PLATINUM-GROUP MINERALS FROM A PLACER … · PLATINUM-GROUP MINERALS FROM A PLACER DEPOSIT IN...

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The Canadian MineralogistVol.48,pp.583-596(2010)DOI:10.3749/canmin.48.3.583

PLATINUM-GROUP MINERALS FROM A PLACER DEPOSIT IN BURWASH CREEK, KLUANE AREA, YUKON TERRITORY, CANADA

YanaFEDORTCHOUK§

Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada

WilliamlEBaRGE

Yukon Geological Survey, Ministry of Energy, Mines and Resources, PO Box 2703, Whitehorse, Yukon Y1A 2C6, Canada

anDREiY.BaRKOV

Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada

lUCaFEDElEanDROBERTJ.BODnaR

Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, U.S.A.

ROBERTF.maRTin

Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada

aBsTRaCT

FiveplacergrainsofPt–Fealloy,ca.0.5 to2mm,roundish inshape,andhithertounconfirmedfromthis locality,wererecoveredfromaplacerdepositinBurwashCreek,Kluanearea,Yukon.Ourresultsofmultipleelectron-microprobeanalyses(EMP)displaythepresenceofelevatedlevelsofPd(0.49–5.84,mean2.38wt.%),Rh(1.05–1.94,mean1.50wt.%),Ir(<0.2to6.91,mean1.33wt.%),andCu(0.17–1.19,mean0.62wt.%)in thesealloygrains;acompositionalseries isobserved, inwhichvaluesofSPGE/(Fe+Cu+Ni)varyfrom3.0to5.7.ConcentrationsofRuareinvariablylow(<0.03to0.11wt.%)inthisseries.ThesegrainsofPt–Fealloyhostmicro-inclusionsofvariousspeciesofplatinum-groupminerals,PGM,diopside[Ca45.7–47.5Mg37.5–48.0Fe4.9–16.8,mg#=100Mg/(Mg+Fe2+)intherange69.0–90.7],plagioclase(Ab57.5–60An35.1–39.9Or2.6–4.8),andsodic–calcicamphiboles(0.6and1.1Naapfu),someofwhicharerichinCl(0.72and1.17wt.%,or0.17and0.36Clapfu,respectively).TheanalyzedinclusionsofPGMaremembersofthebowieiteRh2S3–kashiniteIr2S3solid-solutionseries,miassite(Rh,Pt,Pd)17S15,cooperitePtS,isoferroplatinumPt3Fe(orFe-richplatinum),vasilite(Pd,Cu,Pt)16S7,keithconnitePd20Te7,anunusualPt–Pd–Ni–Cualloy(i.e.,Pd-richplatinum:Pt47.7–52.8Pd29.3–37.9Ni6.3–8.0Cu6.1–7.5),andasulfotellurideofPd,Pd11Te2S2(?),whichmayrepresentanewisomertieite-typecompound.Thetrace-elementcompositionofthesilicatemeltinclusionsdeterminedusinglaser-ablationICP–MSshowsanotableenrichmentinlarge-ionlithophileelements.WeinferthatthereportedassociationofPGMandthetrace-elementcompositionofsilicatemeltinclusionsobservedatBurwashCreekprobablycorrespondtoanAlaskan–Uralian-typemineralization;consideredlesslikelyisarelationwithaTriassicultramafic–maficcomplex(sill-like)ofso-calledKluane-type,alsodevelopedintheplacerarea.

Keywords:platinum-groupelements,meltinclusions,trace-elementcomposition,platinum-groupminerals,Pt–Fealloy,ultra-mafic–maficrocks,placerdeposits,BurwashCreek,Kluanearea,Yukon.

sOmmaiRE

Cinqgrainsarrondisd’unalliagePt–Fed’undiamètreentre0.5to2mm,etnonsignalésjusqu’iciàcetendroit,ontétéprélevésd’ungisementdetypeplacerdanslacriquedeBurwash,régiondeKluane,auYukon.Nosrésultatsd’analysesobtenusavecunemicrosondeélectroniquedémontrentdesteneursélevéesdePd(0.49–5.84,moyenne2.38%,poids),Rh(1.05–1.94,moyenne1.50%),Ir(de<0.2à6.91,moyenne1.33%),etCu(0.17–1.19,moyenne0.62%)danscesgrainsd’alliage;nousinterprétons

§ E-mail address:[email protected]

584 THECanaDianminERalOGisT

cesrésultatsentermesd’unesériecompositionnelledanslaquellelesvaleursdeSPGE/(Fe+Cu+Ni)varientde3.0à5.7.LesconcentrationsdeRusontinvariablementfaibles(de<0.03à0.11%)danscettesérie.Cesgrainsd’alliagePt–Fecontiennentdesmicro-inclusionsd’espècesvariéesdeminérauxdugroupeduplatine(MGP),diopside[Ca45.7–47.5Mg37.5–48.0Fe4.9–16.8,mg#=100Mg/(Mg+Fe2+)dansl’intervalle69.0–90.7],plagioclase(Ab57.5–60An35.1–39.9Or2.6–4.8),etdesamphibolessodiques–calciques(0.6et1.1Naapfu),dontcertainessontrichesenCl(0.72et1.17%,ou0.17et0.36Clapfu,respectivement).LesinclusionsdeMGPanalyséessontdesmembresdelasolutionsolidebowieiteRh2S3–kashiniteIr2S3,miassite(Rh,Pt,Pd)17S15,cooperitePtS,isoferroplatinePt3Fe(oubienplatinericheenFe),vasilite(Pd,Cu,Pt)16S7,keithconnitePd20Te7,unalliagePt–Pd–Ni–Cudecompositioninhabituelle(i.e.,platineenrichienPd:Pt47.7–52.8Pd29.3–37.9Ni6.3–8.0Cu6.1–7.5),etunsulfotellururedePd,Pd11Te2S2(?)quipourrait représenteruncomposénouveaude type isomertieïte.Lacompositiondes reliquatsmagmatiquesen termesd’élémentstraces,établieaveclatechniqueICP–MSutiliséeavecablationaulaser,démontreunenrichissementremarquéenélémentslithophilesàlargerayon.L’associationdesMGPetlesenrichissementsenélémentstracessignalésdanslesreliquatsmagmatiquesaucriqueBurwashtémoigneraientd’uneminéralisationdetypeAlaska–Ourale;nousconsidéronsmoinsprobableunerelationavecuncomplexeultramafique–mafiquetriassiqueenfilons-couchesdetypeKluane,affleurantaussidanslesecteur.

