magmatism,
Keywords:Alaskan+ypeintrusion,ultramaficrocks,platinum-group
Chuck.Alaska.
Sovnaenn
[.e complexeintrusif de Salt Chuck, sur l'lle du Princede Galles,en Alaska,d'6ge pal6ozoiquepr6coce,possddedes
AmafiquesdetypeAlaska,d'ige
pdtrographiques
semblables
h cellesdesmassifsultramafiques
et g6ochimiques
caractdristiques
strucfuraleavecles rochesde la FormationDescon,sur
cr6tac6.Le complexede Salt Chuckmontreune grandediscordance
decumulatsindiquentque
lesquelles
il a impos6uneaur6oledecontactplut6tindistincte.Lesrelationsdetenainet less6quences
possddent
analysds
Tousles6chantillons
fractionnde.
decristallisation
sontli6esparprocessus
lesrochesmafiqueset ultramafiques
et nonuneindicationd'un magma
dansle clinopyroxbne,
dela n6ph€linenormative,r6sultatdela teneur6lev6edu p6leessenei'te
en silice. La valeurde Mg# du clinopyrox0nemodal s'6taleentre0.931et 0.831,et d6finit un trac6normalde
sous-satur6
desmin6rauxfont penserque
et la composition
decristallisation,
ign6.Lesassemblages
demin6raux,la s6quence
fractionnement
faible 1=2 L6-r, h unefugacit6d'oxygdneprochede celledu tampon
le complexeintrusifa cristallis6i pressionrelativement
NNO, e partir d'un magmaparentalhydrat6,saturden silice, et donc i orthopyroxdnenormatif. La pr6dominancedu Pd au Pt
ItV(Ft+Pd) moyen:0.061danscesrochescontrastefortementavecla dominancedu Pt par rapportau Pd dansles autresmassifs
desniveauxplus6lev6sdes6l6mentsdu
detypeAlaska[Pt/(ft+Pd)moyen:0.59].De plus,lesrochesdeSaltChuckcontiennent
groupedu platine @GP)et dessulfures.ks sulfuresici sontplus richesen Cu et en Pd que les sulfuresmagmatiquesdesautres
rochesultramafiques.Nousproposonsun moddledeformationdugisementendeuxdtapes:I ) stademagmatiquepr6coce'aucours
au cours
dansles cumulats,et 2) stadehydromagmatique,
de fagondiss6min6e
duquelles sulfureset les EGPont 6tdddposes
duquella min6ralisationa 6t6concentrdedansla clinopyrox6nitei magn6titep€s de soncontactavecle gabbro.Dansce modble,
les sulfureset lesEGp seraientissusdu magmaparental.
(Traduit par la R6daction)
Mots-cl€s:intrusiondetypeAlaska,rochesultramafiques,6l6mentsdugroupeduplatine,sulfures,magmatismed'arc,Pal6ozoiQue
pr6coce,SaltChuck,Alaska.
r006
THE CANADIAN MINERALOGIST
Ixtnopucnou
genesisofthe Salt Chuckintnrsionbearson the origin
of the ore depositandthat of similar oreselsewhere.
It must be emphasizedthat our work was entirely
confinedto thesurfaceoutcropsand,exceptfor themine
dump,our informationaboutthe undergroundis mostly
derivedfrom the mine mappingof Gault (1945) and
Gault & Wahrhaftig (1992) and chemical data from
Silver GlanceResourcesInc. Otherthan at the mine,
exposuresin the heavily forested area rue extremely
limited.
The Pd-bearingintrusionat SaltChuckon hince of
WalesIsland is spatiallypart of the belt of ultramaJicmafic bodiesthat extendsfrom Klukwan to Duke Island
in southeastern
Alaska(Fig. I ). Thesebodieshavebeen
known for many yearsbecauseof economicinterestin
chromium,iron, nickel, copper,and platinum-group
elements(PGE)(Buddington& Chapin1929,Kennedy
& Walton 1946,Walton l95l). In 1960thesebodies
wererecognized
asa specialclassofintrusions,referred
to as Alaskan-typeultramafic bodies(Noble & Taylor
RscroNaLSsr-nNc
1960,Taylor & Noble 1960).The distributionand age
of the bodies were further refined by Lanphere &
As canbe seenin Figure I, the SaltChuckintrusion
Eberlein(1966),Taylor(1967),Lanphere(1968),Brew lies nearthe westernmarginof the Klukwan-Duke belt
& Morrell (1984),and Himmelberget al. (1936). of 100- to I l0-Ma Alaskan+ypeultramafic bodies
Lanphere& Eberlein(1966) reportedK/Ar agesfor (Lanphere& Eberlein 1966,Brew & Monell 1984),
thesebodies,basedon biotiteandhornblendeo
that range which extendsin Alaska from Duke Islandon the south
from 100to I l0 Ma. Althoughthegeneralized
mapsof to Klukwan on the north, and probablyat least 150km
some of theseinvestigatorsincludedthe Salt Chuck fathernorth to PyroxeniteCreekin the Yukon Territory
intrusionandsmallerultramafic-maficbodieson nearby (Stunock et al. 1980).Among others,the belt includes
islands,noneofthesebodieswasincludedin theorieinal theultramaficbodiesatAlavaBay,UnionBay,Blashke
descriptions
andsamplingfor agedeterminations]Re-Islands,KanePeak,andHaines.This belt cross-cuts
the
cently,Loneyet al. (1987)reporteda mucholderK-Ar terranepattemthat developedin Late Cretaceoustime,
agefor biotite(about429 Ma) in the SaltChuckbody. andis clearlylater(Brew& Morrell 1984,Himmelberg
Similar K-Ar ageswere obtainedon biotite and horn- et al.1986).The SaltChuckbodydiscordantlyintrudes
blende by M.A. Lanphere(written communication, the Alexandertenane (early PaleozoicDesconForma1989)forultramaficbodieson Dall Island(400Ma) and tion), as do the otherroughly coevalultramaficbodies,
SukkwanIsland(440Ma) (Fig. l). Thesebodiesrepre- and their petrogenesisis relatedto the early Paleozoic
sentearlyPaleozoicmagmaticarcactivity similar to the tectonic environmentin which they were formed. Aclate Cretaceousactivity that producedthe betterknown cordingto Gehrels& Saleeby(1987a),the Ordovician
ultrama.ficbodies originally defined as Alaskan-type to Early Silurian plutonic, volcanic, and sedimentary
infiusions(Lanphere& Eberlein 1966,Taylor 1967). rocksof theAlexanderterraneon Princeof WalesIsland
The DNAG Time Scale(Palmer1983)is usedin rhis andnearbyislandsresemble
thelithologicalassemblage,
paper to relate radiometric ages with the geological geochemicalcharacteristics,and regional relations of
time-scale.
modemconvergent-margin
assemblages,
indicatingthat
Otherepisodesof Alaskan-type,magmaticarc em- they were formedin a volcanic-arcenvironment.
placement have been reported from western North
The agerangeofthe SaltChuckandotherPaleozoic
America.Gray et al. (1986)describedthreesuitesof Alaskan-typebodies(400-44! Ma) overlapstheboundpostorogenic,ultramafic to gabbroicrocks in the Kla- ary betweenthe Middle Ordovician to Early Silurian
math Mountains of California and Oreeon that were (438472 Ma) intervalof calc-alkalineplutonismand
intrudedin the Lare Jurassic(142-163-Ma).Findlay theyoungerplutonism
thatis represented
by theyounger
(1969) describedthe Tulameenultramafic-gabbroic Silurian(418Ma) sodicleucodioriteof KassaInlet, as
complexin British Columbiaasan Alaskan-type
by Gehrels& Saleeby(1987b)for southern
body, established
which, accordingto K-Ar ages(175-186 Ma), was Prince of Wales Island. Chemically, the ultramaficintruded in the Jurassic(I-eechet al. 1963"Roddick & mafic bodiesof theSaltChucktypearemorecompatible
Farru 1971,1972).RecentU-Pb zirconages(204-Zlz with theexamplesof volcanic-arc,
plutoncalc-alkaline
Ma) suggestthatit maybe asold asTriassic(Rublee& ism thanwith the sodicplutonic suites.The Salt Chuck
Panish 1990). These occurrencesindicate that the intrusion could well be related to the heterogeneous
petrologicalandtectonicenvironmentthatproducedthe Early Ordovicianto Early Devonianensimaticigneous
Alaskan-typeintrusionswasnot unique.
complexdescribedby
Saleeby& Eberlein(1981),which
Unlike the late CretaceousAlaskan-typeultramafic cropsout nearby(Fig. 2).
