Canadian Mineralogist Vol. 30,pp.289-301 (1992)

ORIGINOF -GROUP-MINERALNUGGETS INFERRED FROMAN -ISOTOPESTUDY

KEIKOHATTORI Ottawa-Carleton GeoscienceCentre and Departmentof Geology, University of Ottawa, Ottawa, Ontario KIN 6N5

LOUIS J. CABRI CanadaCentrefor and EnergyTechnology, 555 Booth St/eet,Ottawo, Ontario KIA OGI

ABSTRACT

Osmium-isotoperatios of individual PGM (platinum-group ) were determined lr sl/a using an ion microprobe. All samplesexcept for one came from placers associatedwith Alpine- or Alaskan-type ultramafic intrusions. Most Alpine+ype samples are Os-Ir-Ru alloys, and most Alaskan-type samples are Pt-Fe alloys (isoferroplatinum or tetraferroplatinum), which contain Os-lr-Ru alloys and /erlichmanite. Osmium-isotope measurementsfor the Pt-Fe alloyswere carried out on thoseOs-bearing inclusions. The Os-isotopevalues are similar betweencores and rims of PGM and among different inclusionsof different phaseswithin individual nuggets.No isotopic variations are observedin PGM that show profound chemicalzoning. The-lack o^!isotopic heterogeneiryin individual nuggetsprecludes a low-temperatureorigin for rhe nuggetsof PGM, 1876t71869,values of all samples showa narrow spread,ranging from 0.99 to 1.12,with most of the valuesbetween 1.00 and 1.06.These values fall within the rangeof l87Os/l86Osvalues of the mantle,suggesting essential derivation of platinum-groupelements (PGE) from the mantle without a significant contribution of crustal Os. The grains of PGM were formed in intrusions' weathered,eroded, and concentratedin placersby mechanicalprocesses. The Os-isotopicdata are consistent with the occurrenceof exsolulionlamellae of PGM, the inclusionsof pristineunwealhered olivine, and the lack of "foreign" mineral inclusions,unrelated to ultramafic rocks, within the nuggets.Os-isotopic values for a placer nuggetand PGM from chromitite in Urals also support the conclusionthat the placer nuggetwas derivedfrom the latter. The irregular shape and nonabraded surfacesof some nuggetsmay simply reflecr their durable nature and a short distanceof transporl from the erodedultramafic intrusions. Keywords;osmium isotopes, placer, platinum nuggets,platinum-group minerals, Alaskan-type, Alpine-type, ultramafic intrusions.

SoMMAIRE

Le rapport desisotopes 187 et 186de I'osmiumdans les min6rauxdu groupedu platine(MGP) a 6t6d6termind in sltu au moyen d'une microsondeionique. Tous les 6chantillonssauf un proviennent de graviers alluvionnaires associ6si desmassifs ultramafiques alpins ou de type Alaska.La plupart des6chanrillons de massifsalpins consistent d'alliagesde Os-lr-Ru, tandisque la plupart des6chantillons de massifsde type "Alaska" sont desalliages de Pt-Fe (isoferroplatineou t6tralerroplatine),qui conriennentaussi des alliages Os-Ir-Ru et laurite ou erlichmanite.Les mesures isotopiquesdes 6chantillonsd'alliage Pt-Fe ont port6 sur leurs inclusionsriches en Os. Le rapport est semblablede la bordure d'un grain de MGP i son coeur, ou parmi les inclusionsde diff6rents min6rauxd'une seulep6pite. Aucune variation isotopiquea 6t6 d6couvertedans les grains de MGP qui sont zon6s.L'absence de h6t6rog6n6it6isotopique dansune p6pite rend impossibleune origine desp6pites par m6canismede faible temp6rature.Les valeursdu rapport l87Os/l86Osne varientque peu, entre0.99 et 1.12;la plupartdes valeurs se siluent entre 1.00 et 1.06,et correspondenl donc ir desvaleurs typiques du manteau. Les 6l6mentsdu groupe du platine seraientdonc d6rivdsessentiellement du manteau,sans contribution importanted'osmium crustal. Les grainsde MGP ont 6t6formes dansdes massifs intrusifs, qui ont subi un lessivageet une 6rosion, pour €tre concentr6sdans des alluvions par processusmdcaniques. Les donn6esisolopiques concordent avec la pr6sencedans les p6pites de lamelles d'exsolution de MGP, la pr6sence d'inclusionsd'olivine saine,et l'absenced'inclusions de min6raux x6nocristiques, sans lien avecles roches ultramafiques. Les valeursdu rapport l8r6r7t86gr dans une p€pite et des MGP d'une chromitite provenant de l'Oural confirment I'hypothbseque la p6pitealluvionnaire a son originedans la chromitite.La formexdnomorphe et la surfacesans signe d'abrasionde certainesp6pites pourraient tout simplementr6sulter de leur tenacit6e1 de la proximit6de la source ultramafique.

