Abstract

Compositional variation of chromite is limited within the massive LG6 Chromitite layer, where chromite is the sole cumulus phase. Compositions vary significantly in chromitiferous orthopyroxenite and in those samples where the immediate contact between the silicate and oxide cumulates has been intersected. These greater variations are attributed to the competition between chromite and orthopyroxene for Al, Fe, and Mg during co-precipitation and subsequent subsolidus re-equilibration. The Cr/Fe ratio, using total Fe, varies between 1.52 and 1.61 in samples from the massive chromitite, but decreases sharply to values below 1.42 in the orthopyroxenitic foot- and hanging-wall. The massive chromitite layer records the highest platinum-group element content (717-945 ppb), whilst the juxtaposed chromitiferous orthopyroxenite records a lower value (304 ppb). Orthopyroxenite with accessory chromite records a value close to background. The relative proportions of the Ru group (Ru, Os, Ir) and the Pt group (Pt, Pd, Rh) remain constant in samples of the massive chromitite, varying around 57.1+ or -4.9% and 42.9+ or -4.9%, respectively. Acid-soluble Cu and Ni contents in the massive chromitite are close to background levels in six samples analysed, with Cu<30 ppm and Ni<70 ppm, while S is invariably <200 ppm. The pronounced regular nature of the platinum-group element mineralization, the low sulphur, and low Cu and Ni contents do not support a sulphide collection or a hydrothermal origin. The relationship between modal proportion of chromite and platinum-group element content strongly suggests a mineralization process triggered solely by the nucleation of chromite.

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