Experimental study on tin partition between granitic silicate melt and coexisting aqueous fluid

Abstract

The distribution of tin between different granitic silicate melts and coexisting aqueous fluids have been determined at 850°C, 100 MPa and fO2 near NNO. Tin distribution coefficients DSnaq.fl./melt are constrained by the composition of melt. When the starting fluid is 0.1 M HCl, DSnaq.fl./melt decreases rapidly from 0.130 ± 0.090∼0.137 ± 0.016 to 4.22 (±0.47) × 10-3 with the Na2O + K2O mole content and ASI of melt varying from 6.55 to 9.39% and 1.37 to 0.66, respectively, and tin concentrations in aqueous fluids and DSnaq.fl./melt decrease from 9.26 ± 0.56 to 0.220 ± 0.013 μg/g and 3.70 (±0.42) × 10-2 to 1.29 (±0.15) × 10-3, respectively with Na/K mole ratio increasing from 0.43 to 1.57 and nearly constant ASI from 1.04 to 1.10. A series of experiments that use the same K-rich peraluminous haplogranitic melt as starting solid whereas investigate the effects of chlorine and fluorine, the presence of chlorine in aqueous fluids increases DSnaq.fl./melt because of tin complexing, the presence of fluorine in the starting fluid does not significantly influence on DSnaq.fl./melt. For the chlorine experiments, DSnaq.fl./melt shows a positive dependence on the concentration of HCl in the starting fluid with log DSnaq.fl./melt = 2.0247 × log[HCl] + 0.6717 ([HCl] unit is M), and SnCl2 is the dominant tin-bearing complex in aqueous fluid. The results of this experiment showed that the peralkalinity of granitic melt and F-bearing in granite can lead to the enrichment of tin in melt phase, and, therefore, they probably serve as a tin ore reservoir or an important transport medium for tin ore formation, whereas K-rich peraluminous granitic melt and high HCl concentrations in the aqueous fluid phase are favorable for tin partitioning in aqueous fluid.