Concentrations of Platinum.Group Elements and Gold in Sulfides from the Strathcona Deposit, Sudburv, Ontario

523

Canadian Mineralogist Vol.30, pp. 523-531 (1993)

CONCENTRATIONSOF PLATINUM.GROUPELEMENTS AND GOLD IN SULFIDESFROM THE STRATHCONA DEPOSIT, SUDBURV, ONTARIO

CHUSI LI, ANTHOI\IY J. NALDRETT ANDJOHN C. RUCKLIDGE Departmzntof Geology,University of Toronto,Toronto, Ontario MSS 3BI

LINASR. KILruS IsoTracelaboratory, Universityof Toronto,Toronto, Ontarto MSS 1A7

ABSTRAc"T

Sulfide mineralsand magnetitefrom the Strathconadeposig Sudbury, Ontario, have beenanalyzed for the platinum-group elements(PGE) and Au with iz siar, ultra-sensitiveaccelerator mass spectrometry (AMS). $rrhotite containsk, Rh and Ru at levels closeto thoseofthe bulk sulfide ores (recalculatedto 1007osulfides), ranging from tensto more than 100ppb; Pt, Pd and Au are presentin the pyrrhotite at much lower levels than in the bulk sulfide ores. Among all the other minerals,including pentlandite,chalcopy'rite, cubanite, bomite and magnetite,pentlandite is the only phasethat accommodatessignificant amounts of Pd, with concentrationsin the rangefrom hundredsto thousandsof ppb. The concentrationsof the other PGE and Au are extremelylow in all ofthe sulfidesand magnetite in comparisonwith thevalues ofthe ores.They rangefrom severalppb to sub-ppb levels.Although pentlanditecontains significant amounts ofPd, its overall contributionto the ore ofthe DeepCopper Zone is not greatbecause the ore containsonly small amountsofthis mineral.Mass-balance calculations indicate that most ofthe Pt, Pd and Au in the ore is not presentin the sulfidesand magnetite,but mustoccur in discreteaccessory rninerals. The partition coefficients ofPGE benveenMss and sulfide liquid havebeen estimated from the compositionsofminerals and ore to be 6.5 x t0-3 to 4.6 l0-2forPt,6.8xl0-3to3.9x10-2forPd,4.7x10-3to8.0x10-2forAu,and5.gto6.Tforlr.Thevariationsintheconcenuations of ft and Ir in the depositcan be explainedby a processinvolving fractional crystallizaiionof monosulfidesolid solution (Mss) from a parentalsulfide liquid andmixing of varyingproportions of variablyfractionated liquid with Mss,if thepartition coefficients betweenMss and sulfide liquid are lessthan 0.1 for Pt andfrom 6.7 to 3.5 for Ir.

Keywords:platinum-group elements, partition coefficient, fractionation. sulfides, acceleratormass sp€ctrometry,Strathcona deposit,Sudbury, Ontario.

SoNrlaerns

Nous avonsanalysd les sulfureset la magn6titeprovenant du gisementde Strathconad Sudbury,en Ontario,pour les 6l6ments du groupedu platineetpourl'orparspectrom6trie de masse avec acceldrateur, m6thode rn silz ultra sensible.I-a pyrrhotitecontient h, Rh et Ru i desniveaux semblablesi ceux dansles €chantillonsde minerai (recalcul6ssur une basede I@Vosulfures), dans I'intervalle de dizainesi plus de l@ ppb. Par contre,Pt, Pd et Au sont pr6sentsdans la pyrrhotite i desniveaux beaucoup plus faiblesque dans le mineraiglobal. De tousles autresmin6raux, y comprispentlandite, chalcopyrite, cubanite, bomite et magndtite, seulela pentlanditepeut accommoderdes concentrations appr6ciables de Pd, dansl'intervalle de centainesi desmillien de ppb. la concentrationdes autres mindraux du groupedu platineet de l'or estextr6mement faible danstous les sulfureset la magn6tite en comparaisondes valeurs pour les 6chantillonsde minerai, de quelquesppb d des niveaux infdrieurs au ppb. Quoique la pentlanditecontienne des concentrations importantes de Pd, sa contributioni la constitutiondu minerai dansla zonedite "Deep Coppe/' n'est pasimportante. Des calculsdu bilan desmasses montrent que la plupart du Pt, du Pd et de I'Au dansle minerai se trouveraitnon dansles sulfuresni la magn6tite,mais plutdt sousforme demin6raux accessoires distincts. Le coefficientde partage des6l6ments du groupedu platineentre solution solide monosulfurde et liquide sulfurda 6t6estim6 i partir de la compositiondes mindrauxet du minerai: entre6.5 x l0-3 et 8.6 x l0-2 oour le PL entre6,8 x l0-3 et 3.9 x l0-2 oour le Pd- entre4.7 x 10-3et 8.0 2 x l0 pourI'Au, et entre5.9 et 6.7 pourI'lr. t-esvariations dans la concentrationde Pt et de Ir dansle gisements'expliquenr par un processusimpliquant cristallisationfractionn6e d'une solution solide monosulfur6eA partir d'un liquide sulfurd parent, et m6langeen proportions variablesd'un liquide variablementdvolud et du monosulfure,si les coefficients de partageentre monosulfureet liquide sulfur6sont inf6rieursa 0.1 pour le Pt, et entre6.7 et 3.5 pour I'Ir.

