Abstract

The stratigraphy, whole-rock composition, and mineralogy from a thick undisturbed section of the Platreef at depth on Turfspruit in the northern limb of the Bushveld Complex have been examined in detail. This section has much more in common with the Critical zone than with the Platreef structure updip although the high-grade platinum-group element (PGE) reefs, located close to the boundary with the Main zone, are much thicker than any known Merensky reef facies. A key feature that marks the conversion of the Platreefstyle mineralization into the Merensky style is the regular appearance of chromite seams, which become more persistent with decreasing contamination westward and downdip. The similar zonal distribution of platinum-group minerals (PGM) in two consecutive thick PGE reefs correlates with changes in silicate compositions and vertical zonation of PGE concentrations in whole rocks and base metal sulfides. This supports orthomagmatic in situ crystallization of ore minerals during mixing of consecutive magmatic influxes. The typical Merensky-style ore assemblages of predominant Pt sulfides and alloys are developed within ~1-m-thick intervals, which correspond to the highest temperature mixing zones between two magmas, whereas the rest of the reefs host Pt-Pd bismuthotellurides and arsenides. PGE content and tenor, as well as Cu/Pd in major base metal sulfides in the Turfspruit reefs, are within the range of the typical high-grade Merensky values, suggesting the same enrichment process.

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