CASSITERITE SOLUBILITY, TIN SPECIATION, AND TRANSPORT IN A MAGMATIC AQUEOUS PHASE

The solubility of SnO2 in chloride +/- fluoride-bearing fluids in equi librium with a number of crystalline, silicate acid buffers, has been measured at 2.0 kbars, as a function of temperature (700-degrees-800-d egrees-C), oxygen fugacity (QFM to QFM + 1.5 log units), and fluid che mistry. Silicate buffers included sanidine-sillimanite-quartz, analbit e-sillimanite-quartz, anorthite-sillimanite-quartz, talc-quartz, and ( ortho)enstatite-quartz. Hundreds to tens of thousands of parts per mil lion of tin can be dissolved in dilute, chloride-bearing acid solution s. Fluoride was found to have no significant influence on the solubili ty of SnO2 at 700-degrees-C. In alkali chloride-bearing, acid solution s, across the entire range of oxygen fugacities examined, tin transpor t is effected by a complex series of stannous chloride-bearing species . These include simple chloride, mixed ligand (hydroxy chloride), and alkali-bearing chloride and hydroxy chloride stannous complexes (SnOHC l, SnCl2, KSnOHCl2, KSnCl3, K2SnCl4, K3SnCl5, K4SnCl6, and NaSnOHCl2, NaSnCl3, etc). Apparent equilibrium constants for the predicted aqueou s tin species have been calculated and species distribution plots deri ved for different solution compositions. The solubility of SnO2 is fav ored by increasing fluid acidity (m(HCl,aq)), elevated aqueous alkali/ acid and K/Na ratios, and reducing conditions, but appears to be only weakly enhanced by temperature increases in the 500-degrees to 750-deg rees-C range.