(Traduitpar la R6daction) Mots-clds:isotopes d'osmium, min6rauxdu groupedu platine, pdpites,graviers alluvionnaires, massifs ultramafiques, type alpin, type Alaska. 289 290 THE CANADIAN MINERALOGIST

INTRoDUCTIoN on the studies of placers in the Urals (Betekhtin l96l), at Tulameen,British Columbia(Rice 1947), Alluvial nuggets of platinum-group minerals and at Joubdo, Ethiopia (Molly 1959).Augustithis (PGM) are historically important. Placers in the (1965) introduced the concept that "nuggets of USSR and Colombia and, to a lesserextent, in native Pt have been formed in the lateritic soil" Canada, were the principal source of the PGE cappingthe Joubdo ultramafic complex.Ottemann through to the first quarter of the 20th century, & Augustithis (1967) suggestedthat they were until the discoveryof lode depositsin South Africa formed "during the processof early alteration or and Siberiaand the increasedefficiency of recovery lateritization". The formation of nuggets was of the PGE from Ni-Cu sulfide ores in Canada. believedto havetaken place"by elementagglutina- The prevailing view on the origin of nuggetsof tion under a phaseof hydration (low temperature) PGM by the mid 1960swas their derivation from of the ultrabasic rocks". These ideas were later ultramafic rocks by weathering and mechanical supported by Cousins (1973) and expandedby processesof transportation.This opinion wasbased Cousins& Kinloch (1976)and Stumpfl (1974)to the "chemical accretion" hypothesis.More recent- ly, Bowles(1988) proposed crystallization of PGM in placers. Rounded shapes were ascribed to processesof chemical accretion, and angular and unabraded textures, to the growth of crystals in sediments(e.g., Bowles 1986, Barker & Lamal 1989).

Frc. I Back-scatteredelectron image of osmian nuggel from the Atlin district, British Columbia (sample number 10120-11).The osmian iridium, havingaverage composition of lr7a.60s1.6Pt1.6(Harris & Cabri 1973), contains a round inclusion of Os-Ir-S-As (OISA). The PGM showedevidence of variable concentrations of Os and Re during the Os-isotopeanalyses. Grey area partially rimming the 1876t71866t nuggetis an intergrowth of irarsiteand osmianiridium Frc. 2. Back-scatteredelectron image and (M-Ia in the diagram). The scalebar represents500 values of iridian osmium nugget from the Atlin pm. Listed here are 1876t71869,ratios (all t 0.006); district, British Columbia (samplenumber 10120-10). the ualues in -parenthesesare the correction due to The nugget contains (Chr) and laurite (Lr) 'o'Refor thepoints shown. Z: 1.081(14.890), Y: 1.083 and is rimmed by irarsite (Ia). The,s-calebar represents 187os/l86os (8.790),X: 1.081(16.2s/o), W: 1.083(9.6V0), V: 1.079 500 pm. Listed here ar" values (all t (5.9q0),U: 1.078(8.890), T: 1.083(l7.SVo), S: 1.088 0.006)for the pointsshown. Z: 1.036,Y:.1.040, X: (18.390),R: 1.082(9.lt/o), P: 1.081(17.590). 1.043,W: 1.036,V: 1.036. ORIGIN OF PCM NUCGETS 291

1879r7t866, Frc. 4. Back-scatteredelectron image and valuesfor an iridianosmium nugget of placerorigin from Teshio,Japan (sample number = Te-l). The grain showedthe highest Re peaks during the Os-isotopeanalyses, but the Re contentswere found to be below the detectionlimit of an electron Frc. 3. Values o1 1879171869,for an osmian iridium microprobe,<0.06 wt.9o.The scalebar represents nugget(Ir5e.6osru.3Pta.t, Harris & Cabri 1973)rimmed 500 m. Listed hereut. 1876t71869sratios (all a by irarsite (Ia) from .the Atlin district, British 0.006);the valuesin parenthesesare the coffection Columbia (samplenumber 10120-9).The,.scale bar dueto r87Refor the pointsshown. Z: 1.054(23r/o), l87os/l86os r$ represents300 pm. Listedhere are ratios Y: 1.043(22t/o),X:1.049 QZi/o), : 1'052(24t/o),Y: (all t 0.006)forthepoints shown.Z: 1.064,Y: 1.061, 1.052Q2a/o), U: 1.048(2090). X: 1.066,W: 1.067,V: 1.061,U: 1.064,T: 1.065,S: 1.065,R: 1.060,Q: 1.068.

