CHROMIAN SPINEL COMPOSITION AND PLATINUM-GROUP ELEMENT MINERALOGY OF CHROMITITES FROM THE MILIA...

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The Canadian MineralogistVol.47,pp.1037-1056(2009)DOI:10.3749/canmin.47.5.1037

CHROMIAN SPINEL COMPOSITION AND PLATINUM-GROUP ELEMENT MINERALOGY OF CHROMITITES FROM THE MILIA AREA,

PINDOS OPHIOLITE COMPLEX, GREECE

ArgyriosKAPsioTis,TAssosA.grAMMATiKoPoULos§,BAsiLiosTsiKoUrAsAndKonsTAnTinHATZiPAnAgioToU

Department of Geology, Section of Earth Materials, Panepistimiopolis of Rion, University of Patras, 265 04 Patras, Greece

FedericAZAccAriniAndgiorgiogArUTi

Department of Applied Earth Sciences and Geophysics, Peter Tunner Str. 5, University of Leoben, A–8700 Leoben, Austria

ABsTrAcT

ThechromititesoftheMiliaareainthePindosophiolitecomplex,inGreece,wereinvestigatedfortheirplatinum-groupmineral(PGM)content.Thechromititesaremassive,morerarelydisseminatedintextureandoccurassmallpods.TheyarecomposedofmagnesiochromitecrystalswithCr#[Cr/(Cr+Al)]between0.80and0.84,andMg#[Mg/(Mg+Fe2+)]between0.54and0.72.Thetotalplatinum-group-element(PGE)contentsinchromititesarerelativelylow(≤170ppb),althoughtheymaylocallybehigher(upto1059ppb).TheIPGE(Os,IrandRu)predominateoverthePPGE(Rh,PtandPd).ThePGMassemblage,consistentwiththegeochemicaldata,isdominatedbylauriteandOs–Ir–Rualloysthatoccurbothassingleandcompositegrains,generallylessthan15mminsize.LauritehasawiderangeofOs-for-Rusubstitution[Ru/(Ru+Os):0.42–0.99],whereasprimaryalloysareenrichedinRu(upto73.80wt.%).Somelauritecrystalsexhibitananomalouspatternofzoning.SuchzoningrequiresaninversionofthenormalT–f(S2)trendinmagmaticsystems,andishereinconsideredtobeduetopostmagmaticprocesses.SomeRu-richalloygrainscontainrelativelyhighRhandPtabundances,similartothoseofresidualsulfidesinmantleperidotites.ThisfeaturesuggeststhatthesealloysmayrepresentresidualPGMphasesfromearlierepisodesofmeltextractionfromtheseverelydepletedmantleunitofthePindosophiolitecomplex.CombinedcompositionaldataindicatethattheMiliachromititesformedfromahydrousboniniticmeltinasuprasubduction-zoneenvironment.

Keywords:chromitites,platinum-groupminerals(PGM),laurite,alloys,ophiolites,Pindos,Greece.

soMMAire

NousavonsétudiéleschromititesdelarégiondeMilia,enGrèce,faisantpartieducomplexeophiolitiquedePinde,dupointdevuedeleurteneurenminérauxdugroupeduplatine(MGP).Leschromititessontmassivesou,plusrarement,disséminéesentextureetseprésententenpetiteslentillesallongées,composéesdecristauxdemagnésiochromiteayantCr#[Cr/(Cr+Al)]entre0.80et0.84,etMg#[Mg/(Mg+Fe2+)]entre0.54et0.72.Lateneurtotaleenélémentsdugroupeduplatineestrelativementfaible(≤170ppb),quoiquequ’ellepeutatteindre1059ppb.Lateneurenélémentsdugroupedel’iridium(Os,IretRu)dépassecelledesélémentsdugroupeduplatine(Rh,PtetPd).L’assemblagedeMGP,enconcordanceaveclesdonnéesgéochimiques,contientsurtoutlalauriteetdesalliagesOs–Ir–Ruengrainsisolésoucomposites,engénéralmoinsde15mmdetaille.Lalauritefaitpreuved’unegrandevariabilitédurapportRu/(Ru+Os),entre0.42et0.99,tandisquelesalliagesprimairessontenrichisenRu(jusqu’à73.80%,poids).Certainscristauxdelauritesontanomalesdansleurschémadezonation,etsemblentindiqueruneinversionquantautracéhabitueldeT–f(S2)d’unsystèmemagmatique;nousl’attribuonsàdesprocessuspostmagmatiques.Certainsgrainsd’alliagesrichesenRucontiennentdesteneursrelativementélevéesenRhetPt,toutcommelessulfuresrésiduelsdepéridotitesdumanteau.Cesgrainsd’alliagepourraientdoncreprésenterdesMGPrésiduelsaprèsl’extractionantérieuredeliquidesilicaté,laissantunmanteautrèsstériledanslecomplexeophiolitiquedePinde.Lesdonnéescombinéessurlacomposi-tionindiquentqueleschromititesdeMiliasesontforméesàpartird’unemagmahydratéboninitiquedansunmilieuaudessusdelazonedesubduction.

(TraduitparlaRédaction)

Mots-clés:chromitites,minérauxdugroupeduplatine,laurite,alliages,ophiolites,Pinde,Grèce.

§ Present address: SGSMinerals Services,AdvancedMineralogyNetwork,LakefieldFacility, SGSLakefieldResearchLimited,185ConcessionStreet,Box4300,Lakefield,OntarioK0L2H0,Canada.E-mail address:[email protected]

1038 THecAnAdiAnMinerALogisT

ophiolitecomplexhasbeeninvestigatedduringthelasttwo decades byEconomou-Eliopoulos&Vacondios(1995),Tarkian et al. (1996), Economou-Eliopouloset al. (1999),Grammatikopoulos et al. (2007), andPrichardet al. (2008).Their results indicate that twodistinctivecompositionalgroupsofchromititescanbedocumented, solely on the basis of chromian spinel,high-Cr and high-Al types, each ofwhich shows arelativelyrestrictedcompositionalvariation.TheyalsoconcludedthatthetotalPGEcontentinthemajorityofchromititesisrelativelylow(<300–400ppb),althougha few samples locally enriched in PGEoccur in theareas ofMilia andKorydallos.More specifically, thehigh-CrchromititesofMiliaareenrichedinOs,Ir,andRucomparedtoPtandPd,whereasthehigh-AlonesintheareaofKorydallostendtobePt-andPd-enriched(Kapsiotiset al.2009).Acleardistinctioncanalsobemadebetween the typesofPGMassemblageshostedineachchromitedeposit.Thehigh-CrchromititesaredominatedbyOs-, Ir-, andRu-richminerals, such asalloys, sulfides, sulfarsenides,diarsenidesandoxides.Ontheotherhand,inthehigh-Alchromitites,Pt-andPd-bearingminerals are the dominant PGM.ThepredominanceofOs-,Ir-andRu-bearingPGMinthechromititeshavingachromianspinelwithhighCr#[Cr/(Cr+Al)]standsincontrasttotheprevalenceofPt-andPd-rich PGM in the chromitites hosting a chromianspinelwithlowCr#.Thisfurtherindicatesthatdifferenttypesofmagmas,havingformedfromvarieddegreesof partialmelting ofmore than onemantle source(Economou-Eliopoulos&Vacondios1995,Prichardet al.2008),wereinvolvedintheformationofthePindoschromitites.

