Evaluating the influence of late-stage fluids and/or melts on the mineralogy and geochemistry of magnetitite layers in the Bushveld Complex

Abstract

Understanding the extent by which layered intrusions have been modified by post-cumulus processes is important for unravelling primary magmatic histories. This study focusses on how upward migrating late-stage fluids or melts may have affected the bases of Bushveld magnetitite layers and their underlying anorthosites. Key observations include dramatic enrichments in the An-contents of plagioclase grains at the magnetitite-anorthosite contact, from An₅₉ to An₉₀, depletion of the lowermost few mm of the magnetitite layer in Cr, and an increase in the extent of ilmenite exsolution in the magnetitite, locally enriching the surrounding magnetite in Cr in some areas. Sr-isotopes from plagioclase are consistent with those recorded for the Upper Zone of the Bushveld Complex, suggesting that the fluids or melts were internally derived. Late-stage melts are unlikely to be responsible for the formation of Cr-rich domal structures at the bases of magnetitite layers because (a) cumulus magnetite underneath magnetitite layers are very poor in Cr, suggesting that late-stage melts were not Cr-rich, (b) where a large xenolith obstructs liquid migration from below, Cr contents within the magnetitite on top and adjacent to the xenolith are indistinguishable, and (c) a small scale protrusion of magnetitite into the underlying anorthosite that would have been submerged in late stage melt are depleted in Cr. While metasomatism at the base of magnetitite layers may have caused some minor redistribution and depletion of Cr, the macroscale Cr-distribution features are inferred as being of primary magmatic origin.