the bulk Earth becausemost Os residesin the mantle (Alldgre & Luck 1980,Walker et al. 1989' on the other hand, havehigh Mineralogical and petrographicstudies support- l99la), Crustal rocks, and varied Re/Os and l87os/l86osratios, with an ing the earlierview, mechanicalderivation of PGM l87os/l86Os nuggets from intrusions, include publications by estimatedaverage ratio of approximate- (1976), (1975), ly 30 (Palmer& Turekian 1986,Palmer et al. 1988). Mertie Cabri & Harris Raicevic& l87os/l86os Cabri (1976), Cabri et a/. (1981), Ford (1981), The large difference in ratios between Hagen e/ ol, (1990),Johan el al. (1990),Nixon el mantle and crustal rocks makes an isotopic study al. (1990),and Slanskyet al. (1991). suitable for examining the contributions from the two sources. Osmium,one of the six PGE, hasseven isotopes l87os/l86os (l84Os,1860S, l87os, l88OS, l8eos, l90OS, le2OS). l87OS This paper presents ratios of PGM is the decayproduct of r87Re,which has a half-life from one lode depositand a largevariety of placers, of 4.56 x l0r0 years(Luck & Alldgre 1983).The including samplesof residualplacer from Joubdo, r87os/r86osratio of the bulk Earth was 0.805 at Ethiopia, which historically generated the con- 4.55 Ga, and the presentratio is believedto be troversyon the origin of nuggetformation. Models approximately1.04 (Alldgre & Luck 1980).The for the formation of PGM nuggetsare discussed values of the mantle have been close to those of in the light of the Os-isotopedata. 292 THE CANADIAN MINERALOGIST

Flc. 5. Back-scatteredelectron image of isoferroplatinumnugget from Tulameen,containing chromite (Chr), olivine (Ol; Foe3-e5;Nixon et al, 1990),laurite (Lr) and iridian osmiuminclusions (sample number C-236). Several thin lamellaeof iridian osmium (Ir21.7Os7a.7Ru3.e;Nixon et ol. 1990).ue presentin the field shown in Figure 5b, but are not apparent in the photo. The dashedarea in Figure 5b (scalebar: I mm) is shown in Figure 5a (scalebar: 500 pm). The laurite in the field 5u, l87gr7l869sratios of 1.087 t 0.008. Other Os-isotopemeasurements from this nuggetare: iridian osmium:1.092 ! 0.008,iridian osmium:1.083 t 0.006,iridian osmium: 1.088+ 0.006, laurite: 1.077J 0.007.

Sevpls DescnrprroN Samplesassociated with Alaskan-typeultramafic intrusionsare from Tulameen(British Columbia), Choco (Colombia), Omutnaya River and the The placer PGM associatedwith Alpine-type Nizhni Tagil dunite massifin the Urals (Russia), intrusions used in this study come from Atlin, Joubdo (westernEthiopia) and GoodnewsBay Ruby, Bullion and Cariboo (British Columbia), (Alaska).Most Alaskan-typesamples consist of a Adamsfield(Tasmania), andTeshio and Onnebetsu Pt-Fe alloy, either isoferroplatinumor tetrafer- (Hokkaido, Japan). Most Alpine-type samples roplatinum.The shapeof the nuggetsvaries from consistof Os-Ir-Ru alloys. Thesealloy samples well rounded (Figs. 5, 6) to irregular with branches were usedfor the Os-isotopeanalyses. The nuggets (Fig. grains vary in shapefrom rounded(Figs. 1, 2) to angular 7). The of alloy commonly contain (Figs. (Figs. 3, 4). They commonly show chemical inclusionsand lamellaeof Os-Ir alloys 5, 6, heterogeneityand zonation,with variableOs and 7, 8), lauriteand erlichmanite(Figs. 5, 6, 8). These Ru contents(Cabri & Harris 1975).Some nuggets inclusionswere used for the Os-isotopestudy. are mixturesof different PGM (Figs. l, 2), and Chemical zonation, similar to that observedin someare coatedby irarsite(Figs. 2, 3, 4) or by a grains of Os-Ir-Ru alloys from the Alpine-type mixture of fine-grainedirarsite and Os-Ir alloys samples,also is observedin the includedphases (Fie. l). (Fie. 6). ORIGIN OF PCM NUGGETS 293

METHODOLOGY

Analyticol procedures

Osmium-isotoperatios were determined on Au-coated PGM grains using a Cameca3f ion microprobeat MIT. Masses185, 186, 187' 188 and 189were determined in order to correctthe ratios of l87Os/t86Osfor the contribution of r87Re, hydrides and mass fractionation. Some grains displayinghigh Re peaksrequired Re corrections of up to 24c/o, i.e., 240/oof mass 187 was contributed by r87Re(Fig. a). Some showed a heterogeieousdistribution of Re. After the correc- tion for Re, calculatedl87os/186os values were found to be consistentwithin the grain (Figs.l, 4), confirming that the Re correction does not introduce additional uncertainty into the measure- ments.Replicate analyses of the samegrains, made overa l0-monthperiod, show a reproducibility(2o) of + 0.6q0. The precisionof each result thus is given as x. 0.60/oor the in-run error, whicheveris larger. Detailed operating condilions and reproducibilitiesof the analysesare describedby Hattori et al. (1991). Becausesignificant amounts of Re were noted in severalsamples during the Os-isotopicanalyses, the Re contentswere checked on the grainscoated with carbon by counting the Relo peak for 60 s using the wavelength-dispersionspectrometer on a JEOL 733 electron-microanalyzersystem. The operating conditions were 20 kV with a beam current of approximately25 nA. Severalgrains of isoferroplatinum and of Os-Ir-Ru alloys were subjected to the examination of Re contents, including an iridian osmium grain that displaysthe highestRe signalsrecorded in the ion microprobe (Fig. a). The ReIa peak was calibratedon a pure Re metal standard, but it was not detectedon any of the grains during the 60 s counting time. Therefore, Re contentsare below the minimum detectionlevel for the electronmicroprobe, 0.06 wt,qo.

