The platinum-group chemistry and mineralogy of the UG-2 chromitite layer of the Bushveld Complex

TLDR: In this paper, the average noble metal content of the UG-2 chromitite layer is about 7 ppm; Pt is most abundant, followed by Pd, Ru, Rh, Ir, and Au, in that order.

Abstract: Individual platinum-group elements, as well as Au, Cu, and Ni, were determined in borehole cores of the UG-2 chromitite layer from different localities in the Bushveld Complex. The average noble metal content of the layer is about 7 ppm; Pt is most abundant, followed by Pd, Ru, Rh, Ir, and Au, in that order. Noble metal contents, recalculated to concentration in 100 percent sulfide, are extremely high, which raises questions regarding the distribution coefficients of noble metals in sulfide and silicate melts, and the initial concentrations of the platinum-group elements in Bushveld magmas.More than 6,000 grains of platinum-group minerals were identified qualitatively in polished sections of the ore, the most abundant species being laurite (RuS 2 ), cooperite (PtS), an unnamed Pt-Ir-Rh-Cu sulfide, braggite ([Pt, Pd, Ni]S), an unnamed Pt-Pb-Cu sulfide, vysotskite (PdS), gold, and electrum (Au, Ag) as well as intermetalloids such as Pt-Fe, Pd-Cu, Pd-Pb, and Pd-Hg. Associated base metal sulfides are mainly pentlandite, chalcopyrite, pyrrhotite, and pyrite. Variation in the platinum-group mineral assemblages can be explained by variations in the fugacity of sulfur between the various localities studied.Compositions of chromite grains show an increase of Fe (super +2) , Fe (super +3) , and Cr upward. Variations in Fe (super +3) /(Fe (super +2) + Fe (super +3) ) (laterally and vertically) may reflect variations in the prevailing fugacity of oxygen.