CHEMICAL AND THERMODYNAMIC CONSTRAINTS ON THE HYDROTHERMAL TRANSPORT AND DEPOSITION OF TIN: I. CALCULATION OF THE SOLUBILITY OF CASSITERITE AT HIGH PRESSURES AND TEMPERATURES

Abstract

Estimation of equation of state parameters for Sn++ and calculation of the thermodynamic properties of other aqueous species and dissociation constants for various stannous and stannic complexes as a function of temperature permit prediction of the high temperature solution chemistry of tin and calculation of the solubility of cassiterite in hydrothermal solutions. The results of these calculations indicate that in the absence of appreciable chloride and fluoride concentrations, Sn(OH)20 and Sn(OH)40 are the predominant tin species in H2O up to 350°C at ~2 pH 7.5. The calculations also indicate that chloride complexes of Sn++ predominate by several orders of magnitude over their fluoride and hydroxide counterparts in 1–3 molal (m) NaCl solutions, except in the presence of geologically unrealistic concentrations of fluoride or a pH greater than ~3.5 at 250°C or ~5.0 at 350°C. At higher pH values, most of the tin in solution is present as hydroxide complexes, even at concentrations of NaCl as high as 3 m. Calculated values of the solubility of cassiterite at high temperatures compare favorably with experimental data reported in the literature. Depending on the fugacity of oxygen and solution composition, the solubility of cassiterite in hydrothermal solutions may exceed 100 ppm under geologically realistic conditions.