Platinum-group element geochemistry of the lower and middle group chromitites of the eastern Bushveld Complex

The results of neutron activation analyses for the six platinum-group elements and Au show that the lower group and middle group chromitites are enriched in them over the silicate-rich host rocks. The platinum-group elements and gold contents increase up the sequence, and the middle group chromitites are richer than the lower group chromitites in these elements. Of the two platinum-group element groups, (Pt + Pd + Rh) and (Ru + Os + Ir), the former occurs in greater abundance in the middle group chromitites, whereas the (Ru + Os + Ir) group only slightly enriches them. The lower group chromitites are enriched in Ru and depleted in Ir + Os compared to the middle group chromitites. This trend is probably due to upward cryptic variation in laurite, the most likely mineral host for these elements in the Bushveld Complex chromitites. There is high interelement correlation between the various platinum-group elements in the (Pt + Pd + Rh) and the (Ru + Os + Ir) groups. Chromitites with more than 80 vol percent chromite are depleted in the (Pt + Pd + Rh) group, due to the effect of postcumulus events. There is generally low correlation between Ni and the platinum-group elements plus gold, and none with Cu.