PrevioUsWorKonTHePgMoFMiLiAcHroMiTiTes

Thefirst studyof thePGMcontent of thePindoschromititeswasmade byTarkian et al. (1996).Theauthors studied (mainly PGE-enriched) samples ofchromititefromseverallocationsofthePindosophio-litecomplex,includingMilia.However,theyobservedPGMgrainsonlyincertainPt-andPd-enrichedsamplesofhigh-AlchromititefromtheareaofKorydallos,butnotfromMilia.TheirstudyrevealedaPGMassemblagedominatedby threedifferentphases, including laurite(twograins),sperrylite(twograins)andanunidentifiedphase,Pt(Ni,Fe)3(fourgrains).Onthebasisoftexturalfeaturesandcompositionaldata,theysuggestedthatthemajorityofthePGM(exceptforlaurite)aresecondaryinorigin.

ThefirstdescriptionofPGMfromtheMiliachro-mititeswasmadebyGrammatikopouloset al. (2007)inthecontextofacomparativestudybetweenthePGMassemblagesofhigh-CrchromititesfromdifferentareasofthePindosmassif.TheauthorsrecoveredthePGMinconcentratesandshowedthatthePGMassemblage

inTrodUcTion

Theroleofchromianspinelasanimportantcollectorofplatinum-groupelements(PGE)hasbeenconfirmedininvestigationsofthePGEcontentofdifferenttypesof chromititesworldwide (e.g.,Augé 1985, Ferrario&Garuti 1990, Pedersen et al. 1993,Garuti et al.1999,Ahmed&Arai 2003).However, there is nowconvincing evidence that thePGE are not carried insolid solution in the chromian spinel, as had beenproposedfromexperimentalstudies(e.g.,Capobianco&Drake1990,Capobiancoet al.1994,Righteret al.2004, Pagéet al. 2009), but formdiscrete platinum-groupminerals(PGM).Onthebasisofin situtexturalobservations,ithasbeenconfirmedthatthePGMinclu-sions in unaltered chromian spinel are near-liquidusphases precipitated from themagma at a very earlystage of its differentiation.The type ofPGMassem-blagehosted inchromitite isalsoconsideredausefulindicatorofthephysicochemicalconditionsprevailingin the earlymagmatic system (e.g.,Augé& Johan1988,Nakagawa&Franco1997,Garutiet al.1999).Inparticular,theparagenesisandcompositionofthePGMareused to estimate the conditions of sulfur fugacityf(S2)and temperature in themagmaduringformationofthechromitites.

Several recentmineralogical studies have shownthattheprimaryPGMassemblagesinchromititescanbecomeunstableandmodifiedwhentheyareaffectedbylow-temperaturealteration(e.g.,Stockman&Hlava1984,Garuti&Zaccarini1997,Zaccariniet al.2005,Ahmed&Economou-Eliopoulos2008).Theseauthorshave shown that secondary processes can crypticallymodifythecompositionofprimaryPGM,whichleadstotheformationofnewPGMphases.AlloysandoxidesrichinRu,OsandIrarethemostcommonsecondaryPGMencounteredinpodiformchromitites(e.g.,Garuti&Zaccarini1997,Ahmed&Arai2003,Gervilla et al.2005,Proenzaet al.2007,2008).

Inthepresentpaper,wediscussthegenesisofMiliachromitites in thePindosophiolitecomplex,andnewdataontheparagenesisandcompositionoftheirPGMassemblageareusedtoconstraintheconditionsoff(S2)andTprevailingin thePindosmantleunitduringtheformationofthechromitites.AcomparativestudyofthePGMassemblagesamongthechromititesinvestigatedinthecomplexismadeinordertofurtherunderstandthemetallogenicfactorsthatcontrolledchromiteandPGMgenesisthroughoutthePindosophiolitecomplex.Theeffectoflow-ThydrothermalalterationontheobservedPGMassemblage is also addressed in order to betterconstrain the conditions underwhich the secondaryPGMhavebeenmodified.

BAcKgroUndinForMATion

The composition of chromian spinel and of theplatinum-groupmineralsinchromititesfromthePindos

MinerALogyoFcHroMiTiTes,PindosoPHioLiTecoMPLex,greece 1039

typicaloftheMiliachromititesisdominatedbylaurite,followedbyOs–Ir–Rualloys.

Morerecently,Prichardet al.(2008)studiedPGM-bearingchromititesfromboththeMiliaandKorydallosareas of the Pindos ophiolite complex.Their in situstudy of theMilia chromitites yielded 30 grains ofPGM in a single PGE-rich chromitite sample.Theydescribed a PGMassemblage rich inOs, Ir,Ru andRh,mainly dominated by sulfarsenides of Pt–Rh–IrwithminorOsandRu (16grains) andOs–Ir–Ru–Rh(±Ni)alloysrarelyalteredtoanoxidephase(10grains),accompaniedbyminorPt–Fe (±Ni,Cu,Au)andAu–Pd alloys (three grains and one grain, respectively).However, theydidnotpresent anyanalyticaldataonthecompositionofPGMintheMiliachromititesandsupported the predominance ofOs–Ir–Ru alloys andirarsite–hollingworthite–platarsite series.Moreover,theysuggestedthatthePGMassemblageintheMiliachromititesissecondaryinoriginbecausethemajorityof thePGMaremottled, inhomogeneous and exhibitoutlines indicating that have been affected by in situprocesses of alteration,which served to replace theoriginalprimarysulfur-bearingPGM.

geoLogicALseTTingAndsAMPLing

The Pindos ophiolite complex is located in thewestern part of northernGreece (Fig. 1) and corre-spondstoapieceofMiddletoUpperJurassicoceaniccrust (Rassios& Smith 2000) that tectonically liesover the autochthonousMaastrichtian–EocenePindosflysch.The Pindos ophiolite complex is subdividedintofourprincipaltectonicunits:theDramalacomplex,theLoumnitsaunitandtheAspropotamoscomplex,allstructurally overlying theAvdellamélange (Jones&Robertson1991).TheDramala unit consists of largemassesofvariablydepletedharzburgite–dunite(>1000km2),whichmayhostsmallbodiesofchromitite(Pefki,Milia,Korydallos, etc., Fig. 1) exhibitingmassive,disseminated,schlierenand,lesscommonly,anodulartexture.ThecrustalrocksoftheAspropotamoscomplexcover a wide spectrum of geochemical affinities,rangingfromMORB(mid-oceanridgebasalts)throughMORB–IAT(island-arctholeiites)toIATand,finally,volcanic rocksof theboninitic series (BSV) (Bizimiset al.2000,Pe-Piperet al.2004).TheLoumnitsaunitrepresentsthebasalmetamorphicsoleofbothDramalaandAspropotamos complexes, comprising low-gradeamphibolite- andgreenschist-faciesmeta-igneousandmetasedimentaryrocksthathaveyielded40Ar–39Aragesof169±5and165±3Ma(Whitechurch&Parrot1978,Spray&Roddick1980).TheAvdellamélange,about1kmthick,containsblocksofvariousrock-typesinatectonizedmatrixandmappablethrust-sheets(Jones&Robertson1991).

