LU3

The Canadian M iner alog is t Vol. 33,pp. 1023-lM5(1995)

STRATIFORMAND ALLUVIALPLAfl NUM MINERALIZATION IN THENEW CALEDONIA OPHIOLITE COMPLEX

THIERRY AUGE lNI PIERREMATIRZOT BRGM'Metallogeny and Geotynarnics", B.P. 6009, 45060 Orldans Cedex 2, France

ABSTRA T

An unusualtype offt mineralizationhas beenfound in the New Caledoniaophiolite complex, associatedwitl stratiform chromite-bearingrocks at the baseof the cumulateseries (close to the transition zone betweendunite and pyroxene-bearing rocks) and with cbromite-richpyroxenite dykes from the samezone. Two placer deposis, both enrichedin Fl also have been recognized one derived from the chromite-rich horizons and the other a cbromitiferous beach sand of unknown origin. Chromite-bearingrocks aremarked by a very strongenrichment in Pt relative to fhe other platinum-groupelements (PGE), $'ith a rrardmunconcentration of 11.5ppm (average3.9 ppm). The Pt enrichmenttakes the form of platinum-groupminerals (PGM) included in chromite crystals and is interpretedas being pdlrar'/. The following minerals have been identified: ft-Fe{u alloys (isoferroplatinum,tulame€nite, tetraferroplatinum and undeterminedphases), cooperite, laurite, bowieite, malanite, cuprorhodsite,unnamed PGE oxides, and base-metalsulfides with PGE in solid solution. The associatedplacers show a slightly different mineral association,with isoferroplatinum,tulameenite, and rare cooperite,sperrylite, Os-k-Ru alloys and PGE oxides. A more complex paragenesishas been identified in the beach san4 with isoferroplatinum,cooperite, laurite, erlichmanite, Os-Ir-Ru alloys, bowieite, ba$ite, hollingworthire, sperrylite, stibiopalladinite, and unnamed Rh sulfide, Pt-Ru-Rh alloy and PGE oxides. This type of mineralization, which differs markedly from the Os-Ru-h mineralization previously recognized in podiform cbromitite in New Caledonia, shows several analogies with Alaskan-type PGE mineralization.The host chromitite,with typical cumulustextwes, is markedby Ti and Feh emichmentdue to derivationfrom an evolved liquid. The PGM have crystallized directly from the melt before being trappedby cbromite. Precipitationof the Pt-Fe alloys was facilitated by a low fugacity of sulfir. The presenceof the PGM in the melt has induced chromite crystal- lization, facilitating nucleation.This episodeof cbromite and PGE mineralizationin the cumulatesindicates an unusualevent in the evolution of the complex. It may correspondto inje.ctionsof a new mapa (probably of boninitic affnity) into the chamber.PGE oxideshave been recognized in the chrornititehorizons and in the placer deposits.The formation ofthese PGE oxides is related to intenselateritization affecting the primary mineralization.Some oxides are the product of oxidation of pre-existingPGM, whereasothers seem to have crystallizedin laterite.This indicatesa certainmobility of the PGE during the lateritic alteration.

Keywords:platinum-group minerals, platinum-group elements, R alloys, ophiolite, ultramafic cumulates,chromitite, platinum mineralization,placer, New Caledonia"

Sol"nuanr

Un type inhabituel de min6ralisationen platine, associdl desamas de cbromite strafiformes,a 6t6 ddcouvertl la basede la s6riedes cumulats de la nappeophiolitique de Nouvelle-Cal6donie,i proximit6 de la zonede transitionentre dunite et cumulats i plroxbnes. On trouve dgalementune min6ralisationen platine li6e i despyrox6nites chromifOres formant desdykes recoupant cettem6me zone. Deux formationsalluvionnaires, enrichies en Pt, ont dgalement6t6 6tudi6es.L'une d6rive d'unemin6ralisation chromifbred P! I'autre, formant des sablesde plage, n'a pas de sourceconnue. I.es rocheschromifbres sont marqu6espar un fort enrichissementen Pt (avecune teneurd6passant cowamment 10 ppm) par rapportaux autres6l6ments du groupedu platine (EGP).La min6ralisationest due I la prdsencede min6rauxdu groupedu platine (MGP) inclus dansles cristauxde cbromiteet est interpr6t6ecomme primaire. On a identifi6: alliagesPt-Fe{u (isoferroplatine,tulameenite, tetraferroplatine et desphases non d6termin6es),cooperite, laurite, bowielte, malanite,cuprorhodsite, des oxydes de diff6rentsEGP et dessulfires de m6taux de baseavec des EGP en solution solide. Le placer d6rivant de cettemh6ralisation reconnueen roche a un contenuen MGP ldgbrementdiffdrent, avecisofenoplatine, tulameenite, et de rarescooperite, sperrylite, des alliages Os-Ir-Ru et desoxydes de divers EGP. La min6ralisationen sablede plage montreune paragenbseplus complexe,avec isoferroplatine, cooperite, laurite, erlicbmanite, des alliages Os-Ir-Ru, bowieite, irarsite, hollingworthite, sperrylite, stibiopalladinite et des phases notr d6termin6es(sulfure de rhodium, alliage Ft-Ru-Rh et des oxydesde diff6rentsEGP). Cesmindralisations diffbrent de manibre significative des mindralisationsen Os-Ir-Ru d6ji reconnuesdans les chromitites podiformes de Nouvelle-Calddonie;par contre,elles montrentdes analogiescertaines avec les mindralisationsd6critss dans des complexesalask6ens. Irs cbromitites h6tes,qui prdsententdes texturesde cumulagmontrent un emichissementen Ti et Feil, traduisantune cristallisationi partir d'un liquide 6volu6.Les MGP ont directementcristallisd i patir du liquide magmatique,avant d'6treinclus dansla chromite. 1024 TI{E CANADIAN MINERAIOGIST

La prdcipitation des alliages Pt-Fe a €tA factlttfe par une faible fugacit6 en soufre. Ia prdsencedes MGP dans le liquide, en facittant sa nucl6ation,a induit la cristallisationde la chromite.Cette min6ralisationen cbromite- 6l6mentsdu groupedu platine correspondh une discontinuit6dans l'6volution du complexe.On proposequ'elle soit li6e i des injections d'un magma non 6volu6 (probablementd'affidt6 boninitique) dans une chambrecontenant un liquide plus diff6renci6. La formation des oxydesde diffdrentsEGP est due d I'intensealtEration lat6ritique qui affectela mindralisationprimaire. Certainsoxydes sont les produits de I'oxydation de MGP pr6existants,d'autres ont directementcristallis6 en lat6rite. Ceci att€steune certainemobilit6 desEGP au coursde l'alt6ration.

Mots-cl4s.' min6raux du groupe du platine, 6l6mentsdu groupe du platine, alliags Pt-pg, ophiolite, cumulatsultrama.fiques, chromitite,min6ralisation en platine, placeg Nouvelle-Cal6donie.

Irrnooucnox GEor-ocvAND GEocnEMsrRY OFIIIE MINERALZATION Platinum-group minerals, described as "platinum flakes"" were first recorded from New Caledonia in TheNew Caledoniaophiolite rwppe 1891(Felatan 1891). They were recognized in alluvial samples, with gold and cbromite, collected in the The ophiolitic nature of the extensive ultamafic Andam River, which drains metamorphicformations rocks covering about 407o of New Caledonia was of the Central Range. They were ascribed to recognizedn 1977by Aubouin et al. T\e nappe,now serpentinites,which were observedas oosheets"in these discontinuous,was emplaced in the Upper Eocene formations (Glasser 1904). More recently, platinum- @aris er al. 1979). It is mainly composedof mantle group mineralls@GM) have been describedas small rocks; threedifferent units canbe distinguished:1) The inclusionstrapped in chromitecrystals from chromitite northern massifs,composed of mantle peridotite, are and from dunite found in the mantle sequenceof the characterizedby the presenceof spinel and plagioclase New Caledonia ophiolite nappe. They consist of lherzolite and by the abundanceof chromite deposits Os-Ir-Ru alloys, Ru and Os sulfides and Ir-Cu (I\4outte1982, Johan & Aug6 1986);2) The inter- sulfides (Johan& Legendre 1980, Irgendre & Augd mediate massifs are composedof harzburgite, with 1986,Aug6 1988).These PGM belongto the mineral minor dunite and rare small occurrencesof chromite. assemblagesclassically found in ophiolitic chromitite Both the northern and intermediate massifs form (Constantinideset al. 1980. Prichard et al. 198L. klippes scatteredalong the west coast (Paris 1981). Stockman& Hlava 1984,Talkington et al. 1984, Aug6 Finally, 3) The "Massif du Sud", forming the southern 1985, 1986, Cocherie et al. L989, Nilsson 1990, extremity of the island and covering one third of its McElduff & Stumpfl 1990). Such mantle-derived surface,is composedof mantlerocks (harzburgitewith chromitite is characterizedby a chondrite-normalized minor dunite and pyroxenitedykes and small bodiesof patternof the PGE, i.e., strongly depletedin Pt and Pd, chromitite), upon which cumulate dunite, wehrlite, typical of podiform chromitite @ageet al. L982). pyroxenite and layered gabbro have locally been Recent descriptions of Pt mineralization in recognized(Paris 1981). Upper gabbros, sheeted dykes ophiolitic rocks and in placersderived from ophiolites and pillow lava are not presentin the New Caledonia (Prichard et al. 1.986,Prichard & Tarkian 1988, ophiolite nappe. Burgath 1988, Hagen et al. 1990, Comivaux & Laflamme 1990,Ohnenstetter et al. l99l) led us to The PiroguesRiver area investigatethe potential for PGE concentrationin the New Caledoniaophiolite. A geochemicalexploration The mineralization of the Pirogues River @g. 1) survey,conducted wifhin the framework of a research was discoveredduring the exploration survey. Very project on natural resources,and s6asis.:ng in the high Pt contentswere recorded in streamsediments and collection and analysis of stream sediment,laterite, river deposits. Detailed mapping of the area and alluvial concentrateand rock samplesfor the PGE, led sampling of the different rock types facilitated the to the discovery of several types of ft-rich PGE discoveryof the primary mineralization. concentrations,unusual in ophiolites. Two of these, The mineralized area correspondsto a transition the Pirogues River mineralization (where high con- zonebetween a unit of massivedunite, interpreted as a centrationsof the PGE were recordedin rock. laterite basal cumulate,and pyroxene-bearingcumulate. The and alluvium) and the Ni Estuary mineralization dunite passestransitionally into wehrlite or cumulus (where high concentrationsof the PGE were found harzburgite, lherzolite and pyroxenite. Gabbro is in beach sand) have been the subject of a detailed invariably in tectonic contact with the cumulates. mineralogicalstudy, the resultsof which arepresented A variety of cumulus textureshave been observedin here. the different rock-types, including coarse-grained P[ MINERALZA'ITON" NEW CALEDONIA OPHIOLITE COMPLEX to25