(Traduit par la Rddaction)

Mots-clds:6ldments du groupe du platine, coefficient de partage, fractionnement,sulfures, spectromdtriede masseavec acc6l6rateur,gisement de Strathcona,Sudbury, Ontario. 524 TTIE CANADIAN MINERALOGIST

INTRoDUCnoN results in the elimination of all moleculeswhose mass could matchthat ofthe selectedisotope. This procedure Knowledge of the distribution of platinum-group reducesthe backgroundto vanishinglysmall levels,and elements(PGE) in sulfideminerals is importantnot only thus givesextraordinarily good sensitivities for the trace for mineralprocessing, but alsofor a full understanding elementsbeing studied. The light group of isotopes, of the fractionationof theseelements during the crystal- l06Pd,I03Rh and lolRu,were analyzed in the 5+ charge lization of a sulfideliquid. hevious attemptsto establish state,energy 12 MeV, whereastheheaviergroup, le7Au, whetherthe PGE,other than thoseoccurring as discrete le6lt, le3lr and l88os,were analyzedin the 6+ charge platinum-groupminerals (PGM), also occur as dilute state,energy 14 MeV. Mean countrales were converted solid-solutionsin the mineralsfrom the Sudburyores, to elementconcentrations with referenceto the sulfide were made with the electron microprobe (Cabri & beadof the SARM-7 standard. Laflamme 1976) and proton microprobe (Cabi et al. The detectionlimits, quoted on the basis of equal 1984).Pt, Pd or Rh were not detectedin chalcopyrite, counting times of 100 seconds,are down to Au 0.003 pe[tlandite and pyrrhotite becausethe analyseswere ppb,I't 0.18 ppb, Rh 0.57 ppb, Ir 0.6 ppb, Pd 2.1ppb, limited by detectionlimits in the ranges30G-500 ppm Ru 4.6 ppb,and Os 48 ppb. (electron microprobe) and l-60 ppm (proton microprobe). Recently,iz siru analysisof sulfidesfor heavy Ne utron- activ ation analys is elementsby acceleratormass spectrometry (AMS) at the IsoTraceLaboratory" University of Toronto, has been PGE and Au were determinedwith a fire-assayand developedto thepoint at which samplescan be routinely neufon-activation technique modified after that of analyzedfor the PGE and Au at the ppb and sub-ppb Hoffrnan et al. (1978). Cu-rich ore samples were levels(Wilson et al. l99l).T\is techniquehas been used reducedto l0 grams, and additional Ni was addedto as a complementarytool to the electron and proton achievea ratio Ni/Cu greaterthan 3 in the fire-assay microprobesin the measurementof concentrationsof bead.The contentof sulfur in themixture was calculated PGE and Au in sulfide minerals from Sudbury (Li & to achievethe stoichiometricratio of NiS andCurS. The Naldrett 1990).As a follow-up to the initial AMS study, sulfide beadswere then dissolvedin hydrochloricacid, base-metalsulfide mineralsfrom different ore-zonesof and the residues were collected in plastic bags for the Strathconadeposit at Sudburywere analyzed.The irradiation.Samples were irradiated for threeminutes at selectedsamples of oresalso were analyzedfor whole- theNuclear Reactor Center of McMasterUnivenity, and rock PGE and Au using the technique of neutron then countedthree minutes for Pd and Rh after a decay activation combined with fue-assaypreconcentration. of threeminutes. For the other elements,samples were The distribution of PGE and Au in individual minerals irradiatedfor onehour andthen counted about 1.5hours and its significanceto the metal zonationin the deposit after one week decay in the Departmentof Geology, form the main subjectofthis paper. Universityof Toronto.Samples with low levelsof Os, Ir and Ru (<0.2 ppb) were countedanew for Os, Ir and Axarvrrcar TScHNIeIJ'Es Ru after approximately four weeks of decay. The dgtectionlimits (threesigma times the squareroot of the Acce le rator mass spe ctrometry background)are down to Ir 0.01,Au 0.02,Rh 0.5, Os 1.0,Ru 3, Pd 3 andft 5 ppb. Specimensused in this analysisare polished mineral surfaces.The preparationtechnique has been described Gsor-octcal BACKGRoUNDAmo Seuplrs SEtsglEn by Wilsonet al. (1991).Inclusion-freeareas of coarse- grained crystals selectedwith an optical microscope The Strathconadeposit is locatedon the North Range werefurther examinedby scanningelectron microprobe of SudburyIgneous Complex (SIC) (Fig. l). Theore lies to assessthe presenceof very small inclusionsof PGM. at the basalcontact of the complex and in the footwall Selectedtargets were then drilled out under water with (Fig. 2). The geologyofthis deposithas been described a hollow, diamond-edgedcore drill of 4 mm internal by Naldrett& Kullerud(1967), Cowan (1968), Abel et diameter at a speed of 4750 rpm. The cylindrical al. (1979),Abel (1981),Coaa & Snddr(1984) and Li "minicores"were washed and driedin air. A setof 12 et al. (1992). Four types of ore are observed.Dissemi- such cores, including standardsof pure copper and natedsulfides of the HangingwallZone occur within the SARM-7 in a sulfide bead,were loaded into the sample Sublayer norite. This zone is underlain by the Main chamber of the acceleratormass spectrometerand Zone, which consistsof massiveand disseminatedore alignedoptically prior to bombardingwith a 30 keV Cs* within the Footwall breccia.The DeepZone consistsof primary ion beam.Negative secondary ions were sput- stringers of massive sulfides that were emplacedin tered,ionized in or nearthe targetgrains, and extracted fractureswithin the footwall gneiss.Pynhotite, chalco- from the sample chamber at 20 keV. Electric and pyrite, magnetiteand pyrite are the major mineralsin magneticfiltering of the ions, followed by acceleration eachofthe precedingore-zones. Copper-rich stringers to 2 MeV andchange in chargefrom negativeto positive, containingchalcopyrite and cubanite (Copper and Deep PCIEAND Au IN SL'LFIDFS.STRATHCONA DEPOSIT 525

