An investigation of the geochemistry of the Middle Group of the eastern Bushveld complex, South Africa Part 1 – the chromitite layers

Abstract

This paper presents whole-rock geochemical and mineral chemical data for chromitite layers of the Middle Group of the eastern Bushveld complex. It reports on compositional variations of chromite from the chromitite layers, and special attention is paid to the platinum-group element (PGE) content and mineralisation within the chromitite layers. The discussion is focused on possible mechanisms by which the association of PGE enrichment within the chromitite layers can be modelled. The MG chromitites from bottom (MG0) to top (MG4C) are characterised by progressive melt evolution, showing decreasing Mg# and Cr/(Cr+Fe) ratio and increasing Cr/(Cr+Al) ratio, TiO2 and V concentrations. An increase in the Mg# together with a drop in the Cr/Cr+Al) and an increasing Cr/(Cr+Fe) ratio at the base of the MG4A chromitite strongly suggests the addition of hot and primitive magma. High Cu concentrations in the chromitite layers with coincident low S values suggest the application of Naldrett et al.36 Fe-loss model during cooling. With the help of the La/Cu and Cu/S ratios it could be shown that Cu is concentrated neither in base metal sulphides nor in trapped silicate melt and thus the model is not applicable. Laser ablation inductively coupled plasma mass spectrometry studies of single chromite grains also have not revealed Cu to be in solid solution and thus another Cu concentrating phase must be found. The PGE patterns of the MG chromitite layers are very similar to the one of the UG2 suggesting that they derived from the same magma and the same style of mineralisation did apply. It furthermore implies the presence of one parental magma only for the entire Critical Zone. Due to low S contents in the MG chromitite layers, it seems unlikely that the PGE have been concentrated by base metal sulphides only. About a sixth of the platinum-group mineral observed is associated with As, Bi and Te, and therefore PGE concentration by the cluster model is favoured. Enrichment of the high-temperature PGE over the lower-temperature PGE in the lowermost MG chromitite layers and the MG4A is probably due to the presence of high temperatures of the chromitite forming melt, and thus temperature could play a role in the fractionation of the two PGE groups.