Abstract

The Platreef is a thick (greater than 100 m) coarse-grained feldspathic orthopyroxenite which transgresses northward across metasediments of the Transvaal Sequence and onto a granitic floor. It is overlain by main zone gabbronorites. Vertical profiles for Rb and Sr isotope ratios through the Platreef and overlying main zone show differences depending upon footwall type. Against a granitic floor the Platreef has initial 87 Sr/ 86 Sr ratios ranging from 0.7107 to 0.7226 (calculated at 2.05 b.y.) suggesting heterogeneous contamination by a melt derived from the granite. Overlying a dolomitic footwall, the Platreef shows lower initial ratios (0.7054 to 0.7147), but high Rb contents again suggest addition of a granitic component. The extent of contamination is least at the base. The role of dolomite assimilation is difficult to quantify. The lowermost rocks of the main zone show high Rb contents, due to contamination, but their initial Sr isotope ratios have not been changed significantly, due to the high proportion of cumulus plagioclase. Orthopyroxene has a much lower initial ratio than its whole rock, indicating that the main cumulus phase formed prior to contamination and that the granitic liquid reacted with the interstitial basic magma to produce the contaminated intercumulus phases.The tenor of platinum-group elements in the Platreef precludes the formation of immiscible sulfide liquid from an interstitial liquid; rather, it must be an early formed phase. Hence the separation of sulfide liquid may predate the main siliceous contamination process.

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