Representativesamplesofchromitite(N=26)werecollectedfromtheareaofMilia,whichislocatedinthesouthernpartoftheDramalacomplex,inanattemptto

examine their compositional characteristics and theirPGMcontent.

descriPTionoFTHeMiLiAcHroMiTiTes

TheMiliachromititesandtheirhostingdunitesarestructurallydiscordantwithrespecttothesurroundingmantle-derivedperidotites(harzburgites)andoccurassmallpodsthatshowsharpcontactswiththeenclosingdunites.Locally,theboundariesbetweentheorebodiesandthehostdunitesmaybegradational,dependingontheamountofchromianspinelcontainedinthedunite.Texturally, the chromitites aremainlymassive (morethan 70 vol.% chromian spinel) and disseminated(10–70 vol.% chromian spinel).The chromitites arestrongly fractured andmylonitized.The interstitialmatrix of the chromitites consists almost exclusivelyofsecondarysilicateminerals,mainlymeshserpentineandchloritewithminortremolite.Onlyrelicsofprimaryolivine are preserved in some samples.The silicatephasesthatoccurasinclusionsinthechromianspinelcrystalsarecompletelyaltered.Rimsofferrianchromiteand decolorized chromite, as alteration products ofchromian spinel, occur sporadically. In thefirst case,theunalteredchromianspinelhaslowerCr#[Cr/(Cr+Al)], higherMg# [Mg/(Mg+Fe2+)] and lowerFe3+#[Fe3+/(Fe3++Cr3++Al3+)]thantheferrianchromiteintherim.Inthesecondcase,chromianspinelhaslowerCr#andFe3+#,butconstantMg#withrespecttodecol-orizedchromite.Accessorybase-metalsulfides(BMS)andbase-metalalloys(BMA)arepresentinthealteredsilicatematrixofthechromitites.

AnALyTicALTecHniqUes

The petrography andmineralogy of the sampleswereinvestigatedwithbothopticalandelectronmicros-copyat250–800magnification.Imageswereobtainedwith a Super JEOL JSM–6300 scanning electronmicroscope (SEM) at theUniversity ofPatras, and aPhilips 40SEMat the Interdepartment InstrumentalCenter(CIGS)oftheUniversityofModenaandReggioEmilia,withanacceleratingvoltageof20–30kVandabeamcurrentof2–10nA.QualitativeanalysesofthePGMinvestigatedwerecarriedoutbyenergy-dispersivespectrometry(EDS)inthetwolaboratoriesmentioned.

Quantitativeanalysesofchromianspinelperformedat theDepartment of Earth and Planetary Sciences,McGillUniversity,Canada,usingaJXAJEOL–8900Lelectronmicroprobeoperatedinwavelength-dispersionspectrometry (WDS)mode at an accelerationvoltageof15kV,beamcurrentof20nAandbeamdiameterofabout5mm.Thetotalcountingtimewas20s.TheZAFcorrectionsoftwarewasused.Calibrationsweredoneusing natural and synthetic referencematerials.Theproportion ofFe3+ in chromian spinelwas calculatedassumingstoichiometry(AB2O4).


Two samples of chromitite were analyzed forPGE+AuattheSGSLakefieldResearchfacilityandActivationLaboratories Ltd.,Ontario,Canada.ThesampleswereanalyzedbyINAA(instrumentalneutron-activation analysis) after a pre-concentration stageof PGE+Auwith nickel sulfidefire-assay collection.Detectionlimitswereinitiallyhigh,andthenloweredinordertodetecteventhelessabundantPGE.AtLake-field’sfacility, thedetectionlimitswere50ppbforIrandRu,and20ppb forRh,PtandPd.AtActivationLaboratories,thedetectionlimitswere10ppbforOs,0.1ppbforIr,5ppbforRuandPt,0.2ppbforRh,2ppbforPd,and0.5ppbforAu.Goldwasfoundtobebelowthedetectionlimitsinallthesamplesanalyzed.

ThePGMwereanalyzedatMcGillUniversitywiththeelectronmicroprobeoperatedinWDS(wavelength-dispersion spectrometry)mode at an accelerationvoltageof20kV,beamcurrentof30nA,andabeamdiameterof2–3mm.Thetotalcountingtimewas20s.ThePRZcorrection softwarewas used.WeusedKaX-raylinesusedforS,As,Fe,Ni,CoandCu,LalinesforRu,Ir,Rh,PtandAg,LbforPd,andMalinesforOs andAu. Puremetalswere used as standards forall thePGEexceptPt, forwhichsyntheticPtAs2wasused,whereasCoNiAs,nativeFe,chalcopyrite,pyriteandPtAs2wereusedasstandardsforNi,Fe,Cu,SandAs,respectively.

Fig.1. SimplifiedgeologicalmapofthePindosophiolitecomplexshowingthelocationoftheareasofMilia,KorydallosandPefki(modifiedafterJones&Robertson1991).TheinsetmapillustratesthelocationofthePindosophiolitesinGreece.


TherecoveryofPGMwasachievedbyapplyingthe“ppmmineralogy” approach (details onwww.natires.com)carriedoutatNATIResearchJSC,St.Petersburg,Russia.AlltheheavymineralsincludingthePGMwererecovered,andconcentrationfactorsofgreaterthan104wereachieved.

sPineLcoMPosiTionAndPgeconcenTrATions

SelectedanalyticalresultsforchromianspinelfromtheMiliachromititesarelistedinTable1.Nosignificantcompositionalvariationsexistbetweencoreandrimofthechromianspinelgrains.Thechemicalcompositionoftheunalteredchromianspinelvariesbetween62.22and 64.87wt.%Cr2O3, 7.91 and 8.36wt.%Al2O3,13.82 and17.09wt.%FeO, and0.06 and0.30wt.%TiO2.TheCr#[Cr/(Cr+Al)]rangesfrom0.80to0.84,and theMg# [Mg/(Mg+ Fe2+)], from 0.53 to 0.72.Thecompositionaldataindicatethatchromianspinelisclassifiedasmagnesiochromite(Fig.2).TheTicontentofmagnesiochromiteissystematicallylow(≤0.3wt.%TiO2),whichistypicalforophioliticchromitites.Onthebasisofthecompositionofchromianspinel,theMiliachromititesareclassifiedashigh-Crchromitites,asalsoindicatedinpreviousstudies(Economou-Eliopoulos&Vacondios 1995,Economou-Eliopouloset al. 1999).