Frc. 6. Back-scatteredelectron images of isoferroplatinum (PF) nuggetwith inclusionsof chromite (Chr), iridian osmium (IO) and erlichmanite(ER) from Goodnews Bay (sample number GDN-I). Dashed square in Figure6a (scalebar: I mm) is the areashown in Figure 6b (scalebar: 200pm). Note zonation from an Os-rich core to a Ru-rich rim in erlichmanite.Values of 1876171866r(all t 0.@6) are: Z (erlichmanite):l'M6' Y (iridianosmium): 1.034, X (erlichmanite):1.030, W (erlichmanite):1.034. 294 THE CANADIAN MINERALOGIST

containing on the order of 6090Os, should be less than 0.001. Thus the r87os/r86osratio of an Os-Ir-Ru alloy would increaseonly by 0.0003 in 500 Ma after crystallizationowing to the decayof 187Re.The contribution of radiogenic l87Os, therefore,is not significant for samplesof Os-bear- ing alloy. The isoferroplatinum nuggetscontain inclusions of Os-Ir alloy. Assumingthat an jsoferroplatinum nuggetcontains less than 0.06wt.9o Re and 5 wt.9o

FIc. 7. Irregularlyshaped Pt-Fe alloy from Goodnews Bay containingiridian osmiumplatelets (light color) (samplenumber I4NMA-NM4). The scale bar repre- sentsI mm.Values of l87os/l86Os.Z (tabular iridian osmium):1.104 t 0.007,Y (tabulariridian osmium): l.lll I 0.008,X oamellariridian osmium): l.ll9 t 0.008,W(thin rod of iridianosmium): 1.121 t 0.002.

Re-Os isotope systemutics

The l87os/l86Osratio of mantle-derivedrocks (Walker et al. 1989,l99la) and the chondritevalues (AllBgre & Luck 1980) define the range of l87os/l86osvalues of the mantleduring geological time (Fig. 9). Although there are detectable variations in 187os/l86osratio in the mantle at a giventime, the differencesare small comparedwith the variations in crustal rocks (Figs. 9, l0). Ultramafic rocks have a low Re,/Osratio (e.9., Walker et al. 1988),and the changein r87os/r86os ratio over geological time is relatively small. However, the t87os/l86osratio of other crustal rocks increasesrapidly owing to generallylow Os and high Re contents in the rocks. For example, l87os/l86os the ratio of tholeiitic rocks, with an Ftc. 8. Photomicrographof a portion of isoferroplatinum averageRe/Os ratio of 35 (Chou e/ a/. 1983),would nugget (PF; Pt74Ir2Fe4Cul) containing iridian os- increasefrom 1.0 to 3.4 in 100Ma. and the ratio mium (IO; Os66lr4Ru3Pt2Rh,)and laurite (Lr) of a black shalewith a Re/Os ratio of 75 (Ravizza ' inclusions, from the Omutnaya River, Urals (sample & Turekian 1989)would changefrom 1.0 to 7.7 in number 53). The nugget is rimmed by thin layer of tulameenite.The scalebar represents500 pm. Values 100Ma. Becauserocks of the uppercrust generally 1879171866r. have a high value of l87os/l86os,the ratio of o1 z: 1,030a 0.012,Y: 1.u21r 0.011. values of different grains of PGM from dissolvedOs in surfacewaters also should be high. Os-isotope the nuggetinclude: Os-ricblaurite rod: 1.033t 0.010, 187Os/t86Os other end of the laurite rod: 1-040 t 0.010, tabular Initial ratio of the PGM iridian osmium inclusion, B: 1.033 t 0,009, tabular iridian osmium inclusion, C: 1.035 t 0.009, tabular The Re contents in the PGM studied are less iridian osmium inclusion, D: 1.037 + 0.007, iridian than 0.06 wt.qo: the Re/Os ralio of the PGM. osmiuminclusion. E: 1.051t 0,010. ORIGIN OF PGM NUGGETS 295

1.4

1.2 oat, @ @ u, (J1n

@

0.8

4.0 3.0 2.0 1.0 present Age(Ga)

Frc. 9. Valueso1 1876.71869rfor the PGM studiedcompared with the valuesof the contemporaneousmantle. Solid circles: PGM in this study and data from PGM from AJpinechromitites from Borneo (Hattori et al. 1992).The range of mantle l87os/l86Osvalues are based on values of chondrite (Alldgre & Luck 1980),mantle xenoliths (Walker al a/. 1989),and komatiite from GorgonaIsland (Walker et al. l99la).