bodies,the Salt Chuck intrusion is the host for an
economicCu-Au-Pd ore deposit(Gault 1945,Holt et
FIELDANDPsrRocRAprilcCHARAcrERIsrrcs
al. 1948,Lnneyet al.1987).The depositis considered
OFTHE INTRUSION
by us to be a magmaticdepositthat wasremobilizedand
concenhatedby later magmaticand hydrothermalproThe Salt Chuck intrusion forms a tadpole-shaped
cesses
duringa singleintrusiveevent.Hence,thepetro- outcroptfiat trendswest-northwestfor a distanceof 7.3
-Y U-K O-N - - - d - ' d
BRITISH COLUMBIA
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chugachrerrane'^*";!#ffi
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Satt Chuck lY
\1 W
Gravinabelt
lffiffil
F..-rf..TF] Mainlandbelr
U{'.;'l.t'{
0
50
sukrcwatt
Islandl
Areaof ultramaficssanpled
100 K[.OME-[R3S
54"
Alask4 showinglocationsof theSaltChuckintrusionandroughlycoeval(earlyPaleozoic)intrusions
Frc. l. Map of southeastem
sampledby the authors,the Duke-Klukwan ultramaficbelt, andmajortectonostratigaphicunits.Map compiledfrom Berg er
al. (1978),Brew & Ford(1984),Himmelberyet al. (1986),andGehrels& Saleeby(1987a).
km; its maximumwidth is about 1.6km at its western who was the first to map the gabbroat North Pole Hill
end(Fig.2). The locationofcontactsshownin Figure2 aspart of the body.The countryrocksconsistof basalt
is virtually the sameas mappedby Sainsbury(1961), flows, pyroclasticrocks,graywacke,andargillite of the
EXPLANATION
E
tEl
ffi
r
Lakes,lide-water,andtidalzones
Qs
Kqd
Surlicialdeposits(Quaternary)--Mainly
alluvium,tidalmudflat,andglaciofluvial
deposits
Quarlzdioriteporphyry(Cretaceous?)--Contains
phenocrysts
(Sainsbury
largeplagioclase
1961
)
DesconFormation(LowerSilurianandOrdovician)*Mainly
graywackeand mudstone
pillowbasahflowsandpyroclastic
whhinterbedded
rocks(Eberlein& Churkin1970;
Ebedeinet al. 198i1)
Ensimaticigneouscomplex(EarlyDevonianto Early Ordovican)--Melaigneous
complex
consislingof massesof dioritic-mafic
migmatite,
and irregularintrusivebodiesof hornblende
and (or)quartzchloriticmagmalite,leucogabbro,
trondhjemite,
and minorpyoxenitecut by
maticandfelsicdikeswarms(Saleeby
& Eberlein1981)
SaltChuckintrusion(Early Devonian?
to EarlyOrdovician)--Sulfide-bearing
clinopyroxenite
and
gabbrowithgradational
contacts, gb - gabbro,cpx - clinopyoxenite,gbcp- undivided
F,r,,,.trn
do6\ad
Shoreline
Stream
{2'Strit
..-/
"
Contact, approximately located
and dip of cumulate layering
-
r
- Fault, approximately located
4ql Strike and dip ol bdding
a
Mine shafl
ALASKA
T'IIESALT CHUCKPd-RICHINTRUSION,
DesconFormation(Eberleinet al. 198T. The actual
contactis not exposed,but samplesftom two outcrops
volcanicrocks,approximately
of clinopyroxene-phyric
l0 metersfrom the nearestoutcropsof the intrusion,
poof matrix to a gramoblastic
show recrystallization
plagioclase,
and
lygonalassociation
of clinopyroxene,
minor biotite. Other outcropsof country rock further
from the intrusion, however,show no evidenceof
thermaleffectsby the intrusion.
In general,thebeddingin the graywackeandargillite
of the Descon Formation strikes noftheast and dips
moderatelyto steeplyto the northwest,with some
reversals,suggestingfolding (Fig. 2). The intrusion,
therefore,is markedly cross-cutttngto the structurein
the countryrocks,althoughthe poor exposuresmakeit
difficult to obtaindetails.
The Salt Chuckintrusionconsistsalmostentirelyof
magnetiteclinopyroxeniteand gabbro (Fig. 2). Our
surfaceobservationsindicatethat the two rock types
gradeirregularly into oneanotherby a gradualincrease
in postcumulusplagioclasein the magnetiteclinopyof cumulusplagioclasein
roxeniteprior to appearance
gabbro, which is consistentwith fractional crystalWithin the mine,
lization of a single magma-batch.
however,magnetiteclinopyroxeniteoverlies gabbro
(Gault 1945,Gault & Wahrhaftig1992,Watkinson&
& Melling(1989)
Melling1989),whichledWatkinson
to proposethat the magnetiteclinopyroxeniteintruded
certainlysuggest
thatthere
thegabbro.Therelationships
may be more than one magnetiteclinopyroxenite gabbrounit. However,ratherthanigneousintrusionas
proposedby Watkinson& Melling (1989),we suggest
overlyingthegabbro
thatthemagnetiteclinopyroxenite
thebaseof a newcyclicunit attributedto
mayrepresent
emplacement
of a new pulseof moreprimitivemagma
into the magmachamber.
Mine mappingby Gault(1945)and Gault & Wahrhaftig(1992)showsthatthecontactbetweenthegabbro
is an inegular
andoverlyingmagnetiteclinopyroxenite
synformwith subverticallimbsthatplungesteeply(45'
to 60') to the southeast.The distancebetweenthis
contacton oppositelimbs is about 100 m. Near this
contact,the magnetiteclinopyroxeniteand gabbroare
intertongued(or interlayered)on a scaleof about0.5 to
1.5 m in thickness.On the surfacenearthe mine, we
observedlenticularcumulatelayers of this scale in
magnetiteclinopyroxeniteand gabbro (Fig. 2), but
were too few to enableus to relatetheir
observations
orientationto that ofthe larger-scalestructuredescribed
in the mine.Gault & Wahrhaftig(1992)constntcteda
vertical sectionthroughthe mine,normalto the trendof
of the main glory
the synform,about170 m southeast
hole.Thesectionshowsadistributionof thickmagnetite
1009
ofalternatandgabbrounitssuggestive
clinopyroxenite
ing antiformsandsynformshavingaxialplanesdipping
steeplyto the southwest,and comparablein scaleand
orientationto the synformdescribedabove.
Most exposuresof magnetiteclinopyroxeniteand
althoughisomogabbroaremassiveandhomogeneous,
dal layers2 to 8 cm thick, distinguishedby different
proportionsof clinopyroxeneand plagioclase,are locally presentin gabbro.The magnetiteclinopyroxenites
cumulateswith postcumuareclinopyroxene-magnetite
Most gabbrosare anhedral
lus biotite and plagioclase.
- plagiogranularor subhedralgranularclinopyroxene
biotite.
clase- magnetitecumulateswith postcumulus
Grainsizeof all therocksis generallyl-2 mm,with the
exceptionof biotite oikocrystsin magnetiteclinopyroxenite,which may be as much as 2 to 3 cm across.
Althoughthe rocks areextensivelyalteredand veined
by epidote,reasonableestimatesof original modal
mineralogyare as follows: clinopyroxene,90Voin
magnetiteclinopyroxeniteto 357oin gabbro;plagioclinopyroxenite,
in magnetite
clase,lessthanl%oto 10Vo
generally5 to l07o
and20 to 657oin gabbro;magnetite,
andgabbro,althoughsome
clinopyroxenite
in magnetite
gabbroscontainas little as 27omagnetite;biotite, I to
l\Vo in magnetiteclinopyroxeniteand gabbro.Only
traceamountsofbrown to gteenhornblendearepresent
andgabbros,which
in somemagnetiteclinopyroxenites
is markedly different from the caseof other Alaskantypeultramaflcrocks,wherehornblendeis a majorand
Apatiteis a ubiquitousaccesconstituent.
characteristic
sory mineralin gabbroand presentin somemagnetite
InterstitialK-feldsparwas noted in
clinopyroxenites.
of sulfidesin the
onesampleof gabbro.Theoccurrence
bemagnetiteclinopyroxeniteandgabbrois discussed
Iow.
The clinopyroxeneis neutralto palegreenin color;
somegrainsshowconcentricopticalzoning.Magnetite
clinopyroxeniteand gabbro show extensivedeuteric
is entirelyaltered
in mostsamples
alteration.Plagioclase
to a mixture of white mica, zoisite or clinozoisite,
epidoteand,in somecases,minor calciteandprehnite.
Biotite is commonlyalteredto chlorite,epidote,and
prehnite.Clinopyroxeneremainsunalteredexceptfor
minor chloritic alterationalong cleavageand fractures
in somesamples.