High-CrchromititesalsooccurintheareasofDramala,KamposDespoti,Trygona,Pefki andKyraKali, andcontainmagnesiochromitewithCr#between0.74and0.83.These chromite deposits are compositionallydifferentfromthehigh-AlonespresentedintheareasofKorydallos,KalambakaandVourbiani,hostingchro-mianspineloflowerCr#(0.40–0.62).AscanbeseeninFigure3,acleardistinctioncannotbemadedirectlybetweenhigh-Cr and high-Al chromitites because ofthe existence of an intermediate compositional groupofchromitites,havingchromitewithanaverageCr#of0.68,intheareaofKorydallos.Moreover,inapreviousstudyoftheMiliachromitites,magnesiochromitewasfoundtohavealowMg#(0.42:Economou-Eliopoulos&Vacondios 1995) (Fig. 3), which is probably tobe attributed to hydrothermal alteration phenomenarelated with the progressive addition of Fe2+ tomagnesiochromite.

The total PGE contents of the chromitites rangefrom 116 up to 170 ppb (Table 2).The chromititesareenrichedintheIPGE(Os,IrandRu)withrespectto thePPGE (Rh, Pt andPd) owing to the presenceof tiny inclusions of IPGE-richmineralswithin themagnesiochromite crystals. The Pd/Ir value showslimited variation that reflects the limited degree offractionationofthemagmafromwhichthechromititesformed.However,themaximumPGEcontentreportedin chromitites fromMilia is 1059 ppb (Economou-Eliopoulos&Vacondios 1995),which indicates thatthesechromitedepositsarelocallyenrichedinPGEandespecially in the IPGE(820ppb).On theotherhand,thehigh-Al chromititesof the areaofKorydallos arelocallyPPGE-enriched(upto6864ppb,Tarkianet al.1996,Prichardet al.2008),showinghighPd/Irvalues,whichindicatesthattheparentmagmaofthehigh-Al

Fig.2. ClassificationofthecompositionofchromianspinelfromtheMiliachromititesintermsofCr/(Cr+Al)versusMg/(Mg+Fe2+).


chromititeswasmoreevolvedthanthatinthecaseofthehigh-Crchromitites.

THePLATinUM-groUPMinerALs

A comparison of PGM discovered in situ with those identified in concentrates

OnlyafewgrainsofPGM(N=10)werediscoveredinthesamplesofchromitites.TheyaremainlyIPGE-basedminerals,which commonlyoccur as polygonalinclusions,orinfillingcrackswithinmagnesiochromite.ThePGMphases recognized include sulfides, alloysandPGE-bearingbase-metalsulfides(BMS).Theyareusuallylessthan5mmacross,althoughexceptionallythey reach15mm in size.Theyoccur as both singleandcompositegrains.

Commonsingle-PGMinclusionsinmagnesiochro-mite include hexagonal crystals of laurite (Fig. 4A)andOs–Ir alloy.CompositePGMgrains enclosed inmagnesiochromite comprise intergrowths of lauriteandOs–Iralloy,aswellasintergrowthsoflauritewithBMS (Fig. 4B).A relatively large composite crystaloflauriteandOs–Iralloywasfoundwithinacrackinmagnesiochromite(Figs.4C–D).

Ruthenium forms themajor constituent in laurite,whereas it is carried in solid solution in erlichmaniteandthePGE-bearingBMS.Osmiumispresentmainly

asOs–Iralloysanderlichmanite,whereasitsubstitutesforRuinlaurite.IridiumiscarriedinsolidsolutioninalltheRu-andOs-basedPGM.

Atotalof51grainsofPGMwererecoveredfromthe chromitite samples.Laurite,Os–Ir–Rualloy, irar-site,secondaryRu-basedoxides,ruarsite,erlichmaniteandRu-bearingBMS constitute the recoveredPGMphases, listed in order of decreasing abundance.Thesizeof the recoveredPGMcommonly ranges from5to15mm,butsinglecrystalsupto61mmareidenti-fied.ThePGMoccurmainly as single (Figs. 5A–B)and,lesscommonly,ascompositeparticles(Fig.5C).SinglePGM include all the above-mentionedphases.Composite PGMgrains include intergrowths of irar-sitewithlauriteandlauritewithOs–Iralloy(Fig.5C).The textural features of the grains recovered havebeen described byGrammatikopoulos et al. (2007),wherePGMareclassifiedasprimaryorsecondaryonthebasisof their internalmorphologyandqualitativecompositionaldata.

It is important to note that some laurite crystalsare cryptically zoned, exhibiting anomalous patternsof internal zoning in back-scattered electron images,a textural feature that is not commonly encountered(Figs.5C–D).

MinerALcoMPosiTions

Laurite–erlichmanite series,ideally(Ru,Os)S2

Laurite,byfarthemostabundantPGMencounteredin thechromitites,presentsarelativelywiderangeofOs-for-Ru substitution (Tables 3, 4).Themajority oftheanalyzedcrystalsareRu-rich(35.25–51.87wt.%),havingrelativelylowOscontents(1.50–20.73wt.%).ThemaximumIrcontentinlauritereaches9.57wt.%,whereas theRh content attains 4.98wt.%.TheAscontent in laurite is generally low (≤0.53wt.%), butsome crystals show greater abundances (up to 5.44wt.%).

ErlichmaniteisrichinOs(44.25wt.%)andischar-acterizedbyrelativelyhighRuandIrcontents(16.78and8.84wt.%,respectively).

ThecompositionsofPGEsulfidesaresummarizedintheOs–Ru–Irdiagram(Fig.6).AlthoughthesulfidecompositionsdonotcovertheentirerangeofOs-for-Rusubstitution,theyfollowageneraltrendofOsenrich-

Fig.3. PlotofCr/(Cr+Al)versusMg/(Mg+Fe2+)inchro-mianspinel from thechromititesof thePindosophiolitecomplex.Thedashedlinediscriminatesbetweenhigh-Crandhigh-Alchromitites,whereas thesolid lineencirclesmagnesiochromite compositions from a previous inves-tigation ofMilia chromitites (Economou-Eliopoulos&Vacondios1995).Symbols:blackcircles,magnesiochro-mite fromMilia (present study);greycircles,magnesio-chromite from the high-Cr chromite deposits ofPindos;open circles, chromian spinel from theAl-rich chromitedepositsofPindos.


ment,extendingfromalmostpureRuS2towardOs-richlaurite[Ru/(Ru+Os)at.%rangingfrom0.76to0.99]andfinallyRu-richerlichmanite[Ru/(Ru+Os):0.42].Inthesamediagram,weshowthatlauritefromMiliadoesnotbearasignificantcompositionalsimilaritywithlauritefromKorydallos(Kapsiotis2008).ThelatterhasrelativelyahighOscontent(24.79–27.83wt.%)andiscompletelydepleted in Ir andRh. In addition, lauritefromKorydallosplotsclosetothelinethatrepresentstheC1chondriticRu/Osvalue(Ru72Os28),butdoesnotfollow the general trend ofOs enrichment of lauritefoundintheMiliachromitites.

Ouranalyticalresultsshowthatthelauritecrystalsare zonedwith respect toOs andRu.Zoned lauriteinvariably preservesOs-rich domains [Os/(Os+Ru):0.16],anddomainsenrichedinRu[Os/(Os+Ru):0.05].Forinstance,inthegrainillustratedinFigure5D,theOscontentdecreases from15.90wt.% in theOs-richareasto4.89wt.%intheRu-richones.

