The role of alkaline silicate-rich fluids in Sn mineralization based on cassiterite solubility experiments.

It is widely accepted that the complexation of Sn with Cl in magmatic-hydrothermal fluids is critical for the formation of endogenic Sn deposits. However, Sn abundance as high as 0.4 wt% has been found in some chlorine-poor but silicate-rich fluid inclusions associated with a Sn-mineralized pegmatite. To examine the role of silicate solutes in Sn mineralization, we measured the solubility of cassiterite in alkaline silicate-rich fluids by direct observation of single crystal cassiterite dissolution in a hydrothermal diamond anvil cell at 540–900 °C and 5.5–13.6 kbar. Experimental results indicated that a significant amount of cassiterite could be dissolved in both Na 2 Si 3 O 7 –H 2 O and Na 3 AlSi 5 O 13 –H 2 O fluids, and SnO 2 solubility was positively correlated with the concentration of silicate solutes, the Na/(Si + Al) ratio of the fluid and temperature. In Na 2 Si 3 O 7 –H 2 O fluids at ∼680 °C with 24 to 45 wt% silicate solute, SnO 2 solubility increased from 0.65 to1.67 wt%. In Na 3 AlSi 5 O 13 –H 2 O fluids at ∼715 °C with 21 to 52 wt% silicate solute, the solubility increased from 0.49 to 1.19 wt%. These values are comparable to cassiterite solubilities in HCl solutions and Li 2 CO 3 solutions, and are much higher than those in NaCl solutions, 0.1 mol/kg NaOH solution, and 14 wt% SiO 2 solution. In situ Raman spectra collected on fluids demonstrated the presence of silicate monomers and oligomers, and the complexation of Sn with these Si-(Al)-Na clusters or with (OH)− could explain the observed high SnO 2 solubility. Comparison with SnO 2 solubility in peraluminous melt suggests that Sn can be significantly enriched in alkaline silicate-rich fluids. Modeling of progressive fluid-melt unmixing successfully reproduces the observed high Sn abundance in silicate-rich fluid inclusions. Alkaline silicate-rich fluids can therefore serve as an effective agent, alternative to chlorine-rich fluids, for Sn mobilization and mineralization, especially in pegmatite systems. [ABSTRACT FROM AUTHOR]