Keywords:élémentsdugroupeduplatine,reliquatsmagmatiques,élémentstraces,minérauxdugroupeduplatine,alliagePt–Fe,rochesultramafiques–mafiques,gisementsplacer,criquedeBurwash,régiondeKluane,Yukon.

grainsobtainedbyW.LeBargefromplacerminerSteveJohnson in 2008 (grainsBR–1,BR–2,BR–3). In thepresent paper,we provide thefirst documentation oftheoccurrenceofplacergrainsofPt–FealloyandofassociatedPGMatBurwash.Ourobjectivesare(1)tocharacterizecompositionalvariations,extentsofsolidsolutionsandlevelsofminorelementspresentintheseplacergrainsofPt–Fealloy,basedonresultsofmultipleelectron-microprobeanalyses(EMP),(2)tocharacterizethe compositions of PGM that occur as inclusionshostedbythegrainsofPt–Fealloy,(3)toexaminethecompositionsofsilicatemeltinclusionsinPt–Fealloygrains, theirmineralogical composition and patternsof trace-element concentrations based on the resultsof laser ablation (LA) ICP–MS, and (4) to discuss apotentialprovenancefortheseplacergrainsofPGM.

analYTiCalmETHODs

TwomethodsofEMPanalysiswereappliedinthisstudytodocumentthecompositionofmineralsinthecollection of placer grains fromBurwash.The bulkof analyses of the PGMwere done inwavelength-dispersion spectrometrymode (WDS)with a JEOLJXA–8900 instrument (McGillUniversity), at 20 kVand30nA, using afinely focusedbeam (2mm)andon-linecorrectionprocedures.Thepeakandbackgroundcountingtimeforalltheelementswas20secondsand10 seconds respectively.Puremetals (Os, Ir,Ru,Rh,Pt, Pd,Fe, andNi), PtAs2, FeS2,CuFeS2 andBi2Te3wereusedasstandards.TheLalinewasmonitoredforIr,Pt,Rh,Ru,Te,andCu,theMalineforOs,theLblineforPdandAs,andtheKalineforFe,Ni,andS.Allpossiblepeak-overlapsamongtheX-rayemissionlinesemployedwerecheckedandcorrected.

Inaddition,tinyinclusionsofPGMwereanalyzedusing aHitachi S–4700 FEGField Emission Scan-ningElectronMicroscope (FESEM) at the InstituteforResearch inMaterials,DalhousieUniversity.Theinstrument is equippedwith an energy-dispersionspectrometer (SEM–EDS) and is usedwith a set of

inTRODUCTiOn

Placers of theYukon host mineral deposits ofvarious types, some ofwhich can be of economicimportance (LeBarge 1996, and references therein).Theareastargetedforplacerdepositsoftheplatinum-groupelements (PGE)areprincipallyassociatedwithAlaskan–Uralian-type complexes and ophiolites, andalsowithTriassicultramafic–maficcomplexes(sill-like)of so-calledKluane type.TheAlaskan–Uralian-typecomplexes, concentrically zoned ultramafic–maficcomplexes typicallywith a dunite–peridotite core,are awell-recognized source of placer platinum.Forexample, ca. 20 tonnes of platinumwas recoveredfromplacerdepositsofGoodnewsBay,Alaska,derivedfromAlaskan-type source rocks (unpubl. report citedinTolstykhet al. 2002). Potential relationshipswithAlaskan-typecomplexeswereinferredforanumberofplaceroccurrencesofplatinum-groupminerals(PGM)documentedinCanada(Nixonet al.1990,Cabriet al.1996,Barkovet al.2005,2008a,2008c).Thesespeciesof PGMare dominantly alloyminerals of the PGE,whichtypicallyoccurin situinanintimateassociationwith chromian spinel.Thus, detrital grains of chro-mite–magnesiochromite and of other oxidemineralsassociatedwithPGMinplacersare important indica-torsoftheprovenanceofthePGM(e.g.,Fedortchouk&LeBarge2008).

Inmany PGM-bearing placers, grains of PGMhaveaccumulatedalongwithgrainsofgold.BurwashCreek, a tributary ofKluaneRiver, is situated in theKluane area in southeasternYukon. Placer goldwasfirstdiscovered there in1904, shortlyafter theKlon-dikeGoldRush,andhasbeenminedintermittentlyinthe area up to the present day.AlthoughPGMhavepreviouslybeenreportedbyplacergoldminersatthislocality, therehavenotbeenanydetailedanalysesorconfirmation of the composition of thePGMgrains.For this study,we used grains donated to theYukonGeologicalSurveyfromtheestateofplacer inspectorGeorgeGilbert (grains BR06–1 and BR06–2) and

plaCERpGmDEpOsiT,BURWasHCREEK,YUKOn 585

well-characterized standards. The results obtainedboth inEDSandWDSmodesare ingoodagreementwitheachother.CompositionsofthegrainsofsilicatemineralshostedbythePt–FealloywerealsoevaluatedusingtheSEM–EDS.