Os–Ir–Ru alloys

Twodistinct compositional groups of alloys havebeenrecognizedinthechromitites(Fig.7,Tables5,6).Thefirst,whichispredominant,comprisesanOs-andIr-richalloythatisgenerallyclassifiedasOsmium.TheOscontentrangesfrom54.95to80.69wt.%,andIrisupto40.65wt.%.TheRucontentissignificantlylowerandvariesbetween1.92and8.17wt.%.Thisgroupischaracterized by low abundances ofRh, Pt and Pd.ThesecondcompositionalgroupofalloysisRu-basedandalsoenriched inOsandIr.TheyareclassifiedasRutheniridosmine andRuthenian osmium.They showhigh concentrations ofRu (15.82–73.80wt.%) andelevatedOs and Ir contents (up to 46.47 and 36.37wt.%,respectively).ThemostdistinctivecharacteristicofthesealloysistheirhigherRhandPtcontentsrela-tivetotheformergroup.TheRhcontentrangesfrom1.05 to 2.71wt.%, andPt from0.78 to 13.78wt.%.

Fig.4. Back-scattered-electronimages(SEM)showingthemorphology,textureandmineralassemblageofthein situPGMintheMiliachromitites.Symbols:Lrt:laurite,Chr:chromite,Cp:chalcopyrite,Srp:serpentine,Os:osmium.


TheRu-basedalloyscontainlowerRhandPtthantheIPGE,buthigherconcentrationsofRhandPtcomparedto the formergroupof alloys. Inaddition, they showhigherPPGE/IPGEvalues(0.02–0.18)thantheformergroupofalloys(0.002–0.010).AlloysofbothgroupsarecharacterizedbylowconcentrationsofBMimpurities.

Sulfarsenides

Weencounteredthesulfarsenidesirarsiteandruar-site(Fig.8,Tables7,8).TheIrcontentis25.42wt.%inirarsite,whereastheconcentrationofRuinruarsiteis20.97wt.%.TheRhandPtcontentsdonotexceed0.58and2.70wt.%,respectively,indicatingnosignifi-cantsolid-solutioninvolvingtheIr–Rh–Ptmonoclinicsulfarsenide end-members. On the other hand, theOs content in ruarsite reaches 25.29wt.%, probablyindicatingextensivemiscibilitybetweencubicruarsiteandosarsite.

discUssion

Origin and tectonic setting of the Pindos chromitites

Although the genesis ofmantle-hosted Cr-richophiolitic chromitites is still amatter of debate, themost recent interpretations for their formation pointto amechanism ofmelt–peridotite reactionwithinconduits in theuppermantle(Arai&Yurimoto1994,Zhouet al. 1996, 1998, 2001).Experimental studiesalso suggest that chromitites can form fromhydrousmeltssaturatedinchromianspinelandolivine(Matveev&Ballhaus2002).

Thechemicalcompositionoftheanalyzedmagne-siochromite,havingahighvalueofCr#(0.80–0.84)anda lowTiO2content, is representativeofCr-richchro-mitites(e.g.,Zhouet al.2001).Previousinvestigatorshaveshownthatbothhigh-Crandhigh-AlchromititescoexistinthePindosophiolitecomplexandthatthesechromititeswerederivedfromseparateparentalmagmas

Fig. 5. Back-scattered-electron images (SEM) showing themorphology, texture andmineral assemblageof the recoveredPGMfromtheMiliachromitites.Symbols:Lrt:laurite,Chr:chromite,Os:osmium,BMS:basemetalsulfide,Awr:awaruite,Os–Lrt:osmium-richlaurite,Cc:chalcocite.


Fig. 6. Compositional variation of laurite intermsofRu–Os–Ir(at.%).Thesolidlinedis-criminatesbetween thefieldsof lauriteanderlichmanite,andthedashedlinerepresentstheRu:OsatomicratioofC1chondrite(afterNaldrett&Duke 1980). Symbols: Blacktriangles:lauritefromMilia,greytriangles:lauritefromKorydallos.


(Economou-Eliopoulos&Vacondios1995,Economou-Eliopouloset al.1999).AccordingtoZhouet al.(1998),themeltsinequilibriumwithCr-richchromitedepositsareofboniniticaffinity,whereas those inequilibriumwithAl-richchromititesareclosetothecompositionofback-arcbasinbasalts(BABB).MeltswithaboniniticaffinityareconsideredtocontainH2Oandareformedintheisland-arcmantle,whereasBABB-typemeltsareformedinnascentspreadingcenters,suchasback-arcbasins(e.g.,Zhouet al.1998).ThedepletednatureofthePindosperidotites,combinedwiththepresenceofMORB–IATandboniniticdykesemplacedinvolcanicrockswith pureMORB affinities in the upper partsof the complex, attest to the generation of hydroustholeiiticandboniniticmelts(producedbysecond-stage

melting), respectively, from thePindos subophioliticmantle(Kostopoulos1989,Bizimiset al.2000,Pe-Piperet al.2004,Saccani&Photiades2005).TheprogressiveevolutionfromCr-richtoAl-richchromititesrecordsthetransitionfromanislandarcthatevolvestoanascentback-arcbasininasuprasubduction-zonesetting.

Inthattransitionalenvironment,Cr-richchromititessuchas thoseof theareaofMiliacrystallizedfromameltproducedbyagreatdegreeofmantlemelting(asisindicatedbythehighCr#ofmagnesiochromite),whichdilutedtheresultantmagmainPtandPdandenricheditinthehighlyrefractoryIPGE(Prichardet al.2008).On the other hand, the high-Al chromitites formedfrom amelt produced by a lower but critical degreeofmelting, thus allowingPt andPd extraction fromthemantle (Prichardet al. 2008).The resultantmeltlocallybecamesaturatedwithS,andminoramountsofimmisciblesulfidemeltinthesilicatemagmaefficientlyscavengeddrastically itsPGEcontent (especially thestronglychalcophilePtandPd),leadingtotheformationofthelocallyPPGE-enrichedchromititesofKorydallos.AccordingtoKeays(1995),high-MgmagmasremainS-undersaturatedduring their evolution, because theyare high-Tmagmas produced by large degrees ofmeltingofamantlesourcealreadydepletedbyearlierepisodes ofmelting,whereas tholeiitic-typemagmascanbecomeS-saturated.Inaddition,Zhouet al.(1998)pointedout that boninites are originally richer in theIPGEthaninthePPGE,whereasMORB-likemagmas


are generally poor in PGE and other chalcophileelements,becausetheyhavealreadylostmostofthemby the time theyreachedS-saturation.TheIPGE-richcharacteroftheMiliachromititesandthelocallyPPGE-and sulfide-rich nature of theKorydallos chromititesareinagreementwiththeirformationattheexpenseofboninitic-andtholeiitic-typemelts,respectively.

Origin of the primary PGM assemblage

ThereisnowabroadconsensusthatPGMinclusionsinchromiterepresentearly-formedmagmaticminerals(e.g.,Stockman&Hlava1984,Augé1988,Ferrario&

Fig.7. CompositionalvariationofalloysintermsofRu–Os–Ir(at.%).Thenomenclatureandmiscibilitygap(greyfield)definedbynaturalOs–IralloysareafterHarris&Cabri(1991).