p16. l. gimFlified geological map of the Pirogues are4 New Caledonia. adcumulate in pyroxenite, and heteradcumulatein of chromite.[n mostcases" the cbromiteis interstitial to wehrlite, with chromiteand olivine as cumulusphases. large grains of orthopyroxene and clinopyroxene. The basal cumulatesare cut by a densesystem of Cbromitite in the pyroxenite dykes has all the charac- dykesconsisting of wehrlite, pyroxeniteand gabbroup teristics of a cumulaterock, and the term oostratiform to 10 m thick, which is apparentlytypical ofthis area. chromite" will also be used for this ore, found in Composite,multiple-injection dykes also are present. the cumulatesequence. It has only been found in the The relationships between the dykes and their host thickest dykes (>1 m). Massive chromitite, without rocks suggestthat some of the dykes result from the interstitial silicates" was discoveredin laterite. This coalescenceof liquid collected from an incompletely peculiar texture could be the result of weatheringof crystallizedpile of cumulates,by a mechanismsimilar chromitite during lateritization, involving complete to filter-pressing.This zonecorresponds probably to an removal of the silicates. No concentrationof sulfide active part of the magma chamber,close to a feeder hasbeen recognized in the mineralizedarea. zo/le. The cumulatesequence is not complete;it is com- Chromite concentrations were obseryed in two plicatedby tectonicimbrication due to its proximity to different positions. In the dunite and wehrtte the base of the nappe. Detailed interpretation of the cumulates,chromite forms thin schlieren and layers relationships between the different rock-types is in (i to 10 cm thick and a meter or so long). This type of many placesimpeded by a thick cover of laterite. ore, referred to as 'ostratiform chromitite". differs In the zonedelimited by a stream-sedimentanomaly distinctly from "podiform chromitite" formed in the discoveredduring the geochemicalsurvey, 93 samples mantle sequence(Cassard et al. 1981, Leblanc & representing the different facies of dykes and Nicolas 1992).It is characterizedby euhedralgrains of cumulateswere analyzedfor Pt and Pd by lead fue- cbromite commonly in a pyroxenite matrix. Chromite assay - direct-current plasma - atomic emission concentationsalso were observedin pyroxenitedykes, spectometry. Table 1 gives the averageand extreme wherethey form either irregular "patches"or layers of values obtained for the various rock-types. Dunite, disseminatedore (with l0 to 50Eocbromite crystals) in wehrlite, pyroxenite and gabbro of the cumulate a pyroxenite mafrix, or as inegular concentrationsof sequencecarry very low Pt values.,Slightenrichment massive ore, also composedof euhedral crystals can be seenin pyroxenite dykes, with a maximum of 1026 TTIE CANADIAN MINERALOGIST

TABLEI. AVEMGE AI.IDEXTREME PI AND Pd CONTENTS (IN PPB)IN TT{E VARIOUSROCK.TYPES OF THE PIROGUES STJITE

Formation Pt min Pd nltn

dunile (8) 20 543 J I wehrlite(6) 2l 540 l5 I 65 pyroxenitecumulote (32) J2 5 140 26 2 t20 pyroxenitedyke (24) 50 5 200 22 4 70 gabbro(4) 23 550 32 5 100 cbromitiferousrocks ( I 9) 3882 230 n500 269 t2 770

Figurein bracketscorresponds to the numberofsamples analyzed.

200 ppb. The Pd distribution is somewhatdifferent, Analyses of the sampleswith highest Pt contents with very low values for dunite (around 3 ppb), and were made by nickel sulfide fire-assay- inductively higher valuesfor the other rocks. The gabbrois richer coupled plasma - mass specffometry(Iable 2). The in Pd than in ft. resultsshow that ft is the most abundantPGE, with a The Pt mineralization seems restricted to the ratio Pt(Pt + Pd + Rh + h + Ru) ranging between chromite-bearingfacies, as all the chromite-richrocks, 4.4 and7.3,again confirming that Pt plays a major role either in the pyroxenite dykes or in the cumulates, in the nrineralization.The pattemof distribution of the show Pt enrichment.The ft content varies between PGE is completely different from that commonly 230 arrd11,500 ppb; it is directly relatedto the Cr2O3 observedin mantlechromitite (Ftg. 3).However, the Ir contentof the rock (Fig. 2), indicating a close associa- tion betweenthe Pt carriersand the chromite crystals. The Pd content of the mineralized samplesremains relatively low (up to 770 ppb) and showsno correlation with the Pt content.

(a v/lJU N li u20

0.001 Os Ir Ru Rh Pt Pd Au Pt Frc. 3. Chondrite-normalizedplots of whole-rock Ftc. 2. Conelation of whole-rock Cr2O3(wtVo) - ft (ppb) group elementsfor the Pt-rich Pirogueschromitite (black in the samples from the Pirogues chromitite, which square:massive chromite, open square: Layered chromite). shows that the PGM are associatedwith the chromite Hatched area corresponds to compositions of New crystals. Caledoniamantle cbromitite Q. Aug6, unpubl. alata). ft MINERALZATION. NEW CALEDONIA OPHIOLITE COMPLEX rcn

TABLE2 PGMAND PGECoNTENT (D,{PPM)OF THEPIROGLIESCHROMITITEFROMULTRAMAFIC CIJMULATE

N' Nature Pt Pd Rh Ru Ir Pt allovs ft oxides cooperite lalrite other ,) R4 massivechromitite (dyke) 4.61 0.55 0.17 0.11 0.04 5 4 1 R5 massivechromitite (dyke) 9.81 0.47 0.42 431 0.t7 5 19 1 4 R3 massivechromitite (dyke) s.94 0.43 0.44 0.28 0.20 4 I 6 J R57 massivechromitite (dyke) 11.50 0.90 0.63 0.19 0.74 5 11 R6 massivechromitite (dyke) 3.90 , I I 4 R1 layeredchromitite (cumulate) 3.30 0.20 0.09 0.07 0.09 1l I I

and Ru content of the ft-rich chromitite is within the fractionsobtained were then treatedwith a supe{panner rangeof samplesof podiform cbromitite. in orderto separateout the densestparticles. The larger Severalheavy-mineral concentates collected in the PGM obtainedwere hand-pickedand then examined area (after yashing about 20 liters of $avel) seemto with the SEM and mounted separatelyin resin to have a high PGE content. One concenfiate,collected preparepolished sections.For the smaller grain-size downstream from the primary mineralization and fractionsothe densestfraction (containing the PGM) containing more than 10 ppm Pt, has been studiedin obtained with the superpannerwas used directly to detail. preparepolished sectionsof the grains. The PGM in these sectionswere then identified in reflected light The Ni estuary and examinedwith the SEM. After the SEM examination.all PGM in rocks and High PGE contents were detected in samplesof coneentrateswere analyzed with a Cameca SX50 cbromitiferousbeach sand collected in the Ni Estuary electron microprobe under the following conditions: (Frg. 1). The Ni River drains the depletedharzburgite acceleration voltage 25 ky, beam current 20 nA, of the Massif du Sud, and there are no indications of counting time 6 seconds.The standardsused were: the potential sourceof the alluvial PGE concenhation. Cr2O3for Cr, AsGa for As, pyrite for Fe and S, stibnite Cbromite crystals on the beach are larger than those for Sb, and pure metals for all other elements.The normally observed.Systematic sampling of the beach X-ray ljnes usedwere Kcr for Ni, CuoFe, S, Co and Cr, showed that the mineralization is discontinuousand lo for Sb, Te, Au, Rh, Ir, Prland Ru, and IB for Os, restrictedto chromite-richhorizons. Pd and As. A PAP correction program was used. Two samplesof sediment containing respectively Several interferencesthat could not be avoided (for 400 and 780 ppb Pt, and very low concentations of example,PdLp and Agtra, or RuZp and Rb,La) were other PGE. were studied. correctedby calculation, Most analysesof PGM includedin cbromitecrystals Saupm PnspARArotIAND revealedthe presenceof Cr and Fe due to excitationof MrcRopnonnTncrnfieues the matrix, becauseof the small size of the particle. This explains the low analytical total in some cases. Polished sectionsof chromitite and chromitiferous Thus Cr, andthe correspondingproportion ofFe dueto rocks with high PGE contents were systematically the chromite (determinedfrom the known Cr/Fe ratio, studied in reflected ligbt at a magnification of 200. measuredin the host chromite)are generally subtracted Base-metal sulfides and platinum-group minerals from the raw analyticaldata, and the atomicconcenta- idenffied optically were studied with a Cambridge tion is calculaledfrom the correctedanalvtical data. Stereoscan250 scanningelectron microscope(SEM) equippedwitl a Si(Li) energy-dispersionspectrometer Sarwle DFJCRTPTIoN (Tracor NorthernTN 2000). Heavy-minsml concenlates and beach sandswere PGM in chrontitite (the Piroguesmineralization) treatedin the laboratoryto separatethe PGM using the technique of J6z6quel(1990). The samples were Six sampleswith ft contentsranging between3.3 separated into five grain-size fractions (<50 pm, and 11.5ppm were examined.Their characteristicsare 50-100, 100-250,250-500, 0.5-1 mm). Eachfraction given Table 2. All the PGM in the six samplesoccur as was separaledwith a hand magnet into 1) ferro- inclusions vrithin chromite crystals. Most PGM are magnetic and 2) pom- and non-magnetic.The ten includedin unalteredand unfractured chromite. Rarelv, TIIE CANADIAN MINERALOGIST

Ftc. 4. Scanningelectron microscope images of PCIM. A, Euhedralgrain of isofenoplatinum from the Piroguesmineraiizerl alluvium; B. composite PGM from the Ni mineralized alluvium; C. isoferroplatinum inclucled in cbromite crystal; D. euhedralcooperite tPtl, with an attachedundeternined Rh sulfide [Rh] in chromite; E. ft-Fe{u alloy containing undeterminedneedles (images C to E are inclusions in chromite from the Piroguesmineralization); F. euhedralgrain of isofenoplatinumcontaining Os latls [Os] and undeterminedF[*Ru-Rh alloy [Rh] (grain from the Ni alluvium). ft MINERALZATION. NEW CALEDONIA OPHIOLITE COMPI-EX t029

