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STRATIFORM and ALLUVIAL Plafl NUM MINERALIZATION in THE 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
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