Abstract

New and recently published platinum-group element data of chromite-rich layers in the Bushveld Complex are evaluated. Chondrite-normalized platinum-group element plots of these data show that trends for chromitite layers differ from that of the Merensky reef. The data furthermore indicate remarkable consistencies in compositional trends for many kilometers along strike of individual layers, suggesting that the conditions of formation of chromitite layers and their associated platinum-group element mineralization were very uniform over large areas of the magma chamber. Significant differences in the proportions of platinum-group elements within a particular layer are only evident on a regional scale.The platinum-group minerals of the middle group of chromitite layers in the Marikana area of the western Bushveld are mainly sulfides and arsenides. Laurite is the most common platinum-group mineral and is mostly enclosed by chromite. A variety of other platinum-group minerals occur in close association with base metal sulfides. Extremely low bulk sulfur content in the chromitite layers is ascribed to a postcumulus loss of sulfur. Postcumulus loss of sulfur and the existence of laurite enclosed within chromite casts doubt on the validity of any petrogenetic models for platinum mineralization in chromitite-rich layers based on partition coefficients and the R factor.

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