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 phases throughout the Bushveld Complex. Interstitial apatite is near end-member chlorapatite below the Merensky reef (Lower and Critical Zones) and has increasingly fluorian compositions with increasing structural height above the reef (Main and Upper Zones). Cl/F variations in biotite are more limited owing to crystal-chemical controls on halogen substitution, but are also consistent with a decrease in the Cl/F ratio with structural height in the complex. A detailed section of the upper Lower Zone to the Critical Zone is characterized by an upward decrease in sulfide mode from 0.01-0.1% 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 Main Zone is associated with a shift to more radiogenic Sr isotopic signature, implying that the unusually Cl-rich Lower and Critical Zones are not due to assimilation of crustal rocks. Nor is the Main Zone more Cl rich where it onlaps the country rocks of the floor, suggesting little if any Cl was introduced by infiltrating country rock fluids. Instead, the results are consistent with other studies that suggest Bushveld volatile components are largely magmatic. This is also supported by apatite-biotite geothermometry, which gives typical equilibrium temperatures of 750°C. The increasingly fluorian apatite with height in the Upper Zone can be explained by volatile saturation and exsolved a Cl-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 the Lower and Critical Zones magma(s) were unusually volatile rich and could easily have separated a Cl-rich fluid phase during solidification of the interstitial liquid. The stratigraphic distribution of S, Cu and the PGE in the Critical Zone cannot readily be explained either by precipitation of sulfide as a cotectic phase or as a function of trapped liquid abundance. Evidence from potholes and the PGE-rich Driekop pipe of the Bushveld Complex imply that migrating Cl-rich fluids mobilized the base and precious metal sulfides. We suggest that the distribution of sulfide minerals and the chalcophile elements in the Lower and Critical Zones reflects a general process of vapor refining and chromatographic separation of these elements during the evolution and migration of a metalliferous, Cl-rich fluid phase.

Duke Scholars

Author Alan E. Boudreau Earth and Climate Sciences