The petrology and geochemistry of the Merensky reef in the Rustenburg area.

Abstract

Four Merensky reef underground exposures of different reef thickness, representative of the lithological variations exposed in mining, have been drilled and mapped. The relationship of the Merensky reef to the underlying rocks is paraconformable, and a broad-based definition of the highly variable Merensky reef, on the basis of detailed mine-wide mapping, is presented. Fifty two whole-rock samples from one drill intersection were analysed for major and trace elements by X-Ray Fluorescence spectrometry, and for platinum-group elements (PGE) by Neutron Activation analysis. The remaining three drill intersections were analysed for trace elements, and for PGE in one instance. Orthopyroxene and plagioclase mineral separates from one intersection were analysed for major elements by XRF, and the mineral compositions determined. The results of the whole-rock and silicate mineral chemistry are presented and discussed. Whole-rock geochemistry is controlled by modal composition, as are most trace elements. Incompatible elements such as Nb, Zr, Ba, Y and Rb occur in elevated abundances in the feldspathic pyroxenites and show systematic low-correlation relationships with Cu, Ni, Sand the PGE. These patterns are ascribed to the pore space competition between incompatible element enriched silicate melt and sulphide melt. The PGE are systematically associated with the base metal sulphide elements, with some localised decoupling, with Pd and Au showing the greatest chalcophile nature. The other PGE are highly correlated. Deviations in the geochemistry relative to the mode are analysed and discussed. Orthopyroxene and plagioclase mineral compositions consistently define three-way lithological associations, demonstrating limited geochemical relationship between the lithologies. These lithologies represent the footwall and hangingwall norites/anorthosites and the Merensky reef/Merensky pyroxenite sequence. The Merensky reef pegmatoid and the overlying Merensky pyroxenite have more evolved Mg# and Ca# than the norite or anorthosite. Ni in orthopyroxene correlates with whole-rock Ni, providing evidence of re-equilibration. A systematic relationship exists between plagioclase and orthopyroxene mineral compositions, with Ti in both phases defining primary and re-equilibrated trends. Apart from the elevated PGE abundances in the Merensky reef, the Merensky reef and overlying Merensky pyroxenite are geochemically indistinguishable. Certain evolved element distribution patterns coincide with the zone of elevated PGE, Cu and Ni abundances. Abundant geochemical evidence is consistent with late-stage in situ hydromagmatic alteration and modification to the Merensky reef, and in part, the Merensky pyroxenite. In contrast, the texturally similar footwall and hangingwall norites show very little evidence of hydromagmatic effects, where magmatic and submagmatic processes are well preserved. The whole-rock and mineral geochemistry defines and characterises the Merensky succession and provides certain constraints for petrogenetic modelling. A multi-stage process is envisaged for the petrogenesis and evolution of the Merensky succession, involving, 1) a magmatic stage, 2) a submagmatic stage, and 3) a hydromagmatic stage.