The magnetiteclinopyroxeniteexposedat the glory
holein theSaltChuckmineis intrudedby a gabbrodike
with diabasicto subophitictexture.An exposureof
clinopyroxene- hornblende- biotite diorite with a
subhedralgranulartextureoccursnearthe contactof the
intrusionnorth of the Salt Chuck mine. The possible
geneticrelationshipof the diabaseand diorite to the
Frc.2. Geologicalmapof theSaltChuckarea.MapunitsaroundminemodifiedfromGault( 1945);geologyawayfromtheintmsion
modifiedfrom Sainsbury( 1961)andEberleiner ai. ( I 983).
l0l0
THE CANADIAN
MINERALOGIST
magnetiteclinopyroxenite
andgabbroofthe SaltChuck The Mgf showsa substantial
range,from 0.99to 0.94
intrusionis discussed
below.
in magnetiteclinopyroxeniteand from 092 to 0.82 in
gabbro, which is consistentwith a normal trend of
CoMposITroN
oFRocKsANDMrr.rERALs
fractional crystallization. The lower Mg# (0.80) in
magnetiteclinopyroxenitesample86GH8A andgabbro
The chemicalcompositionandCIPW norm for rocks sample86GH7Ais attributedto thepresence
of bornite
of the Salt Chuckintrusionare given in Table l. The and chalcopyrite,which also accountsfor the slightly
analyseswere obtainedby using X-ray fluorescence low oxide totals.The diabasedike (sample86cH8B)
spectroscopy.FeO and Fe2O3were determined by that intrudes the magnetite clinopyroxenite and the
independent
measurement
of Fe2nusinggravimetricand clinopyroxene- hornblende- biotite diorite (sample
wet-chemicalmethods.A petrogeneticallysignificant 86GH14A)arehypersthene-norrnative
andhavea norfeatureof the intrusion is that all the samplesanalyzed mativesilicateMg# of 0.78and0.57,respectively.
The
are feldspathoid-normative;four samplesof magnetite possiblegeneticrelationshipof thesetwo rocks.Jsthe
clinopyroxeniteandtwo of gabbroareevenlamite-nor- SaltChuckcumulatesis discussed
below.
mative. The presenceof silica-undersaturated
Chemicalcompositionsand structuralformulasof
componentsin thenormis a resultof thehighproportionofihe clinopyroxene,hornblende,and biotite are given in
esseneite
component(CaFe3*AlSiO6)
of clinopyroxene Tables 2 through 4. All mineralswere analyzedusing
in theserocks(seebelow).Because
at Washington
Univertherocksofthe Salt theJEOLmodel733Superprobe
Chuckintrusionarecumulateswith variablepercentages sity, St. Louis, Missouri.Naturalmineralsand oxides
of magnetite,the relationshipof whole-rockchemistry were usedas standards.Matrix correctionswere made
to chemicaltrendsdueto fractionationis bestindicated by the methodof Bence & Albee (1968) using the
by the Mg# [Mg(Mg+Fe2+)]of thenormativesilicates. correctionfactorsof Albee & Ray (1970).Becauseof
TABLE.I. CHEMICALCOMPOSITION
AND CIPWNORMOF SAMPLESOF THE SALTCHUCKINTRUSION
samplo
86GH
234
86GH
254
86GH
20A
Si02
TlOz
Al2Og
Fe2O3
Feo
MnO
Mgo
Cao
Na2O
Kzo
Peos
H20
Sum
40.40
0.92
5.67
13.89
8.00
0.21
12.00
1 7. 7 0
0.00
0.28
0.02
0.63
99.72
41.30
0.90
5.54
12.89
6.57
0.18
12.10
19.60
0.25
0.06
0.00
0.49
99.88
41.40
0.83
4.72
12.56
7.49
0.20
12.40
19.10
0.23
0.00
0.00
1.05
99.98
86GH
98
40.40
0.92
15.40
8.58
5.68
0.22
8.00
16.80
0.65
0.76
0.09
2.09
99.59
86GH
8A
36.00
1.31
12.70
10.16
9.11
0.26
8.13
15.00
0.40
1.56
1.12
1.40
97.15
86GH
11A
4 1. 1 0
1.02
7.35
11 . 3 3
7.16
0.23
I 1.40
17.80
0.45
0.41
0.10
1.21
99.56
86GH
7A
40.70
0.94
16.10
6.70
6.65
0.34
7.03
13.70
1.25
1.48
0.81
2.85
98.55
86GH
1A
86GH
27A
40.50
0.74
|8.50
7.54
4.91
0.21
6.30
17.00
0.65
43.30
0.84
17.00
6.79
5.40
0.23
6.44
14.50
1.62
noa
noe
0.06
2.25
99.62
86GH
38
86GH
88
86GH
144
0.40
1.86
99.36
47.20
0.61
19.70
4.26
3.62
0.24
3.67
10.40
1. 7 3
4.83
0.51
2.47
99.24
46.80
0.71
14.80
3.20
6.93
0.18
10.30
11.90
1.24
0.99
0.07
2.46
99.58
51.40
0.54
19.00
1.44
6.48
0.16
4.10
9.76
3.30
1.36
0.26
1.37
99.21
5.94
10.03
37.15
29.49
6.73
32.78
6.02
10.80
32.84
8.21
28.54
33.74
2.18
26.95
4.55
13.66
11.59
1 1. 4 4
2.62
CIH / Norm
or
ab
an
lc
di
hy
ot
cs
rnt
I
ap
1.33
14.77
0.27
0.01
57.79
13.90
0.28
1.15
60.29
1 1. 9 4
0.05
1.07
62.76
37.80
3.61
3.06
36.85
?9.50
7.5s
1 . 9r
28.18
17.11
1.93
2.10
56.20
3.70
1.82
2.02
18.80
1.72
3.00
1.19
18.41
1.59
3.83
0.08
12.76
't.79
0.21
10.49
1.67
15.38
2.60
2.7'l
3.61
0.14
16.70
1.97
0.23
20.32
1.76
nnF
9.14
4.78
35.47
3.40
24.05
45.93
4.57
3.06
30.36
9.19
2.72
5.07
4.00
22.06
13.23
a.72
10.15
1.87
1.96
11.23
1.44
0.14
10.10
1. 6 4
0.95
6.38
1.20
1.22
4.78
1.39
0.17
2.19
1.05
0.61
alomicntb
- ___l!L
noE
doo
0.94
0.92
0.80
0.94
0.80
0.91
0.86
0.a2
0.78
0.57
1.00
1.00
1. 0 0
1.00
1.00
1. 0 0
0.88
1.00
0.76
0.82
4.74
0.53
(Mg+Fd+)
Rocktyp€sol analfzedsamplosin Tables1-4. 86GH23Aclinopyroxenite:
86GH20A
86GH25Aclinopyroxenite;
86GH34Bclinopyroxenite;
clinopyroxenite;86GHlB
clinopyroxenite:
86GH35Agabbro;86GH9Bgabbro;86GHBAclinopyroxenils;86GH11A
clinopyroxenite;86GHl24
melagabbro;
86GH7Amelagabbro;
86GH1Agabbro;86GH27Agabbro;86cH3Amelagabbro;
86cH30Agabbro;86GH3Bgabbro;86GHBB
diabase:86GH14A diorite.