Flc. 5. Scanningelectron microscope images of PGM. A. Sectionof the compositegrain shownin Fig. 48, composedofPt-Fe grain' alloy tptl dth *O"trrmioed Pi-Ru-Rh alloy tRhl and "interlayered" Os alloy and Pt alloy [Os-ft]; B. detail_of-the showingthe relationshipbetween the ft-Fe alloy [R] and Pt-Ru-Rh alloy [Rh]; C. complex grain composedof_isoferro- and an platinui fftl containing inclusions of laurite tRul, a lath of bowieite [r], a large zoned grain of laurite [Ru], grain ima"termineann suffide Rhl; O. euhedralgrain of isoferroplatinumcontaining inclusions of laurite [Ru]; E. showing isoferroplatinumin a synplectitic associati,on*i11 6 mineial that has not been preserved;F. two-phasegrain of iridium (white) a:rderlichmanite (grey) containinginclusions sf ssmirrm(Os). All grainsme from the Ni mineqalizedalluvium. 1030 TIIE CANADIAN MINERALOGIST

the PGM is found at the boundarybefween chromite Mineral assemblages and a silicate inclusion (still mainly enclosedby the chromite grain). In some cases,PGM appearwithin Table 3 summarizesthe different minslal species small cracks related to serpentinizationand are occurringin the cbromitite of the PiroguesRiver andin obviously not geneticallyrelated to them. Rather,the the Pirogues and Ni mineralized alluvium. Data for presenceof the PGM seems to have controlled mantle chromitite for the Massif du Sud and Ti6bagfii the direction of the cracks. massif (New Caledonia)are includedfor comparison. The size of the PGM grains is very variable, from In the Pirogues chromitite, the disnibution of the less than 1 pm (too small to be accuratelyanalyzed), PGM varies from one sample to another. Whereas up to 200 Um in largest dimension. Most of the ft-Fe-(Cu) alloys are systematicallypresent (Table 3), grains are in the range 10 to 40 g.m (Fig. 4). pGM Pt sulfides (cooperite)are irregularly distributed;they inclusionsare generally single-phase and isometric, but are abundantin only three samples,and are lacking in some are composedof two phases,both isometric; fwo samples.Pt-Fe oxides were analyzedfor the first othersappear elongate (tabular crystals). Aggregates of time (Aug6 & Legendre L994); thet proportionsvary PGM crystals trapped in chromite also have been considerably.The main sourceof ft in the chromitite observed. appearsto be the Pt-Fe alloys and oxides and the pt Some grains show intergrowths of two phases, sulfides.Laurite alsois widespread.Bowieite, malanite which could be due to exsolution.Others enclose verv and cuprorhodsitewere found in t}ree samples of fine needlesthat havenot beenidenffied (Frg. 4E). chromitite. Base-metalsulfides containing exsolution bodies of PGM and PGE in solid solution were PGM in alluviam observedin one sample.Os-h-Ru alloys, which are the most commonPGM in chromitite from the mantle Ninety per cent of the PGM from the pirosues sequenceof the Massif du Sud (Legendre & Aug6 mineralized alluvium occur in the <100 pn fracf,on, 1986,Aug6 & Johan1988), have not beenrecorded in which representsonly 0.I wt.Voof theconcentrate. The the Pirogueschromitite. rest of the PGM are in the 10G-250pm fraction, in The Piroguesmineralized alluvium is also charac- which most of the grains are ferromagnetic,whereas terized by the abundance of members of the only 30Voof the grains in the <100 Fm fraction is ft-FdCu) system,but with a different distribution: ferromagnetic.About 150 grains from the pirogues isoferroplatinum, rare in chromitite, is the most concentrateswere separatedand studied. In the Ni common PGM (Table 3). ft-Fe oxides are also beachsand, most PGM werein the 50-100 pn fraction abundant in the Pirogues alluvium. The rarity of (no magnetic separationwas done). About 70 grains cooperiteand the lack of laurite arenoteworthv. On the were studied. Concentratesfrom both localities are other hand,Pt arsenide(sperrytite) and separategrains composedmainly of chromitegrains, varying in shape of Os-Ir-Ru alloys, lacking in the chromitite samples, from euhedral crystals to rounded and weathered havebeen discovered in the alluvium. grains.There are also rare iron pisolites and accessory magnetite. Alluvial PGM generally are euhedral, showing TABLE 3. Pq\{ DISCOVERED IN THE PIRO

The mineralizedalluvium of the Ni Estuarycontains (L.54.3 wt.VoFe2O),lowTi (around0.1 wt.7o TiOr) the largestnumber 6f mineralspecies. Like the Pirogues and a high Mg/(Mg+Fe) ratio (0.60-0.75). Mantle alluvium, it is composedmainly of isoferroplatinum. depositsfrom the Massif du Sud are slightly emiched However,numerous alloys of the Os-k-Ru series,PGE in Al and imFoverishedin Cr, as indicatedin Figure 6. sulfides(cooperite, laurite, erlichmanite),sulfarsenides Chromite concenffationsin the cumulateseries of the (hollingworthite and irarsite) and antimonides(stibio Piroguesare4 emichedin Pt, differ significantly from palladinite)were found eitheras small inclusions in other those of samplesof mantle chromitite in having rela- PGM, as separategrains, or asparts of compositegmins. tively low but variableCr (36.349.4 vrt.VoCrp), low PGE oxides have been recognized in the tbree Al (8.9-11.3wt.Vo At2O),high Fe3+ (10.6-2l.2wt.Vo mineralized zones. They comprise minerals of the FerOr), high Ti (0.5-0.9 wt.Vo TiO) and a_low Pt-Fe-O systern,with various contentsof O, but also Mg/(Mg+Fe) value (0.30-0.41). No significant more complex ft-h-Fe-Rh oxides and Ru-Mn-Fe difference was observedbetween cbromitite in dykes oxides (or hydroxides).Their detailed characteristics and chromitite in cumulates' Cbromite in mantle andthe analyticaltechniques employed were described chromitite is characterizedby Al-for-Cr substitution' by Aug6 & Irgendre (1994). with constant Fe3+,whereas chromite in stratiform chromitite is characterizedby Fe3+-for-Crsubstitution, CHEtvfisrRYor rnn Host Cnnourrng with constantA1. Dsseminated chromite in cumulate rocks host'ing The compositionof chromitefrom different samples fte mineralization is marked by large compositional of ft-rich chromitite of the Piroguessuite is plotted in variations that overlap the field of the PGE-rich Figure 6, togetherwith disseminatedchromite from the chromitite. The trend of Fe3+enricbment is strongly cumulateseries, and chromitite from mantle deposits marked, suggestingcrystallization in more oxidizing (Ii6baghi and Massif du Sud). Mantle depositsfrom conditions. The differencesin chromite composition the Ti6baghi massif are composedof chromite of very betweenstratjforrn and podiform depositsindicate that constantcomposition, with high Ct (55.543.5 wt.Vo they derivefrom liquids of very different compositions Cr2O3)and low Al Q.8-14.2 wr.VoN2\),1ow Fe3+ (Roeder1994). Fe

Cr AI Frc. 6. Compositionof cbromitefrom PGE-richcbromitite of the Piroguesmineralization (closediircles) and of disseminatedcbromite in cumulateshosting the mineralization iopen circles).Fields correspondto mantlechromitite from Ti6b:ghi (1) andMassif du Sud (2) deposits. 1032 THE CANADIAN MINERALOGIST

TrE Pr-ATT{rrM-GnoupMnERALS isoferroplatinum, except in two grains, where it reaches2.6 w.Vo. The Pt content of isoferroplatinum Pt-Fe-{Cu-Pd) phases varies between 82.0 and 92.0 wt.Vo [the latter value correspondsto the pure PhGe,Cu) end-memberl.The Two types of h-Fe mineral have been idenffied: deficiencyin Pt is balancedby the incorporationofpd, alloys and oxides,both in the chromitite and alluvirtm. as indicated in Figure 8, Pd reaching a maximum of Although the existence of natural pt oxide had 4.5 wt.%o.Isoferroplatinum plots just below the ideal previouslybeen mentioned (Cabri et al. 1977,1981, substitutionline; the role of other PGE in substitution Loucks1978, Ixer & hichard 1989,Nixon et al. 1990, for Pt is thus minor, and ft is rarely replacedby Fe or Legendre & Aug6 1993, hichard et al, 1994), tke Cu. Iridium, however, enters significantly into abundanceand the size ofthe minerals observedhere isoferroplatinum(maximum 4.7 vtt.Vo),and Rh also is are quite uncollmon. present(up to 2.1,wt.Eo). Isoferroplatinumfrom the Ni mineralizedalluvinm Isofenoplatinum and.undeternxined pt-Fe-Cu alloys; showsless variation in Pt/Fe(Fig. 7B). Similaxly,its pd Isoferroplatinumrepresents the majority of the pdM content is lower. On the other hand, there is a clear of the Pirogues and Ni Estuary alluvial suites. It Rh-for-h substitution,with a maximumRh contentof generally occurs as euhedralgrains, with or without 6.l%[email protected]). Notethat the differentcompositions plot trappedphases @g. 5D). Itr the Piroguescbromitite, on the L:L substitution line, with only local enrich- isofenoplatinum is rare (only three crystals were ments in Os, Ru and h. Only Cu (average0.3 vt.Vo) identified). Representativecompositions of isoferro- and Ni (0.1 wt.Vo)ue generallypresent. platinum are given in Table 4. The composition of the Pt-Fe{u alloys in the The composition of isoferroplatinumfrom the Pirogueschromitite is plotted in Figure l0A. The com- Piroguesmineralized 6llgyirrm is shownin Fisure 7A. position of the ft-Fe oxides is plotted in Figure 10B. No significant compositional zoning ha-s been There are two compositionaltypes of Pt-Fe{u alloy: observed,and only slight variations in the F/Fe ratio a compositionclose to the Pt Fe (with a Pt contentof are recordedfrom one grain to anotler. In contrastto about88.9 wt.Vo), and,acompositional range from ptFe observationsmade elsewhere(Aug6 & Legendre (with a Pt contentof about 62 v,a.Vo)to a composition 1992), Ct does not enter in significant amountsinto closeto ftFeCu (or ftrFeCu, with an excessof Fe).

TABLE4. SETECTED COMPOSITIONS OFPI-F+CU, OS-h.RUAND PI.RU.Rh ALLOYS

Wdght per cmt Os o.o0 0.o0 0.00 0.oo o.o0 o.oo 0.oo 0.oo 61.39 13.15 27,97 41,74 23.58 59.45 2,15 2,22 Ru 0.00 0.06 0.oo 0.38 0.12 0.OO 0.06 0.00 17.32 0.45 4.69 30.59 8.99 0.99 25.96 25.52 Pt 85.75 91.81 82.02 A2.93 78,aa 71,20 74,76 75.82 0.99 7.80 5.49 1.30 15.01 1,32 54,14 54.2A o.o2 0.03 0.00 0,oo 0.63 1.14 0.13 0.o0 0,00 0.01 0.o2 0.o2 0.12 0.OO o.10 0.05 AS o.o7 0.oo o,22 0.oo o.oo 0,o4 o.o4 0.26 o.o9 0.10 0.22 0,06 0.O0 0.o9 0,00 0.oo lr 4,67 0.36 1,72 0.11 0.13 0.oo 2.34 0.29 1A.A3 7A.22 60.83 26,15 49.81 37.53 2,93 3,11 Pd o.42 0.00 6.81 0.60 5.4a 0.94 0.34 0.00 0.00 0.o0 0.20 0.24 0.00 0,00 o.00 0.o0 0.00 0.o2 0.00 0.oo 0,03 0.01 0.00 0.00 0.04 0.o2 0.00 0.00 0,o2 o.oo 0.@ o.00 o.oo o.o1 0.09 0.00 0.o0 0.12 o.o8 0.03 0.00 0.03 0.00 0.03 0.00 0.oo o.ol 0.00 Fe 7,94 7,72 7.96 8.60 11.00 17.19 13.25 9.2a 0.39 0.56 0.00 0.@ 0,69 0.oo t.l9 0,76 Rh 0.56 0.51 0.37 6.07 0.42 0.23 o,27 0.80 0.46 1.61 1.17 0.36 1.55 0.07 13.68 14.O0 't3,24 Cu o.2't o.12 0.36 0.49 0.82 6.00 4,70 0.17 0.86 0.57 0,2A 0.52 0.40 0.10 0.r8 NI 0.o4 0.03 0.o7 0.07 1.01 1,99 0.16 0.27 0.11 0.O2 0.00 0,06 0.oo o.o1 o,o4 0.12 Total 99,67 100.65 99.62 99.25 98.61 98,86 100.14 99.99 99.78 102.80 101.15 100.82 100.29 99.99 loo,31 100,23 Atomicprgponlons os 0'o0 o.@ o.oo o.@ o.oo o,@ o.oo o.oo s2,49 12.go 26.11 s2.zs 21.19 sa.67 1.s7 1,64 Ru 0.00 0.09 0.oo 0.57 0,17 o.OO O.O7 o.OO 27,a6 0.78 8.24 44.66 15.21 1.83 35,68 36.36 ft 70.83 76.93 64.39 64.68 59.52 45,73 48.86 49,72 0.83 7,11 6.00 O.98 13.15 1,27 38.57 38.97 cr o.o5 0.o8 0.00 0.00 0.o0 0.oo 0.33 o.oo o.oo o.oo o.@ o.oo o.@ o,oo o.oo o.oo & 0.15 0.00 0,45 0,@ o.oo o.o7 o.o7 0.45 0.20 0.24 0.5r o.r 1 o.oo o,24 o.oo o.oo lf 3.91 o.29 1.37 0.O8 0.10 o.oo 1.55 0.19 15.93 72.39 56.19 20,08 /14.30 36.04 2,11 2.27 Pd 0.63 0.o0 9.80 o.8o 7.68 l.tl 0.41 o,oo o.oo o.oo 0.33 0.33 0.OO O.OO O.OO O.OO co 0.0o o.o5 0.01 o.@ 0.06 o.o2 o.oo o.oo o.'11 o.o5 o.oo o.oo 0.06 o.oo o.@ o.oo s 0.00 0.06 0.45 0.o1 o.oo 0.46 0.33 0j4 o.OO O.r5 o.OO 0.15 o.OO O.OO O.O4 0.OO Fe go.z5 22,90 22.30 21.a2 23,43 27.48 sa.24 21.26 1,12 1,72 o.oo o.@ 2.11 o.oo 2,95 r.9o Rh 0.88 0.80 0.55 8.98 0.6r O.2A O.34 0.99 0.73 2.78 2.O2 0.51 2.58 0,13 18.47 19.06 cu 0.52 o.31 o.87 1.17 1.89 1i,92 17.46 26,96 o.4s 2.3a r.sg 0.66 1.4o t,l8 o,2z 0.39 Ni 0.12 0.09 0.17 0.19 2.54 4.24 0.35 0.69 0.30 O.O5 0.oo 0.15 o.oo o.o4 0.10 o.29

