The Merensky-Bastard interval at Hackney, eastern Bushveld Complex: results of a combined Sr–Nd-isotopic investigation

Abstract

The Merensky and Bastard reefs of the Bushveld Complex occur within what has been called a transitional macro-unit along the boundary of the Critical and Main zones. The transitional unit is characterised by a geochemical hiatus recording distinct inflections in mineral chemistry and isotopic compositions. Previously these inflections in mineral chemistry and changes in isotopic compositions were attributed mostly to the influx of a magma that was compositionally distinct from the resident magma and that was parental to the Main Zone of the complex. Sr-isotopic variations across this interval have been particularly well-studied, but despite this, little consensus exists regarding the petrogenesis and metallogenesis of this economically important interval. Here we report whole-rock Sr–Nd-isotopic, major- and trace element geochemical and mineral chemical data across the Merensky-Bastard interval as intersected by borehole BH8172 on the farm Hackney in the eastern Bushveld Complex. Variations in whole-rock Cr/MgO values and initial Sr isotopic ratios across the interval are consistent with the results of previous studies that argued for the co-accumulation of minerals from compositionally and isotopically distinct magmas, of Critical and Main Zone lineages, respectively. In our model, a magma of Critical Zone affinity enters the chamber causing erosion along the chamber floor. Orthopyroxene and plagioclase crystallise from the Critical Zone magma to form the Merensky Reef, as suggested by high whole-rock Cr/MgO ratios (> 80) and unradiogenic Sr-isotopic compositions (⁸⁷Sr/⁸⁶Sr_i < 0.7068). A plagioclase-laden magma of Main Zone affinity subsequently intruded the chamber as a basal flow, elevating the resident Critical Zone magma. Plagioclase within the former floated, forming a solid raft onto which the Bastard Reef was deposited, a model that is entirely consistent with density considerations and an upward increase in the An-content of plagioclase as observed in the anorthositic package between the Merensky and Bastard reefs. From a metallogenetic viewpoint, this would imply that the Main Zone could not have been the source of the PGEs within the Merensky Reef.