Fluid inclusion and stable isotope evidence for interaction between granites and magmatic hydrothermal fluids during formation of disseminated and pipe-style mineralization at the Zaaiplaats tin mine

Tin-tungsten-rare earth element mineralization at Zaaiplaats is hosted by the Lease and Bobbejaankop Granites. Pervasive hydrothermal alteration of the granites resulted in replacement of original biotite by chlorite, sericite, carbonate and Ti oxides, development of coarse vein and patch perthite and antiperthite in alkali feldspar, and precipitation of hydrothermal minerals in miarolitic cavities. Modeling isotopic exchange between granites and a magmatic fluid phase in isotopic equilibrium with average Bobbejaankop Granite quartz at 600°C indicates that whole-rock compositions result from final exchange at temperatures less than 400°C, with fluid compositions in the range 4.2 to 5.2‰ at 300°C and 0.8 to 1.8‰ at 200°C. These results are consistent with fluid compositions calculated from δ ¹⁸O _quartz and pressure-corrected fluid inclusion homogenization temperatures, and indicate that oxygen isotope variation at Zaaiplaats reflects exchange between the granite and the magmatic fluid phase at temperatures down to 200°C. -from Authors