OneplacergrainofPt–FealloyfromBurwashCreekwas found to contain four inclusionsof silicatemelt,whichareconsidered tobeprimaryandrepresent themelttrappedinthePGMduringgrowth.Theinclusionscontainmultiplecrystals,ordaughterminerals,whichis characteristic ofmelt inclusions that have cooledrelatively slowly after trapping (Bodnar& Student2006).Thegrainwasmountedinepoxyandpolishedtoexposetheinclusions.Thecompositionoftheindi-vidualmineralphasesineachinclusionwasexaminedusingEDSanalysesontheFE–SEM.AstheinclusionsinanopaquegrainofPt–Fealloyhavetobeexposedbypolishingprior to the analyses, they could not behomogenized to obtain the compositionof thewholeinclusion by EMP analyses.As a result, individualphaseswere analyzedusing theEDS–SEMapproachandassumedtocontain50wt.%SiO2,arealisticesti-mateformaficmagmas.Thisestimatewasusedasaninternal standard for theLA–ICP–MS analyses. Forthisreason,theabsolutevaluesoftheelementconcen-trations obtained byLA–ICP–MS analyses are onlyaccuratetoapproximately±10%,whereastherelativeconcentrationsandratiosoftheelementsaredeterminedwith highprecision.TheLA–ICP–MSanalyseswereconducted atVirginiaTech using anAgilent 7500cequadrupoleICP–MSandaLambdaPhysikGeoLas193

nmExcimer laser-ablation systemwithHe gasflow.TheNIST610glass,usedasanexternalstandard,wasanalyzedtwicebeforeandaftertheanalyses.Thesizeofthelaserspotwasadjustedtothelargestpossiblesize,toincludethemaximumamountoftheinclusionmaterialbut avoiding the host phase.The time-resolvedLA–ICP–MSdatawerereducedusingtheAMSanalyticalsoftware(Mutchleret al.2008).

GEOlOGYOFTHEplaCERaREa

BurwashCreek,anortheast-flowingtributaryoftheKluaneRiver,islocatedintheKluanemafic–ultramaficbelt along the easternmargin ofWrangellia terrane(Fig. 1).TheDenali fault divides the associations oftheWrangelliaterranefromthoseoftheYukon–TananaterraneandKluanemetamorphicassemblage(KMA).The oldest rocks in the drainage area belong to theSkolaiGroup,aPennsylvanianandPermianvolcanic-sedimentary sequence of the intruded byTriassicsill-likesubvolcanicmafic–ultramaficintrusivebodieswithknownNi–Cu–PGEmineralization; thesebodiesconstitutetheKluanemafic–ultramaficbelt.Theintru-sivebodiesservedasmagmaconduitsfortheoverlyingbasalts of theNikolaiGroup (Hulbert 1997),whichhavegeochemicalfeaturesoffloodbasaltsformedasapartofanoceanicplateau.Thebasementforthewholesequenceisnotexposed,exceptperhapsforagabbroiccomplexwest ofDonjekRiver (Hulbert 1997).Thesmall creeks that are tributaries of BurwashCreekcut throughCretaceous granodiorite–diorite of the

FiG.1. GeneralizedmapshowingtheregionalgeologyandthelocationofBurwashCreekintheYukon(source:filesoftheYukonGeologicalSurvey,Whitehorse).


KluaneRangessuiteandMioceneandPlioceneflowsofbasalticandesite.Ourearlierstudyofprovenanceofdetritalchromianspinelfromtheheavy-mineralconcen-tratesrevealedacompositionallydiversepopulationinBurwashCreek (Fedortchouk&LeBarge 2008, andreferencestherein).Thechromianspinelwithmorethan5wt.%ofTiO2issimilartothatfoundinKluaneintru-sions.GrainswithananomalouslyhighZncontent(upto1wt.%ZnO)mayberelatedtothemassive-sulfideoreassociatedwithKluaneintrusions.Therestof thechromian spinelpopulationhasTi (<0.5wt.%),Fe3+/R3+(<0.10)andFe2+/(Fe2++Mg)(between0.2and0.6)lowerthanchromianspinelfromcontinentalintrusionsorAlaskan-type complexes, andwere likely derivedfromoceanicperidotites.Wefoundnomaterialintheheavy-mineral samples that couldclearly indicate thepresenceofanAlaskan-typeintrusion.However,detritalchromian spinelwith a highCr/(Cr+Al) value alsoshowslightlyelevatedvaluesofFe3+/R3+,comparableto compositions of chromite from theAlaskan-typeintrusions reported in the literature (Fedortchouk&LeBarge2008,andreferencestherein).

plaCERGRainsOFTHEHOsTpT–FEallOYFROmBURWasHCREEK

Five placer grains of Pt–Fe alloy fromBurwashCreekwereanalyzed;theyareca.0.5to2mminsize,roundishordroplet-likeinshape,andcontaintinyinclu-sions(typically10–20mmthick)ofvariousspeciesofPGM(e.g.,Figs.2A–E),asdescribedbelow.