For minerals inoluded in chromitg raw analyses aro given and atomio ooncenrtetiom are elculated on the baso ofthe mn€cted ealys€s. AP andNi'minsals ftom the mineralized allwium oftle Pirogues mdNi zone repectivoly, R - minenls fiom tho pirogues chromitito. PI MINERALZATION, NEW CALEDONIA OPHIOLIIE COMPLEX 1033 Pt

Fe

Cu Frc. 7. Compositionof Ft-Fe{u minerals from the mineralizedalluvium. A. Pirogues (closed circles: isoferroplatinum;open circles: nrla.meenite;squares: Pt-Fe oxides); B. Ni (closedcircles: isoferroplatinum:squares: Fl-Fe oxides).

70 70

Pt eo Pt uo

50 50

40 40 o246E Pd Rh Ftc. 8. Conelation of Pt and Pd (atomic proportions) Ftc. 9. Correlation of Pl and Rh (atomic proportions) in isoferroplatinum (closed circles) and tulameenite in isofenoplatinum (circles) and Pt-Fe oxides (squares) (opencircles) from the Piroguesmineralized alluvium. fromtle Ni alluvium. ro34 THE CANADIAN MINERALOGIST Pt

Fe Cu FIc. 10. Composition of Ft-Fe{u minerals. A. Alloys from chromitite, pirogues mineralization,B. oxidesfrom cbromitite.Pirosues rnineralization.

Very low contents of Os, Ir, and Ru have been explainedin terms of the size of the PGM studied.In detectedin all the ft-Fe-{u alloys. Rh and pd are the the alluvium, isoferroplatinum(Cu-poor) coexists with only PGE invariably present,but no clem substitution tulameenite (as discrete grains), whereas in the trend of PGE for [t has been recorded.Cu reachesa chromitite mineralization, the intermediatecomposi- maximumvalue of 17.0 vrt.Vo(Fig. 10A). Figure 11 tions conespond,in most cases,to closeassociations of shows substitution of Cu for Fe, from close ro tulameeniteand ft-Fe alloy in very small particles. tetraferroplatinum to a composition close to hrla- meenite(with a deficiencyin ft). However,tlis result is difficult to interpret, *inss dstailed SEM investiga- tions have shown that most of the ft-Fe-(Cu) alloys includedin chromititeare rwo-phase pGM, the grains being too small to be correctly analyzed.A Cu-rich phase(tulameenite?) commonly occursas needlesin a JU ft-Fe alloy @g. 4E). The compositions obtained probably reflect the different proportionsof the phases in the volume analyzed.Bowles (1990) suggestedthe possibility of solid solution betweentulameenite and Cu zo tetraferroplatinum. Observations made here are consistentwith low-temperatureexsolution for some \.rl compositions. Mineral compositionsreveal appreciable differences among localities sampled.Isoferroplatinum from the Pirogues alluvial suite is enriched in pd" whereas isoferroplatinum from the Ni Estuary suite shows Rh ?. .t^:: ) enrichment, both alluvial settings being marked by PtFe' isofenoplatinumwith a low level of Cu. ln contrasl the 50 Pirogueschromitite is characterizedby the scarcity of isoferroplatinumand the abundanceof Cu-rich ft-Fe Fe alloys. This apparent difference between pGM-rich FIc. 11. Correlation of Cu and Fe (atomic proportions) alluvium andchromitite of the PiroguesRiver could be in Pt-Fe{u alloys frorn the Pirogueschromitite. ft MINERALZATION, NEW CALEDON'IAOPHIOLITE COMPLEX 1035

UnnamedPt-Fe ortdes:Ttrc compositionsof the ft-Fe complex elementalsubstitutions (Os + h for Rh + Ru) orides from the chromititemineralization are plotted in are observed.Similarly, incorporationof Ru is marked Figure 10B. They show a wide Pt-Fe compositional by an increase in Rh. Variations in Pt and Fe are range and Cu enrichmentat a constantPt/Fe ratio. Pd more difficult to evaluate,since they may be due to andRh arethe only PGEpresent in significantamounts contaminationfrom the host isoferroplatinum. in tlese minerali (ranging, respectively, befween The generalformula of this phasecan be written: 0.2arrd4.2'trt.Vo,and0and0.5wt.Vo).T\tecucontent Pb.s6-0.aaRu0.z6-o.rrR\.0*zFee.oz+.rho.or+.mOso.01-o.0a' reaches76.8 wt.Vo,and Ni also is invariably present. Taking only the major elements and the lowest Pt The proportion of Pt-Fe oxide (over alloy) in content,this canbe simplified to PtRUR\q This phase chromititevaries from one sampleto another(Table 2). is probably a new mineral species.One of its modesof The composition of the ft-Fe oxide from the Ni occurence @gs. 5A, B) suggestshigh-temperaftre Estuary alluvium is plotted in Figures 78 and 9. In exsolution from a complex phase(to give this phase contrait to the alloys, oxides are marked by a Pt/Fe and an isoferroplatinumcomposition)' The pr,oportion atomic ratio close to unity and a low Rh content.The of the ftRuRh phaseis low, andestimated at l1Vo.Thts compositions of Pt-Fe oxides from the Pirogues gives an initial compositionthat is not very different allu;ial suite are plotted in Figure 7A. A wide from an isoferroplatinumcomposition (with ft 82.02' range of oxygen contentshas been recorded,from Fe 6.77,Cu 0.76,Tr L.2L,Rh 4.36,Ni 0.17 ut.Vo),but 1.0 wt.Vo, wnicn corresponds to the formula with Os (0.36)and Ru (4.09).The lattertwo elements, to L0.4 wt.Vo,corresponding however,have so far not beenrecorded in significant @t9.rrFe9.31Pdn.ot)>r.rOo.z,"(Pt0.5;ii;oj6P4;i"rrbr., to The increasein-oxygen amounts in isoferroplatinum.This could explain the doesdiii miidify tfie nffe'iatio, which remains,with instability of this phaseat a low temperature,resulting few exceptions,close to 1. High Pd contents(average in exsolution. 2.9 wt.Va,maximum L3.4 tryr.Vo)have been recorded in the oxides (Aug6 & Legendre1994). Os-Ir-Ru alloy

Talameenite:Tulameenite was discoveredin severalof Os-Ir-Ru alloy is considered characteristic of the isoferroplatinumgrains from the Piroguesalluvial podiform chromitite from ophiolites and placers suite. It occurs as small gnins either attachedto the derived from ophiolites. Surprisingly, it has not been host isoferroplatinum, or included in it. The Fe{u found in the Piroguessuite of chrcmitite, but it should ratio of the Pt2FeCugrains is variable (Table 4), one be emphasizedthat the chromitite sampleshave very particle having a compositionclose to the ideal com- low concentrationsof Os, Ir and Ru, lower than mantle position (Frg. 7A). No Cu-rich ft-Fe alloy has been chromititein somecnses (Frg. 3). However,lamellae of found in the Ni mineralized alluvium. In contrast"a Os-h-Ru alloy have been identified as products systematicCu-enrichment is observedin ft-Fe alloys of exsolution in isoferroplatinum from the Pirogues in the chromitite, with some compositions plotting alluvium. close to the Pt2CuFeend-member, which could be Os-k-Ru alloy is more commonin the Ni Estuary interpretedas being tulameenite@gs. 10A, 11). suite. where it occun either as: 1) discrete grains' 2) two-phase grains with erlichmanite @g. 5F1, or UnnamedPt-Ru-Rh alloy 3) laths in complex grains of isoferroplatinum.In the third case, the Os-k-Ru alloy contains very thin A mineral with Pt, Ru and Rh as major constituents (<1 pm) lamellae of Ft, too ttrin to be analyzed has been discovered in several grains of isoferro- (Figs.4F, 5A). Most compositionsplot in the osmium platinum from the Ni Estuarysuite. This phaseoccurs field, closeto the rutheniumor iridium fields Qlarris & in two different environments:1) it forrns an apparent Cabi l99L; Table 4, Fig. 12). Individual grains are matrix of very small rounded grains of Pt3Fe homogeneousin compositionand carry L.2-I5.0 vtt.Vo composition @gs. 5A, B); this texture suggestsa Ft, which is presentin solid solution or as thin laths. process of high-temperatureexsolution, and 2) Several compositionsplot in the miscibility gap as oodroplets"(2 or 3 pm. across,Fig. 4D, or irregular defined by Harris & Cabri (i973) and correspondto larger crystalsapparently trapped in the host isoferro- laths (with up to 15 ,lut.VoPt in solid solution) adjacent platinum. to large grainsof osmium. The largest crystals are those that give the lowest The analytical data presentedhere fall in the range ft and Fe contents, around 53.3-56.9 wt.Vo and of compositions given by Hagen et aI. (L990) fot 0.8-2.9 wt%o, and the highest Ru and Rh content alluvial alloys derived from an ophiolite. They differ, Oetween2L.6 and27.9 w.Vo,and 6.3 ard L5.9wt.Vo, however, from compositions of osmium in mantle respectively;see Table 4). The other elementsdetected chromitite (Legendre& Aug6 1986) in the systematic in significant amountsinclude Os (between0.9 and presence of ft. Cumulate chromitite from Albania 5.L wtVo),h (1.0-9.4wtVo), atd Cu (up to 0.6 tttt.Vo). contains inclusions of Os-h-Ru alloy with up to In spite of the small number of analysesconsidered, 20 wt.VoPt Q.{ezirajL992). Peck et al. (L992) gavea 1036 THE CANADIAN MINERALOGIST Ru

a Osmium J

Os Ir Ftc: 12. Composition of Ir-Os-Ru alloys from mineralizerl alluvium. Closed circles: Ni, opencircles: Pirogues."l: field of rutleniridosmine, 2: miscibility gap.

