Chromitite formation—a key to understanding processes of platinum enrichment

Abstract

The mafic layered suite of the 2050 m.y. old Bushveld Complex hosts a number of substantial platinum-group element (PGE)-bearing chromitite layers, including the UG2, within the Critical Zone, together with thin chromitite stringers of the platinum-bearing Merensky Reef. Until 1982 only the Merensky Reef was mined for platinum, although it has long been known that chromitites also host PGE-bearing minerals (PGM). Three groups of chromitites occur: (i) a Lower Group of up to seven major layers hosted in feldspathic pyroxenite; (ii) a Middle Group with four layers hosted by feldspathic pyroxenite or norite; and (iii) an Upper Group, usually of two chromitite packages, hosted in pyroxenite, norite or anorthosite. There is a systematic chemical variation from the bottom to the top chromitite layer in terms of Cr : Fe ratio and the abundance and proportion of PGE.

Detailed studies of ⁸⁷Sr/⁸⁶Sr isotope variations undertaken on interstitial plagioclase from chromitites and different silicate host rocks show that the magma from which the chromitites formed (interstitial plagioclase Sr_i <0.7099) usually differed radically from the resident liquid from which the immediate footwall rocks crystallized (Sr_i, ca 0.7060-0.7064). These high Sr isotope ratios can only have been produced by sudden and extensive contamination by an extremely radiogenic component. The only viable source for this component in the chamber is the felsitic roof rocks or a granophyric roof-rock melt. It is suggested that such contamination occurred when a new magma influx penetrated the residual liquid and interacted with the overlying roof rock as well as mixing with the resident liquid. It is envisaged in this model that chromite cascaded to the floor together with a small amount of magma adherent to the chromite or entrained within the slurry to produce interstitial silicates with enriched isotopic ratios.

The close correspondence of chromitite and PGE enrichment strongly suggests that the contamination process that resulted in chromite formation also triggered precipitation of the PGE. The base of each major chromitite layer marks the point where there was a substantial injection of new magma into the chamber, which resulted in erosion of the cumulate pile, interaction with roof rocks and inflation of the chamber. Thus, the major PGM and chromitite ore deposits of the Bushveld Complex are unconformity-related and are associated with mixing of new magma, coupled to simultaneous contamination by granophyric roof-rock melt. The chromitites represent, therefore, the products of a roof contamination and magma mixing process.