Garuti1990,Torres-Ruizet al.1996)ratherthanphasesexsolved from the host chromite during subsoliduscooling.Infact,suchinclusionsareconsideredtorepre-sentearlyprecipitatesfromthemeltbeforeorduringtheprecipitationofchromite(s.l.).ThecommoneuhedralshapeofPGMinclusionsinchromiteandtheirsporadicdistributionevenwithinthesamechromitepodindicatethat they represent high-T phases on the liquidus ofprimitivemelts.As they are considerednear-liquidusmineralstrappedinthegrowingchromitecrystals,theirconditionsofcrystallizationshouldbesimilartothoseofchromite.

The appropriatemechanism for the direct forma-tionofPGMfroma silicatemelt hasbeen explainedby the “metal clusters”model proposed byTredouxet al.(1995).Accordingtotheirmodel,thePGEtendto coalesce intometallic clusters in the silicatemelt.These polyatomic aggregates of PGE act as nucleifor other early-formed phases like chromite (andolivine).DependingonthephysicochemicalconditionsprevailinginthemeltatthetimeofPGEcoalescence,especiallyTandf(S2),alloysorsulfidesmayformpriortochromitecrystallization.

According to certain experimental studies (e.g.,Brenan&Andrews2001,Andrews&Brenan2002)at

highTandlowf(S2)conditions,arelativelyoxidized,low-FeO silicatemeltwill crystallize an alloywith arutheniridosminecompositionpriortoRu-richlaurite.By increasing f(S2) and decreasingT, the two-phase(alloy and sulfide) stability fieldwill expand,withbothlauriteandalloydissolvingmoreOsandIr.Anyfurtherincreaseoff(S2)ordecreaseofT(orboth)willpromote the precipitation ofOs-rich sulfide (namelyerlichmanite)becauseOspartitionspreferentiallyintothesulfidephasewithfallingtemperature.

Grammatikopoulos et al. (2007) showed that theprimaryPGMassemblageofthechromititesfromtheMilia area is dominated by laurite and not by IPGEalloys and sulfarsenides (e.g., Prichard et al. 2008),followedbyOs–Ir–Ru alloys.The present analyticaldata suggest thatRu-rich alloys precipitated duringan initial stage of crystallization characterized byhighTandlowf(S2),followedbyRu-richlauritewithincreasing f(S2).At amore advanced stage at higherf(S2)orlowerT,alloyandlauritestartedtoaccommo-datemoreOsandIr,whereastheRucontentofthemeltwasexclusivelyconsumedbytheformationoflaurite.Atafinalstageofevolution,anincreaseoff(S2)allowedthe precipitation of erlichmanite,whereas the forma-tionofOs–Iralloyshadalreadyceased.Althoughthe

Fig.8. CompositionalvariationofsulfarsenidesintermsofRh–Pt–Ir(at.%).


Ru–Ossulfidesanalyzedinthepresentstudyshowanapparentcompositionalgapbetweenlauriteanderlich-manite, the coexistenceofRu-rich alloy,Os–Ir alloy,lauriteanderlichmaniteinthePGMassemblageoftheMiliachromititesindicatescrystallizationunderawiderangeofT–f(S2)conditions.Therefore,thecoexistenceofRu-rich alloys,which are considered indicators ofextremely low f(S2) (Nakagawa&Franco1997),anderlichmaniteindicatesthatPGMcrystallizationspannedawiderangeofT–f(S2)conditions,startingathighTandprogressingtorelativelyhighf(S2)approachingtheOs–OsS2buffer(Fig.9).

The PGM of Milia chromitites compared to PGM from other chromitites

The common presence of both IPGE alloys andsulfidesasthemajormineralconstituentsofthePGMassemblageofophioliticchromititesindicatesthattheir

formationtookplaceunderarelativelywiderangeofT–f(S2) conditions.Themajority of ophiolitic chro-mititesworldwide ischaracterizedby thecoexistenceof alloys and sulfides of laurite type. Such occur-rences include theVourinos chromitites,Greece andtheTiébaghi ophiolite,NewCaledonia (Augé 1985,1988),ThetfordMines,Quebec,Canada (Corrivaux& Laflamme 1990), Samar ophiolite, Philippines(Nakagawa&Franco1997),Omanophiolite(Ahmed&Arai 2003),Kraubath ultramaficmassif,Austria(Malitchet al.2003),Ortacaultramaficmassif,Turkey(Uysal et al. 2005),Mayarí–Baracoa ophiolitic belt,easternCuba(Gervillaet al.2005)andtheKopultra-maficcomplex,northeasternTurkey(Uysalet al.2007).

The PGMassemblages that have been identifiedin the chromitites from two other localities in thePindosophiolitecomplexarecompletelydistinctfromthePGMparagenesishosted in theMiliachromitites.Grammatikopoulos et al. (2007) reported a PGM

Fig.9. Metal–sulfideequilibriumcurves forRu,Os, Ir andNiasa functionof sulfurfugacity,expressedaslogf(S2)andtemperature(T);modifiedafterStockman&Hlava(1984),Melcheret al.(1997)andreferencestherein.GreyfieldindicatestheestimatedT–f(S2)conditionsforthecrystallizationofRu–Os–Iralloys,lauriteanderlichmaniteinthechromititesofMilia.


were transported to the uppermantle, and extremelyhigh estimatedmelting temperature (>3000°C in thedeepmantle) inferred by extrapolating binary phase-diagrams(Os–Ir,Os–Ru,andIr–Ru)toanassumedbulkcompositionoftheinitialmeltcombinedwithapressurecorrectioncorrespondingtolowermantle–outercoreconditions.More recent studies on themineralogicalcontentofmantle-hostedophioliticchromititesprovideconfirmation of the view that IPGE-rich alloys formas early cumulates during crystallizationof primitivemagmasunderconditions typical in theuppermantle(e.g.,Talkingtonet al.1984,Garutiet al.1999,Matveev&Ballhaus 2002,Brenkeret al. 2003).AmagmaticoriginisstronglysupportedbytheintimateassociationofIPGE-richalloyswithchromite,bothasahostphaseandasinclusionsinmassiveandnodular-texturedchro-mitites,thecommonoccurrenceofinclusionsofolivineand pyroxenewith typical compositions ofmantleassemblages in IPGE-based alloys, and the observedhigh-Texsolutionpatternsinthesealloys(e.g.,Shiet al.2007).Inaddition,thesimilarrangeofvariationin187Os/188OsvaluesamongIPGE-richalloysandmassivechromitites favorsamagmaticoriginfor thesehighlyrefractoryalloys(Brenkeret al.2003).

