The Halogen Geochemistry of the Bushveld Complex, Republic of South Africa: Implications for Chalcophile Element Distribution in the Lower and Critical Zones

Abstract

solidification of the interstitial liquid. The stratigraphic distribution Halogen-bearing minerals, especially apatite, are minor but ubiof S, Cu and the PGE in the Critical Zone cannot readily be quitous phases throughout the Bushveld Complex. Interstitial apatite explained either by precipitation of sulfide as a cotectic phase or as is near end-member chlorapatite below the Merensky reef (Lower a function of trapped liquid abundance. Evidence from potholes and and Critical Zones) and has increasingly fluorian compositions with the PGE-rich Driekop pipe of the Bushveld Complex imply that increasing structural height above the reef (Main and Upper Zones). migrating Cl-rich fluids mobilized the base and precious metal Cl/F variations in biotite are more limited owing to crystal-chemical sulfides. We suggest that the distribution of sulfide minerals and controls on halogen substitution, but are also consistent with a the chalcophile elements in the Lower and Critical Zones reflects a decrease in the Cl/F ratio with structural height in the complex. general process of vapor refining and chromatographic separation of A detailed section of the upper Lower Zone to the Critical Zone is these elements during the evolution and migration of a metalliferous, characterized by an upward decrease in sulfide mode from 0·01– Cl-rich fluid phase. 0·1% to trace–0·001%. Cu tends to correlate with other incompatible elements in most samples, whereas the platinum-group elements (PGE) can behave independently, particularly in the Critical Zone. The decrease in the Cl/F ratio of apatite in the