Atotalof95WDSanalyseswerecarriedoutalongEMPtraversesindifferentpartsandzonesofthefivegrains of Pt–Fe alloy.The observed levels of solidsolutioninthesegrainsare,onaverage,Pd(0.49–5.84,

mean2.38wt.%),Rh(1.05–1.94,mean1.50wt.%),Ir(<0.2to6.91,mean1.33wt.%),Cu(0.17–1.19,mean0.62wt.%),Os(<0.1to0.90,mean0.40wt.%),andNi(<0.06to0.12,mean0.05wt.%).ConcentrationsofRuareverylowinalloftheseplacergrains(Tables1,2),rangingfromthelowerlimitofdetection(<0.03wt.%;WDS) to0.11,withameanvalueof<0.03wt.%Ru.Incontrast,themaximumcontentofPdisnotablyhigh(5.8%),althoughhigherconcentrations,upto11wt.%Pd,weredocumentedinaPt–(Pd)–FealloyfromArchCreek,Yukon,forexample(Barkovet al.2008a).Ourresults(EMP)yieldanextensivecompositionalseries,inwhichvalues ofSPGE/(Fe+Cu+Ni) vary from3.0 to 5.7.These compositionswould seem to implythatagapexistsbetweenPt3Fe,corresponding to thestoichiometry of isoferroplatinum, and aPt–Fe alloyricher inPt (Fig.3).However, thesedataobtainedatBurwash are likely not representative of the entirespectrumof compositions observed for Pt–Fe alloysfromother placers (e.g.,Barkovet al. 2005, 2008c).In terms of Ir–Rh–Pd compositional space, the datapoints tend toplot along the Ir–RhandRh–Pd joins;however,arelativeenrichmentinPdischaracteristicofmostofthesecompositions,whichgenerallyplotalongthe Ir–Rh trend (Fig.4).The twograinswithSPGE/(Fe+Cu+Ni)�3 (BR–1,BR–3onFigs. 3 and4)haveasimilarIr–Rh–Pdcontent,withelevatedIr(Fig.4),whereasthegrainswithSPGE/(Fe+Cu+Ni)>5(BR–2,BR06–1,BR06–2onFigs.3and4)plotalongtheRh–Pd join (Fig. 4). In Ir–Rh–Pd compositionalspace, the Ir-enriched grains show a trend towardthe Ir corner.However, the Ir content of these twograinsdoesnotconsistentlychangefromcentertothemargin,implyingnocompositionalzonation,butratherlocalheterogeneities.Thecompositionalvariationsof


PGMgrains are also reflected in the composition ofthe inclusions that theyhost.The Ir-richBR–3graincontainsIr-richinclusionsofbowieite(Fig.2),themorePd-enriched grainBR–2 has inclusionswith Pd-richphasesincludingmiassiteandvasilite,andBR06–2hascooperiteandsilicateinclusions(Fig.2).

THEinClUsiOnsOFpGm

Members of the bowieite–kashinite series, Rh2S3–Ir2S3

ComparedwiththeotherspeciesofPGM,bowieiteis relativelycommonin theanalyzed inclusions.Twocompositionalvarietiesareobserved:Pt-rich(poorinIr)

FiG.2. A–E.Themorphologyofplacer grains of Pt–Fe alloyfromBurwashCreek(inback-scatteredelectronimages)andinclusions of PGM hostedby thesegrains (in secondaryelectron images). Symbols:Co: cooperite,Mi:miassite,Va:vasilite,Bo:bowieite.


FiG.3. CompositionalvariationobservedinplacergrainsofPt–FealloyfromBurwashCreek,Yukon. Plot of values of ratioSPGE/(Fe+Cu+Ni) in thePt–Fe alloy incompositional profiles through the grains (15–20 data points from each grains; 95point-analysesintotal;WDSdata).Symbols:BR–1,BR–2,BR–3,BR–06–1,BR–06–2.

FiG. 4. The Ir–Rh–Pddiagram (atom%) showingvariations in content of Ir,Rh andPdincompositionsoftheplacergrainsofPt–FealloyfromBurwashCreek(n=95;WDSdata).


andIr-rich(anal.1–9,Table3).Bowieiteandkashiniteareisostructural;thus,theexistenceofasolid-solutionseries between these end-members is not unexpected(Desborough&Criddle 1984,Begizovet al. 1985).TheincorporationofPtismoreuncommon.Thephase“Pt2S3”reportedinresultsofearlyexperimentsdoesnotexistinthesystemPt–S(Grønvoldet al.1960).Specialconditionsareperhapsrequiredtostabilizethe“Pt2S3”component,whichmayhaveastructuralformulaPt2+2(S2)2–S2– or,with amixed-valence state ofPt atoms,(Pt2+Pt4+)2S2–3.

Miassite, (Rh,Pt,Pd)17S15

The namemiassite is IMA-approved; the typelocalityforthisspeciesisaPGM-bearingplaceroftheMiassRiverintheUrals(Britvinet al.2001).Thisnamethus replaces “prassoite”, reported from theTiébaghimassif,NewCaledonia (seeCabri 1981); it appearsthatmiassite and “prassoite” refer to the samephaseRh17S15(cubic,Pm3m),whichexistsintheRh–SandFe–Rh–Ssystems(Okamoto1992b,Makovickyet al.2002).Agrainofmiassite,enclosedbyaPt–Fealloyandanalyzedinthisstudy,hasaformulaRh17S15;PdandPtareminor(anal.10,Table3).

Cooperite, PtS

TheEMPdata suggest thatadeviation from idealproportionsexists in theanalyzedgrain; its composi-tion recalculated to a total of two atomsper formulaunit(apfu)isclosetoPt0.9S1.1(anal.11,Table3).Theobserveddegreeofnonstoichiometry,whichisminor,mayrepresentacompositionalfeature(cf.thesyntheticanalogofcooperite,PtS1.1:Grønvoldet al.1960).