(Naldrett & Kullerud 1967, Cowan 1968, Keays & Crocket1970, Chyi & Crocket1976, Abel et al.1979, Naldrett et al. 1982\. Farrow & Watkinson (1992), however,suggested that the sulfide veins of the Deep CopperZone are ofhydrothermal origin becauseofthe occurrenceof hydrous alteration associatedwith the ores. Li et al. (1992) proposedthat the Deep Copper Zone is the end-product of the fractionation of an original sulfide magmagiving rise to the entire deposit, becausethe composition of the Deep Copper Zone follows the trend of the compositionalvariations in the deposit. They argued that the thin alteration-induced selvagesalong the sulfideveins represent typical contact 'Sr 0 smlle8 alterationsresulting from the emplacementof a sulfide Yl H 'dkn 6l o a liquid in the footwall. Most samplesin this study were collecled from the Deep CopperZone. Chalcopyriteis the most abundant ETTcrmptvre - Ffi cretss sulfide mineralin this zone.It occursas mosaic patches ilr,u"xo,r' li$B"tr El quruie |a MaficNode lcoMPtE( that usually encloseother sulfide and oxide minerals. la ud Sublayer Rets J Etr{ffitu" ffiE ctetncoarmaoo Volcadc R@k8 Coarsegrains are up to 3 mm across.Cubanite is the next fll onvadnromation -.-. Fadtg most abundantsulfide mineral.It occun as coarselaths m odapilgForEado! a Sulfi&Depdt up to 0.5 mm acrossin chalcopyriteor asmassive round Ftc. 1.Geological map of theSudbury region (after Pye et al. patches together with chalcopyrite. Ilnhotite and 1984). pentlanditeconstitute only a small proportion of the sulfides.Pyrrhotite occurs invariably as small inclusions in chalcopyrite or cubanite.Pentlandite is presentas massiveblocky crystalsup to 5 cm across.Bomite and Copper Zones), with minor magnetite, pentlandite, millerite areminor sulfides.Millerite is presentas coarse pynhotite andsphalerite, occur 100-500meters into the grainstogether with quartz,usually at the terminationof footwall. veins. Magnetiteoccurs tbroughout the sulfide assem- The Strathconadeposit has been interpreted as a blageas anhedral to euhedralgrains up to severalmm in typical magmaticsulfide depositby many investigators diameter.

Ftc. 2. Vertical cross-sectiontbrough the Suathconadeposit (6280 E, looking East). 526 TTIE CANADIAN MINERALOGIST

Additional sampleswere collected from theHanging- TABLE 1 CONCENIRATIONS OF METATSTIN TgE BULK ORES (lm% wall Zone for the analysisof pyrrhotite. The blocky, sulFtDEs) oFTUE STRATHCONA DEPOSIT coarse-grainedpynhotite, together with pentlandite, Sanple IIRuRbP!PdAUGMS magnetite,chalcopyrite and pyrite, are interstitial to H@glngeallZde silicateswithin the Sublayernorite. H90{ 34 80 n r25 9' 30 ll 02t 4.3 39.96 H9&9 43 100 r19 108 79 X 8 0.32 3.9 38.79 H9GIO 4694. t07 t36 47 t6 4 02b 4.2 3t.tt REsuLTs Aw6ge 39 ot tu t23 74 y I 02t 4.r 3921