Recently, Luguet et al. (2007) established thatmm-size PGM, such as Ru–Os±Ir sulfides and Pt–Ir±Osalloys,likelylocatedintheintergranularmatrixofdepletedharzburgites,account for50–100%of thebudgetofhighlysiderophileelementsintheseperido-tites.ThePGMhavebeen interpreted to be residual,having formed by complete consumption ofmantleBMS under a high degree ofmelting (23–24%). Inaddition, Shi et al. (2007) discovered residualRu–Os–IralloysintheDongqiaochromititesinTibetthatappearedtohavebeenscavengedfromthelithosphericmantleduring themelt–rock reactionprocess respon-sibleforchromititeformation(Zhouet al.1996).

Therefore, although themagmatic origin ofOs–Iralloys from theMilia chromitites is undoubted, theorigin of some alloys of the second compositionalgroup,withtheelevatedRhandPtcontentsisnotclear.Grainsofthesealloysareeuhedralinshapeanddonotpresent any textural or compositional characteristicsindicativeofasecondaryorigin(i.e.,porousstructure,rugged andmottled surfaces, inhomogeneities).Theyare also characterized by very low concentrations ofBMimpuritiesandalmostcompleteabsenceofS,Asandother elements that are commonly interpreted tohaveahydrothermalorigin(i.e.,Sn,Te).Itisintriguingthat in primitive-mantle-normalized PGE patterns,theseRu-richalloysdonotshowtheexpectedsteppedprofilesattributed to fractionationbetweenOs, Irandthe other PGE. In fact, someRu-rich alloys exhibitnormalizedPGEpatterns(Fig.11)thatarecharacterizedbyacontinuousdecrease fromOs towardPd,similartoresidualmss(monosulfidesolid-solution)inmantle(Lorand&Alard2001)andabyssal(Luguetet al.2001)peridotites.The elemental signatures of these alloys

assemblagedominatedbyanduoite,lauriteandruarsiteintheCr-richchromititesfromPefki,whereasPrichardet al. (2008) andKapsiotiset al. (in press) reporteda PGMassemblage dominated byPd–Pt alloyswithbasemetals in theAl-rich chromitites from the areaofKorydallos.The predominance of diarsenides andsulfarsenidesinthechromititesfromPefki,combinedwiththeabsenceofIPGE-richalloys,indicatethatf(As)andf(S2)conditionsratherthanf(S2)andThaveplayedthemostimportantroleduringtheformationofPGMatPefki(Kapsiotiset al.,inpress).Ontheotherhand,theassociationofPt-andPd-bearingPGMwithbasemetalsatKorydallossuggests that thesePGEwerecollectedbysmallquantitiesofanimmisciblesulfideliquidthatseparated from the tholeiiticmagma (Prichard et al.2008,Kapsiotiset al.,inpress).ThedifferencesinthePGMassemblages hosted in the Pindos chromitites,combinedwith thewidevariations in compositionofthechromitedeposits throughout thecomplex, reflectdifferencesintheparentalmeltsofthechromititesandinthedegreeofmantlemelting.Inaddition,thedissimi-larities in thePGMassemblagesbetween thehigh-Crchromitites (MiliaandPefki)of thecomplex indicatethatthemantlesourcefromwhichtheirparentalmeltsformedwasrelativelyinhomogeneous.

In terms of laurite composition, our chromititesbear a resemblance to the chromitites from easternCuba(Gervillaet al.2005)and theOrtacaultramaficcomplex, Turkey (Uysal et al. 2005) (Fig. 10A).Hydrousmaficmeltsgenerated ina suprasubduction-zonesettinghavebeen linked to the formationof thechromitites from those two latter areas. In addition,lauritecompositionsinourchromititesamplesfromthePindosophiolitecomplexdifferfromthoseintheotherophioliticoccurrencesinGreece.Forexample,lauritecompositions from theMilia chromitites are distinctfromlauritefromtheVourinoschromitites(Augé1985,1988,Garuti&Zaccarini1997,Kapsiotiset al.2010),theOthrys chromitites (Garutiet al. 1999),Rhodope(Tarkian et al. 1991) and Skyros chrome deposits(Tarkianet al.1992;Fig.10B).LauritefromtheMiliachromititesspanstherangeofOs-for-RusubstitutionoflauritefromVourinos,RhodopeandSkyros,butshowsamore restricted rangewith respect to that from theOthrysmassif.

Implications on the origin of the IPGE-rich alloys

EarlyscenariosproposedfortheformationofIPGE-richalloyspurported that they formedunder extremeP–Tconditionsinthelowermostmantleorintheoutercore, andhavebeen incorporated asmicroxenocrystsintotheupperconvectivemantlebydeep-rootedmantleplumes (e.g.,Bird&Bassett 1980).These inferenceswerebasedontexturalobservationsofcomplexinter-growthsofPGEalloysandaninterpretationthatthesealloys crystallized fromamelt, ahypothesizedphasechangearguedtoreflectpressurereleaseasthealloys


suggest that theymay represent breakdownproductsof residualsulfides in theperidotitesduring thechro-mitite-formingmelt–rock reaction process prevailingintheuppermantle.Becauseoftheirhighlyrefractorycharacterandtheextensiveoverlapofdorbitalsinthemetallicbonds,phasesurfacesofthesePGMwouldbeverydifferentfromthemorerigidcovalentstructuresinthesurroundingsilicatemelt,andsotheinteractionbetween these PGM and themelt would be veryrestricted. In fact, theywill remain unmodified evenafterexperiencingseveralepisodesofmantlemelting.However,owingtoacompletelackofOsisotopicdata,theresidualoriginofthesealloyscannotbepositivelyconfirmed.

Origin of the inverse zoning in laurite and conditions of alteration

As inferred above, theOs content of laurite is adirect indicator of theT–f(S2) conditions prevailing

Fig. 10. Comparison of laurite compositions fromMilia chromititeswith those from(A)theeasternCubaophiolites(Gervillaet al.2005)andtheOrtacaultramaficmassif(Uysalet al.2005) in the ternarydiagramRu–Os–Ir (at.%),and from(B)Rhodope(Tarkianet al. 1991),Skyros (Tarkianet al. 1992),Othrys (Garutiet al. 1999) andVourinos(Augé1985,1988,Garuti&Zaccarini1997,Kapsiotiset al.2009,2010).Symbols:Blacktriangles:lauritefromMilia,greytriangles:lauritefromKorydallos,greycircles:opencirclesandopentrianglesrepresentlauritefromVourinosgivenbyAugé(1985,1988),Garuti&Zaccarini(1997)andKapsiotiset al.(2010),respectively.

Fig.11. PGEpatternsofselectedRu–Os–Iralloys(anal.M4andM7)fromMiliachromitites,normalizedtoprimitivemantle(McDonough&Sun1995).


in the silicate magma during crystallization. Thesolubility ofOs in laurite increaseswith decreasingTorincreasingf(S2)(orboth).Sincef(S2)isexpectedto increasewith decreasingT in amagmatic system(e.g., Stockman&Hlava 1984, Ferrario&Garuti1990,Melcheret al.1997,Garutiet al.1999),lauritecanprogressivelyaccommodatehigheramountsofOs(+Ir).Ifchromitecrystallizationisdelayed,thechemicalexchangeofOsbetweenlauriteandthemeltisfurtherpromoted,givingrisetotheformationofzonedcrystalsoflaurite.However,lauritegrainswithanOs-richcoreandaOs-poorrim(reversezoning)havebeendescribedin the literature (e.g.,Barkovet al. 2004,Gervillaet al. 2005,Zaccariniet al. 2005,González-Jiménezet al.2009).