Isoferroplatinum, Pt3Fe (or Fe-rich platinum)

Isoferroplatinum, ideally Pt3Fe, has an orderedprimitivecubic(pc)structure,spacegroupPm3m,andtypicallycontains25to35at.%Fe.TherelatedspeciesofPt–FealloyisFe-richplatinum(20–50at.%Fe)or“nativePt” (Fe<20 at.%andPt>80 at.%), having adisordered structure (fcc), space groupFm3m (Cabri&Feather1975).

The compositionof some inclusions in the placergrainsofPt–FealloycorrespondtoaPt3Fe-typealloy,whichhasthestoichiometryofisoferroplatinum(anal.12,Table3);noXRDdatacouldbeobtainedowingtothetinygrain-sizeoftheseinclusions.Thustheymayrepresent Fe-rich platinum, not necessarily isoferro-platinum.Theprefix“ferroan”(and“ferroanplatinum”)are avoided in accordancewith the recommendationapprovedbytheIMA(Baylisset al.2005).

Vasilite, (Pd,Cu,Pt)16S7

OurresultsofEMPanalysesobtainedforinclusionsof aPd–(Cu)-rich sulfide are consistentmost closelywithatomicproportionsofvasilite,Pd16S7(anal.13–15,Table 3).An alternative interpretation, based on theexistenceinthesystemPd–SofaphasePd3S(Okamoto1992c),seemstobelessprobable.IncontrasttovasilitefromthetypelocalityatNovoseltsi,Bulgaria(Atanasov1990),inclusionsofvasiliteinoursamplesarevirtuallydevoidofTe,thelevelofwhichislowerthanthelimitof detection in theWDSanalysis (<0.06wt.%; anal.15,Table3).However,elevatedlevelsofCu(1.9wt.%or0.6apfu)arepresent,incommonwithcompositionof the type-localitysampleofvasilite.Tolstykhet al.(2000)describedaphase,alsoCu-bearingandpoorin


Te, related to vasilite from thePustayaRiver placerdeposit, associatedwith anAlaskan-type complex inKamchatka,Russia.

Pt–Pd–Ni–Cu alloy (Pd-rich platinum)

Inclusions of aPt-dominant alloy enclosedwithingrainsofPt–FealloyareunusuallyrichinPd,Ni,andCu,andpoorinFe(minorconcentrationsofS,ascribedto contaminationby a sulfidephase,were omitted inthese results).The three inclusionsanalyzedgave thefollowing ranges (expressed inwt.%):Pd 20.8–27.7,Ni2.5–3.1,Cu2.7–3.2,andFe0.7–1.0,or,intermsofatom%:Pt47.7–52.8Pd29.3–37.9Ni6.3–8.0Cu6.1–7.5.ValuesofSPGE/(Fe+Cu+Ni)vary from4.5 to5.9.Wenotethat themaximumvalue in this range (5.9) is closeto themaximumof5.7observed for placer grainsofPt–Fe alloy.This correspondence implies that thesealloy phases, the host and inclusions, have attainedequilibriuminthedistributionofPGEandbasemetals.

Keithconnite, Pd20Te7 (or Pd3–xTe; 0.14 < x < 0.43)

KeithconnitewasfirstdescribedasPd3–xTe(0.14<x<0.43),inassociationwithtelluropalladinitePd9Te4,from theStillwater layeredcomplex,Montana (Cabriet al. 1979).An alternative formula for keithconnite,Pd20Te7,isbasedontheobservedanalogywithsyntheticPd20Te7(Wopersnow&Schubert1977,Bayliss1990).

ThecompositionofaPd-richtellurideenclosedbyagrainofPt–FealloyatBurwashis:Pd71.71,Pt0.63,Te27.66,foratotalof100.0wt.%,whichcorrespondsto (Pd3.01Pt0.01)S3.02Te0.97, or to (Pd20.3Pt0.1)S20.4Te6.6,calculatedonthebasisofatotalof4apfuand27apfu,respectively.NoXRDdata could be obtained owingtoitssmallgrain-size.Interestingly,onemorephaseisrelevant and exists in the systemPd–Te:Pd3Te (bcc)crystallizedbyeutectic reactionL +(Pd)→Pd3Teat780°C(Okamoto1992c).Thus, thepossibilitycannotbeexcludedthatwearedealingwithunnamedPd3Te.

Unnamed Pd11Te2S2 (?)

An unusual phase of Pd-rich sulfotelluride (?),enclosed by a grain of Pt–Fe alloy,may representunnamedPd11Te2S2, possibly related to isomertieitePd11Sb2As2,miessiite, Pd11Te2Se2, and to unnamedPd11(Te,Sb)2As2.The composition of the presentlyanalyzed phase (SEM–EDS) is: Pd 73.65, Pt 1.28,Au1.37,Te19.83,S3.87,a totalof100.0wt.%; theformula is (Pd10.57Au0.11Pt0.10)S10.78Te2.37S1.84 (basis:15apfu).NoXRDdatacouldbeobtained.Analterna-tive interpretation leads to the formula (Pd2.82Au0.03Pt0.03)S2.88(Te0.63S0.49)S1.12(basis:4apfu),orPd3(Te,S),whichcouldpertaintoaS-richvarietyofkeithconnite,or, perhaps, to a compositional variant of the “high-temperaturePd3Te”(cf.Okamoto1992c).

It is known that various substitutions involvingnonmetals (chalcogens)may occur in isomertieite-typecompounds.Miessiite,Pd11Te2Se2,anewmineralspecies isostructuralwith isomertieite, Pd11Sb2As2,wasdiscoveredatMiessijoki,Finland(Kojonenet al.2007).ATe-richisomertieite(Pd10.96Fe0.03)S10.99(Sb1.13Te0.94)S2.07As1.93,was reported from a placer depositofBritishColumbia (Barkovet al. 2008c).A relatedPGM,describedas“Te-richisomertieite”fromBurma(Hagenet al.1990,Cabriet al.1996),correspondstoaTe-dominant (unnamed) analogue of isomertieite:(Pd10.44Pt0.55)S10.99(Te1.08Sb0.86)S1.94As2.08.