The concentrationsof PGEand Au in the ore samples D€€.pcoplgTt@2 0.ll 47t9 5213 296 26.2t 2.2 32.40 are given in Table l, and their concentrationsin 1007o I POB ud Au e b ppb, C!, M ed S @ tn fl pe@t 2 sulfidesare listed in Table 2. The samplesselected from Ile @@trali@ of PCE @ the arenge val6 of 32 typtal trciw slide G gim by U .t al (1992),tie cm@mdm of Nl Cu ed S @ Ue arengE val6 of 1577|rcire the Deep Copper Zone for the AMS single-crystal $nnde @ sralyred by Fatenbddge Lrd. analysisare enrichedin one particular mineral. Those samplesenriched in accessoryminerals such as millerite (R-3, R-68, L18,L23 andP-4) (398-3) andbornite ous distribution of these elementsin different grains. have unusualcompositions with respectto the typical When comparedwith the concentrationsof theseele- massiveores of the Deep CopperZone (Table 2). The mentsin the bulk sulfide ore,ft, Pd andAu in pyrrhotite averageconcentrations of PGE and Au in the sulfide are very much lower, whereasOs, h, Rh and Ru in fraction of the Deep Copper Zone given by Li et al. pyrrhotite areclose to the levelsin the bulk ore. (1992)we usedin the following discussionwhere t}re compositionof the zoneis concerned. Pentlandite The concentrationsof PGE and Au in mineralsare listed in Table 3. The valuesin parenthesesin Table 3 As in pyrrhotite,inhomogeneous distribution expressthe ranges offour analyses.They vary from less of PGE in different grains are also presentin the than l07o up to 10Voof the means.The variationsare samplesof pentlanditefrom the Deep Copper primarily attributedto the inhomogeneousdistribution Zone. Pentlandite of thePGE in a singlegrain. containssignificant amounts ofPd, rangingfrom 33I to 2088ppb. These values, however, are less than one half Pyrrhotite ofthe valuesofthe bulk sulfide ore.Ru wasnot detected in any ofthe grains.Os and Rh are abovethe detection limits in only one grain. Half of the sampleshave Ir Au and all of the PGE except Os were detectedin higher than the detectionlimits. Except for Pd, all the pyrrhotite. The concentrationof Os in most grains of other elementsin pentlanditeare very much lower than pynhotite is below the detectionlimit; grain only one their levels in the bulk ore. was found to containmeasurable Os at 52 ppb, slightly higher than the detection limit. Tbree grains were Chalcopyrite,cubanite, millerite, bornite analyzedin eachof two samples(H9G-9 and H90-10). and lnagnetite The variationsof the PGEamong different grainsftrnge from4Voup to morethan 507o, indicating inhomogene- No significant amounts of PGE or Au have been measuredin anyof thoseminerals from theDeep Copper Zone. Ru, Rh, k and Os in all of the samplesare lower TABLE I. CONCENTRATTONSOF PGB AND BASE METAIJ INT1IE ORESOFTHESTRATHCONADEFOSTT than the detectionlimits. t evelsof Pd in theseminerals arein therange 4 to 4 1 ppb.ft rangesfrom less S!6DI9 o3lrRuRnR Pd Au MCU than0. I 8 to 5.6 ppb"and Au, from 0.07to 58 ppb. These Hegingul Zo@ values H90-t 15.4 37 39.9 58 45 l4 5 1.98 0.12 t8.l are much lower than those of the bulk sulfide ore, for H9S9 t7.4 40 47.3 43 32 23 3 l.t 0.0t r53 which typicalvalues are 4719 ppb ft, 5213ppb Pd and H9GIO 6 145 33 12 3 /C7 0.M 21,3 296 ppb Au. D€p Copler Zoe R-3

TABLE 3. CONCENTRATIONSOF PCIEAND Au IN SULI'IDFS AND MAGNETITEFROM TTTE STRATT{CONA DEPOSIT

Sanple

Po H90-8 9.0(5.1) 0.14(0.03) 8l (8) nd 6.E(l:7) 109(3) 206(r9) H90-9 6.0(1.4) 0.15(0.01) 6r(8) nd 4.8 (l.E) 54 (3) 114(3) 8.3(2.8) 0.2(0.0t s3 (8) nd 7.3(4.0) 53 (10) 154(12) Average 7.2(1.6) 0.lE(0.04) 57(6) nd 6.1(1.8) s3.5(0.5) 134(28) H90-10 0.6(0.16) 0.15(0.03) 105(4)52(21) 3.7(1.1) l7l (5) 114(26) nd 0.27(0.0E) 95 (17) nd 5.7Q.6) 138(3) 154(26) 0.4(0.06) 2.86(r.34) I (4) nd 5.9(3.2) s8 (14) 170(19) Average <0.5 l.l(1.5) 100(t <52 5.1(1.2) rz2(s8) r46Q9) Average (Po) <5.6 0.47(0.54) 79 (n) <52 6.0(o.e) 95 (36) 162(39)

