MINERALOGY AND GEOCHEMISTRY OF PLATINUM-RICH CHROMITITES FROM THE MANTLE-CRUST TRANSITION ZONE AT OUEN ISLAND, NEW CALEDONIA OPHIOLITE

Abstract

Micropods, irregular bodies and schlieren of chromitite occur within a unit of layered plagioclase-bearing wehrlite on Ouen Island, southern New Caledonia. The primary chromian spinel is Al-rich, with Cr# [atomic (Cr/Cr + Al)] between 0.52 and 0.62, and Mg# [Mg/(Mg + Fe 2+ )] between 0.57 and 0.72. Contents of minor and trace elements (Ga, Ni, Zn, Co, Mn, V, Sc) in primary spinel, obtained by LA–ICP–MS, are comparable to those reported in chromian spinel from podiform chromitites in other ophiolite complexes. The chromitites are strongly enriched in Pt and Pd (up to 11.5 ppm Pt + Pd), reflecting the presence of abundant platinum-group minerals (PGM), including alloys, sulfides, oxides and compounds containing Te, Sb or Hg. Also present are Ru–(Os–Ir) phases, in accessory amounts: sulfides (mainly laurite, two unidentified Os–S and Ir–S compounds) and sulfarsenides of Ir and Rh. Abundant base-metal minerals (BMM) occur together with the PGM; they include Ni–Cu–Fe sulfides (pentlandite, millerite, heazlewoodite, chalcopyrite and bornite), awaruite (Ni 3 Fe), and a phase with a composition close to NiAsS (probably gersdorffite). The geochemistry and mineralogy of the chromitite and the PGE mineralization show that the Ouen Island chromitites have a different origin than those found in the mantle and crust sequences of the New Caledonia ophiolite complex. The distribution, morphology and composition of the PGM and BMM in the chromitites define two distinct paragenetic assemblages: i) primary or magmatic, consisting of PGM and BMS grains enclosed in the unaltered core of chromian spinel grains and in primary anhydrous silicates (olivine and pyroxene) of the matrix; ii) secondary or postmagmatic, consisting of PGM and BMM grains found along fractures, enclosed or in contact with high-temperature secondary hydrous silicates (amphibole) and low-temperature silicates (serpentine, chlorite). Microtextural and compositional data for the secondary PGM and BMM mineral assemblages suggest that after the crystallization of chromian spinel, an excess of H 2 O generated a residual high-temperature hydrothermal fluid that modified the magmatic PGM mineralogy, producing small-scale remobilization of the PGE. Late serpentinization overprinted this hydrothermal alteration, and contributed to the degradation of the primary PGM and BMM. At this stage, PGE-bearing alloys and sulfides were replaced by new stable alloys and oxides of the PGE.