Characterising the “Transvaal Jade” alteration front: Evidence for fluid-driven critical metal mobility in the Bushveld Complex

Abstract The world-famous and culturally significant “Transvaal Jade” hydrogrossular assemblage developed through two genetically distinct fluid events associated with chromite-bearing lithologies of the Bushveld Complex. The first event involved infiltration by a platinum-group element (PGE)-bearing, Ca-rich, moderately alkaline, silica-undersaturated brine that metasomatically replaced primary anorthitic plagioclase with hydrogrossular garnet, while partially oxidising chromite to ferri-chromite-magnetite. A second, later fluid pulse, markedly more basic and derived from serpentinisation of surrounding ultramafic rocks, introduced a pervasive serpentine meshwork, cross-cutting and overprinting the earlier alteration. Petrographic, mineral chemical, and geochemical analyses confirm that the Transvaal Jade represents a rodingite-like metasomatic system formed at upper greenschist to lower amphibolite conditions (inferred from mineral assemblages) and near-neutral to alkaline, moderately oxidising fluids, with selective mobility governed by fluid composition and redox-controlled Fe fixation. These preliminary findings resolve a distinct reaction-front signature of fluid alteration within the Bushveld Complex, herein referred to as the “Transvaal Jade Alteration Front.” Furthermore, this study refines and expands earlier models, linking the cultural gemstone value of this rock to a complex fluid-rock interaction history with implications for critical metal mobility in layered intrusions.