A Reactive In-situ Crystallisation Origin for the UG2 Chromitite of the Bushveld Complex, South Africa?

Abstract

The UG2 Chromitite of the ~2 Ga Bushveld Complex (South Africa) is one of the largest platinum-group element (PGE) deposits on Earth. It is ~1 m thick and can be traced for virtually the entire circumference of the eastern and western lobes of the Bushveld. Despite its economic importance and a plethora of studies devoted to understanding its petrogenesis, there is no consensus on how the UG2 body formed. Models invoking fractional crystallisation, in-situ crystallisation, gravity settling within crystal-rich slurries and crustal contamination have all been proposed. Recent studies on chromitite formation in other layered intrusions have demonstrated significant mineral chemical and textural disequilibrium features associated with chromitite layers, attributed to a reactive origin for these bodies. This study aims to evaluate if reaction between incoming magma and anorthositic footwall may have triggered crystallisation of some or all of the massive UG2 Chromitite. Specifically, we focus on microtextural variations at the mm-to-cm-scale across silicate to massive chromitite contacts in the UG2 and associated leader seam from a drill core sampled on the Magazynskraal farm in the western lobe of the Bushveld intrusion. We have carried out petrographic analysis and quantitative textural approaches including crystal size distribution (CSD), dihedral angle measurements, as well as mineral chemical and in-situ 87Sr/86Sr laser ablation inductively coupled-plasma mass spectrometry (LA-ICPMS) in plagioclase samples leading up and into the base of the UG2 Chromitite. We also studied materials at the contacts of the overlying leader seam. Petrographic observations reveal disequilibrium textural relationships in the footwall pyroxenite; e.g., chromite is typically separated from orthopyroxene by thin (μm-scale) rims of plagioclase. Interstitial plagioclase is chemically zoned and the anorthite content of the plagioclase shows consistent increases (e.g., An58 to An92) near chromite. CSD analysis yields mainly log-linear plots suggesting in-situ crystallisation with some evidence for postcumulus textural modification. Apparent chromite-chromite-plagioclase dihedral angle measurements reveal median values (for a given ~1 cm thick interval) of 65-75° and suggest that textural equilibrium has not been achieved. The 87Sr/86Sr variability points to the interaction between isotopically distinct signatures at the postcumulus stage. Our combined observations suggest disequilibrium between mineral phases and reactive (dissolution-reprecipitation) crystallisation in the UG2 Chromitite crystal mush.