THEsiliCaTEinClUsiOns

Clinopyroxene, Cl-bearing amphibole, and plagioclase

ThreeinclusionsofclinopyroxenehostedbyPt–Fealloy,analyzedinthisstudy,arediopside,whichvariesincompositionfromhighlymagnesiantomoderatelyrichinMg:Ca47.1Mg48.0Fe4.9[100Mg:(Mg+Fe2+)=mg#90.7,Ca47.5Mg43.9Fe8.6 (mg# 83.6)], andCa45.7Mg37.5Fe16.8(mg# 69.0).The calculated formulae, Ca0.84(Mg0.85[6]Al0.15Fe0.09)S1.09Si1.99O6,Ca0.84(Mg0.77Fe0.15[6]Al0.15Mn0.01)S1.08(Si1.99Al0.01)2O6, and (Ca0.85Na0.06)S0.91(Mg0.69Fe0.31[6]Al0.10)S1.10(Si1.92Al0.08)2O6,demonstratethepresenceof[6]Al.

Twoinclusionsofsodic–calcicamphiboles(0.6and1.1Naapfu)enclosed inplacergrainsofPt–Fealloyare relatively enriched inCl (0.72 and1.17wt.%, or0.17and0.36Clapfu,respectively).Intermsoftheirenrichment inNa andCl, these inclusions are some-what similar to inclusions of sodic–calcic amphiboleobservedinplacergrainsofaPt–FealloyfromFlorenceCreek,Yukon.Thelatteramphiboleisnotablyricherin

Cl,however(cf.Barkovet al.2008a).Inaddition,twograinsofplagioclase,hostedbygrainsofPt–FealloyatBurwash, correspond to andesine:Ab60An35.1Or4.8andAb57.5An39.9Or2.6. Interestingly,nearlypurealbite(Ab97.8)occursasaninclusioninPt–FealloyatFlor-enceCreek,Yukon(Barkovet al.2008a).

Trace-element compositions of silicate melt inclusions

Trace-elementcompositionsofthefourinclusionsofsilicatemeltweredeterminedusingLA–ICP–MS.TwoinclusionsarelocatedclosertothemarginofthegrainandtwoareintheinteriorpartofthePt–Fealloyhostgrain.Themineralogicalandchemicalcompositionofallfourinclusionsisidentical(Fig.5,Table4).Concen-trationsoflightandheavyrare-earthelements(LREEandHREE)normalized to the chondritic abundances(McDonough&Sun1995)showslightlynegativeslopeforREE(Fig.5a).TheslopeoftheREEpatternisnotsteep,withtheratioofLa/Lu�3andawell-developedEudepletion.Depletion inHREE, if present, is veryminor, consistentwith a garnet-absentmantle sourcefor themagmas.Themulti-element “spider-diagram”(Fig. 5b) shows significant decoupling between thetwo important groups of incompatible elements.Thelarge-ion lithophile elements (LILE) Sr,K,Rb andBa are enriched andbehavedifferently than the highfield-strength elements (HFSE),Th–Y,which showmuchlowerconcentrations(Fig.5b).AnotherimportantfeatureonthisdiagramisthelargenegativetroughatNb andTi.TheNb trough shown for Incl. 1 is evenlarger for the other three inclusions,which haveNbconcentrationsbelowthedetectionlimit(Table4).

DisCUssiOn

Potential provenance

Three types of potential source-rocks: ophiolites,Alaskan-type complexes and sill-like “Kluane-type”intrusions, could account for the reportedoccurrenceatBurwashCreek of detrital grains of Pt–Fe alloy.Theophiolite-typeprovenanceappearstobeunlikely,however,basedontheobservedpatternsofenrichmentinCu,Ir,RhandPd,andofthestrongdepletioninRuincompositionsoftheanalyzedgrainsofPt–Fealloy(Tables 1, 2).These characteristics rather point to anAlaskan-typesource.Thevirtualabsenceinthisasso-ciationofgrainsofOs–Ir–Ru–PtalloysenrichedinRu(cf.Weiser&Bachmann1999),andtheobservedPt–Irassociation (up to6.9wt. Ir,Tables1,2), implyinga“M”-shaped chondrite-normalized pattern of abun-dance,with peaks at Pt and Ir, are consistentwithanAlaskan-typemineralization. InNorthAmerica,examplesofRu-richalloysofthePGE,presumablyofophioliteorigin,werereportedfromCalifornia,amongotherlocalities(Barkovet al.2008b).


Ingeneral,thespeciationofPGMpresentasinclu-sions in placer grains of Pt–Fe alloy cannot providedistinctivemineralogical criteria to recognize theirprovenance.Forexample,Rh–Irsulfidescanoccurinassociationwithapodiformchromitite(Malitchet al.2001), a layered intrusion (Oberthüret al. 2004), orwith anAlaskan-type complex (Stanleyet al. 2005).Nevertheless, the observed assemblage of PGM inthe analyzed inclusions at Burwash Creek closelycorresponds to that expected in anAlaskan–Uralian-typemineralization. Indeed, the same association ofbowieite–kashinite,miassite, vasilite, cooperite, andkeithconnitewasdocumentedininclusionsingrainsofisoferroplatinum-typealloyfromaplacerintheMiass

River, in theUrals (Britvinet al.2001).Whereas theoriginoftheMiassplacerremainspoorlyunderstood,andtheMiassRivercutsvariousophiolitecomplexesthatcouldbethesourceoftheplacerPGM,Alaskan–Uralian-typecomplexesalsoarearecognizedsourceofplacerplatinumin that region.Besides, thefollowingspecies of Rh–Ir sulfideswere first discovered inassociationwithAlaskan-typecomplexes:bowieite inAlaska(Desborough&Criddle1984),andkashiniteandmiassiteintheUrals(Begizovet al.1985,Britvinet al.2001).Phasescorrespondingtovasiliteincompositionwere reported in associationwith Pt–Fe alloy fromAlaskan-typecomplexesoftheKamchatka–Koryakbelt(Rudashevsky&Zhdanov1983,Tolstykhet al.2000).

