Abstract

Pt, Pd, Ir, and Au were determined by neutron-activation analysis in a drill core from the Merensky Reef of the Bushveld Complex. The core intersects economic platinum mineralization on the farm Middelkraal (Western Platinum Limited), some 25 miles east of Rustenburg, western Transvaal, South Africa.The host rock is a feldspathic pyroxenite consisting of orthopyroxene and plagioclase with accessory clinopyroxene, biotite, chrome spinel, and sulfides. The highest noble metal values are associated spatially with a thin (approximately one-eighth inch) band of chrome spinel and with pronounced hydrous alteration of primary silicates.One-inch whole-rock samples of split core were taken sequentially along the entire core length. Chrome spinel and sulfide mineral separates (approximately forty separates) were obtained from the other half of each split, where possible.The whole-rock analyses yield maxima for Pt, Pd, and Au immediately above the chromite marker seam and coincident with maxima for sulfur, nickel, and copper. Ir is high in the samples showing Pt-Pd-Au maxima but exhibits somewhat higher values at other points along the core.Preferential association of Pd and Ir with sulfides is strong and consistent over the entire core. Pd correlates well with pentlandite-rich sulfides. In general, neither Pd nor Ir is consistently enriched in oxide phases, although there are exceptions. Approximately 35 and 40 percent of total whole-rock Pd and Ir, respectively, are associated with sulfides and oxides. The remainder is presumably contained within discrete platinoid minerals.Pt and Au do not show clear enrichment in separated sulfides and oxides, the latter phases accounting for only 10 and 5 percent of the whole-rock total for these metals, respectively.

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