Geochemical and structural indicators for hydrothermal fluid migration: a case study in the Bushveld Complex

Abstract. The Bushveld Complex is a renowned crystalline igneous intrusion, with various hypotheses regarding its origin. However, the role of post-magmatic hydrothermal fluids in its evolution remains underexplored. The present hydrogeological study, based on data derived from a deep exploration well drilled into the eastern limb of the Bushveld Complex as part of the Bushveld Complex Drilling Project (BVDP) funded by the International Continental Scientific Drilling Program (ICDP) and which intersects with a lamprophyre dyke at depth, highlights geochemical and structural indicators of deep-seated hydrothermal fluid migration within the crystalline formation. A distinct hydrogeochemical transition is observed at 450 m below ground level, coinciding with a severely fractured anorthosite layer, with evidence of mineral crystallisation from post-magmatic hydrothermal fluid migration. The zone is characterised by a significant increase in total dissolved solids (TDSs), sodium, chloride, sulfate, calcium, potassium, zinc, fluoride and boron concentrations, as well as elevated concentrations of redox-sensitive trace metals such as iron and manganese, in addition to stable isotope deviations. These signatures suggest hydrothermal fluid–rock interactions facilitated by structural conduits for fluid migration, such as dykes and fractured zones. The hydrogeological data and geological features suggest deep-seated, saline and potentially volatile-rich fluid migration, which has implications for post-magmatic hydrothermal mineralisation and groundwater evolution. The potential association of these fluids with volatiles poses exploration risks and highlights the need for integrated gas monitoring. Real-time drilling fluid analysis and isotopic characterisation are recommended to better elucidate fluid sources and migration pathways. These findings contribute to an updated understanding of post-magmatic processes using geochemical indicators to inform future exploration and groundwater management strategies in these fractured crystalline formations.