Twopossiblemechanismshave been proposed inthe literature to explain the second type of composi-tional zoning of laurite.Barkovet al. (2004) recog-nizedthatthezonedlaurite–erlichmanitegrainsdonotimply uniform conditions of crystallization,whereasGonzález-Jiménezet al.(2009)concludedthatzoningpatternsinlaurite–erlichmaniteinclusionsinfreshchro-mitereflectopen-ratherthanclosed-systemconditions.Rapidfluctuationsofmagmaticconditions,whicharemore likely tooccur inanopensystem,especiallyT,f(S2)andf(O2),canaccountfortheobservedrhythmiczones in laurite. Since sudden changes inT (of theorderof100°C) in theuppermantlearenot likely tooccur, fluctuations in f(S2) and f(O2) aremore likelyresponsible for the development of reverse zoning inthecrystallizinglaurite(Gervilla et al.2005).Changesinf(S2)andf(O2)maybecausedbymagmaminglingintheuppermantle(Arai&Yurimoto1994,Zhouet al.1996)orbyassimilationofSi-richrocksbyaprimitivemelt(Bédard&Hébert1998).Mixingoftwomeltswithdifferentactivitiesofsilica(onedifferentiatedandoneprimitive)inturbulentmagmasinconduitscanpromoteacontinuousincreaseinTanddecreaseinf(S2),whichcan explain the reverse pattern of zoning in laurite(González-Jiménezet al.2009).

Asanalternative,Zaccariniet al.(2005)proposedthat the commonly observed trend ofOs increasingfromcoretorimdoesnotrepresenttheresultofgrowthzoningduetoanincreaseoftheOspartitioncoefficientbetween silicatemelt and lauritewith decreasingT(Brenan&Andrews2001).Theysuggestedthatduringthepostmagmaticstage,primary lauritecanadjust itscomposition in response to the expected decrease off(S2).Undersuchconditions,lauriteexpelsOsandIr,whicharereadilyincorporatedintonewlycrystallizingphases.

Ourdatasuggestthatthesecondmechanismislikelyresponsible for the formationof zoning in the lauritecrystalsstudied.AspecificfeatureoftheselauritegrainsisthealternatingOs-richandRu-richzones,whichdonotseemtofollowcrystal-growthfaces,whichwouldprobably reflect themagmatic origin of the zoned

laurite.As illustrated in Figure 12, laurite is invari-ablyassociatedwithOs–Iralloysandrarelywithotherphases(e.g.,BMS).QualitativeanalysesshowthatthesealloysareenrichedinFe,andcanbeporousintexture.TheyarealsocommonlyattachedtotheOs-richpartofthezonedgrains(Fig.12),whichisnotinaccordancewith theequilibriumTand f(S2)conditionsforalloysand sulfides determined experimentallyby Brenan&Andrews (2001).TheBMS is rich inCu (namelychalcocite) and of secondary origin, probably afterchalcopyrite.Thus,we speculate that the compositegrains of primary laurite andBMS, found as inclu-sionsinmagnesiochromite,aretheprecursorsofthesecompositecrystals,inwhichlauriteischemicallyzoned.It seems that these precursorswere exposed to post-magmaticprocesses,probablyasaresultofextensivecrackingof thehost grains ofmagnesiochromite.Wesuggest that f(S2) decreasedduring the postmagmaticstage,thusenablingOsandprobablyIrtobereleasedfromtheprimarylauriteandtoformsecondaryalloys.However, the selective removal ofOs from laurite isquiteintriguing.Underthereducingconditionsusuallyprevailingduringsecondaryalteration,desulfurizationisthemostcommoneffectonsulfides.ThepreferentialremovalofOsisprobablycontrolledbytheprevailingEh conditions.Theoretical predictionson the relativestabilityofOscompoundsundervariableEh–pHcondi-tionsinthesystemOs–S–O–H(Brookins1987)showthatOsbecomesmoremobile thanotherPGEunderoxidizingconditions.Suchconditionsmusthavebeenreachedlocally,asthereisnodirectevidenceforhighactivityofO2duringthealterationofourchromitites.

The close association of zoned grains of lauritewith secondaryOs–Ir alloys indicates thatOs and Irwere notmobilized on a large scale.Consequently,secondaryprocessesdidnotchangethebulk-rockPGEconcentrationsignificantly.Prichardet al.(2008)alsosuggestedthatthemottledandinhomogeneousIPGE-bearingPGM(Os–Ir–Ru–Rhalloysandsulfarsenides)in theMiliachromititeshavereplaced in situoriginalprimary sulfur-bearing PGM.The predominance oflaurite as inclusions inmagnesiochromite and in theconcentratesleadstotheconclusionthattheprecursormineralwaslaurite,andthatithashaditssulfurcontentremovedduringalteration.

concLUsions

The investigation of theMilia chromitites of thePindosophiolitecomplexhasrevealedthattheyseemtohaveformedfromaboniniticmelt.TheirPGMassem-blageisdominatedbyIPGE-richphases,andmainlybylauriteandOs–Ir–Rualloys,withminorsulfarsenides.ThecoexistenceofRu-richalloys,Ru-richlaurite,Os–Iralloysanderlichmanite in thePGMassemblage indi-catesthatTand f(S2)conditionsvariedwidelybeforeandduringchromititeformation.TheextensiveOs–Ru


substitution shown by laurite reflects a significantincrease in f(S2), attaining theOs–OsS2 buffer, untilthe beginning ofmagnesiochromite crystallization.TheRh-andPt-richnatureofsomeprimaryRu–Os–Iralloysmaybeindirectevidenceoftheirresidualratherthantheirmagmaticorigin.Theinternallyzonedlauritecrystalsdocumentedinthisstudyaredirectevidenceofaninversionof thenormalT–f(S2) trendinmagmaticsystems,andhereattributedtopostmagmaticprocessesunderlocallyoxidizingconditions.

AcKnoWLedgeMenTs

This paper presents part of thePh.D. thesis ofA.KapsiotisattheUniversityofPatras,Greece.WethankProf.M.Economou-EliopoulosandDr.G.T.Nixonfortheirconstructivecriticismandhelpfulcommentsonthemanuscript.R.F.Martin isalso thankedforhisedito-rial review,which substantially improved the paper.We also thankBillKotsopoulos at theUniversity of

Patras for his helpwith the scanning electronmicro-scope.ResearchwaspartlysupportedbytheUniversityof Patras,Karatheodoris Project B097, toTAG.A.KapsiotiswasalsosupportedbytheStateScholarshipFoundationofGreece(IKY)duringhisPh.D.study.

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Received March 15, 2009, revised manuscript accepted Sep-tember 30, 2009.

CHROMIAN SPINEL COMPOSITION AND PLATINUM-GROUP ELEMENT MINERALOGY OF CHROMITITES FROM THE MILIA...

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