l0l I
THE SALT CHUCK Pd.RICHINTRUSION.ALASKA
IN THE SALTCHUCKINTRUSION
OF CLINOPYROXENE
TABLE2. CHEMICAL
COMPOSITION
Ssrtplg
Sl02
'IlOz
Al2q
C1203
FeO'
MnO
Mgo
Na2O
Sum
86GH
23A
86GH
25A
86GH
348
48.60 48.90
0.53 0.50
4.7A 4.47
0.00 0.00
6.91 6.69
0.16 0.18
14.15 'f4.31
23.72 23.57
0.23 0.22
99.08 98.83
86GH
20A
86GH
rB
86GH
35A
86GH
98
86GH
8A
E6GH
12A
86GH
1 1A
48.82 49.45 48.88 49.96 49.64 49.53 48.51
0.64 0.42 0.61 0.44 0.59 0.62 0.59
4.65 4.16 4.47 3.69 4.10 4.35 5.07
0.00 0.00 0.00 0.00 0.00 0.00 0.01
7.37 6.69 7.69 7.38 6.78 7.23 7.44
0.35 0.14 0.34 0.41 0.37 0.34 0.24
14.16 14.34 13.75 13.82 14.56 14.11 13.39
23.00 23.53 23.22 22.71 22.74 23.25 23.34
0.35 0.27 0.42 0.71 0.34 0.36 0.38
99.32 99.00 99.36 99.1.| 99.12 99.79 98.98
86GH
7A
86GH
1A
86GH
27A
86GH
3A
86GH
30A
86GH
38
86OH
8B
86GH
14A
Eo,
nO2
A2q
Cr2O3
FeO'
Mno
MSO
CaO
Na2O
Sum
4a.29 49.64 49.27 50.09 48.85 50.06 50.02 50.06 50.79
0.60 0.50 0.67 0.55 0.65 0.51 0.58 0.62 0.14
5.30 3.61 4.66 4.05 4.89 3.31 3.78 4.19 1.10
0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.16 0.01
8.04 7.27 7.10 7.17 7.49 7.31 8.10 8.4'111.16
0.24 0.42 0.33 0.36 0.26 0.35 0.60 0.21 0.58
12.92 14.15 13.91 14.30 13.44 14.47 13.40 15.51 13.51
23.55 22.96 23.05 22.96 23.16 22.22 22.04 20.15 20.70
0.46 0.38 0.38 0.40 0.43 0.47 0.58 0.24 0.24
99.40 98.92 99.38 99.89 99.,|7 98.69 99.10 99.53 98.20
si
M
S!m
1.799 1.853 1.830 1.850 1.A21 1.A70 1.871 1.854 1.938
o.zo't 0.147 0.170 0.1500.179 0.130 0.129 0.146 0.049
0.032 0.012 0.034 0.026 0.036 0.016 0.037 0.037 0.000
0.0r7 0.0r4 0.019 0.0150.018 0.014 0.016 0.017 0.004
0.ooo 0.000 0.000 0.000 0.000 0.000 0.000 0.005 0.000
0.168 0.135 0.127 0.123 0.138 0.120 0.102 0.088 0.058
0.083 0.092 0.094 0.099 0.095 0.109 0.152 0.173 0.298
0 . 0 0 7 0 . 0 1 3 0 . 0 1 0 0 . 0 11 0 . 0 0 8 0 . 0 1 1 0 . 0 1 9 0 . 0 0 7 0 . 0 1 9
0.718 0.788 0.77'l 0.7aa 0.747 0.a06 0.74a 0.857 0.769
0.941 0.919 0.918 0.910 0.926 0.890 0.884 0.800 0.847
0.033 0.027 0.028 0.029 0.031 0.034 0.042 0.017 0.017
4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000
Fomla per4 Cdlo6
Forola mr 4 Catio6
NA
Sum
'1.808 1.822 1.8r3 1.839 1.818 1.859 1.844 1.832 1.811
0.192 0.178 0.187 0.161 0lA2 0.'t410.156 0.168 0.189
0.017 0.019 0.017 0.022 0.014 0.021 0.024 0.021 0.035
0.015 0.014 0.018 0.012 0.017 0.012 0.017 0.017 0.017
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
0.162 0.147 0.160 0.135 0.165 0.147 0.124 0.138 0.147
0.053 0.062 0.069 0.073 0.074 0.082 0.087 0.085 0.085
0.005 0.006 0.0r1 0.004 0.011 0.013 0.012 0.011 0.008
0.785 0.795 0.784 0.796 0.763 0.767 0.A07 0.77A 0.746
0.946 0.942 0.916 0.939 0.926 0.907 0.906 0.923 0.935
0.017 0.016 0.025 0.019 0.030 0.051 0.025 0.026 0.027
4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000
(Mg+Fe2+)
0.937 0.928 0.919 0.916 0.911 0.903 0.903 0.901 0.S98
Mo
(Ms+Fsz+)
0.897 0.895 0.892 0.889 0.887 0.881 0.831 0.832 0.721
ESr
0.t45 0.131 0.134 0.1150.134 0.096 0.098 0.113 0.120
E8T
0.135 0.107 0.098 0.094 0.107 0.086 0.060 0.070 0.041
En
FS
0.401 0.404 0.395 0.411 0.404 0.42',t0.405 0.408 0.404
0.562 0.553 0.555 0.539 0.543 0.524 0.537 0.535 0.535
0.038 0.043 0.050 0.050 0.053 0.056 0.059 0.058 0.061
En
FS
0.404 0.414 0.407 0.410 0.407 0.411 0.415 0.353 0.412
0.535 0.525 0.528 0.525 0.526 0.519 0.4A7 0.538 0.424
0.062 0.062 0.065 0.066 0.067 0.070 0.099 0.109 0.164
3t
I
Fel
Fe2*
MN
Mg
-Ti
Fe*
Fe2+
Mn
Mg
Totafiron calculatedas FeO;Fe3+and Fe2+ inmineralformulaecalculatedfrom chargebalance.
t Es, Wo, En and Fs calculatedaccordingto the proceduresol Lindsley& Andersen(19s3). See
Table1 lor rock typ6.
the high oxygenfugacity of the magma,asindicatedby
crystallizationof abundantmagnetite,structuralformulas for clinopyroxenewerecalculatedby normalizingto
four cations,and Fe3*and Fe2*were calculatedfrom
chargebalance.AlthoughcalculationofFe3* and Fe2*
by this procedureis subjectto biasresultingfrom elrors
in the analysis,particularlyfor Si, it is superiorin this
caseto assumingall iron as FeO or Fe2O3.For hornblende,thebestformulawasobtainedby normalizingto
13 cationsexclusiveof Ca, Na and K, and assigning
cations to sites accordingto the method outlined by
Robinsonelal. (1982).Biotiteformulaswerecalculated
by normalizingto I I atomsof oxygenwith all iron as
FeO,which obviously introducessomeelror.
With the classificationschemeof the International
MineralogicalAssociation(Morimoto et al. 1988), all
the clinopyroxenecompositionsin the Salt Chuck
infiusionwouldbe classifiedasdiopside.However,the
clinopyroxenein magnetiteclinopyroxeniteandgabbro
Fe3+content;in all but onecase,Fe3*is
hasa substantial
greaterthanFe2+.Most of the Fe3*in the clinopyroxene
component,which is not takeninto
is in the esseneite
accountin the IMA classificationscheme.Thus we
of clinopycalculatedthe Wo, En, andFs components
roxene(Table2) usingthe projectionschemeofl-indsley & Andersen(1983).This procedureshowsthat
makeup only 76.5to 80.3%
quadrilateralcomponents
oftotal quadrilateralandothercomponents,andthat the
componentrangesfrom 14.5to 6.0Vo.When
esseneite
projectedonto the pyroxenequadrilateral,the clinopyroxenesamplesplot asMg-rich augite(Fig. 3).
The Mg# of the modal clinopyroxenein magnetite
clinopyroxeniteand gabbrorangesfrom 0.94 to 0.83'
indicating a substantialchemicaltrend due to differentiation. The Mg# of pyroxenein the diorite is 0.72.
Although not regular,thereis alsoa generaldecreasein
of clinopyroxene
component
proportionof theesseneite
Inspectionof TablesI and2 shows
with differentiation.
that thereis generallya good correlationbetweenthe
Mg# of the modal clinopyroxeneand the normative
clinopyroxene.Exceptionsare samples86GH8A and
86Gli?A, which have a lower Mg# in normative
clinopyroxeneowing to the presenceof significant
borniteandchalcopyrite.Thegenerallygoodcorrelation
betweenMg# of modal and normative clinopyroxene
ratio in clinopyroxenecalcuindicatesthat theFe2+lFe3+
latedby chargebalanceis consistentwith theFeO/FerO3
ratio determinedby analysisof the rocks. The general
agreementof Mg# alsosuggeststhat the FeOandFerO3
values determinedfor the rocks are close to primary
values and have not been significantly affected by
alteration.
1012
THE CANADIAN MINERALOGIST
TABLE3. CHEMICAL
COMPOSITION
OF
HORNBLENDE
IN GABBROANDDIORITEOF
THESALTCHUCKINTRUSION
TABLE 4. CHEMICALCOMPOSITIONOF
BIOTITE IN THE SALT CHUCK INTRUSION
86GTEA
Sarnpb
86Gf€B
SiO2
TiO2
AleQ
FeO'
MnO
Mgo
40.o4
2.15
13.19
13 . 0 0
0.25
12.97
12.31
2.56
1.31
97.79
Na2O
Kzo
Sum
SI
lVAl
Sum T
vlAl
Ti
Fe&
Fe2+
Mn
Mg
Sum C
t\,la
SumB
M
K
Sum A
86GH144
41.79
lot
9.48
20.43
0.32
9.31
10.90
1.74
noo
96.94
5.939
2.06't
8.000
0.245
0.241
o.429
1.185
0.031
2.869
5.000
'1.959
0.041
2.000
0.697
0.249
0.946
5.000
1.774
o.226
2.000
0.286
0.192
0.47a
0.708
0.558
6.338
8.000
0.034
0.924
1.668
0.041
'Total ion
calq,thted as FeO. Fs& and F#+ in mlnoral
fomula calculatedtrom chargebalanco. Minsraltonnrla
nonnalizod lo 13 cations oxclusfueot Ca, Na, and K.