Os in inclusions, the l87os/l86osratio of the sulfides. The isotopic ratios of all these PGM, inclusionswould increaseby lessthan 0.003in 500 independentof mineral species,are similar (Figs. Ma. If the inclusionsare 0.5 wt.9o of the total 5, 6, 7, 8). It is remarkablethat, in addition,PGM mass,the increasewould be 0.03.The contribution from widely separatedgeographic localities also of the radiogenicr87Os must still be small. The havesimilar l87os/l86osratios. observedl87os/l86os values are, therefore,safely The PGM from the Nizhni Tagil dunite massif, assumedto be the initial valuesat the time of their Urals, Russia,have an averagel87os/r86os value formation. of 1.029 t 0.006. The value is identical to the averagel87os/l86os value of 1.035 a 0.006for a RESULTS nugget from the Omutnaya River, Sissertj, near . Sverdlosk,Urals, which is locatedabout 180 km to the south. The nugget was derived from the r87os/r86os The ratio of all samplesfalls in the Omutnaya dunite massif, which belongs to the rangefrom 0.99 to 1.12,with most of the values same ultramafic belt as the Nizhni Tagil massif between1.00 and 1.06. Severalgrains that show (Betekhtin1961). chemical heterogeneitywere examined in detail. The data, togetherwith the descriptionof PGM Iridian nuggets from Atlin osmium the district from Joubdo @thiopia), appearin Table l. The (Figs.I , 2, 3) showvariable Os contents,but values rest of the data and descriptionof PGM analyzed of the Os-isotopicratio were found to be similar are available from the Depository of Unpublished within each grain. There is no evidence of l87Os Data, CISTI, National ResearchCouncil, Ottawa, overgrowth of PGM enriched in crustal nor CanadaKIA 0S2. of a trend of l87Osenrichment toward the rim of PGM grains. The Os-isotoperatio of the core and rim of the grains that are coated by irarsite is DISCUSSION similarwithin the limits of precision(Figs. l, 2, 3), indicating that the alteration of the Os-Ir alloy to Os-isotope rotios of PGM nuggets irarsite was not accompaniedby the introduction of radiogenicOs. The observedvalues of t87os/t86osfrom placer Some nuggets contain several PGM, such as PGM (0.99 to l'l2) show a narrow spread,and PGE alloys of contrasting compositions and they aregenerally lower than the l87os/r86osvalues 296 THE CANADIAN MINERALOGIST

2.0 4.0 o.u 8.0

PGMnuggets I presentmantle I Largelntrusions Bushveld I Stillwater I Freetown I Sudbury

Mn-nodules -

Continental Crust

BlackShales

l 10.0 20.0 30.0 40.0 1u7os/186os

Frc. 10. Valuesof 18u6r71869,for the PGM nuggelsstudied compared with those of largemafic intrusionsand crustalrocks. The 1876171866rvalues of the mafic intrusionsare initial values.The valuesof sedimentsand manganesenodules are present-daymeasured values. Sources of data: PGM nuggets(this study), presentmantle (Walker et al. 1989,l99la, Martin 1991),Bushveld Complex (Hart & Kinloch 1989),Stillwater Complex (Lambert et al. 1989,Martin 1989), FreetownComplex (Hatt ori et al. I 99I ), SudburyComplex (Walker el a/. I 99I b), manganesenodule (Luck & Turekian 1983,Palmer & Turekian 1986,Esser & Turekian 1988),continental crust (Palmer& Turekian 1986),Paleozoic black shale(Ravizza & Turekian1989).

reportedfor the rocks and grains of PGM from Moopr-sFoR THE FoRMATToN or PGM NuGGErs largeintrusions (0.88 to 7.55)such as the Bushveld (Hart & Kinloch 1989),Stillwater (e.g., Lambert et Formation of PGM by high-temperature (Hattori al. 1989, Martin 1989), Freetown et al. hydrot hermaI processes l99l) and Sudburycomplexes (Walker et al. l99lb; Fig. l0). The highervalues for the largeintrusions many are attributedto contaminationof the masmasbv Oxygen-and hydrogen-isotopestudies of epizonal intrusions suggestthat the emplacement crustalOs. l8:9r7186Os of igneousrocks in shallowlevels commonly results 15. valuesof the PGM nuggetsfall (e.9., r87Os/l86os in large-scalemeteoric hydrothermal activity within the rangeof valuesof themantle Taylor & Forester 1979).The presenceof fluid r87os/r86Os (Figs. 9, l0). The low valuesof the inclusions in PGE-rich dunite pipes from the nuggetssuggest that the PGE in the nuggetswere BushveldComplex seemsto support the involve- derived from the mantle without a significant ment of a high-temperature(>700oC) fluid for contributionfrom crustalOs. PGE mineralization in large intrusions (e.9., ORIGIN OF PGM NUGGETS 297

TABT,E 1. leo"/r&o" lATIos oF pc![ IN BrrttDUAr, PLAcf,B NUcGEf,s FaoM TsE JouBDo ULTMI'AFIC COMPLEK, BIBBIB DISTBIdI, I{XSTERN EISIOPIA

