Tin transport and cassiterite precipitation from hydrothermal fluids

Cassiterite (SnO₂) is the main ore mineral of tin in magmatic–hydrothermal tin deposits, but tin transport and precipitation mechanisms from hydrothermal fluids remain poorly understood. We critically evaluated aqueous tin speciation in hydrothermal fluids from extensive experimental data and thermodynamic modeling. Sn(Ⅱ) chloride complexes in hydrothermal fluids exist mainly as SnCl⁺, SnCl₂(aq), and SnCl₃⁻. The revised Helgeson–Kirkham–Flowers model parameters of these three tin species and two tin ions (Sn⁴⁺ and Sn²⁺) were derived from the correlation algorithms among these parameters, and the standard molar properties of cassiterite were optimized to be internally consistent with the available thermodynamic dataset. These thermodynamic parameters, together with the available equilibrium constant equation of Sn(Ⅳ) chloride complexes, could reproduce the available solubility data of cassiterite in acidic solutions at 400–700 ℃ under oxygen fugacity (f_O2) levels buffered by hematite–magnetite (HM) or nickel–nickel oxide (NNO). These comparisons allow modeling chemical systems of SnO₂–NaCl–HCl–H₂O (liquid phase) to examine tin transport and cassiterite precipitation mechanisms under tin-mineralizing conditions: 300–500 ℃, 50–150 MPa, 2 molal NaCl, and f_O2 levels from QFM (quartz–fayalite–magnetite) to HM. Sn(Ⅱ) chloride complexes are commonly interpreted to dominate in aqueous tin speciation under f_O2 = NNO, but our modeling results indicate that considerable contents of Sn(Ⅳ) chloride complexes also exist in those reduced fluids with high HCl contents, consistent with recent in situ high-temperature experiments and molecular dynamic simulations. The Sn(Ⅱ)/Sn(Ⅳ) ratios in fluids depends on f_O2, temperature, and HCl contents. A considerable amount of Sn(Ⅳ) possibly exist in an early mineralization stage even under f_O2 = NNO; if so, redox reactions are unnecessary to precipitate cassiterite from these mineralizing fluids. We find that even if the f_O2 levels are constant, simple cooling can alter mineralizing fluids to be more oxidized (e.g., from QFM to HM) and cause cassiterite precipitation, indicating that oxidizing agents are not necessary as previously thought. This explains why cassiterite can precipitate in host rocks (e.g., sandstone or quartzite) that do not provide oxidizing agents. A simple rise in f_O2 levels and pH neutralization (e.g., greisenization) also cause cassiterite precipitation. Cassiterite solubility in oxidized acidic hydrothermal fluids (NNO < f_O2 ≤ HM) is high enough to account for the tin contents of fluid inclusions from typical tin deposits, but the mineralization potential of oxdized fluids (f_O2 ≤ NNO) under the same conditions.