The halogen geochemistry of the Bushveld Complex, Republic of South Africa: Implications for chalcophile element distribution in the lower and critical zones

Halogen-bearing minerals, especially apatite, are minor but ubiquitous phas es throughout the Bushveld Complex. Interstitial apatite is near end-member chlorapatite below the Merensky reef (Lower and Critical Zones) and has in creasingly fluorian compositions with increasing structural height above th e reef(Main and Upper Zones). Cl/F variations in biotite are more limited o wing to crystal-chemical controls on halogen substitution, but are also con sistent with a decrease in the Cl/F ratio with structural height in the com plex. A detailed section of the upper Lower Zone to the Critical Zone is ch aracterized by an upward decrease in sulfide model from 0.01- 0.1% to trace -0.001%. Cu tends to correlate with other incompatible elements in most sam ples, whereas the platinum-group elements (PGE) can behave independently, p articularly in the Critical Zone. The decrease in the Cl/F ratio of apatite in the Main Sone is associated with a shift to more radiogenic Sr isotopic signature, implying that the unusually Cl-rich Lower and Critical Zones ar e not due to assimilation of crustal rocks. Nor is the Main Zone more Cl ri ch where it onlaps the country rocks of the,floor, suggesting little if any Cl was introduced by infiltrating country rock fluids. Instead, the result s are consistent with other studies that suggest Bushveld volatile componen ts are largely magmatic. This is also supported by apatite-biotite geotherm ometry, which gives typical equilibrium temperatures of 750 degreesC. The i ncreasingly fluorian apatite with height in the Upper Zone can be explained by volatile saturation and exsolved a CI-rich volatile phase. The high Cl/ F ratio inferred for the Lower and Critical Zone magma(s) and the evidence for volatile saturation during crystallization of the Upper Zone indicate t he Lower and Critical Zones magma(s) were unusually volatile rich and could easily have separated a Cl-rich fluid phase during solidification of the i nterstitial liquid. The stratigraphic distribution of S, Cu and the PGE in the Critical Zone cannot readily be explained either by precipitation of su lfide as a cotectic phase or as a function of trapped liquid abundance. Evi dence from potholes and the PGE-rich Driekop pipe of the Bushveld Complex i mply that migrating Cl-rich fluids mobilized the base and precious metal su lfides. We suggest that the distribution of sulfide minerals and the chalco phile elements in the Lower and Critical Zones reflects a general process o f vapor refining and chromatographic separation of these elements during th e evolution and mig-ration of a metalliferous, Cl-rich fluid phase.