FiG. 5. A)Chondrite-normalized (McDonough&Sun1995)REEabundances.B)N–MORB-normalized(Hofmann1988,Hartet al.1999)LILEandHFSEabundancesinmicro-inclusionsofsilicatemeltinahostPt–Fealloyplacergrain.


Theorigin of the type-locality vasilite atNovoseltsi,Bulgaria,isnotwellunderstood(Atanasov1990).

Theothertypeofpotentialprovenancetoconsideris so-calledKluane-typemafic–ultramafic sill-likeintrusive bodies, emplaced during a period of upliftandextensionassociatedwithbasalticvolcanism.TheBurwash property is located in the east half of theQuillCreekmafic–ultramafic complex, amultiphasesill-like intrusion ca. 20 km long and up to 1 kmthick.ThecomplexhostsanumberofoccurrencesofNi–Cu–PGEmineralization, including theWellgreendeposit,which iseconomically importantandadjoinstheBurwashproperty to thewest (Cabriet al. 1993,Hulbert 1997).Theproximitywould seem to favor aKluane-type intrusion as the prospective source forthe reported placer grains of Pt–Fe alloy observedat Burwash Creek. However, such a source is notcorroborated by the following observations: (1)TheQuillCreek complex isLowerTriassic (232.3± 1.0Ma:U–Pbdating of zirconbyMortensen&Hulbert1991),close inage to thePGE-richPermian–TriassicNoril’sk complex of Siberia (251.2± 0.3Ma:U–PbdatingofzirconandbaddeleyitebyKamoet al.1996).Noevidenceisreportedintheliteraturetoindicatethatmafic–ultramaficsillsassociatedwithfloodbasaltsanddevelopedinNoril’skorinotherareascouldrepresentlode-sources for placer grains ofPt–Fe alloy.On theotherhand, theAlaskan-typecomplexesare thewell-recognized sourceof both in situ andplacer depositsof platinum, aswas reported fromvarious localitiesworldwide,especiallyfromAlaska, theUrals,Siberia(Aldan Shield), andKamchatka (e.g., Cabri 1981,Nixonet al.1990,Cabriet al.1996,Garutiet al.2002,Malitch&Thalhammer2002,Tolstykhet al.2002).(2)TheWellgreendepositischaracterizedbyaverysmallgrain-sizeofmostof thePGM,whichareprincipally Pd–(Pt)–Ni-richstibiotelluridesandbismuthotellurides.PGEalloymineralsareveryrareinthisdepositandinrelateddepositsaswell(Cabriet al.1993,Barkovet al.2002).Thus,aKluane-typesourceseemstobeamoreremotepossibility.

Implications based on the inclusions in placer grains of Pt–Fe alloy

Thevalueofmg#of inclusionsofdiopside in thedetritalgrainsofPt–Fealloyattainsamaximumof90.7,which is high and consistentwith highlymagnesiancompositionsof their lode source-rocks.These inclu-sionsarenotablyenrichedinoctahedrallycoordinatedAl.The content of [6]Al in pyroxene increaseswithincreasingpressure(e.g.,Aoki&Kushiro1968).Thus,theseinclusionsofdiopsidemaywellhavecrystallizedunder high-pressure conditions, similar to inclusionsof omphacite-rich clinopyroxene hosted by grains ofPt–Fe alloy fromNizhniyTagil,which is a “classic”Alaskan–Uralian-type complex in theUrals,Russia

(cf. Johan2006). Inaddition, theobserved inclusionsofNa–(Cl)-rich amphibole and of sodic plagioclase,documentedatBurwashCreek,aresimilartoinclusionsofaNa-richamphiboleandsodicplagioclaseinnuggetsofFe-bearingplatinumlikelyderivedfromanAlaskan-typecomplexinPapuaNewGuinea(Johanet al.2000).

