PLATINUM-GROUP ELEMENT PROPORTIONS IN CHROMITITES OF THE BUSHVELD COMPLEX - IMPLICATIONS FOR FRACTIONATION AND MAGMA MIXING MODELS

Chromitite layers in the Critical Zone of the Bushveld complex display a systematic change in the chondrite normalized platinum group elemen t (PGE) concentration patterns. The bell-shaped pattern of the lower l ayers of the lower group of chromitite layers suggests a very low sulp hide content and that ruthenium, osmium and iridium are largely contai ned as magmatic platinum group minerals (PGM) in the chromite grains. Elevated rhodium concentrations compared to platinum and palladium sug gests a possible solid solution component of rhodium in chromite. The upper of the lower group of chromitite layers and the middle group of layers have gentle positive patterns, which suggest the presence of a base metal sulphide component. Pt/(Pt+Pd) ratios within these layers c an be related to the thickness of the chromitite layer and total PGE c ontent. Positive slopes for the upper group of chromitite layers and t he Merensky Reef reflects sulphide dominant PGE mineralization of samp les from these layers used for this investigation. It is also demonstr ated that all chromitite layers of the Bushveld complex obtained their PGE content largely by a combination of enclosure in chromite of earl y, magmatic PGM's and subsequent sulphide collection from a smaller vo lume of magma as that from which the chromite crystallized. The result s also indicate that the Ir/Os ratio seems to be a sensitive petrogene tic indicator. Variations in this ratio over the sequence of chromitit e layers suggest that the B2/B3 parental melts of the Bushveld had a h igher Ir/Os ratio than the earlier B1 parental magma.