PGE distribution in base-metal alloys and sulfides of the New Caledonia ophiolite

An Upper Eocene ophiolite nappe covering some 40% of New Caledonia consists of depleted mantle harzburgites, locally covered by mafic and ultramafic c umulates. These mantle harzburgites in places show an enrichment in sulfide and alloy (heazlewoodite, pentlandite, awaruite, millerite and native copp er), accompanied by anomalous concentrations of the PGE, as revealed by the whole-rock analyses of three sulfide-enriched samples and by the heavy-min eral concentrates obtained from these samples (up to 1835 ppb Ir, 1527 ppb Ph, 9718 ppb Pt, 11,494 ppb Pd and 988 ppb Au). Although studies to determi ne the PGE-carrier minerals by SEM were unsuccessful, SIMS depth profiles s how that Pt is strongly enriched in awaruite, albeit varying widely among g rains, e.g., from 4 to 1210 ppm for 21 grains in one sample. In contrast, 1 1 grains of pentlandite from the same sample do not exceed 10 ppm Pt, where as 10 out of 12 grains of heazlewoodite show no detectable Pt, and the Pt c ontent of the other two grains is estimated to be very low (<1 ppm). SIMS i on images thus confirm that awaruite is a preferential host for Pt and that the Pt concentration could be related to the mineral's Fe, Cu, Ni and Co c ontent. These images also show that the awaruite, whether or not Pt-bearing , contains some Au, whereas the heazlewoodite is Au-free. Textural features and trace-element distributions of the ore minerals suggest that the miner alization occurred in a two-step process. In the initial mineralizing event , primary PGE-bearing base-metal sulfides (BMS) crystallized in intergranul ar positions among the silicates. The second step was related to serpentini zation and involved transformation of the primary EMS assemblage to the pre sent secondary assemblage of heazlewoodite, awaruite, and pentlandite, but with a different pattern of PGE distribution.