Inmanycasesreportedintheliterature,inclusionsof species ofPGMhosted byPGEalloyminerals orbymembers of the spinel group (commonly chro-mite–magnesiochromite) are considered to be of ahigh-temperature,magmaticorigin.Thisinterpretationis corroborated by experiments in fluid-free “dry”systems.Forexample,asyntheticanalogofcooperite(PtS)crystallizedunder“dry”conditionsofsynthesisatatemperatureupto1100°C(Cabriet al.1978,Verryn&Merkle 2000).The other examples of species of“high-temperature”PGM include alloys of refractoryelements,suchasanOs–Iralloy,orlaurite–erlichmanite(RuS2–OsS2).ThelatteriscommonlyconsideredtobeoneofthefirstPGMtocrystallize,anditscompositionmayevenreflecttheRu:Osratiooftheprimitivemantle(Zaccariniet al.2004).Ontheotherhand,influid-richsystems, these andother species ofPGMcouldhavecrystallizedatalowertemperature,correspondingtoapostmagmatic-hydrothermalstage,e.g.,cooperitewasobservedasaproductoflow-temperaturehydrothermalsynthesis(Plyusninaet al.2000),andlauriteofhydro-thermaloriginwasdescribedfromtheImandralayeredcomplex,Russia(Barkov&Fleet2004).NativeosmiumdepositedfromalateNa–H2O–(Cl)-bearingfluidphaseat a postmagmatic–hydrothermal stage, and aW-richalloyofOs–Ir,presumablyofmetasomaticorigin,alsowere reported (Barkovet al. 2008a, b). In the “dry”condensedsystemCu–Rh–S,phasesRh2S3andRh17S15,relatedtobowieiteandmiassite,whicharepresentasinclusionsatBurwashCreek,wereobservedat900°C(Karup-Møller&Makovicky 2007). In contrast, alow-temperaturecrystallizationofthesePGM,bowieiteandmiassite,ispossibleinenvironmentsrichinvola-tile species, as indicated by results of hydrothermalsynthesis,whichyieldwell-formed crystals ofRh2S3andRh17S15at400°C(Zhanget al.2009).Onthebasisofexperimentaldataobtainedin“dry”systems,grainsof keithconnite could formvia a peritectoid reactionPd3Te+ Pd8Te3→ Pd20Te7 at 754°C, and grains ofvasilite,viaaperitecticreactionL+PdS→Pd16S7at639°C(Okamoto1992a,1992c).However, theoccur-renceofsodic–calcicamphiboles(someenrichedinCl),associatedwithPt–Fealloyclearlyimpliesthepresenceof a fluid phase in the system.Thus the associationdescribed of bowieite–kashinite,miassite, vasilite,cooperite, keithconnite and of Pd-rich sulfotelluride(?), included inPt–FealloyatBurwashCreek, couldwell have crystallized at a lower temperature, fromtrappedmicroportionsoffluidphaseorameltenrichedinvolatilecomponents.


Tectonic setting based on trace-element geochemistry

Concentrationsoftraceelementsinsilicatemeltaredeterminedby thenatureof themantlesourceunder-goingpartialmeltingtoproducemaficmagmaandtheprocessesofmagmadifferentiation,crystallizationandcontamination. Patterns ofREE and other groups ofincompatibleelements(LILEandHFSE)varygreatlyasafunctionoftectonicenvironments,andtheirratiosarecommonlyusedasindicatorsofthetectonicenvi-ronmentofmagmageneration.Inthisstudy,thethreemaincandidatesforthetypeofsourcerockforthePt–Fealloy placer grains belong to very different tectonicsettings.TheophiolitesorAlpine-typeperidotites areproducts of oceanic-rifting-relatedmagmatism, conti-nentalmafic–ultramafic complexes and small bodiessuchasKluanesillsaretheresultofcontinentalrifting,andAlaskan-typecomplexesrepresentsubduction-zonemagmatisminarcenvironments.

Ophiolites and oceanic peridotites typically showa positive or slightly negative slope of REE.Thefractionalcrystallizationofplagioclse resulting in thedevelopmentofaEudepletioninREEpatternsisrare.Thesemagmasareequallydepleted inall the incom-patible elements and show similar behavior ofLILEandHFSE.SuchbehaviorofLILEandHFSEisverydifferentfromthesignificantdecouplingbetweenthesetwogroupsof incompatibleelementsobserved inoursamples(Fig.5,Table4).ThelargeLILE/HFSEratioofsilicateinclusionshostedinPt–FealloyfromBurwashCreekcanbeusedtoruleoutophiolitesorAlpine-typeperidotites as the sourceof the placer platinum.Thisis also supported by the complex patterns of REEin the inclusions studied.The pattern of highLILE/HFSEvalueisadistinctivefeatureofsubduction-zonemagmas; it results from themuchhighermobility ofLILE in aqueous fluids involved inmagma genera-tionat subductionzones.Tosomeextent,decouplingbetween LILE and HFSEmay also be present inmagmasgeneratedduring continental rifting, such astheKluanesills,owingtothecontributionoffluidsinthemantle-sourceregionundergoingmelting.However,theextremelyhighLILE/HFSEvalue,overtwoordersofmagnitude,isinbetteragreementwithasubduction-relatedAlaskan-type complex as the source of theplacerplatinum.ThisisalsosupportedbythepresenceofanegativetroughforNbthatischaracteristicofarcmagmatism, lowTiandthecomplexpatternsofREEtypical formagmas derived from themantlewedgeabovethesubductionzone,withtheircomplexdeple-tion and enrichment history.The trace-element ratiosinthesilicateinclusionssuggestthatthesourceofthePt–Fealloyplacergrains inBurwashCreek isnotofophiolitetypenoroceanicperidotites.Thesedatacannotundeniably distinguish between theKluane sills andfragmentsofanAlaskan-typecomplex,butaremuch

moreconsistentwithanAlaskan-typecomplexas thesourceofthisPt–Fealloyphase.

aCKnOWlEDGEmEnTs

ThisstudywasmadepossiblewithfinancialsupportfromtheYukonGeologicalSurvey,whichisgratefullyacknowledged.We thank Lang Shi,Department ofEarthandPlanetarySciences,McGillUniversity,forhistechnicalassistancewithelectron-microprobeanalyses.We acknowledgePatricia Scallion for the helpwithSEMwork,andtheInstituteforResearchinMaterials,DalhousieUniversity,foraccesstotheFE–SEMfundedbyCanadaFoundationforInnovation.WearegratefultoJacobHanleyandthereviewers,EvgenyPushkarevandAberraMogessie,whosecommentshelpedtoimprovethemanuscript.

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Received September 8, 2009, revised manuscript accepted June 3, 2010.

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