Abstract

The Tiebaghi ultramafic massif, northern New Caledonia, has been the main chromite producer of the island. It is a small part of a huge ultramafic nappe emplaced during the Upper Eocene and is mainly composed of peridotites with tectonite fabrics; it has suffered a complex tectonic history involving several phases of folding and fracturing. The different ultramafic facies, ranging from dunite to plagioclase lherzolite, appear to form a coherent lithostratigraphic succession, the Tiebaghi Series, which reflects the successive appearance, upward in the series, of olivine and spinel, then orthopyroxene, and then clinopyroxene. Plagioclase results from metamorphic reequilibration of the upper part of the series. In the field the Tiebaghi Series is a succession, on a centimetric to metric scale, of dunite, peridotite, and pyroxenite, with a sequential organization evidenced by cyclic units of decametric thickness. Geochemically, there is a continuous enrichment in Ca, Al, Si, Fe-Mg from harzburgite and dunite to plagioclase lherzolite. The mineral compositions also show evolution, by a decrease of the Mg/Fe and Ni content of silicates and the Cr/Al of chrome spinel, and by an increase of the Al, Ti, and Na contents of pyroxenes, with conspicuous geochemical cyclic units.The chromite deposits appear at three levels of the host series: as schlieren in thick dunitic layers near the base of the series; as small lenses in the lherzolite unit; and, for most of the deposits, in or near the transition zone between harzburgite and lherzolite, where they may have formed a small number of chromite-rich layers of large lateral extension. The three most important deposits (Tiebaghi, Chagrin, and Fantoche) display typical podiform morphologies and ores, with the elongation directions of the orebodies parallel to the tectonic structures of the country rocks. In contrast, the Alpha and Vieille Montagne 1 deposits, situated higher in the series, show chromite-rich layers of regular thickness, with large open folds and with well-preserved cumulate textures. Geochemically, ore-forming chromite has a rather uniform, Mg- and Cr-rich composition, except for Al-rich ores rarely found within the lherzolitic unit. Within a given deposit, the Mg/Fe (super +2) ratios of chromite and olivine increase with the chromite/olivine modal amount; the mineral compositions otherwise compare well with those of dunites of the host series.The Tiebaghi massif shows typical features of both Alpine-type ultramafic massifs and ultramafic zones of stratiform tholeiitic complexes. The differentiation of the Tiebaghi Series is explained by fractional crystallization of a mixture of basaltic liquid and Mg-rich minerals, mainly olivine. Accumulation of chromite-rich layers during differentiation of the series is due to Cr supersaturation of the liquid, which may result from drops in total pressure.

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