THE PGE AND BASE-METAL ALLOYS IN THE PODIFORM CHROMITITES OF THE LUOBUSA OPHIOLITE, SOUTHERN TIBET

Abstract

PGE and base-metal alloy minerals from podiform chromitites of the Luobusa ophiolite, southern Tibet include Os–Ir, Os–Ir–Ru, Pt–Fe, Ir–Ni–Fe, Fe–Ni–Cr, and Fe–Co with highly variable compositions. Small amounts of diamond, graphite, SiC, an unnamed chromium carbide, iron silicide (Fe–Si), and native Fe, Ni, Cr, Au, Cu and Si also are present. The alloys and native elements were recovered primarily from heavy-mineral separates of the chromitites, but some are included in, or attached to, magnesiochromite grains. They are mostly subhedral to anhedral and range in size from about 0.05 to 0.5 mm. Some grains have euhedral crystal forms and others have round shapes, suggestive of melt droplets. Fe–Ni-bearing, Ru-rich PGE alloys, Fe–Ni and Fe–Co alloys and native Fe and Ni in the Luobusa chromitites are interpreted as secondary minerals formed by alteration of PGE sulfides. The Pt–Fe, Os–Ir, and Ir–Ni–Fe alloys, Fe silicide and perhaps the native Si are considered to be xenocrysts from the mantle, transported to shallow depths by a rising plume and then captured by the melts from which the Luobusa chromitites crystallized.