Abstract

Mineralogical examination of borehole core from the weathered zone of the UG2 Chromitite reef at the Union Section of AMPLATS in the northwestern Bushveld Complex, South Africa, reveals that the mineralogy of the reef has undergone a number of changes. The chromite particles have an outer rim which is cracked and slightly enriched in chromium and depleted in iron compared to the core. The base-metal sulphide (BMS) content of the original unweathered UG2 reef is only trace, but in the weathered zone no residual BMS are present. Chemical analyses of the borehole core indicate that the platinum values do not change with depth. Palladium however shows depletion at the top and enrichment at approximately 25 m, which is near the base of the weathered zone. Microscopic and SEM examination shows that the platinum-group minerals (PGM) in the weathered zone have undergone varying degrees of alteration. Pt-Rh-Cu-S, an unnamed but a common PGM in the UG2, shows distinct alteration characteristics. Where a PGM, usually cooperite (PtS), was observed, it was noticed on the back-scattered electron (BSE) image on the SEM that the nearby silicate was of a higher brightness than expected. Energy dispersive X-ray analysis of the silicates reveals a high palladium, and occasionally rhodium, content of these silicates. Also present are significant levels of copper and occasionally nickel and manganese. In two cases "PGE-oxides" were observed; a Pd-oxide containing Pd-Cu-Fe and a Pt-oxide containing Pt-Mn-Ni.

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