'IlOz
AL2Og
FoO'
MnO
Mgo
Na2O
Kzo
Sum
8@H9B
86GH12A
86GHJ4A
36.26
4.03
16.55
10.99
0.17
17.59
0.04
0.62
8.56
95.18
3.70
16.93
12.06
0.19
17.47
0.15
0.39
7.90
94.53
3.58
17.10
13.04
0.20
16.00
0.10
0.27
7.72
94.26
3.73
14.68
23.89
0.18
9.43
0.04
0.13
a.31
95.47
Cationsp€r11
Oxygsns
Sum
2.883
1.317
0.111
0.222
0.673
0.01
1.919
0.003
0.088
0.799
7.425
2.642
'f .358
0.117
0.205
0.745
0.012
1.924
0.012
0.056
0.744
7.815
2.689
1.311
0.183
0.200
0.809
0.013
1.768
0.008
0.039
0.730
7.749
2.730
1.270
0.076
0.218
1.554
0.012
1.094
0.003
0.020
0.A24
7.A01
Mq
(MglFe)
0.740
0.721
0.686
0.413
lt
FE
Mn
Mg
NA
'Iolal iron
€bclded as FeO.
S@ Iabl€ I for mk typ€s.
grains commonly show concenfic zoning, with Mg#
decreasing slightly toward the rim. The maximum
difference in Mg# between the interior and rim of a
single grain is about 0.05, and generally it is less than
0.02.
Hornblendeis rareandgenerallyoccursonly in trace
amounts.Using the classificationof Leake(1978) as
modified by Hawthorne(1981), rhe one amphibole
analyzedin a gabbro,is magnesio-hastingsite;
hornblendein thedioriteis tschermakitic
(Table3). Mg# for
The pyroxenecompositionsgiven in Table 2 and the biotite in magnetiteclinopyroxeniteand gabbro
plotted in Figure 3 are averagesof severalgTainsper rangesfrom about0.74 to 0.69; in diorite,the Mg# is
sample.Thereis no significantvariationamongaverage about0.41(Table4). TheTiO2contentof biotiteranges
grain compositionsin a given sample,but individual from 3.73to 4.03 wt.%o.
En
Fs
Ftc. 3. Compositions of clinopyroxene in the Salt chuck intrusion projected onto the
pyroxene quadrilateral.Wo, En, and Fs componentswere calculatedusing the projection
schemeof Lindsley & Andersen ( 1983).
THE SALT CHUCK Pd-RJCHINTRUSION, ALASKA
PETROGENESIS
Conditionsof crystallization
1013
duction of the magmaduring fractional crystallization
would be expectedin responseto magnetitecrystallization. Similar oxidizing conditions near the NNO
buffer were reported for ultramafic xenoliths in arc
similarto Alaskanbasaltthathavemineralassemblages
type ultramaficrocks(Arculus & Wills 1980,Conrad&
Kay 1984).
Theoccurrenceof cumulusmagnetiteandtheabsence
of cumulusplagioclasein themagnetiteclinopyroxenite
suggesta temperatureof crystallizationbelow 1090"C
but above1025"C;thesevaluescorrespondto the upper
stabitty limits of magnetiteand plagioclase,respectively, in hydrousbasaltmagmaat2 kbarandattheNNo
buffer (Holloway & Burnham 1972). ln those rare
gabbroswherehornblendeis aminorcumulusphase,the
to at
temperatureof crystallizationmusthavedecreased
least975oC,which is the upperstabilityofhornblende
at2kbar andNNO (Holloway& Burnham1972).T\e
upper stability of magnetiteand plagioclasedecreases,
and that of hornblendeincreases,with increasingpressure but" as discussedabove,the sequenceof cryst4llization suggeststhat the pressureof crystallizationwas
probablynear2 kbar.
Thetemperature,pressure,watercontent,andoxygen
fugacity during crystallizationof the Salt Chuck intrusion can be qualitatively estimatedby comparingthe
natural mineral assemblagesand crystallization sequenceto products of hydrous melting and crystallization experimentsusing natural basaltic composi
tions.Theorderof crystallizationindicatedby magnetite
clinopyroxeniteand gabbroin the Salt Chuckintrusion
is clinopyroxene+ magnetitefollowedby plagioclase.
Hornblendeoccurs only in trace amounts,and biotite
nearly alwaysis postcumulus.The abundanceof magnetiteandthecommonoccurrenceof biotite indicatethat
the magmathat crystallizedto form theultramaficrocks
was substantiallyrich in Fe3*and H2O. Experimental
studies have shown that under water-saturatedand
-undersaturated
conditionsandanoxygenfugacityatthe
NNO buffer, plagioclase and hornblende crystallize
after clinopyroxeneand magnetite(Holloway & Burnham 1972,Helz 1973,Allen & Boettcher1978).At
moderateto high pressures,hornblendecrystallizesat a
temperaturesubstantiallyhigherthanthe temperatureat Nature of parent magmn
which plagioclasecrystallizes,but at low pressures(2-3
The Alaskan-typebodieshavelong beenrecognized
kbar),the upperstabilityofplagioclaseandhornblende
are relatively close,and below about 2.5 kbar, plago- asa distinct classof ultramafic-maficintrusions(Noble
clasecrystallizesbeforehornblende(Holloway & Burn- & Taylor 1960,Taylor& Noble1960,kvine 1974),and
ham 1972).Furthermore,at pressuresbelow 2 kbar, the the natureof theparentalmagmahaslong beendebatetl.
upper stability of hornblendeis very nearthe tempera- No chilled margins representingthe parental magma
ture of the solidus (Holloway & Burnham L972), a have been found; thus the compositionof the original
condition that would restrict crystallization of horn- melt could only be infened from considerationof
andfrom rock andmineralchemisblende.Thus, the sequenceof crystallization and the cumulatesequences
of hornblendein the Salt Chuckintrusion try relative to experimentaland theoretical studies.
near-absence
g et al. (1986)
suggestthat it crystallizedunderlow pressure,probably James( I 97I ), Munay ( I 972),Hi mmelber
ofhorn- and, more recently,Loucks (1990) arguedthat at least
2 kbar or less.Alternatively,the near-absence
blendecouldindicateawatercontentbelowthatrequired someof the Alaskan-typeultramaficrocks fractionated
island-arc
for saturationof hornblende.but in view of the common from subalkaline,orthopyroxene-normative,
occurrenceof biotite, this interpretationseemsunlikely. basalticparentalmagmas,with the resultantproduction
The occurrenceof cumulusmagnetitewith cumulus of residualbasalticand andesiticliquids. Subalkaline,
clinopyroxenesuggeststhatthe stateof oxidationof the island-arcbasaltic parental magmasfor Alaskan-type
magmaat thetime of crystallizationwasat leastashigh ultramafic rocks also were implied by Conrad& Kay
as the NNO buffer. For basalticmelts under oxidizing ( 1984) andDeBui et al. ( 1987) in equatingxenolithsin
conditions equivalent to the NNO buffer, magnetite Aleutian arc volcanic rocks with Alaskan-type ulon the otherhand,
crystallizesafter clinopyroxenebut prior to plagioclase tramaficrocks.kvine (1967,1,974),
(Holloway & Burnham 1972, Helz 1973). At less argued that the Alaskan-type ultramafic rocks were
oxidizing conditions,the FMQ buffer, ilmenite is the fractionatedfrom high-Ca,high-Mg, alkalineultrabasic
near-liquidusphase.At conditions of the HM buffer, parental magmas, with the resulting production of
alkali basalts.Irvine's
Fe-Ti oxide phasesarenmongthe earliestto crystallize residual,critically undersaturated
but they include abundanttitaniferous hematite(Helz (1974) argument for a silica-undersaturatedalkaline
1973)"which was not observedin the Salt Chuck ultrabasicparentalmagmawasbasedin part on nonnaintrusion. The fact ftrat Fe3+exceedsFe2*in clinopy- tive nepheline in rocks with abundantpostcumulus
roxenealsosuggestsa relativelyhigh fugacityofoxygen hornblende.Loucks(1990),however,pointedout that
studiesof Bowen(1928),Helz,(1973)
during crystallization.However,the generaldecreasein theexperimental
the esseneitecomponentof clinopyroxenewith differ- and Wones(1979) all showedthat hornblendeprecipientiation suggeststhat the redox state of the magma tatedfrom orthopyroxene-or quartz-normativemeltsis
decreasedas fractional crystallizationproceeded.Re- generally nepheline-normative.Thus hornblende- or
t0t4
TT1ECANADIAN MINERALOGIST
biotite-bearingcumulatesthat crystallized from satu- early-crystallizedclinopyroxenefrom suchmagmasis
ratedhydrousmagmaswill commonlybe feldspathoid- characteristicallyaluminous,and particularly enriched
normative.