Pn 89&5 0.54(0.58) 3.1(0.6) nd nd 311(122) nd nd R-3 3.1(0.9) 2 (0.3) nd nd 1502(nq nd nd R.68 0.7(0.04) 1.2(0.01) nd nd 2088(167) nd nd 2.2(1.0) 1.3(0.9) nd nd 1126Q99, nd nd 1.5(0.14) 1.3(0.15) nd nd 77r (Y5) 0.6(04) nd Average (Pn) l.6l (1.1) 1.78(0.8) nd nd ll70 (680) <0.6 nd

Mil L-18 0.86(0.23) 0.1(0.02) 0.9(0.5) nd 36 (9) nd nd L-23 0.9(0.2) 58 (12) nd nd 20 (8) nd od P4 0.16(0.02) 7.21 nd nd 9ndnd 1.2(0.3) 6.0(0.6) nd nd r5 (s) 0.7(0.2) nd Average(Mil) 0.78(0.4) 17.8(n) <0.9 nd 20 (12) <{.7 nd

Ccp 898-6 4.1(0.6) 14.8(1.9) nd nd 4l (12) nd nd R-4 5.6(0.3) l5 (0.4) 5.E(1.5) nd 28 (5) 0.7(0.2) nd R-10 1.2(0.2t 3 (0.2) nd nd n6) nd nd R-15 l.l4 (0.08) 0.3(0.02) nd nd s (2) 0.6(0.3) nd 0.46(0.26) 0.72(0.21)1.4(0.3) nd 23(4, nd nd Average(Ccp) 2.5(2.2) 6.8(7.s) 4.6 nd 25 (13) <0.65 nd clb R-15 0.25(0.05) 0.46(0.08) nd nd 20 (10) nd nd Mag MT-l nd 0.07(0.02) nd nd 4(3) nd nd Bn E9E-3 1.24(0.06) 0.09 nd MDLt 0.18 0.003 0.6 48 tl

AI elementsin ppb. (+/-): ra4es of concentration;MDL: dstectionlimiq nd: not d€tected"Po: pynfiotit€ Ccp: chslcopydteiPn: pentlandite;Cub: cubanitsi Bn: bomitq Mil: millerite; Mag: magnetits. have beenestimated to be 88.4, 10.8 and 0.8 wt.7o, two neighboring deposits, the Levack West and (ftTq), respectively.Using averagevalues for both pyrrhotite Coleman"indicate that Pt occursas moncheite calculation insizwaite and niggliite (ftSn) (Cabri & and for the bulk sulfide ore, a mass-balance '1976).ItGtBi) indicatesthx l\OVo of the Ru, 77 Vo of the lr, and68Vo Laflamme is likely thatPt in the Hangingwall phases' of the Rh occurin pyrrhotite.Qnly 77oof the Pt, 1'67oof Zoneat Strathconaalso occurs primarily asthese the Pd and 5Voof the Au in bulk ore can be accounted for by pynhotite. Becausethe samplesare apparently PGE distribution in the Deep CopperZone free of drscretemicrograins of PGM at the resolutionof an SEM analysis,it is likely that thesePGE in pynhotite Ir, Os, Rh andRu in both the sulfide mineralsand the occur either in solid solution or as submicrometer-size bulk oresin the DeepCopper Zone were not detectedby exsolved particles similar to the particles and atomic either AMS or neutron-activationanalysis. Among the clusters of gold describedas occuring in pyrite by other elements,only Pd is detectedat significantlevels Bakkenet al. (1989).Only smallamounts of Pt' Pd and in pentlandite.Concentrations of Pt and Au in all the Au occur in the pynhotite. The majority of the ft and minerals are extremely low in comparisonwith their Au in the bulk ore must occur as discrete precious concentrationsin the bulk sulfide ore. The composition mineralsor in other phases.No detailedmineralogical of the Deep Copper Zone, calculatedfrom results of investigationhas been carried out in this zone.Data from 1577analyses ofmassive ore (analyzedby Falconbridge 528 THE CANADIAN MINERALOGIST

