Garnet geochemistry reveals late-stage oxidation of tin-bearing fractionated granite

In hydrothermal tin (Sn) systems, it remains unclear whether cassiterite precipitates from reduced or oxidized fluids. To resolve this issue, the geochemistry of magmatic garnet and cassiterite separated from fractionated muscovite-garnet granite in the Paleocene Bawapin Sn-W deposit was systematically investigated. CaO contents in Mn-rich garnet (spessartine) decrease from core to rim in single crystals and are negatively correlated with MnO/(MnO + FeO) ratios. These features suggest that garnet CaO content may be a good differentiation index for granitic magma evolution. Moreover, the Sn content in Mn-rich garnet increases with increasing Ca content and then decreases sharply at Ca contents of approximately 4300 ppm. Combined with evidence of Ta-rich magmatic cassiterite, the decreasing Sn content likely reflects the crystallization of magmatic cassiterite from the more evolved Sn-rich melts under oxidized conditions (fO(2) > Delta FMQ +1.5), in contrast to the reduced characteristics of the less-fractionated biotite monzogranite (fO(2) = Delta FMQ - 0.5). Late-stage oxidation might be attributable to fluid exsolution in the water-rich and Fe-poor granitic melts. This further indicates that hydrothermal cassiterite could precipitate in exsolved Sn4+-bearing fluids without Sn oxidation. This conclusion may provide a new perspective to our understanding of granite-related hydrothermal Sn systems worldwide.