in the esseneitecomponent.The esseneitecomponent
ln the caseof the Salt Chuck intrusion"the mineral calculatesin the norm as magnetiteplus silica-underassemblages,
the sequenceof crystallization,and the saturatedcomponents.Clinopyroxenecompositionsrerock and mineral chemistry all indicate fractionation portedby Holloway& Burnham(1972) andHelz(1973)
from a silica-safurated,
orthopyroxene-normative
paren- from their experimentalcrystallizationof hydroussilicatal magmasimilar to thosethat yield island-arctasats. saturatedbasaltsare generallynot feldspathoid-normaAs discussedabove,theSaltChuckmineralassemblaees tive; however,the clinopyroxeneencountered
in experiand sequenceof crystallizationare duplicaredby cr!s- ments generally coexists with hornblende, which
tallization experimentson hydrous,silica-saturaied6a- incorporatesthosecomponentsthat calculateasnonnasaltic melts at low pressures(= 2 kbar) and oxygen tive feldspathoids.The virmal absenceof hornblendein
fugacity near that of the NNO buffer (Hollowav & the Salt Chuck crystallizationsequenceresultedin the
Burnham 1972). T\e experimental studies of Helz normative feldspathoidcomponentsentering clinopy(1973) on the hydrous system, although ar higher roxene.
pressure(5 kbar), alsosupportour interpretation.
Crystallization of the Salt Chuck mineral assem, The Salt Chuckmagnetiteclinopyroxenitesandgab- blagesfrom anhydrousisland-mcbasalticmagmaalso
bros arefeldspathoid-normative
(Table 1). However,as is supportedby the studiesof Loucks (1990),which
mentioned above, this is a result of the substantial showed that clinopyroxene in arc cumulateshas a
esseneite,component
(CaFe3+AlSiO6)
of the clinopy- distinctivetrendin termsof AVTi ratio. Figure4 shows
roxene,which is a resultofthe hydrous,oxidizing nature that the Salt Chuck clinopyroxenesfall on the arc
of the magma(Loucks1990).tnucks arguedthat in a cumulatetrend.
If the isolated exposureof diorite is an evolved
lldrous and oxidizing basalticmagma,the activity of
the enstatitecomponentwould be depressed,and the productof fractionationofthe SaltChuckparentmagma,
activitiesof thealuminouspyroxenecomponentsin both thenthat parentmagmawasunquestionablysilica-satumelt and clinopyroxenecrystalswould be raised.Thus rated. The rock and mineral chemistry of the diorite.
16
Per-alkaline
12
arccumulatetrend
t
*
\
Normal-atkatine
#
! - Q
<s
riftcumulatetrends
v
Sub-alkaline
0
0
2
3
Ti02 , wl o/o
4
FIc. -4. Plot of IvAl content,-expressedas a percentageof total Al, against TiO2 for
clinopyroxenein the Salt chuck intrusion.Field boundariesarefrom Lebas (1962f,and
trendsarefrom Loucks (1990).
THE SALT CHUCK Pd.RICHINTRUSION.ALASKA
comincludinga significantproportionof theesseneite
ponentin thepyroxene,is supportiveof this interpretation, but unfortunatelyno contactsbetweenthe diorite
and gabbro or magnetiteclinopyroxeniteare exposed,
andthusa geneticrelationshipcannotbe unequivocally
demonstrated.
As mentionedabove,no chilled marginsor other
representatives
of parentalmagmafor the Alaskan-type
ultramaficbodieshaveeverbeenfound. However.it is
plausiblethat the diabasedike (sample86GH8B)that
intrudes Salt Chuck magnetiteclinopyroxenitemay
representthe nearly direct crystallization of the Salt
Chuckparentalmagma.The diabasedike hasa nofinato bein exchange
tive Mg# of 0.78,whichis appropriate
equilibriumwith clinopyroxenewith a Mg# of 0.90to
(Table2).
0.94typicalof themagnetiteclinopyroxenite
The Fe2*lIFeatomicratio of the diabasedike is 0.707,
which corresponds
to a silicateliquid with an oxygen
tugacityof NNO plus I or 2 log units(Sackel a/. 1980,
Carmichael& Ghiorso 1986);this value is aboutthe
sameas the oxygen fugacity indicatedfor the magma
that crystallizedthe magnetiteclinopyroxenitesand
gabbros.
and
Thediabasedikeis hypersthene-normative
not too dissimilarin chemicalcompositionfrom the
1921 Kiluea olivine tholeiite used in the hydrous
experiments
of Holloway& Burnham(1972)andHelz
(1973).Thusthediabasedikehasanappropriate
chemical composition,Mg#, and redox stateto representa
plausibleparentalliquid to the mineralassemblages
at
SaltChuck.
We believethatthe SaltChuckdataindicatethatthe
magnetiteclinopyroxeniteandgabbrofractionatedfrom
a hydrousbasalticliquid andthat this parentalliquid is
by the compositionof the diabase
closelyrepresented
These
dike that intrudesthe magnetiteclinopyroxenite.
conclusionshaveimplicationsfor the origin of highbasalts.
Loucks(1990)argued
aluminaandcalc-alkaline
thatfor hydrousbasalticliquidsunderoxidizingcondi
tions,early fractionationof clinopyroxeneenrichedin
hastheconsequence
ofenhanctheesseneite
component
residual
ingthealuminaandsilicacontentofprogressive
liquids,thus leadingto crystallizationof high-alumina
basaltsand calc-alkalinebasalt.Our study of the Salt
Chuck intrusion,especiallyour documentationof crysclinopyroxenefrom a parentallizationof esseneite-rich
tal magmathat most likely had a hydrous,highly
oxidizing basalticcomposition,supportsthe arguments
of Loucks ( I 990) for the importanceof fractionationof
subsilicicclinopyroxenein the derivationof calc-alkaline
andhigh-aluminabasalts.
OREDEPos[S
The commercial (Au+Pd)-bearingcopper sulfide
depositassociatedwith the Salt Chuck intrusiondiffers
markedly from the known PGE occurences in other
Alaskan-type ultramafic bodies in southeastAlaska.
Typically, the Alaskan-type ultramafic bodies have
1015
of ft-dominantPGEthat
concentrations
noncommercial
with olivine-richrocksin which sulfides
areassociated
aregenerallyscarce.Iron, in the form of magnetite,is
the only commodity that has attracted commercial
interest (Ruckmick & Noble 1959). These characultramaficintrusions
teristicsalsoapplyto Alaskan-type
suchas in the KlamathMountains(Grayet
elsewhere,
a/. 1986)andin BritishColumbia(Findlay1969).Where
commercialPGE depositsdo occur in Alaskan-type
intrusions,asin theUrals(Razin1976,Naldrett& Cabri
with olivine-richrockscon1976),they areassociated
taining concentrationsof either chromite or magnetite.
It seems,therefore,thatthe abundanceof sulfideandthe
absenceof olivine in the Salt Chuck intrusion are
anomaliesamongthe Alaskan-typeintrusionsin general.
The Salt Chuckore depositis locatedin the eastern
middlepartof theintrusion,asindicatedby thelocation
of theSaltChuckmine(Fig.2). This mine,theonly one
thathasworkedthedeposit,operatedintermittentlyfrom
1905to I 94I andproducedabout300,000metrictonnes
of milling ore (chiefly bornite and chalcopyritein
magnetiteclinopyroxenite)estimatedto haveaveraged
0.95wtToCu, 14.91g Au, 7.04g Ag, and26.1g Pdper
tonne(Holt et41.1948,Gault 1945).Recentexploration
Inc'
hasbeenundertakenby Silver GlanceResources
(formerlyOrbexIndustriesInc.).
are
Gradesarehigherand sulfidemineralaggregates
larger in the magnetiteclinopyroxenite,in which ulevenlydisfibuted, irregular,interstitialgrainsof bornite
occur in modalamountsas much as 15 vol.7o.In the
generallylower-gradegabbroore,thebomite grainsare
smaller and more evenly distributed.Chalcopyrite,
of the
althoughwidelydisributed,is a minorcomponent
ore, being largely confined to fault zones.Alteration
productsare chalcocite,covellite,and digenite.Small
butof little economic
flakesof nativeCuarewidespread,
importance.Other elementsassociatedwith the Cu
sulfidesare recoverableamountsof Au, Ag, Pd, and
minor ft. In addition, ft-group minerals(PGM) probably form an important source of PGE in the ore.