Samplo 2.5 mm x 1 mm' 8M192&247-10 fisf€roDlalirum with ir€aulil surta@ with brmch6' - '10 ;ontainlio abundarnorisnted irldia Gmlum lmellae, pm wide' trd tabuiariridim osmlum. Rimsot sme iridlm €mium inclwiom are partally replacod by unidentfrod lr-rich phs€) labular lridhn osmium grain A in lhe nuggot 1.022r .006 1.022a .@6 tabular lridia cmium grain B in ths nuggel '1.027 iridian 6mium larnellas r .006 BMlg&247-12 (R-Fs allovwilh very lreqular surtffi, 0.8 mm x 1.3mm, @ntaining ioa snam'd Iridhn cbmiummelal. 10/m x "moltm trd roundedlridiil @miuni ind6lon) iridim 6mlum rd lnclusion 1.027a .006 (roundedR-Fe alloy,0.7 mm, @ntajningshort rods of mlum, BMl92&247-13 -6orrm, 2otm x md l@nd cmium mstalinclBlom) '1.023 lddlan Grnium rod I .00€ roundsd lridla Gmium inclBion 1.0223 .006 BMlg*247-14 (rounded boturoplatnum nqgget @ntaining genkinite, 5}*''%';ff#''k?3lgd.iil'fl H*".F,fl ,n,il,";lf f LTH'j' '1.034 lridlan osmium rod in th€ c€ntre ol the Pt-F6, I .0O8 iridian osmium rod in tha matgin ot tho nugget Pt-Fs 30tm x 70tm irldim Bmlum thin lamellae 1.023a .007 lridian osmlum rod in the othsr mtrgin ot ihs nugg€t 1.019I .010 iridim osmium in the @nral part ot ths nugget 1.t2.!.@7

(a) Fouwtog tls rc@clatue of f,auis & Cabtlt (1991), (b) Alalyd€l tlata gtyo h Cablt st al. (1981).