Ltd.),is 26.2lVoCu,2.l77o Ni and32.357o S. If all the content of Pt in pyrrhotite may not represent the Ni occurs as pentlandite,this amountsto 5.7 wt.Vo concentrationin Mss at the time of its crystallizationbut pentlandite.This is a maximum value becausethe ore may well be low. Therefore,the partition coefficient is also contains minor amounts of another Ni-bearing a minimumvalue. mineral,millerite. Although pentlanditeaccommodates If, on the other hand, the presentPt content of the appreciableamounts of Pd, its contributionto the bulk HangingwallZone is takento be that of the originalMss ore is not great becauseof its low concentration.A (7a ppb),the partitioncoefftcient would be 8.6 x 10-2. mass-balancecalculation indicates that only 1.37oof the This suppositionalso representsa simplification, since Pd of the ore occursin pentlandite.More-than 98Va of the sulfidesof the HangingwallZone are likely to have the Pd, as well as almost l00Voof the Pt and Au, must startedas a mixture of original Mss andtrapped sulfide occur in other phases.A detailedmineralogical investi- liquid. Sincethe partitioncoefficient is lessthan one in gation has shown that much of the Ft and Pd occur as either case, the trapped sulfide liquid would have discreteminerals involving Te, Bi, Sn and As in containedmore Pt thanthe Mss. so that the Pt contentof microftacturesand along grain boundariesof sulfide the Mss obtainedin this secondway is a maximurq and mineralsin theDeep CopperZone (Li & Naldrettl993l. so is the partition coefficient.Thus the actual value of the coefficient, Dp, (the concentrationof ft in Mss Partitioning of PGE betweenMss and suffideliquid divided by the concentrationof ft in the liquid), is between8.6 x lf2 and6.5 x 1fr3.The formerseems ro If we acceptthe fractional crystallizationmodel for agree with the experimentally determined partition the grosszoning of the Strathconadeposit (Naldrett ar coefficientfrom the systemFe-Cu-ft-S at 900'C, from al. 1982,Li et al. 1992),wecan calculate the composi- 0.2to 8 x l0-2 (Makovickyet at.1986). tion parental of the sulfide liquid giving rise to the Application of a similar reasoningto Pd, Au and Ir deposit from the ore reservesand compositionsof the leadsto the partition coefficientslisted in Table 5. The different ore zones in the deposit. The results of the partition coefficientsof Pd andAu are similar to that of calculationsare given in Table4. ft. The value of Dt. is more than 5. The partitioningof PGEbetween monosulfide solid solution(Mss) and sulfide liquid canbe estimatedin nro ways.First, let us considerft. If the presentPt content Modelfor the origin of the ore zonation of thepyrrhotiteintheHangingwall Zone (5.6 ppb) were that of the original Mss crystallizing from the parental The HangingwallZone has been ascribed to theresult sulfide liquid, the partition coefficientwould havebeen of gravitationalsettling of an immiscible sulfide liquid 6.5 x l0-3. However,the experimentaldata of Mako- within the Sublayernorite, andits subsequentfractional vicky et al. (1986) have shown that a considerable crystallizationto yield Mss cumulateand a fractionaied amountof PGE can enterinto the Mss at high tempera- sulfideliquid (Naldrett& Kullerud1967 ,Naldrett et al. ture and exsolve at lower temperature.The present 1982).The ore zonesin the footwallare considered ro

TABLE 4. CONCENTRATIONSOF PGEIN TIIE STRAT}TCONADEFOSM

Ore re.serve Massofsulfides proDortioa ElementslCn 100%sulndes) Ore zong (tonnex 106) (toonex 106) ofsullides Ir Pt Pd Au

Ni Zone2 3t.226 14.01 90?o l5 447 4r0 7E CopperZone3 0.n6 0.28 2% 0.08 3927 4800 2n .DeepCopperZonC 7.6n t.22 8% 0.ll 4719 52t3 296 StrathconaDeposit 15.51 lA0Vo 13.5 858 882 100

I All elementsare in ppb. 2 Ni Zone = HBnginW8[ Zone + Maitr Zone + Deep Zone; their proponions are hom Cowan (1968), the concentrarionsof PGE in the sulfide fractionsof rhe Main and Deep Zonesare from Nddrc0 er 4L (1982),and thoseof rhe Hangingwal Zone from this studyl ore reservesare from CJ.A. Coats (supednt8ndenl,Falconbridge Ltd., SudburyOperation, personal comm.). 3 Ore rcserveis ftom Abel (1981);rhe concentralionsof FGE in the sulfide fracrion of the ore arc froml"i et aL (1992t. a Ore reserveis from FalconbridgeLd. (Intem.Rep., l9E6); the concantrationsofFGE in the sulfido fraction ofthe orc arc fuoml.i et aL (1992\. PGEAND Au IN SULFIDES,STRATHCONA DEPOSIT 529

