The Formation of the PGE Bearing Bushveld Chromitites and the Merensky Reef by Magma Mixing: A Combined Re-Os and Rb-Sr Study

Abstract

The Critical Zone of the Bushveld Complex is characterized by repeated cyclic crystallization series (Cyclic Units), each comprising a chromitite layer, enriched in PGE-bearing sulphides and platinum metals, at its base. It is now widely accepted, among workers in the Bushveld Complex, that injections of fresh melt followed by mixing with residual liquid forced the composition of the crystallizing hybrid magmas into the stability field of chromite (Scoon et al. , 1994 and references therein) and thus lead to the accumulation of prominent chromitite layers. However, details of this process, like the composition of the magmas involved and the origin of the crustal component displayed by initial isotope ratios (particularly Sr isotopes, Kruger and Marsh, 1982) are still under investigation. From inverse geochemical fractionation trends (e.g. Mg number of whole rocks and pyroxenes, Ni/Yi of pyroxenes) at stratigraphic levels, where new melt was injected, Eales et al. (1990) and others concluded that the new influxing magma must be of primitive composition. Based on immediate increases in initial Sr ratios at the same stratigraphic horizons, Kruger & Marsh (1982) and others concluded that the new magmas were derived from a partial crustal source or were crustally contaminated prior to the influx into the magma chamber. In contrast to chromite accumulation, the PGE enrichment within chromitites is still ascribed variously to (1) the orthomagmatic model: PGE are collected by an immiscible sulphide melt, being formed after magma mixing, (2) the fluid flow model: PGE are transported by upward-migrating hydrothermal fluids into the chromitites and platinum reefs (Ballhans et al., 1986) and (3) the chromitite control model: PGE are concentrated in sulphide-poor chromitites, probably due to local S saturation (Scoon et al., 1994). Gruppe Isotopengeologie, Mineralogisch-Petrographisches Institut, Universitiit Bern, Erlachstrasse 9a, 3012 Bern, Switzerland