Stumpfl & Rucklidge 1982,Schiffries & Skinner rocks,the mineralizationof PGE would havealso 1987).Fluids derived from hydrous magmaswere occurredoutside the intrusivebodies. r87os/186os involved in the PGM mineralization in the ratiosof PGM obtainedin this study Alaskan-typeintrusions near Fifield, Australia, alsosuggest that the contributionof crustalOs was according to Johan et al. (1989). It is, however, insignificant. Though current data and evidence unlikely that the relatively small Alpine- and cannot reject the possibleinvolvement of high- Alaskan-type ultramafic intrusions generated temperaturefluids in the Alaskan-and Alpine-type meteoric-hydrothermal activity. Alpine-type in- intrusions, the overall evidence indicates that trusions are believedto be either obductedslices of hydrothermalactivity in the caseof theseintrusions upper mantle or cumulates of boninitic magmas was not significantin the formation of PGM. solidifiedin the lower crust (e.9.,Serri 1981).The origin of Alaskan-typeintrusions is controversial, Formqtion of PGM during serpentinization but the dunite bodiesare consideredto represent cumulatesformed in the feederpart of arc magmas Platinum and Pd are consideredto be more (Conrad & Kay 1984)or pre-existingAlpine-type soluble in hydrothermal fluids comparedwith the ultramafic intrusions (Kelemen& Ghiorso 1986). rest of PGE (Mountain & Wood 1988).The lack In either case,there was little opportunity for them of variation in Pt and Pd contents during the to produce hydrothermal activity at an upper serpentinizationof ultramafic rocks (e.9., Ross& crustal level. Keays 1979,Oshin & Crocket 1982)suggests that Postulatedhigh-temperature hydrothermal ac- the less soluble Os would not be mobile during tivity should have altered the primary silicate serpentinizationProcesses. chemistry,if such fluids were responsiblefor much Many Alaskan-type intrusions presentlydisplay of PGM crystallization.However, pristine silicate pristine compositionsof the primary minerals, mineralogy common in most Alaskan-type in- including olivine and clinopyroxene, beneath the trusions counters this hypothesis.Furthermore, weatheredzones. Examples include the Goodnews olivineenclosed in PGM hasidentical compositions Bay complex(e.g., Southworth & Foley 1986)'the to the olivine in chromitites. but distinct from Tulameen complex (Findlay 1969)' and several olivine in the rest of intrusions(e.g., Ford 1981, zonedcomplexes in the Urals (Betekhtinl96l). On Nixon e/ al. 1990). the other hand, Alpine-typeintrusions have com- In addition, the occurrenceof PGM confined monly undergoneextensive serpentinization. If within ultramafic bodiesand their absenceoutside serpentinizationis responsiblefor the formation of ultramafic bodies indicate that the postulated nuggetPGM, one would expectto observePt- and hydrothermal activity did not take place on a large Pd-bearingnuggets associated with highly serpen- scale.If the hydrothermalactivity involved country tinizedAlpine-type intrusions, because Pt and Pd 298 THE CANADIAN MINERALOGIST are more solublethan the other PGE. Empirical they were protectedby the Pt-Fe alloys during observations, however, suggest that relatively sedimentaryprocesses. unserpentinizedAlaskan-type intrusions are accom- The occurrenceof crystallographicallyoriented panied mostly by nuggetsof Pt- and Pd-bearing lamellae Os-Ir in Pt-Fe alloys also is phases,whereas highly serpentinizedAlpine-type of alloys difficult to accountfor by the postulatedformation ultramafic intrusions are associatedwith Os-Ir-Ru of PGM at low temperatures. The texture is alloy nuggets(e.g., Cabri & Naldrett 1984).Some Alpine-type intrusions are associatedwith Pt-bear- attributed to exsolution of a once-homogeneous high temperatures (Cabri et al. 1987, ing nuggetsas well as Os-Ir-Ru phases(Burgath alloy at Slanskyet al. l99l). 1988), but these casesare not common. These observationsappear to suggestthat the nugget-sized None of the nuggets in our study contain PGM werenot formed by the hydrothermalactivity inclusionsthat are foreign to ultramafic rocks. In that causedserpentinization. addition, the compositions of the olivine and As mentioned earlier, Re,/Os values of chromite are typical of the ultramafic source-rocks ultramafic rocks are low compared with those of (e.9.,Nixon et al. 1990).If the nuggetswere indeed many other crustal rocks. The values, however, formed by element"agglutination" or accretionof particles vary within ultramafic rocks and should be very fine in sediments, it is statistically low in chromititesbecause of their high Os contents reasonableto assumethat the nuggetmatrix would (e.9., Crocket 1981, Table 2). This implies that enclose minerals from adjacent lithologies and silicates in ultramafic rocks have high Re,/Os minerals abundant in placers, such as quartz, values. Therefore, radiogenic l87Os should be zircon, and monazite. The lack of such foreign releasedduring the breakdownof silicateminerals, inclusionsalso is consistentwith the high-tempera- resulting in high r87os/r86osin serpentinizing ture formation of the nuggets. fluids. If PGM were indeed formed during PGM nuggetsconsist of crystallographicallyand serpentinization,high l87os/l86osvalues should be compositionallydistinct mineral species;they are observedin some nuggets, but the lack of such not noncrystalline, "amorphous" mixtures of evidenceprecludes serpentinization as the principal metals. The phases observed in the nuggets are processof the formation of PGM. isoferroplatinum (cubic Pt3Fe),tetraferroplatinum Tulameenite and some Cu-Pt phases, which (tetragonal PtFe), hexagonalosmium, commonly rim PGM nuggets, are consideredto and rutheniridosmine.and cubic iridium. Some have been formed during serpentinization form euhedralto subhedralcrystals. These nuggets (Betekhtin 1961,Nixon et al. 1990,Cabri & Genkin are very diffegent from "minute" "colloform" l99l). Owing to the low concentrationof Os in aggregatesof native platinum documented by theseminerals, Os-isotopic measurements were not Wagnerand Schneiderhcihn(cited in Wagner 1929). possibleduring this study. However,these minerals The noncrystallineaggregates are believedto have may recorda l87os/l86osratio higherthan that in been formed during weathering of Pt- and the early-formedPGM. Pd-bearing sulfides at low temperatures(Wagner 1929). Formation of PGM at low temperatures during Geochemicol evidence: Laboratory experiments weothering or sedimentation and thermodynamiccalculations demonstrate that PGE may be significantly soluble in acidic saline This model includes the formation of PGM solutions,even at low temperatures(e.9., Mountain nuggets by "element agglutination" (Augustithis & Wood 1988). Surface waters in ultramafic 1965)or "accretion" of fine particles (Cousins terranes cannot dissolve a significant amount of 1973,Cousins & Kinloch 1976),and their formation PGE becausethey usuallyare alkaline(pH >10) by dissolution and precipitation processes(Bowles owing to serpentinizing reactions of silicate 1986,1988, Barker & Lamal 1989).The modelmay minerals(e.g., Barneset al, 1978).Surface run-off be discounted on two different grounds: may become acidic upon completion of serpen- mineralogicaland textural evidenceand geochemi- tinization. This is not the case,however, for most cal data, including Os-isotopevalues obtained in Alaskan-type intrusions, becauseserpentinization this study. rarely is pervasive. Mineralogical and textural evidence: Some Pt-Fe If "agglutination" were involved in the forma- nuggets contain inclusions of igneous silicate tion of nuggetsof PGM, nuggetswould be expected minerals and oxides, exsolution lamellae of other to show isotopic heterogeneitywithin individual PGM, and sulfides(Figs. 2, 5, 6,'7, 8). Olivineand grains becausethey would contain PGM formed sulfides are not stable during weathering and from PGE derived from different parts of an sedimentaryprocesses. Their presence indicates that ultramafic intrusion or from different intrusions. OR]GIN OF PGM NUGGETS 299