TABLE 5. PARTTIIONINC OF PCiEBETWEBN Ms AND SULFIDE UQUID tie-lines are not proportionalto their weight volumesin the mixture. For instance,point M (representingthe lies Id@d p@tal lquidr 13.5 E5E 882 100 Main Zone)in the shadedparallelogram, although it Pynhodte (Po) 5.6 6 0.41 closeto the middle of the tie-line (F = 60), actually H8ryilgsal Zo@ (gZ) 9l 7434E representsa mixture of 68VoMss and 32Vobquid. T\e DPafi@rl 5.9 6.5X lO3 6.8X 10.3 4.7 X t0:3 model with higher D1 (unshadedparallelogram) ac- D Izllatdd 6,7 8.6xto2 3.9x102 E.ox102 counts better for the compositionof the Hangingwall Zone. The Main and DeepZones can be interpretedas . A[ elemera e h ppb (s Table 4 fu @lcdsti@). the mixtures of cumulateMss with liquid. The Copper and Deep Copper Zones can be interpreted as the havebeen emplaced by injection ofthe residualsulfide residual liquid after abott 857oof crystallizationwith liquid into fracturesin the footwall (Naldretter al. 1982, resp€ct to their Pt contents,but their Ir contentsare Li et al. I 992).The valuesof Dp,and Dk estimatedabove higherthanthepredicted values inthe model.The model maybe usedto testthe hypothesis. with thelowerDo (shadedparallelogram) can satisfy the The compositionof the Strathconaore is such that compositionofthe Copperand Deep Copper Zones, but Mss would be the first mineral to crystallize,according it fails to explainthe concentrationoflr in the Hanging- to phaserelations in the systemFe-Ni-S (Kullerud eraL wall Zone. The problem with the content of Ir can be 1969).If the residualliquid was ableprogressively to eliminatedif Do is greaterthan 3.5 at a high temperature drain down into the footwall, perfectfractional crystal- and decreaseson cooling. Available experimentaldata lization could result. Irt us consideran incompatible on the PGE otherthan Ir indicatethat their solubility in element ft and a compatible element Ir. Figure 3 Mss decreaseswith temperatureQVlakovicky e/ a/. illustratesthe compositionalvariations of Mss and its 1986).If the solubilityof Ir in Mss alsodecreases with coexistingliquid during this process.The parallelogram temperature,the value of D1 would decreaseduring on the left is a model for a Da of 6.7 and a Dp, of 8.6 x crystallization. Itr2. The shadedparallelogram is anothermodel, for a In summary,if thepartition coefficientsbetween Mss D1 of 3.5 anda Dp,of 8.6 x 10-2.The compositionof and sulfide liquid are about 8.6 x 1fr2 for Dp,and from any mixture of Mss and its coexistingliquid is defined 6.7 to 3.5 for Dp the ore zonationin the Strathcona by thetie-lines (F) bet'weenthe coexisting phases. Notice deposit can be interpretedas the consequenceof frac- that both axesare drawn on a logarithmic scaleand that tional crystallizationof Mss from a sulfide liquid with the relative distancesof a composition point to the respectto the concentrationsof Pt and h of the ores.

€*l a ! 0.1

0.01

0.001 lm 10m 10000

Pt, PPb

Flc. 3. Fractionalcrystallization of Mssfrom a sulfide liquid. Remainingliquid @) is in wt. percent Solid circles labeled with H, M, D, C and DC are the projections of the Hangingwall,Main, Deep,Copper and DeepCopper Zones, respectively. The compositionsof the Main and DeepZones are from Naldrett et al. (1)32). 530 THE CANADIAN MINERALOGIST