range of 0 to 2.3 wt.Vo PI in grains of alloy from iridium as a two-phasegrain, the other is included in westernTasmania. isofenoplatinum).Laurite is more common;it hasbeen found asinclusions in cbromitefrom the Piroguessuite Cooperite (whereit occursas isolatedcrystals or, in rare cases,in a complex associationwith other PGE sulfides) and Pt sulfidesoccur almost exclusively in the Pirogues in the Ni Estuary alluvium, where it fonns large dis- suite of chromitite, where they are irregularly dis- cretegrains or occursas inclusionsin isofenoplatinum tributed within 15emineralized zones (Table 2). T\ey or, more rarely, in osmium alloy. Note the absenceof occur as small monophaseinclusions in the chromite laurite from the alluvial suite of the Pirogues zone crystals;in a few cases,small grains of Rh sulfide are (Table3). adjacentto cooperite(Frg. aD). Only one 100-pmgrain The compositionsof laurite and erlichmanite are of cooperite has been discovered in the pirogues given in Table 5. Laurite from the Pirogueschromitite alluvium, and one in the Ni Estuarvalluvium. whereit tends to have a more restricted composition than forms small (10 pm) inclusionsirra grain of isoferro- alluvial laurite from the Ni beachsand. both environ- platinum. mentsbeing chwacteized by a low content of k. On The compositions obtained are close to ideal the other hand,laurite from the Ni suiteis markedby a cooperite(Table 5), with low Pd (from 0 to2.78 wt.Vo) low but significant enricbmentin As (up to 6.3 wt.Eo), and Ni contents(from 0.05 to 0.82 fi.7a). Rh also with an As-for-S substitution correspondingto the was detected in some grains, with a maximum of replacementof one atom of S by one atom of As. LL5 wt.%a.The analytical results tend to show an A positive linear correlation between Rh and As excess of S, with an average formula close to @h reachingL2 wt.Vo)could indicatethe incorporation Mo.gqSr.06. of RhAsS in the structure of laurite. Other elements entering the laurite are Ft (0-13.1 wt.Vo), Cn Lourite-erlic hmanites erie s (A4.4wt.Ea) andNi (H.2 wt.7o). Pt, almost systematicallydetected here, does not Two crystalsof erlichmanitewere discovered,botl occur in Ru-Os sulfide from mantle cbromitite. Aug6 from the Ni Estuary alluvium (one is associatedwith & Legendre (1992) noted that the presenceof Pt in ft MINERALUATION, NEW CALEDONIA OPItrOLITE COMPLEX 1037

TABLE 5. SELECTED COMPOSMONS OF PGE SULFIDES

orlichmanite laurhe coooerhe bowtefte ffi NilNi3N|4RlANi6Ni8R1BBlcR.|DR5AR1DNigNiloNi1l Wolghtper csnt Os 64.90 40.95 6.13 11.88 5]7 0.87 o.OO O.OO 0.00 0'o0 0'00 0.00 0.oo 0.@ Ru 1.01 20,64 42.85 41.3A 49.96 57.44 o.OO 0.o0 0.o0 0.00 0'00 0.oo 0.00 0.00 ft 1.59 0.22 1.06 0.14 4.29 O,41 81.59 79.51 81.91 7A.78 0'25 11.84 4.24 4,27 o.oo o.oo o.0o 1.21 0.13 0.12 1.35 2.09 0.86 2.O1 2.AA 0.OO 0.07 0.12 As o.35 o.oo 8.40 0.15 0,74 0.34 0.o8 0'01 0.00 0.01 0.o0 0.oo 0,11 0.06 ll 5.79 A,21 0.92 1,75 1.33 o.l4 o.O8 0.03 0.36 0.o0 32.01 4.77 1.36 1.36 Pd 0.35 o.oo o.oo o.oo 0.28 0.oo 1.15 1.10 1.16 1'49 0.00 0.o4 o.23 0,63 Co o.oo o.@ o.o3 o.o4 o.oo o.o3 o.o3 0.00 0.03 0.01 0.00 0.@ 0.10 0.06 15.40 15.45 15.11 15.60 25.9A 2A39 20.03 19.97 s 25.56 2A,72 33.00 35,74 31.45 36.73 'I FE o,oo o.o5 o.0o 1.20 0.45 0.37 0.81 1.46 0.60 1.62 1.88 0.53 .16 0.9S Fh o.5o 0.66 12.29 A.71 3.93 2,40 0.21 0.35 0.32 0.36 36.72 67.',1o 71,O2 70.42 o.o9 o.o8 o.o3 0.oo o.ol o.oo o.oo o.@ 0.oo o.oo o.41 0.03 1.06 0.92 NI o.o5 0.o2 0.06 0.04 0.07 0.@ o.1o 0.63 0.o7 0.48 0.o2 0.o4 0.84 0.91 Total 100.19 99.55 101,70rcA.24 98.40 99.01 100.79100.62 100.41 100.36 [email protected] 100.74 100.26 99.65 Atomicproportions Os 28.39 15.74 1.59 3.75 1'91 0.26 O'OO O.@ O.@ 0.0O O'0O 0'@ O'O0 o'oo Ru O.83 14.e3 24.gg 24.56 31.04 32.39 0.OO O.OO O.OO O.00 0.0o 0.0o 0.0o 0'00 ft o,o8 o.o8 o.32 o'04 1.38 o.12 45.72 44,s9 46.21 44.06 0.09 4.1o 1.57 1.58 A3 0.39 o.oo 5.03 0.12 0.62 0.26 0.12 o.ol o.oo o.o2 0,00 0'oo o'11 0'06 b 2,51 5j2 O.28 O.55 O.43 O,O4 O.O5 O.O2 O.21 O'00 12'42 1'68 O'50 o'51 Pdo.27o.ooo.@o,0o0.160.o01.181.131'19',1,520.000'240'100'42 co o.oo o.oo o.o3 o.o4 0.oo o.o3 o.o5 o.oo 0.06 0.03 0.o0 0.00 0.00 0.oo s 66.33 65.48 60.66 60.88 61.60 65.30 52.49 52,71 51.86 53.10 60.39 55.66 44.64 44.42 Foo.ooo.o7o.ooo.11o.g7o.27o.0oo'[email protected].@o.&1.4a1.2o Fh 0.41 0.47 7.OO 3.91 2.40 1.33 0.22 0.37 0.34 0.38 26'6/0 37.62 49.31 49.24 Cu 0.1'l O.O9 O.O3 O.OO O.O1 O.OO O.OO O.OO O.OO 0.00 0'48 0'O3 l'19 1'o4 NioioSo!o20'060i040.07o.o00.181.17o.13o.890.020.051.o31.12

Samelegend as Table4.

laurite characterizesminerals derived from a Pt-rich P6.ot)>a.szS+.o2,the ideal composition being RhrSa. environment.This is confirmedhere. This gives a metal: sulfide atomic ratio of 1.25,which doesnot correspondto prassoiteGh17S1s or Rh3S). Rh-Ir suffides This phasecould be the Rh sulfide equivalentof the compoundPt5Sea, described as luberoite by Jedwab Bowieite: Two grains close to the ideal formula et al. (1992). Palladseite@d17Se15), a Pd-Se equiva- (Rh,hft)2s3 (bowieite)have been discovered. The first lent of prassoite,has also been described@avis et al- (anal. RlD, Table 5), in the Piroguescbromitite, is a 1977). thin, 20-pm-long, tabular crystal trappedin chromite, of composition (Rh136\.62Cuo.o)>z.oS:.0,with minor Thiospinels amountsof Pt (about 0.4 wt.Vo).The other gfain of bowieite was discovered in a complex grain of SeveralPGE sulfidescontaining Pt, h or Rh as PGE isoferroplatinum,with small inclusionsof laurite, from and a base metal @e, Cu or Ni) were discoveredin the Ni Estuaryalluvium (anal. Ni9, Table 5, Fig. 5C). the Piroguessamples of chromitite. They occur either This grain, G.h1.e3Ph.16h0.oe)E2.rzSz.ar,differs in com- as discretecrystals or as fwo-phasegrainso associated position from that discoveredin chromitite by its higtt with cooperiteor with base-metalsulfide. Becauseof Pt content and much lower Ir content.It also contains their small slze and the fact that they are generally minor Pd (0.1-0.4wtVo) andFe (0.1-1.4wt.7o). S is associatedwith other phases, it is difficult to depletedwith respectto the ideal formula. characterize their composition precisely. However, thesephases appear clearly to belong to the thiospinel UndetermincdRh suffide: In the alluvial grain from (linnaeite) group, with the general formula the Ni Estuaryalluvium that containsbowieite, another (Fe,Ni,CuXIr,Rh,Pt)2Sa(Table 6). Figure 13A shows Rh sulfide (about20 pm across,Fig. 5C) was analyzed. two compositionalgroups: the malanite field (ideally Its Rh contentis high, about71 wt.Vo.Italso contains CuPt Sr, but markedby extensiveft-for-Rh substitu- Pt (4.4 wr.Vo),h (1.4),Fe (1.0),Cu (1.0),Ni (0.9)and rion, and the cuprorhodsitefield (CuRh2sr. Similarly, Pd (0.4), and about20 wt%oS (Iable 5). The grain Figure 138 showstwo types of composition,one close corresponds to (Rha.aoPt1.1aFeq.13Cus.1,Niq.eelre.etto the Cu apex,and the otherrxiith a Fe{u composition 1038 TTIE CANADIAN MINERALOCIST

