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The Halogen Geochemistry of the Bushveld Complex, Republic of South Africa: Implications for Chalcophile Element Distribution in the Lower and Critical Zones 总被引：16，自引：5，他引：11
Halogen-bearing minerals, especially apatite, are minor butubiquitous phases throughout the Bushveld Complex. Interstitialapatite is near end-member chlorapatite below the Merensky reef(Lower and Critical Zones) and has increasingly fluorian compositionswith increasing structural height above the reef (Main and UpperZones). Cl/F variations in biotite are more limited owing tocrystal-chemical controls on halogen substitution, but are alsoconsistent with a decrease in the Cl/F ratio with structuralheight in the complex. A detailed section of the upper LowerZone to the Critical Zone is characterized by an upward decreasein sulfide mode from 0·01–0·1% to trace–0·001%.Cu tends to correlate with other incompatible elements in mostsamples, whereas the platinum-group elements (PGE) can behaveindependently, particularly in the Critical Zone. The decreasein the Cl/F ratio of apatite in the Main Zone is associatedwith a shift to more radiogenic Sr isotopic signature, implyingthat the unusually Cl-rich Lower and Critical Zones are notdue to assimilation of crustal rocks. Nor is the Main Zone moreCl rich where it onlaps the country rocks of the floor, suggestinglittle if any Cl was introduced by infiltrating country rockfluids. Instead, the results are consistent with other studiesthat suggest Bushveld volatile components are largely magmatic.This is also supported by apatite–biotite geothermometry,which gives typical equilibrium temperatures of 750°C. Theincreasingly fluorian apatite with height in the Upper Zonecan be explained by volatile saturation and exsolved a Cl-richvolatile phase. The high Cl/F ratio inferred for the Lower andCritical Zone magma(s) and the evidence for volatile saturationduring crystallization of the Upper Zone indicate the Lowerand Critical Zones magma(s) were unusually volatile rich andcould easily have separated a Cl-rich fluid phase during solidificationof the interstitial liquid. The stratigraphic distribution ofS, Cu and the PGE in the Critical Zone cannot readily be explainedeither by precipitation of sulfide as a cotectic phase or asa function of trapped liquid abundance. Evidence from potholesand the PGE-rich Driekop pipe of the Bushveld Complex implythat migrating Cl-rich fluids mobilized the base and preciousmetal sulfides. We suggest that the distribution of sulfideminerals and the chalcophile elements in the Lower and CriticalZones reflects a general process of vapor refining and chromatographicseparation of these elements during the evolution and migrationof a metalliferous, Cl-rich fluid phase. KEY WORDS: Bushveld Complex; chlorine; platinum-group elements; layered intrusions 相似文献
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