Watkinson& Melling (1989) describeda complex
texture,consistingof abundant,large,mostly isolated
grainsof PGM, consistingmostlyof kotulskite(PdTe),
which are either intergrown with hessite(Ag2Te),or
rimmedor entirelyreplacedby temagamite(Pd3HgTef.
Also occurring as free aggregatesor within sulfide
grains are complex intergrowthsof Au-bearing sper+ Au,
rylite (PtAs) +kotulskite+PQAsSb,temagamite
andkotulskite+ temagamite+ merenskyite(PdTeJ.
Based on megascopicobservations,Gault (1945)
suggestedthat the ore-bearingfluids followed the contact and spreadinto the fractured magnetiteclinopyroxeniteandthe less-fracturedgabbro.The influenceof
the gabbro- magnetiteclinopyroxenitecontacton ore
depositionhasbeenrecentlyaffirmed by Watkinson&
Melling (1989).Accordingto Gault (1945),the more
prominent faults (some containing chalcopyrite)may
1016
THE CANADIAN
MINERALOCIST
havebeenthe main channelfor dispersionof copperbearingfluidsto smallerfaultsandfractures,andthence
to microfractures
observedin therock at SaltChuckby
Mertie (1969). Mertie suggestedthat, although the
sulfidesappearmegascopically
to bemagmaticsegrcgations,numerousmicroscopiccrackswereadequate
for
circulationof ore solutions.Accordingto Mertie, the
degreeofalterationappears
to beafunctionofore grade,
supporting
a hydrothermal
origin.
It is our view that the predominantlydisseminated
characterof the sulfidesand PGM, togetherwith the
igneoustexturesof the host rocks, indicatethat the
segregationswere producedby late magmatichydrothermalprocesses
in a singleintrusiveevent,probablythesameeventthatproducedtheextensivedeuteric
alterationof the silicates.Veins and fracture-fillings
were producedat this laterstagewithin the magmatic
environment.
The presence
of abundantPd-richminerals(Wa&inson& Melling 1989)suggests
latestagesof
PGEmineralization,
in whichpalladiumwentintoPGM
rather than into sulfides(Stumpfl & Tarkian 1976).
Late-stage,lower-temperaturePGE mineralization
(<720'C) in the SaltChuckdepositalsois indicatedby
the presence
ofthe Pd bismuthotelluride
mineralssuch
(Sweenyet
as merenskyite"
kotulskite,andtemagamite
al.1989.
RecentlyWatkinson& Melling (1989) described
aggregatesof sulfide mineralsat Salt Chuckwith relict
magmatictexturesthat rangefrom disseminatedto net
textures.Thesearecomposedof complexintergrowths
andconcentricassemblages
of chalcopyrite- bornitedigenite- chalcocitewith euhedralepidote.
In our view, the Salt Chuck depositwas probably
formedby a two-stageprocess:( 1) a magmaticstageof
crystallizationin which the sulfidesandPGEaccumulated in a disseminated
mannerin cumulusdeposits,
possiblylargelyin thegabbro,and(2) a latermagmatichydrothermalstageduring which the sulfidesand PGE
wereremobilizedandconcentrated
in veinsandfracturefillings, largely in the magnetiteclinopyroxenitebut
controlledby the complexmagnetiteclinopyroxenite
gabbrocontact.In this model,thesourceofthe sulfides
andPGEwasthe magmathatproducedthe SaltChuck
intrusion,in agreementwith the suggestionof Gault
(te4s).
Table5 givesa summaryof the analy'ticaldataon the
PGE for both the Salt Chuckintrusionand the CretaceousAlaskan-typeultramaficintrusionsof southeast-
TABLE 5. PTATINUM.GROUPELEMENTANALYSESOF ULTRAMAFICAND MAFIC ROCKS IN THE
SALT CHUCK INTRUSION
AND CRETACEOUSALASKAN.ryPE INTRUSIONSIN SOUTHEASTEHNALASKAT
L@ ury
BocKTypE- R
DUKEISLAND'Max.(22) Prd/Cpx
PrdrCpx
DUKEISLAND'AV.(22)
Prdlopr
UttttOt!geY'lrax. (50)
Prd/cpx
uNloN BAY. Av. (50)
Prd/cpx
eLASHfe tS'lvlax.(tO)
Prdlcpr
BLASHKElS'Av. (10)
KLUKWAN'Max.(10)
Cpx
KLUKWAN'Av. (10)
cpx
cpx
KLUKWAN{
HAINES*
cpr
HAINEStr
Hbd
UNIONBAY87GH27A
Du
UNIONBAY8TGH34A
Whr
UNIONBAY8TGH44A
G.b
UNIONBAY87GM7A
cpx
UNIONBAYETGH4OA
Hbd
UNIONBAYSTGH41A
Cpx
AVERAGEALASKAN-TYPERATIOS
SALT CHUCK
87GH42A
Cpx
87GH43A
cpr
SALTCHUCK'Max.(6)
cpx
SALTCHUCK'Av.(6)
Cpx
SCMI Av.(32)
Cpx
sCM Av.(35)
Gb
AVEMGE SALTCHUCKRATIOS
2OO
37
1600
93
20
IO
100
46
9.5
48
44
't2
8.5
2.3
7.3
0.9
7.9
pd
Rh fb
140
33
2OO
29
20
IO
100
40
14
27
51
0.8
0
9.5
3.9
1.1
11
1O
0
62
19 840
4.4
51
160 2900
57 1010
37.7 1230
7.17 201
tl
Ni
Ar
Cu
(Cu+Ni)
Pl
(ft+Pd)
Pd
(Pd+RtD
Rh
(Rh-Rr,)
0.50
0.00
1.00
0.87
1.00
1.00
0.73
0.35
0.68
0.59
0.53
nqo
0.80
0.50
v.ou
0.53
0.40
0.64
0.46
0.94
1.00
0.19
u.oc
0.45
o.42
0.59
0
no
1.2
0.8 1.5 2.1
'1.7
0.8 6.1
000
0.600
000
00.5
0
0
0
0
0
0
0
0
0
0.02
0.08
7001 24 314
1575 18 181
1.00
0.99
0.99
0.03
0.03
0.05
grlgpt cu, Ni, and A! in wr %. Av. = ayerago;Max. . maxirum; (22) = ru,mberot sp€cimens.
L ll"TlnT,ll!:1:
rrc, penoollo; cpx,ry,1!'liol:
ctinopyroxenitei
Hbd, homblendito;Du, dunite;Whr, wshrlito;Gb, gabbo.
' Analysss
lrom Clarkand Grgenwood(1972).
#^Analysesfrom A.B. Ford (writtenconiunicalion,1991).
tSCM-. Analysestrom Salt ChuckMine,courtssy
' Silver'clancaResourceslnc. (tomgrly Orbextndustrioslnc.).
87GH"' Specimenscolocted by the adhors.
1.00
0.00
1017
THE SALT CHUCK Pd.RICHINTRUSION,ALASKA
a
Merensky
..o
dunite
O LaPerouse
0.6
Pechenga +
jludburv
*
5
E+ o.o
o.
r
o La Perorseau.
trtr
Komatiites
tr
4.2
Bushveld
A
Noril'sk
ta'
o
Stillwater
La perousegb O lNorit,sk
,aa
SaltChuck
'l'l'l'll
v
0.2
0
0.4
0.6
0.8
1
Cu/(Gu+Ni),rvt %
Flc. 5. Plot of Pt(Pt+Pd) for magnetite clinopyroxenite and gabbro of the Salt Chuck
intrusion (see Table 5 for data) compared with world-wide data adaptedfrom Naldren
& Cabri ( I 976). Data for the La Perousegabbro from Czamanskear a/. ( I 98 I )'
PGBChondrites(C1)
a
tr
T
A
A
scM GBAV.(35)
scM cP Av.(32)
87GH42A
STGHztalA
SALTCHUCK"m
SALTCHUCK-a
0.1
OslrRuRhPdRAu
Pd, ft, andAu data(Table5) for the Salt Chuckintrusion.