TABLE6. SELECTEDCOMPOSMONS OF VARIOUS PGE SPECIES

Weight per cant Os o.o0 0.00 0.o0 0.00 0.00 0.36 o.41 0.oo 1.62 1.24 0.01 0.00 0.@ o.01 Ru o.oo 0.00 0.00 0.69 0.00 0.04 o.84 0.99 4.58 0.@ o.13 0.00 0.00 0.00 Pt 42.48 12.46 2.10 0.52 1.39 1.33 6.46 1.39 0.AO 17.26 56.38 57.34 3.46 4.50 Ct 1.16 1.71 2.60 2,36 o.48 0.41 0.00 0.@ 0.0s 0.00 0.00 0.00 0.16 0.09 As 0.oo 0.11 0.o9 0.00 31.97 33.23 26.A3 26.29 24.24 27.86 41.94 41.48 o.34 3.62 It 5.57 24.21 18.92 17.53 0.42 0.r9 49.39 40.20 r6.66 41.48 0.26 0.10 o.22 0.21 Pd o.44 0.17 0.00 0.20 o.34 0.44 0.00 0.]6 0.00 0.00 0.o2 0.oo 66.69 68.23 Co o.21 0.06 0.40 0.28 0.00 0.o7 0.00 0.00 0.01 0.@ 0.o0 0.@ 0.@ o.o2 s 26.21 27.82 30.37 30.18 14.43 16.2A 11.25 13.95 16.23 7.43 0.66 0.44 o.o8 0.07 Fe 1.04 6.67 16.82 12.54 o.a7 0,50 o.00 0.oo o.o0 0.38 o.09 0.oo 0.09 0.93 Rh 16.12 21.84 18.97 20.40 49.29 47.09 4.47 17.17 32.90 0.61 0.81 0.16 0.09 0.00 0.00 0.00 0.00 0.00 o.00 0.00 0.o0 0.oo 0.00 0.00 0.o0 0,00 o.35 1.03 Cu 9.06 7.69 6.11 9.92 0.06 0.00 4.64 0.40 0.19 0.62 0,o0 0.00 1.78 0.09 sb o.o0 0.00 0.00 0.00 0.96 0.19 0.00 0.10 0.00 0.oo 0.00 0.00 28.88 21.13 Ni o.18 0.67 7.91 5.18 0.o7 0.o1 o.o2 0.@ 0.02 0.05 o.oo o.oo 0.o4 0.01 Total 102.47 103.30 102.29 99.67 100.30100.14 99.43 100.63101.4't 97.41 100.22 99.52 102.07 99.94 Atomicproportions Os 0.o0 0.00 0.00 0.00 o.o0 0.13 o.21 0.00 0.63 0.71 0.01 0.00 0.00 0.01 Ru o.o0 0.00 0.00 0.35 0.00 0.03 0.79 0.83 3.36 0.00 0.15 0.00 0.00 0.00 n 15.85 4.27 0.62 0.16 0.51 0.rl8 3.16 0.60 0.30 9.32 32.89 34.06 1.91 2.61 As o.oo o.10 0.07 0.oo 30.61 30.99 33.05 29.56 27.93 39.19 63.77 64.14 0.49 6.24 It 2.11 8.42 6.72 A.M 0.16 0.o7 24.M 17.62 6.42 22.74 0.16 0.06 0.13 0.12 Pd o.30 0.11 0.00 0.11 o.23 0.29 0.oo o.12 0.00 0.00 o.o2 0.00 67.68 69.98 Co 0.26 0.o7 0.40 0.26 o.00 0.17 o.@ 0.00 0.o2 0.00 0.o0 0.00 0.00 0.0o s 59.49 57.99 55.10 56.16 32.28 36.13 33.38 36.64 37,44 25,74 1.93 1.67 o.27 0.26 Fo o.o0 6.13 15.06 11.12 1.12 0.63 0.00 0.00 0.oo 0.72 o,17 0.oo o.18 1.42 Rh 11.40 14.18 9.69 11.83 4437 31.97 4.13 14.06 23.66 0.62 0.89 0.1a o.o!t 0.oo Cu 10.37 8.09 5.59 9.31 0.07 0.00 o.81 0.52 0.22 0.36 0.00 0.@ 3.O2 0,15 le 0.00 0.@ 0.@ 0.@ 0.@ 0.00 0.@ o.00 0.00 0.00 0.00 0.00 0.36 0.88 sb 0.00 0.00 0.o0 0.00 0.67 0.11 0.00 0.07 0.00 0.00 0.00 0.00 26.81 19.01 Ni o.22 0.65 7.84 6.27 0.o8 0.01 0.04 0.00 0.03 0.o9 o.oo o.oo o.o7 0.o2

Samelqgedd as Table 4.

(with a slight enrichmentin Ni). The former corre- tion (Table6). Os (up to 0.3 *.Vo), Ru (0.2),Pd (0.2) spondsexactly to the malanite composition,whereas and Rh (1.2) havebeen detected sporadically, whereas the latter correspondsto the minerals plotted in the h (up to 0.8) is systematicallypresent. Fe (up to cuprorhodsitefield, which areenriched in Fe. A similar 0.7 ,lt.Vo) also was detectedin most grains, as was S observation was made by Johan et al. (1990) fot (0.2to0.7 wt.Vo). cuprorhodsitefrom the DuranceRiver alluvium. Srtbiopalladinite Ir anl Rh sulfarsenides Stibiopalladinite (ideally Pdssb, occurs in two Irarsite (hAsS) and hollingworthite @bAsS) have grains of isoferroplatinumof the Ni alluvium as small been identified in the Ni alluvial samplesonly. They crystalsat the boundarybetween harsite and isoferro- oscur as small crystalsadjacent to isofenoplatinumor platinum. The compositions obtained (Table 6) are included in it. One grain is a complex associationof similar: (Pda.76Cun.21\.13h0.01)x.11(Sbr.rrAso.orTeo., hollingworthite, irarsite and sperrylite. harsite and So.oz)>r.rs.The Cu content seemsto be particularly hollingworthite present a large compositionalrange. hieh. Ottrerelements detected in significant amountsinclude Os (up to 4.76 wt Vo),Ru (up to 7.2), Pd (up to 0.44), DrscussloN Fe (0.96)and Cu (0.54,see Table 6). Two types of chromitite havebeen identified in the Sperrylite New Caledonia ophiolite complex, with which two types of PGE enrichment are associated: mantle- Sperrylite is present as discrete gfains in the derived chromite deposits (such as Ti6baghi) with a Pirogues mineralized alluvium and in a complex long history of exploitation, containing Os-h-Ru in irarsite-hollingworthite grain in the Ni Estuary. The the form of alloy and sulfides, and cbromite concen- sperrylitegrains are close to the ideal PtAs, composi- frations in the cumulate series, containing the Pt ft MINERALZATTON. NEW CALEDONIA OPHIOLTTECOMPLE'X 1039 Rh

Ir Pt FIc. 13. composition of PGE thiospinelsfrom the chromitite mineralizationA) in terms of the triangular diagramRh-h-Pt (closed symbols correspondto Cu-rich minerals), and B) in terms of the triangular diagram Cu-Fe-Ni (closedsymbols correspondto Pt-rich minerals).

enrichmentdescribed here. is characterizedby Cr-rich, Fe3+-poorchromile, typical The two types of chromitite - PGE mineralization of podiform chromite, crystallizing from a primitive show similarities. such as in the mode of tle occur- melt at low oxygenfugacrty. Stratiform chromitite and rence of the PGM, systematically as inclusions in chromitite in dykes,marked by a relatively low content chromite crystals, and the coexistenceof PGE alloy of Cr and strong Fe3+ and Ti enrichment, have and sulfides. On the other hand, the two types are crystallized from a more evolved (Al-Ti-enriche4 distinguishedby the natureof the PGM, their size and Cr-Mg-depleted) melt under msls qxidizing condi- abundance,and the textureand compositionofthe host tions. The presenceof interstitial silicates, olivine in chromitite, suggesting major differences in the mantlechromitite andpyroxene in cumulateore, points environmentin which they crystallized. in the samedirection.

Host chromitite Origin of the PGE enrichment

Typical textures of mantle chromitite indicate The origin of PGM in mantle chromitite has been crystallizationin a turbulentmilieu (Lags et aI. L982), ascribedto the early crystallizationof small Os-k-Ru whereasthe textures of stratiform chromitite suggest PGM in a conduit (accordingto the model of Lago crystallizationin quieterconditions. Mantle chromitite et al. L982), the PGM then being tapped, acting as 1040 TIIE CANADIAN MINERALOGIST

nuclei, during chromite formation (Aug6 1985). The (Frg. 14). This hypothesishas been proposedby absenceof PGM in the surroundingsilicales and of Amoss6et al. (1990) to explain the presenceof Pt-Fe sulfide phasesrelated to the PGE mineralizationleads alloy included in chromite crystals in Alaskan-type us to suggestthe same mechanismfor the Pirogues complexes.Roeder & Jarnieson(1992) have deter- chromitite, i.e., the crystallization of chromite in an mined experimentallythe compositionof chromiteand environment that already contained the PGM, the coexisting ft-Fe alloy under magmatic conditions. chromitetrapping the PGM. They showedthat a decreasein.(Or), increasingthe The abundanceof PGM in a thin layer (represented activity ofFe in the alloy relative to that ofthe oxidized by the chromitelayer) suggeststhat the mineralization iron species,could stabilizethe alloy. resultedfrom the suddennucleation of the PGM at a Other authorshave investigatedthe partitioning of cerlain level of the magmachamber. Crystallization of PGE betweenspinel and silicate melt. Capobianco& chromik could have been induced by the presence Drake (1990) concludedthat Ru and Rh are strongly of the PGM in the magm4 which facilitatesnucleation compatible in spinel, whereasPd is snongly incom- becauseof the property of the PGM of reducing the patible. However,the role of PGE partitiening into the activation energy for crystallization (Capobianco& spinel structure on the formation of PGE concentra- Drake 1990). tions has not been clarified. gimilafly, the hypothesis A complex assemblageof PGM is presentin the that PGM are formed by exsolution from the host Piroguescbromitite (Table3). In terrnsof conditionsof chromite has been rejected(Aug6 1988), and there is crystallization, the coexistence of PGE alloy and no reasonto reconsiderthis possibility. sulfide is difficult to interpreq as this would require a The distribution of the PGE-chromite-enriched largerange of temperatureandflS), incompatiblewith zonesassociated with dykes suggeststhat the crystal- their presencein a single sample.A possibleexplana- lization of the PGM andthe associatedcbromite results tion is that the different speciesare not in thermo- in complex and suddenlyoccurring processes.Seams dynamicequilibrium. The milieu from which the pGM of chromite and concentrations of chromite in have crystallized evolved rapidly in a closed system cumulatesand dykes emichedin orthopyroxenein an local increasein due to chromite crystal- lwith "(S, ophiolite are uncommon and require special condi- lizationl; the entapment of the early PGM in chromite tions. We suggestthat crystallizationof cbromite (and did preclude any subsequentre-equilibrationo thus of the included PGM) results from influx of a mafic, preserving a complex assemblageof minerals. The Cr-rich magmainto the chamber,with interaction of presenceof base-metalsulfides (BMS) in chromitite, the two liquids. The abundance of dykes in the probably due to oversaturationin sulfur, illustratesthe Piroguesarea and the discontinuity of the mineralized incompatibility. We consider that the assemblage zonesat a meter scaleargue for numerousinjections of included in chromite is a high-temperatureone, PGM magma at the base of the chamber. The initiation having crystallized above 1050'C (temperatureesti- of mixing betweenthe new magma and its host will matedusing pyroxenegeothennometry in chromitite). promotechanges in the physicalconditions [increase in Clearly, the platinum mineralizationand the crystal- flO), decreasein Tl.These new conditions,together lization of chromite are genetically linked; the with low fugacity of S in the magm4 will promote concenfrationfactor (in a hypotheticalmagma) for both sudden crystallization of the PGE (Amoss€ et al. ft and Cr is of the sameorder of magnituds.Qa ths 1990), which will be accompaniedor followed by basisof a simplified figure of 30 ppb Pt in the magma precipitationof chromite (dueto the samevariations in and 3 ppm in tle ore, and A.4VoCr2O3 in the the physicalconditions, see kvine 1981).The close rnagmaand 40Vo in the chromitehorizons, the concen- associationof the two suggeststhat this processoccurs ffation factor can be estimatedto be 100.It is thus not in a restricted zote. at the interface betweenthe two necessaxyto envisage two different processesfor magmas, representedby the mineralized layer. A concentrationof chromite and PGE. Both Cr and PGE similar mechanism has recently been proposed by could have been collected from a volume of magma Pedersenet al. (1993) to explain Pt mineralizationin severalhundred times that of the ore. ophiolitic cumulatesof the Leka ophiolite, Norway. The effect of oxygen fugacrty on the solubility of The nature of the rocks mineralized in PGE (i.e., PGE is importan! as shownexperimentally by Amoss6 abundanceof orthopyroxene,presence of chromite et al. (1990) and Amoss6 & Allibert (1993). An concentrationscrystallized from an evolved liquid) increasein oxygen fugacity resultsin a decreasein Ir suggeststhat they could have been derived from a solubility, whereasthe solubility of ft also decreases Cr-Mg-Si-rich magma,of possibleboninitic afftnity. but remainshigher than that of h. This could explain the observation made here, of crystallization of k PGM in chromitite versusplacer and lnterite (associatedwith Os and Ru) as an alloy in mantle chromitiie at low oxygen fugacity, and crystallization The differences between primary and alluvial of ft alloy at a higher oxygen fugacity, higher in mineralizationin the Piroguessuite are readily inter- the ophiolite sequence,i.e,, in the cumulate series pretedin lerms of weatheringand different methodsof ft MINERALZATION. NEW CALBDONIA OPHIOLITE COMPITX LMI

