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The contrast between platinum-group element abundances in chromitites of ophiolites and layered intrusions is illustrated in Figure 1. Typical ophiolitic chromitite is characterized by Os, Ir, and Ru abundances in the range of 0.1 to 1.0 times chondritic abundance, and Pt and Pd abundances about 0.01 times chondritic. The Bushveld Middle and Upper Group chromitites are very different, with Ir and Ru 0.5 to 1.0 times and Pt and Pd 0.5 to 4 times chondritic abundance. With the exception of the A chromitite, the Stillwater chromitites are intermediate between those from ophiolites and those of the Bushveld Upper and Middle Groups. The A chromitite is similar in its concentrations to the Upper and Middle Groups. The Bushveld LG-6 (not plotted) falls with the majority of the Stillwater chromitites. Typical concentrations in the Merensky and J-M reefs are also plotted in Figure 1 and they illustrate the very high Pt and Pd and low Ru, Ir, and Os that characterize sulfide-dominant ores. Naldrett et al. (1987) discussed the reasons for the difference between the A and other Stillwater chromitites (Fig . 2) and showed that if0.06 percent of the sulfide that characterizes the J-M reef is added to the J chromitite, a reasonable match for the A is obtained (Fig. 3) . This led them to suggest that the reason for the A being so much richer in Pt and Pd than the others was its original higher sulfide content. Talkington and Watkinson (1986) reached a similar conclusion for platinum-group elements in chromite-rich rocks of both ophiolites and layered intrusions. It is suggested in this paper that the original abundance of platinum-group element-enriched base metal sul-fides is one of the reasons for the difference between chromitites from ophiolites and those from layered intrusions. 0361-0128/89/907/180-8 $ 3.00 modified after Naldrett et al. (1987). Chondritic abundances as for Figure 1. ß •! J'--10 a-•a A ½HOMITITE J e.----e MODEL ,'• / .,7% Os Ru Pt Au FIG. 3. Chondrite-normalized plot of platinum-group element content of the Stillwater A chromitite and a model composition obtained by adding 0.06 percent of J-M reef sulfide to the Stillwater J chromitite. Diagram after Naldrett et al. (1987). Chondritic abundances as for Figure 1.

A systematic investigation of the exsolution textures in titanomagnetite grains of massive magnetite layers and their associated host rocks from the entire upper zone sequence has revealed noticeable differences that can be related to changes in the oxidation state of the magma during crystallization and cooling. The observed oxidation exsolution intergrowths indicate a higher fo2 during formation of the magnetite layers than during crystallization of the disseminated titanomagnetite, as well as a decrease in the fo2 required to precipitate successive titanomagnetite-rich layers upward in the sequence.Peculiar composite lameliar intergrowths of magnetite and ilmenite in the uppermost magnetite layers and in some magnetite plugs are ascribed to a subsolvus oxidation of ulviispinel in ulviSspinel-rich magnetite. The configuration of the magnetite-ulviSspinel solvus dictates that ulviispinel in Ti-rich magnetite-ulv•ispinel solid solution high in the succession exsolves at considerably higher temperature than in Ti-poor solid solution lower in the sequence. Subsolvus oxidation of ulviispinel to ilmenite at higher temperatures near the top of the intrusion facilitated diffusion of ilmenite to produce the variety of different composite exsolution textures. The textures are not developed where abundant oxidation exsolution of ilmenite has taken place at temperatures above the solvus.

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