Abstract

The Stillwater, Fiskenaesset, and Bushveld Complexes are all composed of layered ultramafic and mafic rocks in which the cumulus phases generally appear in a regular order, which have similar petrologic and chemical characteristics, which are all Precambrian in age, and which contain platinum-group elements that vary widely in their abundances and relative proportions. Two of the complexes, the Stillwater and Bushveld, contain platinum-group element enriched horizons not yet found in the Fiskenaesset. The ratio of Pt/(Pt + Pd) in rocks from the Stillwater tends to increase from the basal zone to the middle of the ultramafic zone, decreases to the beginning of the banded zone, and then increases again through the banded zone; many fluctuations, however, are superimposed upon this generalized trend. The trends of Pt/(Pt + Pd) ratios appear to show a positive correlation with the Mg/(Mg + Fe (super +2) ) ratio in olivine. In the Fiskenaesset Complex, the Pt/(Pt + Pd) ratio increases rapidly from 0.3 to 0.7 from the lower gabbro and ultramafic units through the leucogabbro units, then increases slowly through the anorthosite and upper gabbro unit. Superimposed on this generalized trend are increasing and decreasing patterns within each unit.Analyses of platinum, palladium, and rhodium for 69 rocks from the main and upper zones of the eastern Bushveld from the Roossenekal area show generally low concentrations of less than 1 ppb Pt; 0.1 ppb Pd; and 0.1 ppb Rh, but larger amounts occur in some magnetite layers and sulfide-bearing zones. Within the main zone up to the pyroxenite marker, from the pyroxenite marker to below lower magnetite layer 2, and in the upper zone, the Pt/(Pt + Pd) ratio decreases with stratigraphic position, although with minor fluctuations in these intervals. The concentration levels of the platinum-group metals appear to be directly related to the amount of sulfide material present and the proportion of intercumulus material. Variations in Pt/(Pt + Pd) ratios in the Bushveld and Fiskenaesset Complexes are presumably related to changes in major mineral compositions as they are in the Stillwater Complex.

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