Stratifurm mineral izat i on

- ft,Fe,Cu alloys - Pt sulfides PGM Chromite - Ru,Ossulfides - Pt,RlLIr sulfides $roxene Ridgeaxis I B t Magmatic ,A ^' ,4, .- Pyroxenite Sequence :/a ={r:ft&1 : :n:n- n:5- * Wehrlite at =-=:'_-:* f ._ Dunite

Mantle Peridotite

- Os,Rusulfides Olivine PGM - Os,k Ru alloys Chromite

Podilorm mi neralizati on

Flc. 14. Location and characteristicsof "podiform" (A) and "sfatiform" @) cbromitite - PGE depositsin the New Caledonia ophiolite. 1. Emplacementof podiform chromitite at the ddge axis, 2. Deformation of the podiform bodiesdue to mantle flow. 3. Emplacementof the chambenand dykeswhere PGM and stratiform chromitite crystallize.

sampling. PGE sulfides (cooperite, laurite and Piroguessuite (Iable 3) in the nature and diversity of thiospinels),representing more than 50Voof. the PGM PGM. Pt-Fe alloys arethe most commonminerals, but grains in chromitite, are lacking in the alluvial represent only 4OVoof the grains, followed by mineralization.This could be explainedby a sampling Os-h-Ru alloy (l0%o), lawite' (L07o), and holling- bias, sincethe PGE sulfidesin chromite,in contrastto worthite (10Vo).T\e location of the mineralization the Pt-Fe alloys, form very small grains,which cannot (beachsand at the Ni Estuary)gives no indicationof its be recovered in the heavy-mineral concentrates. origin. However,the zone as a whole correspondsto a However, the reducedpresence of Os-h-Ru alloy, a typical harzburgitic mantle series. Sequencesof mineral classically describedin placers deriving from cumulateappear 20 km farther south (in a zonethat is ophiolites (Cabri & Hanis 1975, Hagen et aI. 1990) not drainedby the Ni River). The mins6[i241ie11111131 aud recognizedin the mantle cbromitite of the Massif haveoriginated in anothertype ofunit, probablypart of du Sud (Iable 3), must be emphasized. the mantle series.It shouldbe notedthat alluvial ft-Fe Many PGM from the alluvial suite show textures mineralswere alsofound in the Ti6baghimassf where attributedto solutionduring surfaceweathering; "near- cumulate rocks are totally absent (Augd 1993). A perfect crystal form", such as those describedfrom preliminary suwey indicates the possibi[ty that alluvial nuggets (interpretedby some authors as the pyroxenite dykes are a possible soruceof the grains. result of mineral growth in the surface environment: Similar dykes, which are very abundant in the Ni seeBowles 1986) are very raxe. region, could be the source of the mineralized The Ni atluvial suite differs significantly from the alluvium. L042 TTIE CANADIAN MINERALOGIST

Comparisonwith other environments paragenesisto the mineralized alluvium described here. However, an important difference lies in the Among the ophiolite complexeswhere significant presence of silicate and glass inclusions in ft-Fe Pt enrichmenthas been described,two show strong nuggets from Alaskan-type complexes(Nixon et a/. analogieswith the Piroguessuite; theseare the Eshie- 1990,Johan et al. 799I, Aug6 & Legendre1992), the Beauceophiolite in Quebec(Gauthier et a\.7990) and natureof which suggesta high partial pressureof H2O the Tropoja ophiolite in Albania (Obnenstettsret al. in the melt. Such inclusions were not found here. 1991). Another, the Leka ophiolite complex in However, we wish to emphasizethat a mineralogical Norway, showsdifferences that deserveto be stressed. study of the Pirogues and Ni mineralized alluvium Pedersen et al. (1993) described PGE-enriched alonewould certainly not have suggestedan ophiolitic rocks in the Leka complex.They identified ft and Pd source,but rather an "Alaskan-typeinfrusion that has enrichment in mantle chromitite, which appearsin yet to be idenffied". some ways 5imil6 fe some of the Unst chromitite Certain samplesof stratiform chromitite show very (Prichard& Tarkian 1988).In contrastto the observa- high enrichment in PGE, e.g., the cbromitite in the tions madehere, the stratifom chromitite in the Leka Middle andUpper groups of the Bushveldcomplex and ophiolite is depleledin ft (and Pd). Such depletionis the A chromitite of the Stillwater complex. Lee & explainedby the fact that the ft (and Pd) concentra- Pany (1988) and Teigler (1990)have shownthat PGE tions are displacedrelative to chromite:Pt emichment concenffationincreases with stratigraphicheight, and is observedabove the chromitite. and Pd enrichmentis Von Gruenewildt et al. (L986) emphasizedthe observed in sulfide-rich horizons. Pedersen et al. differencesin the PGE disribution betweenthe various (1993) suggestedthat the PGE were scavengedfrom layen of cbromite in the Bushveld complex and the the parentalmagma shortly after it enteredthe magma Merensky reef. They found that the PGE mineraliza- chamber,and before it mixed with the residentmaema. tion occurs as sulfides Qaurite)and arsenideincluded The lack of Pd mineralization in New Caledoniais in chromite crystals, but in many cases Pt and Pd easily explained by the lack of sulfides, which are minerals are in close association with base-metal known to collect Pd. The main differencebetween the sulfides.interstitial to the chromite. Leka and New Caledonia ophiolites is that PGM Thus, in contrast to ophiolitic or Alaskan-type crystallizationwas not accompaniedby chromiie crys- mineralization, base-metal sulfides have played a tallization in the Leka ophiolite. This differenceshows major role in PGE mineralization associatedwith that the presenceof chromite is not necessaryto chromite layers in stratiform complexes.Naldrett & preserve the PGE mineralization. However, the Pt Yon Gruenewaldt(1989) suggested that in a first stage, content recorded in the New Caledonia samples is the PGE in the chromite-richlayers were collectedby much higher than that observed the stratiform Pt a sulfide liquid. In a second stage, desulfurization mineralizationin the Leka ophiolite. This indicatesthat leaves PGE-rich chromitite containing very sparse chromite is an efficient PGE 'tollecto/'. Attention sulfides. Such a process cannot be invoked for shouldbe focussedon and abovethe chromitite in the mineralizationin an ophiolitic setting. context of prospectionfor 'ostratiformFt deposits" in ophiolite. CoNcLUsIoNs The ophiolitic mineralizationof the PiroguesRiver also shows striking similarities to Alaskan-type (1) The New Caledoniaophiolite containstwo typesof mineralization. Samples of chromitite at Tulameen, cbromite deposits (podiform deposits in the mantle British Columbia, are stongly enriched in ft witl sequence,and stratifom deposits in the cumulate respectto the other PGE. Their composition,marked sequence)with which two typesof PGE mineralization by Ti and Fe3+emichmeng is very similal to 16s are associated:a typical Os-Ru-h associationin the chromitite described here, and the primary high- podiform chromitite, and a ft-dominant associationin temperature PGM paragenesis is composed of a the sfatiforrn chromitite. In both cases. the PGE complex Pt-Fe{u alloy, laurile and erlichmanite emichmentis due to the presenceof PGM trappedin (Nixon et aI. L990), the concentration of PGE in cbromite after crystallizationfrom the melt. chromitite being interpreted as taking place by the (2) The constant associationof chromite and PGM accumulation of PGM directly segregated from (dominatedby Pt minerals)in cbromiteof the Pirogues the magma. cumulates suggeststhat PGM induced chromite Iohan et al. (L989) describedPGE mineralization nucleation.Thus chromitecrystallization plays a major mainly composedof ft-Fe alloys, cooperite, erlich- role in trappingand preserving the PGEmineralization. manite, sper4'lite, malanite and Pr-Pd antimonidesin (3) The levels enriched in chromite and PGE corre- clinopyroxenite of an Alaskan-type complex. spond to influxes of new magma into the chamber. Similarly, nuggets derived from Alaskan-type p614 srystallizationis promotedby the new conditions complexes Q.{ixon er al. 1990, Johat et al. 1991, in which the melt finds itse[ and is facilitated by the Slansky et al. 1991) are very similar in terms of low fugacityof S prevailingin fle maglna.A possible ft MINERALZA'TION. NEW CALEDOMA OPHIOLITE COMPLEX LA43 explanation for the complex PGM paragenesis FrscrR, W. & Pmour,q M. (1990): describedin the Piroguessuite (witl isoferroplatinum, Experimental study of the solubility of platinum and in basic silicate melts - implications for the tulameenite, and undetermined Pt-Fe-Cu alloys, iridium of platinum-group elements during malanite, differentiation cooperite,Os-h-Ru alloy, laurite, bowieite, magmaticprocesses. Chem- GeoI. 8l, 45-53. cuprorhodsite,sperrylite, and BMS with PGE in solid solution) is that they were formed under thermo- AnBou[{, J., Marraus& M. & Au.Ecnn, C. (1977): Ia dynamic disequilibrium because tle environment couronne ophiolitique p€riaustralienne: un charriage evolvedrapidly in a closedsystem, with local chauges oc6aniquereprdsentatif des stadespr&oces de 1'6volution inflO),.(S) and proportion of PGE, without possible alpine.C.R. Acad- Sci. Paris 235,sCr.D,953-956. re-equilibrationafter entrapmentin the chromite. (4) The PiroguesRiver Pt-enrichedsuite showsseveral Auc6, T. (1985): Platinum-group-mineral inclusions in analogies with Alaskan-t'?e PcE-effichment, but ophiolitic chromitite from the Vourinos complex,Greece. differs significantly in mode of Pt mineralizationfrom Can.Mineral. 23. 163-17L. chromitite layers in stratiform complexes.A sulfide (1986): Platinum-group-minerallinclusions in phasehas probably played a major role in the PGE chrornititesfrom the Oman ophiolite. Bu|.l.Miniral. 109, concenhationin the latler. but this is not the casein 301-304. ophiolite and Alaskan-typesuites. (5) The study of mineralized alluvium derived from (1988): Platinum-groupminsrals in the Tidbaehi pri:nary mineralizationhas revealedsolution features and Vourinos ophiolitic complexes:genetic implications. affecting certain grains, confirming a cerlain mobility Can.Mineral. 26. 177-I92. of the PGE during lateritic weathering.This hasled to the formation of PGE oxidesin lateritic soils. (1993): Min6raux alluvionnaires du groupe du (6) In the Ni mineralized alluvium, a complex para- platine en Nouvelle Cal6donie.C.R. Acad. Sci.Paris 316, genesis of the PGM includes isoferroplatinum, an s6r.1I,91-97. unnamedPt-Ru-Rh alloy, cooperite,Os-Ir-Ru alloy, laurite, erlichmanite, bowieite, irarsite, holling- - & Josar,Z. (1988):Comparative study of chromite depositsftom Troodos,Vourinos, North Oman and New worthite, sperrylite, stibiopalladinite and unnamed Caledonia ophiolites. /n Mineral Deposits within the PGE oxide and Rh sulfide. This association was European Community (J. Boissonnas& P. Omenetto, (not derivedfrom a source identified) that differs from eds.).Springer-Verlag, Berlin, GermanyQ67 -288). that of the Piroguessuite, since cumulates are absentin ttris zone. & Lncm.ronr, O. (1992): Pt-Fe nuggets from (7) The presenceof a wide spectrumof PGM, there- alluvial depositsin eastemMadagascat. Can. Mineral. N, fore, suggeststhat ophiolitic PGE mineralizationneed 983-1004. not be confined to a limited group of PGM, and indicates that ophiolites should be reinterpretedfor & - (1994): Platinum-group-element their potential in PGE enrichment. oxides ftom the Piroguesophiolitic mineralization,New Caledonia: origin and significance. Econ. Geol. 89, Acr

