A Petrological investigation of the Rustenburg layered suite and associated mineralization South of Potgietersrus

Abstract

A sequence of 3250 m of the Rustenburg Layered Suite and its associated mineralization south of Potgietersrus was investigated. Four episodes of faulting have deformed the area. This resulted in very early differentiates, not seen elsewhere in the Bushveld Complex, to outcrop together with economic concentrations of the best metallurgical grade chromite presently being mined in the Republic. The Mg:Fe:Ca ratio of theCa-poor pyroxenes varies from 89,5:8,8:1,6 in the lower zone to 44,2:52,3:3,37 in the upper zone. The latter composition demarcates the Fe-rich end of the two pyroxene limit. Textural evidence implies that there is a peritectic reaction between the ironrich Ca-poor pyroxene and the melt and that this may account for the termination of the two pyroxene field. A significantly higher mean Ko~~~F~p; for the study area (0,822) than for the other sectors of the Bushveld Complex (0,782) suggests that the pyroxenes of similar composition crystallized at higher temperatures in the Potgietersrus limb. Examination of the Al :Si ratio in Ca-rich pyroxenes from a variety of magmatic environments confirms that this variable can be used to monitor relative changes in the a ~~~t. Chemical data of the Ca-rich pyroxenes suggest that this phase define~ an Fe enrichment - Ca depletion trend during differentiation uhlike that for most other tholeiitic intrusions. The V205 content of the main magnetitite layer and the cr203 and the Cr/Fe2++Fe3+ values in the upper and lower chromitite layers in the study area are the highest encountered in the Bushveld Complex. Textural evidence in these layers show that they have been up-graded to dense monomineralic layers by postcumulus sintering. Calculated intensive parameters for the Potgietersrus magma suggest that it crystallized over a temperature interval from 1276°C in the lower zone to 1022°C in the basal portion of the upper zone. Oxygen fugacity conditions for the lower zone ranged from 10-6,21 to 10-4,98 atm whereas lower values of 1o-11 to 1o-9 atm were operative in the upper zone. The study area contains abundant concentrations of sulfides at several levels in the sequence. The separation of the sulfide liquid is related in most cases to new influxes of metal-rich magma and mixing with the residual magma in the chamber. Several definite sulfide facies occur in the layered sequence. Sulfur isotope investigations indicate that all the sulfur in the study area is mantle derived and that the isotopic composition of the sulfur was controlled by the prevailing fo2, which in turn controlled the partitioning of S02 and HzS between sulfide melt and magma.