Canadian Mineralogist Vol. 30,pp.289-301 (1992) ORIGINOF PLATINUM-GROUP-MINERALNUGGETS INFERRED FROMAN OSMIUM-ISOTOPESTUDY KEIKOHATTORI Ottawa-Carleton GeoscienceCentre and Departmentof Geology, University of Ottawa, Ottawa, Ontario KIN 6N5 LOUIS J. CABRI CanadaCentrefor Mineral and EnergyTechnology, 555 Booth St/eet,Ottawo, Ontario KIA OGI ABSTRACT Osmium-isotoperatios of individual PGM (platinum-group minerals) 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 laurite/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 iridium 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 chromite (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 iridian osmiumnugget 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