Partitioning of Sn and W between granitic melt and aqueous fluid

Hydrothermal granite-related Sn and W deposits typically show predominance of one metal over the other. At the intrusion and deposit scale, this separation may reflect contrasting behavior of these elements during late-stage magmatic processes. In our experimental study, we determined fluid-melt partition coefficients of Sn and W at 750 degrees C, 200 MPa as function of the alumina saturation index (ASI) of the melt for NaCl + KCl +/- HCl fluids. Two sets of data were obtained, one from analysis of fluid inclusions trapped in glass, and the other based on analysis of the quenched fluid. Our data indicate slight partitioning of Sn into the melt and, at ASI of 0.99-1.16, strong partitioning of W into the aqueous fluid. The effect of quenching was evident for tungsten, which indicates that fluid-melt partition coefficients of W reported in the literature are underestimated. The contrasting fluid-melt partitioning of Sn and W implies that hydrothermal Sn and W mineralization is rarely cogenetic. Furthermore, contrasting fluid-melt partitioning accounts for the separation and zonation of these metals on the deposit scale and the different style of predominant mineralization. Tin precipitates from the granitic melt as cassiterite or is incorporated in biotite and other minerals. Later exsolution of HCl-bearing magmatic fluids can efficiently mobilize Sn from the granite or the wall rock of the intrusion, but can transport Sn only over relatively short distances due to reaction of the fluid with feldspar (greisenization), which results in precipitation of cassiterite and formation of tin greisens. Tungsten is partitioned into the magmatic fluid and will be lost from the intrusion along with this fluid. Because greisenization reactions do usually not result in tungsten mineralization, W can be transported farther away from the intrusion than Sn and is deposited in scheelite-bearing skarns or, upon cooling, in quartz-wolframite veins.