CABRT,L.J., CRDDLE,A.J., Larumm, J.H.G., BSARNE"G.S. & Hannrs, D.C. (1981): Mineralogical study of complex RmmsNcES Pt-Fe nuggetsfrom Ettuopiu BulL MintrallM, 508-525.

AvossE,J. & Ar,rumr, M. (1993):Partitioning of iridium - & HApsrs,D.C. (1975):Zoning in Os-h alloys and andplatinum between metals and silicate melts: evidence the reLationof the geologicaland tectonic environmentof for passivafionof themetals depending on fO2.Geochim. the sourcerocks to the bulk ft:Ft+h+Os ratio for placen. Cosmachirn.Acta {1, 2395-2398. Can M ineral. t3. 266-274. 1044 THE CANADIAN MINERALOGIST

Lanar"nvn, J.H.G. & Srswanr J.M. (1977): Cliff, Shetland./r Fifth Int. Platinum Symp. (Helstuki). Platinum-groupminerals from Onverwacht.tr. Platarsir€, Bull. Soc. Geol. Finland 61, 40 (abstr.). a new sulfarsenide of platinum. Can. Mineral. 15, 385-388. JEDWAB,J., CRvEr.E, 8., Gour-r, G., Hwaur, X. & PRE[, P. (1992): The new platinum selenideluberoite PqSeofrom CApoBrANco,C.J. & DnaKs, M.J. (1990): Paftirioning of the Lubero region (Kilu hovince, Zaire). Eur. J, Mineral. ruthenium, rhodium, a:rd palladium betweenspinel and 4.683-692. silicate melt and implicationsfor platinum group element fractionation nends. Geochim. Cosmochim.Acta 54. ftzfAuu-, P. (1990): M4tfutdes al:tuzlles de sdparation de 869-874. phases mindrales en laboratoire. Editions BRGM, Maauelset m6thodes,Or16ans, France. CASSARD,D., Nrcor-as, A., RABn{ovffcH,M., MounE, J., LEBr-aNc,M. & Pnnmrorm., A. (1981): Structuralclass! JoHAN,Z. & AucE, T. (1986): Ophiolitic mantle sequences fication of chromite pods in southern New Caledonia. and tleir evolution: mineral chemistry constraints..an Econ.Geol. 76. 805-831. Metallogeny of Basic and Ultrabasic Rocks (M.J. Gallagher,R.A. Ixer, C.R. Neary & H.M. Prichar4 eds.r. Cocffinre, A., AucE, T. & METER,G. (1989): Geochemistry Inst.Mining Metall.,Iondon, U.K. (305-317). of platinum-group elementsin various types of spinels from the Vourinos ophiolite complex, Grxne. Chzm & LnonNone, O. (1980): Min6ralogie des GeoL.77.27-39. platinoldes dans les chromites massives du feuillet ophiolitique de la Nouvelle-Calddonie./n R6sum6 des Consrawrwrors, C.C., KnrcsroN, G.A. & Flsrnn. p.C. principaux rdsultats scientifiqueset technique du SGN. (1980):The occurrenceofplatinum group mineralsin the Ed. BRGM, Orl6ans,France (80). chromitites of the Kokkinorotsos chrome mine, Cyprus. In Proc, Int. Ophiolite Symp. (A. Panayiotou, ed.7. Ouxexsrmrm, M., Frsamn, W & ArrrossE,J. GeologicalSurvey Deparment, Cyprus (93-101). (1990): Platinum-groupminerals from the DuranceRiver alluvium, France.Mineral. PetroL 4,287-306. CoRRIvAux,L. & LartamlIs, J.H.G. (1990): Min6ralogie des 6l6mentsdu groupe du platine dans les chromitites de SLANSKv,E., BannoN,L.M. & Supps., I'ophiolite de Thedord Mines, Qu6bec.Can. MfuzraL ?A, D. (1989): Platinum mineralization in the Alaskan-type 579-595. intrusive complexes near Fifield, New South Wales, Australia. l. Platinum-groupminerals in clinopyroxenites Davrs, R.J., CLARK, A.M. & CruDpLe, A.J. (19711: of the Kelvin Grove Prospecl Owendale intrusion. Palladseite,a new mineral from ltabira. Minas Gerais. Mineral. Pe trol, 40. 289-309. Btaz;i,Mineral. Mag.4l, 123. Sunsrv; E. & OnNsNsrErrER,M. (1991): Germmn, M., Conp.rveux,L., T!.orrrun, L.J., CaspJ, J., Isoferroplatinumnuggets from Milverton (Fifield, NSW, LaFl-Allnrm,J.H.G. & Bm.cnnoN,M, (1990): Chromitites Australia): a contributionto the origin of PGE mineraliza- platinifdres des complexes ophiolitiques de lEstrie - tion in Alaskan-type complexes.C.R, Acat, Sci. Paris Beauce, Appalaches du Sud du Qu6bec. Mineral. 312, sdr. II, 55-60. DepositaE, 169-178. LAco,8.L., RABrNowrcz,M. & Nrcor-As,A. (1982):Podiform GIAssR, E. (1904): Richessesmin6rales de la Nouvelle- chromite ore bodies: a senetic model. J. Petrol. 23. Cal6dorrre.Ann dzs Mines, 5, s6r. 10, 540-541,. 103-125.

HAcEN,D., WErsER,T. & HrAy THAN(1990): Platiirurn-group Lnaranc, M. & Ntcous, A. (1992):Les chromititesophioli- mineralsin quatemarygoldplacers in the Upper Chindwin ttqlues.Chronique recherche mini\re SVI, 3-25. areaof northernBwma. Mineral. Petrol. 42.265-286. LE, C.A. & Pannr, S.J. (1988): Platinum-group element Hanrus, D.C. & CasRr, L.J. (1973): The nomenclatureof geochemistryof tle lower and middle group chromitites natural alloys ef esmiur! iridium and ruthenium based of the Eastern Bushveld complex. Econ. Geol, 83- on nerr compositional data of alloys from world-wide tLn-L1,39. occtlrrences.Can. Mineral. 12. 1.A+In. LEcENDRE,O. & Auc6, T. (1986): Mineralogy of platinum- & - (1991): Nomenclarureof platinum- goup minerall inclusions in cbromitites from different group elementalloys: review and revision. Can. Mineral. ophiolitic complexes. 1z Metallogeny of Basic and 29,231-23',7. lllfabasic Rocks @iI.J.Gallagher, R.A. Ixer, C.R. Neary & H.M. Prichar4 eds.). Init. Mining Metall., London, Invnn, T.N. (1981): A liquid-density conrrolled model for u.K. (36r-372). chromitite formation in the Muskox Intrusion. Carnegie Inst. Wash, YearBook8l.317-324. & - (1993): Pr6senced'un oxyde dlridium naturel dans les concentrdsplatinifbres de la rdgion L

Loucxs, R.R. (1978): Platinum-groupminerals in the New PH,ATAN,L. (1891): Constitution g6ologique de I'archipel Rambler copper-nickel deposit Wyoming. /n Eleventh n6o-cal6donien,Dans l-,es Mines de la Nouvelle- General Meet", Int. Mineral. Assoc. (Novosibirsk) 1, Cal6donie.Editions "k GdnieCivil", Paris,France. 152(abstr.). Pnrcrano, H.M., I)

Ne nr, N. (1992): Etudep4trologiqw et inetallogdniquedu Porrs, P.J.& NEARy,C.R. (1981): Platinum group massifophiolitique de Tropojq Albanie. Rdf4renceparti- element minerals in the Unst chromite. ShetlandIsles. culi?re aw gisernentsd.e chrornite et AAmentsdu groape Trans.Inst. Mining Metall.90, B186-8188. du platinc. Thbse de doctorat, Universitd d'Orl6ans, Orl6ans,France. & Tennar, M. (1988): Platinum and palladium minerals from two PGE-rich localities in the Shetland Nnssott, L.P. (1990): Platinum-groupmineral inclusions in ophiolite complex. Can Mineral. 26,979-990. cbromitite from the Osthammerenulfamafic tectonite body, southcentral Norway . Mineral. Petrol. 42,249-263. RoEDER,P.L. (1994): Chromite: from the fiery rain of chondrulesto the Kilauea Iki lava lake. Can Mineral.32, ND(oN, G.T., CagRI, L.J. & Len-emm, J.H.G. (1990): 729-7M. Platinum-group-elementmineralization in lode and placer deposits associated with the Tulameen Alaskan-type - & JAlflFsoN,H.E. (1992):Composition of chromite complex,British Columbia.Can, Mineral.28, 503-535. and co-existing ft-Fe alloy at magnatic tempefifures. Awt J. Earth ici.39.419426. Orwmrwrr:rm, M., KARAJ,N., NEmAJ, A., Jonar, Z. & Co{A, A. (1991): Le potentiel platinifere des ophiolites: Sr-ANsIs,E., JoHAN,2.,OHNENSTETTE& M., BannoN,L.M. & min6ralisationsen 6l6mentsdu groupe du platine @GE) Surrel, D. (1991): Platinum mineralization in the dans les massifs de Tropoja et Bulqiz4 Albanie. C.fi. Alaskan-type intrusive conplexes near Fifiel4 NSW Acad- Sci.Paris 313. 56r. 2.201,-208. Australia.2. Platinum-groupminerals in placer depositsat Fifield. Mineral. Petrol. 43, 161-180. Pecn, N.J., CASSARD,D. & HerrTv, J. (1982):Palladium, platinum, rhodium, ruthenium and iridium in chromitites Srocrcruax, H.W. & III-AVA, P.F. (1984): Platinum-group from the Massif du Sud and Ti€baghi Massif, New mineralsin alpine chromititesfrom southweslernOregon. CaledoniaEcon. Geol. 77. 157I-1577. Econ. Geol. 79, 491-508.

PARIS,J.P. (1981): G6olagiede la Noavelle-Cal6ilonie.M€m. Term.rcrorq, R.W., Warr