ABSTRACT

The distribution of tin between different granitic silicate melts and coexisting aqueous fluids have been determined at 850°C, 100 MPa and f_O2 near NNO. Tin distribution coefficients D_Sn^aq.fl./melt are constrained by the composition of melt. When the starting fluid is 0.1 M HCl, D_Sn^aq.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 D_Sn^aq.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 D_Sn^aq.fl./melt because of tin complexing, the presence of fluorine in the starting fluid does not significantly influence on D_Sn^aq.fl./melt. For the chlorine experiments, D_Sn^aq.fl./melt shows a positive dependence on the concentration of HCl in the starting fluid with log D_Sn^aq.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.