CoNcLusroNs platinum-group elements from some copper-nickel depositsof the Sudburyare4 Ontario.Econ. Geol.7l, l. Mostof theRu, Rh andk in theHangingwall Zone I 1s9-1195. is locatedin pyrrhotite. Cuyr, L.L. & Cnocrrr, J.H. (1976):Partition of platinum, 2. Pentlanditein the Deep Copper Zone containsa palladium, iridium, and gold among coexisting minerals significant amountof Pd, but its overall contributionto from the Deep Ore Zone, Strathcona mine, Sudbury, ttle ore is not great,because only a minor amountof this Ontario.Econ. GeoI.7l, 119G1205. mineral is presentin this zone. 3. Chalcopyrite,cubanite, bornite and magnetitedo CoArs,C.J.A. & SNaron,P. (1984):Ore deposits of theNorth not containappreciable amounts of PGEand Au. Range,Onaping-Levack are4 Sudbury./n The Geology 4. Most of ft, Pd and Au in the Deep CopperZone and Ore Depositsof the SudburyStructure @.G. Pye, A.J. mustoccur as discrete grains of platinum-groupminerals Naldrett & P.E. Giblin, eds,).Ontaio Geol. Surv,, Spec, andgold. Vol.1.327-346. 5. The partition coefficiena of ft andIr betweenMss CowAN,J.C. (1968):Geology of the Strathconaore deposit. and sulfide liquid estimated from compositions of Can.Inst. Min. Metall.,Bull.6l(669),38-54. mineralsand ore arebetween 8.6 x l0-2 and4.7 x lf for Pt, Pd andAu, andbetween 5.9 and6.7 for Ir. Fannow, C.E.G. & WarrwsoN, D.H. (1992): Alteration 6. The variationsof ft and h contentsin the deposit and the role of fluid in Ni, Cu and platinum-group can be explained by a process involving fractional elements deposition, Sudbury Igneous Cornplex crystallizationof Mss from a parent sulfide liquid and contact, Onaping-Levackarea, Ontario. Mineral. Petrol. mixing of varying proportionsof variably fractionated 6.67-83. liquid with Mss if the valuesof Do andD,, betw*n Mss HoFEfr4AN,E.L., NALDRE"TT,A.J., VAN LooN, J.C., HANcocK, andthe sulfide liquid areabout 8.6 x 1fr2 and lessthan R.G.V.& MeusoN,A. ( 1978):The determination of all tlte 6.7,respectively. platinum group elementsand gold in rocks and ore by neutron activation analysis after preconcentrationby a AcrNowt-sDcsMEr.trs nickel sulfide fue-assaytechnique on large samples.ArnL Chim.Acta 102. 157-166. We aregrateful to Dr. G.C.Wilson forhis assistance (1970): precious with the AMS analysisand Dr. C.J.A. Coats for KEAys, R.R. & Cnocrr-r, J. H. A study of providing metalsin theSudbury nickel imrptive ores. Econ Geol.65, us data on the ore reserves.The paper has 438450. greatly benefittedfrom critical and constructivecom- mentsfrom Drs. M. Zientek, R.R. Loucks and, espe- Kur-mnuo, G., Yrnio, R.A. & Mou, G.H. (1969): Phase cially,S.-J. Barnes. Editorial suggestions and comments relationsin the Cu-Fe-S, Cu-Ni-S, andFe-Ni-S systems. from R.F. Martin also are acknowledged.This project Econ. GeoI.,Monogr. 4, 323-343. wasfinancially supportedby OGSgrant 326 to AJN. CL is gratefulto the H.V. Ellsworth GraduateFellowship in Lr, CHUsr& Nalnnrrr, A. J. (190): PGEsstudy at Sudbury: Mineralogy during the academicyear 1990-1991. their concentrationby sulfide-liquid fractionation..ln Geo- scienceResearch Programme, Summary of ResearchI 989- 1990 (V.C. Milne, ed.). Ontario Geol. Sun., Misc. Pap. REFERENcES tso.3746.

Anrq M.K. (1981):The structure ofthe Strathconamine copper - & - (ID3): Platinum-groupminerals from the zone.Can. Inst. Min, Metall., Bull. 74(826),89-97. Deep Copper Tnne of the Strathconadeposit, Sudbury, Ontario. Can M ineral. 31, 3144. -, BucHAN,R., Coars,CJ.A. & Pnnsrorr, M.E. (1979): Coppermineralizationin theFootwallComplex, Strathcona , CoArs, CJ.C. & JoHANNESSEN,P, (1992): mine, Sudbury,Ontario. Can Mineral. fi,n5-285. Platinum,palladium, gold and copper-richstringers at the Strathconamine, Sudbury:their emichmentby fractiona- BAKKEN,B.M., Hocunna, M.F., Jn., Mansnar.r-"A.F. & tion of a sulfideliqrid. Econ.Geol.87,1584-1598. TURNER,AIVI. (1989):High-resolution microscopy ofgold in unoxidized ore from the Carlin mine. Nevada.Ecorz. MAKovrcKy,M., Merovrcry, E. & Rosn-Hersel,J. (1986): Geol.M.171-179. Experimentalstudies on the solubility and distribution of platinumgroup elements in base-metalsulfides in platinum CesRr,L.J., BLANK,H., EL GoREsy,A., Lamewrn, J.H.G., deposits./n Metallogeny of Basic and Ultrabasic Rocks NoB[-trr{c,R., Szcoruc, M.B. & TRAXEL,K. (1984): (M.J. Gallagher,R.A. Ixer, C.R. Neary & F.M. Prichard, Quantitative trace-element analyses of sulfides from eds.).Inst. Min. Metall.,London (415-425). Sudbury and Stillwater by proton microprobe. Caz. Mineral.22.52l-542. NALDRFm,A.J.,INNES, D.G., SowA, J. & GonroN,M.P. (1982): Compositionalvariations within andbetween five Sudbury - & Lan-aMIr,tr,J.H.G. (1976): The mineralogy of the ore deposils.Econ. Geol. 77, 1519-1534. PGE AND Au IN SULFIDES,STRATTTCONA DEPOSIT 531

-& KwmRUD,G. (1967):Astudyof theStrathconamine WusoN,G.C., Kuus, L.R. & RUcKLIDGE,J.C. (1991): In situ and its bearingon the origin of the nickel-copper ores of analysisofprecious metals in polishedmineral samples and theSudbury district, Ontario. J. Petrol.8,453-531. sulphide "standards"by acceleratormass spectrometry. Geochim.Cosmochim" Acta 55, 2241-2251. Pvr, E.G.,NalnnE'rr, A.J. & GtnuN,P.E. (1984): The geology and ore depositsof the Sudburystructure. Ontario GeoI. Received August 9, 1991, revised manuscrtpt accepted Sun., Spec.Vol.l. September15, 1992.