Lack of isotopic heterogeneityin individual grains of Kagoshima University for the samples from also rejects considerationof the "agglutination" Japan.LJC receivedsamples over a periodof many hypothesis as the principal process for nugget yearsfrom J.A. Mandarinoand R.I. Gait (Royal formation. Ontario Museum), A.J. Criddle (The Natural If the nuggets were precipitated in sediments History Museum, London)' J. Haight (Univ. from dissolvedPGE, they would show high and British Columbia), J.G. Symons (Tasmanian varied l87os/l86osvalues. They would haveincor- Departmentof Mines),A.D. Genkin (Institutefor porated 187Osreleased from decomposingsilicate Geologyof OreDeposits, Petrography, Mineralogy and sulfide minerals of the ultramafic rocks and and Geochemistry,Moscow), and J. Cochraneof the accompanyinggabbros and volcanic rocks. In Mallorca, Spain. Careful reading and helpful addition, dissolvedOs in surfacewaters would not commentsby D.C. Harris, B. Ballantyne,R.R. necessarilyhave a constantl87os/l86os ratio. If the Keaysand R.F. Martin are appreciated.Financial nuggetsgrew by overgrowth on pre-existingPGM assistanceof NSERC to KH is acknowledged. grains, the marginsshould be high in l87Os.One would expecta zonedprofile in l87Os,/l80sratio. RrrrneNcrs The lack of a high l87Oscontent in the rim or in any nuggetsprecludes the low-temperaturegrowth ALLEcne,C.J. & Lucr,J.-M. (1980): Osmium isotopes of nuggetsof PGM as the principal processof their as petrogeneticand geological ftacerc. Earth formation. Planet.Sci. Lett. 4E,148-154.

CoNcLUSIONS AucusrrrHrs,S.S. (1965): Mineralogical and geochemi- cal studiesof the platiniferrousdunite - birbirite - The 187Os/186Osvalues of PGM within individual pyroxenitecomplex of Yubdo,Birbir, W. Ethiopia. A, $9-196. nuggets are consistentlylow and uniform. Some Chem.Erde nuggetsof PGM show prominent chemical varia- & Lavel, K. (1989):Offshore extension consist of different phases,but BenrEn,J.C. tions, and some of platiniferousbedrock and associated sedimenta- they are homogeneousin Os-isotopiccomposition. r87os/l86Os tion of the GoodnewsBay ultramaficcomplex, Low values reject a postulated low- Alaska. MarineMining E, 365-390. temperature formation of PGM in sedimentary environments. The data indicate that PGE were BenNEs,I., O'Nrll, J.R. & TnEscesrs,J.J. (1978): derived essentially from the mantle without a Present-dayserpentinization in New Caledonia, significant crustal component. PGM were formed Oman and Yugoslavia. Ceochim, Cosmochim. in the intrusions,eroded, transported and mechani- Acta 42, 144-145. cally concentratedin placers. For eluvial PGM, they remained in the residual soil on ultramafic BrrrrurrN, A.G. (1961): MikroskopischeUnter- intrusions during sedimentaryprocesses. The ir- suchungenan Platinerzenaus dem Ural. Neues regular surface of some PGM may simply be Jahrb. Mineral. Abh. 97, l-34. attributed to their durable nature and the short platinum- distancesof transportation of the grains from the Bowlrs,J.F.W. (1986): The development of group in laterites. Econ. Geol. 81, source. minerals 1278-1285. This conclusion is in agreement with the petrological mineralogical and studies carried out (1988):Further studiesof the developmentof by Cabri & Harris (1975) and Ford (1981) for platinum-group minerals in the laterites of the nuggetsassociated with Alpine-type intrusions, and Freetown layered complex, Sierra Leone. /n by Raicevic & Cabri (1976), Cabri et c/. (1981), Geo-Platinum '87 (H.M. Prichard, P.J. Potts, Hagen et al. (1990), Johar et al. (1990), Nixon el J.F.W. Bowles& S.J. Cribb, eds.). Elsevier,New al. (1990) and Slansky et al. (1991) for PGM York (273-280). nuggetsassociated with Alaskan-type intrusions. BuncarH, K.-P. (1988): Platinum-group minerals in placer ACKNOWLEDGEMENTS ophiolitic chromitites and alluvial deposits, Meratus-Bobaris area. southeast Kalimantan. 1n Geo-Platinum '87 (H.M. Prichard' P'J. Potts, We thank J.H.G. Laflamme of the Canada J.F.W. Bowles& S.J. Cribb, eds.).Elsevier, New Centre for Mineral and Energy Technology for York (383-403). checking the detection limit of Re and providing someof the photos,and S.R. Hart and N. Shimizu Casnr, L.J., CntooLE, A.J., Lerlavrrle, J.H.G., of the Woods Hole OceanographicInstilution for BEanNa,G.S. & Hennts,D.C. (1981):Mineralogi- their assistanceduring the Os-isotopeanalyses of cal study of complex Pt-Fe nuggets from Ethiopia. the samplesat MIT. KH is grateful to Y. Urashima Bull. Mindral. 104, 508-525. 300 THE CANADIAN MINERALOGIST

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Srnnr, G. (1981): The petrochemistry of ophiolite Received June 21, 1991,revised manuscript accepted gabbroic complexes:a key for the classificationof November 20, 1991.