Rc. 6. Chondrite-normalized
Chondrite(C-l) valuesfrom Naldrett& Duke(1980);plottingprogramfrom Wheatley
& Rock(1988).
l0l8
TTTECANADIAN MINERALOGIST
1000
100
10
I
0.0
-
0.00
Os
T
lr
---'
Ru
Rh
Pd
ft
Au
PGErChondrites(C1)
1 0 0- -l
B.
ll
10
Os
Ru
Rh
Pd
Pt
Au
Ftc. 7. chondrite-normalized
PGE diagramsfor Alaskan-typeultramafic intrusions,
southeastern
Alaska. Construcdonof diagramsas in Figure 6. A. Union Bay:
. 87GH27A, tr 87GH34A,I 87GH44A,A 87GH47A, A 87GH4OA,V 87GH41A,
V UnionBay* Max.,+ UnionBay* Av. B. o Dukelsland*Max.,r DukeIslandEAv..
n BlashkeIs.*Av., A Klukwan*Max.,A KlukwanoAv., V Klukwan#,y Haines#cpx,
+ Haines#hbl. SeeTable5 for dataandkev
THE SALT CHUCK Pd.RICHINTRUSION,ALASKA
r0l9
em Alaska.From thesedata,it is clearthat the domi- UnionBay bodyareespeciallyconflicting,andsuggest
nanceof Pd overPt [averagePt/(ft+Pd)= 0.06] in the a needfor furtherstudy.
Salt Chuckintrusioncontrastssharplywith the general
CONCLUSIONS
dominanceof ft overPd [averagePt/(ft+Pd)= 0.59]in
otherAlaskan-typebodies."Alsoevidentarethe generin the Salt
ally higher levels of PGE concentrations
The early PaleozoicSalt Chuck intrusionhas petChuck deposit,which also containsa much higher rographicandchemicalcharacteristics
thataresimilarto
concentration
of sulfide.The Salt Chuckdataare also those of CretaceousAlaskan-type ultramafic-mafic
compared(Fig. 5) to theCu/(Cu+ Ni) andPt(ft t Pd) bodies.Theintrusionconsistsalmostentirelyof magnetwith other ultramafic ite clinopyroxeniteand gabbrocumulates.Field relaratios of sulfideoresassociated
rocks (Naldrett& Cabri 1976).Sulfidesin the Salt tionshipsand sequences
of cumulusmineralsindicate
moreCu- andPd-rich that the two rock typesarerelatedby fractionalcrystalChuckintrusionaresubstantially
than magmaticsulfidesin otherultramaficrocks,and lization.Relationships
theexistence
in theminesuggest
plot at thehigh-Pdandhigh-CucornerofFigure 5. The of cyclic unis producedby multiple emplacement
of
atypicallyhigh Cu/(Cu+Ni)ratio in the magnetitecli- magmabatches
significantfeatureof
. A petrogenetically
mightbeanributedto thetwo-stagemodel the intrusion is that all the samplesanalyzedare
nopyroxenite
proposedabove.The high fugacity of oxygenof the feldspathoid-normative;
four samplesof magnetitecliearly precipitationof nopyroxenite
magmawould have suppressed
andtwo ofgabbroareevenlarnite-nornasulfides(Kasura& Nagashima1974,Carroll& Ruther- tive. The presence
components
of silica-undersaturated
ford 1988)and allowedNi to enterearly-crystallizing in tlre norm is a result of the high proportion of the
silicatesand oxides. Consequently,the sulfides or esseneite
in clinopyroxene
component(CaFe3*AlSiO6)
sulfide liquid that precipitatedfrom the evolvedgab- in theserocks,and not a indicationof crystallization
Subsequent from a silica-undersaturated
broicmagmawouldhavebeenCu-enriched.
magma.TheMg# valuesof
remobilizationof the sulfidesor sulfideliquid into the the normativeand modalclinopyroxeneshow a good
magnetiteclinopyroxeniteby magmatic-hydrothermal correlationand a substantialrange,consistentwith a
processes
wouldthenyield sulfideswith a Cu/(Cu+Ni) normaltrendof fractionalcrystallization.
Althoughnot
ratiothatwouldbe atypicalfor thehostrock.
regular,thereis alsoa generaldecreasein theproportion
in of the esseneitecomponentof clinopyroxenewith
Naldren(1981)concludedthat PGE concentrate
sulfidedropletsby a factor of 100to 1000over their differentiation,suggestingreductionof the magmaas
work by crystallizationproceeded.Comparisonof the natural
levelsin thehostmagma.Recentexperimental
Stoneera/. ( I 990)hasshownthatPGEtAu arestrongly assemblages
of crystallization
of mineralsandsequence
partitionedinto sulfideliquid. Accordingto them,the to productsof experimentson hydrousmelting and
sulfide liquid - silicatemelt partition coefficients(D crystallizationusingnaturalbasalticcompositions
sugvalues)for the noblemetalsare9 (+7) x lff for Pd, 9 geststhattheSaltChuckintrusioncrystallizedunderlow
(10.7)x 105forIr, g (t6) x 103forft, and I (10.9)x 103 pressures,probably 2 kbw or less, with oxidation
for Au. Accordingto thesedata,theaffinity of Pd andIr conditionsnearthoseof theNNO buffer.Temperatures
for sulfideliquid is about50 timesgreaterthanthat of of crystallization probably decreasedfrom about
to approxi
ft and about500 times greaterthan that of Au. This 1090'Cfor the magnetiteclinopyroxenites
gabbros.
relationis supportedby the datafrom the Salt Chuck mately975'C for somehornblende-bearing
and
of crystallization,
sequence
Mineralassemblages,
mine,whererocksricherin sulfidesarericherin Pd and
alsoAu (Table5, seeCu contentversusPdl'Holt et al. rock and mineralchemistryall indicatefractionation
orthopyroxene-nonnaprocesscould from a hydrous,silica-saturated,
I 948,Gault 1945).This sulfide-collector
explainthe higherPGE+Aucontentin the sulfide-rich tive parentalmagmasimilarto thosethatyieldisland-arc
diabasedikethat
Salt Chuck intrusion than in the other, sulfide-poor basalts.The orthopyroxene-normative
Alaska.Figures intrudesmagnetiteclinopyroxenitehas an appropriate
intrusionsin southeastern
Alaskan-type
Mg#,andredoxstateto represent
6 and7 graphically comparethe PGE patternsof Salt chemicalcomposition,
Chuck sampleswith other Alaskan-typebodies of a plausibleparentalliquid to the Salt Chuck mineral
The commercialAu+Pd-bearingcopper
Alaskaby meansof normalizeddiagrams assemblages.
southeastern
with theSaltChuckintrusion
basedon the computerprogramof Wheatley& Rock sulfidedepositassociated
(1988)and the chondritedatafrom Naldrett& Duke differs markedlyfrom theknown occulrencesof PGEin
( I 980).Thesefiguresshowthegeneraldominance
of Pd other Alaskan-type ultramafic bodies in southeast
of PGE Alaska. ft-group minerals (PGM) probably form an
overP[ andthe generallyhigherconcentration
in the Salt Chuck intrusion.However,data for the importantsourceof PGE in the ore. The Salt Chuck
of
less-abundant
PGE are limited,andthe analyticaldata depositis Pd-dominant,has higher concentrations
more
are not of uniform quality. For this reason,it is not PGEandsulfides,andthesulfidesaresubstantially
possibleto makea detailedcomparisonof the normal- Cu- and Pd-rich than magmaticsulfidesin other ulizedPGEpatternswithin theregion,muchlesswith the tramaficrocks.We concludethattheSaltChuckdeposit
rest of the world. For example,the PGE datafrom the wasprobablyformedby a two-stageprocess:( I ) a stage
1020
THE CANADIAN
MINERALOGIST
of magmaticcrystallizationin which the sulfidesand
PGEaccumulated
in a disseminated
mannerin cumulus
deposits,possiblylargelyin the gabbro,and (2) a later
magmatic-hydrothermal
stageduringwhich thesulfides
and PGE were remobilizedand concentrated
in veins
and fracture-fillings,largelyin the magnetiteclinopyroxenitebut controlledby the complexmagnetitecli- gabbrocontact.In thismodel,thesource
nopyroxenite
of the sulfidesand PGEwasthe magmathatproduced
the SaltChuckintrusion.
AlthoughtheSaltChuckintrusiongenerallyhasbeen
consideredas an Alaskan-typeintrusion and shows
many similaritiesto otherAlaskan-typebodies,we do
not proposethat the conditionsof crystallization,
compositionof parentmagma,and ore-formingprocesses
infened for the Salt Chuck intrusionare universally
applicable
to all intrusionsclassifiedasbeingof Alaskan
type.
ACKNowLEDcEMEIITS
We thank Dr. PeterE. Fox of SilverGlanceResources
(formerly Orbex Indusries Inc.), of Vancouver, B.C.,
for his courtesy in granting us accessto the Salt Chuck
property, and for providing us with chemical data.
Thanks also are due to the following colleaguesat the
U.S. Geological Survey: G.K. Czamanske and T.E.
Keith for their helpful reviews of the paper, and Floyd
Gray for providing us with information on the mine. We
especially thank R.F. Martin, G.T. Nixon, and an
anonymous reviewer whose thorough and constructive
reviews provided new insights and led to a much
improved paper.
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