Hydrothermal thallium mineralization up to 300 °C: A thermodynamic approach

Abstract Thermodynamic properties of solid and aqueous thallium species are essential in understanding of the hydrothermal thallium mineralization. However, these data are largely lacking. To remedy this situation, complete thermodynamic properties including entropy, heat capacity, enthalpy and free energy for a number of ternary and quaternary thallium-bearing sulfosalts, and secondary thallium minerals formed in supergene environments such as lanmuchangite, are estimated according to the revised estimation methods proposed in this study. The reliability of these estimation methods is demonstrated by the good agreement between the estimated values and the measured values for a number of sulfosalts with known thermodynamic properties. In the aspect of aqueous thallium species, the stability constants involving several thallium complexes including TlHCO3o, TlCO3−, TlSO4−, TlHSo, TlOHo and TlAco at 25 °C are critically evaluated at first. Then, their stability constants at elevated temperatures up to 300 °C at vapor saturation pressures are obtained based on the isocoulombic approach. Using the data from these two aspects, one may be able to assess quantitatively the solubility of thallium-bearing minerals and the relative importance of various thallium aqueous species in transport of thallium up to 300 °C. In the relatively oxidizing environments where sulfate dominates, the results from this study suggest that free thallium ion (Tl+) is important at 100 °C from pH 1 to pH 9. At 200 °C, Tl+ is the dominant species from pH 1 to pH 7. At 300 °C, Tl+ and TlClo predominate from pH 1 to pH 5. At higher pHs, TlSO4−, TlHCO3o, TlCO3− and TlOHo become increasingly important. In the reducing environments where sulfide dominates, Tl+ is important in acidic pH region, whereas TlHSo is dominant in neutral to alkaline pH range, from 100 °C to 200 °C. At 300 °C, Tl+ and TlClo are dominant species in acidic pH region, and TlHSo predominates in the pH range from ∼ 5.5 to ∼ 9.5. At higher pHs, TlCO3− and TlOHo are dominant. Thermodynamic calculations suggest that lorandite is the solubility-controlling phase in low temperature (∼ 100 °C to 200 °C) hydrothermal fluids. Thallium constituents in form of solid solutions in sulfides, for instance, as Tl0.5As0.5S2 in pyrite, may be the solubility-controlling phase for thallium in hydrothermal solutions up to 300 °C or higher rich in reduced sulfur. Solubilities of both carlinite alone and lorandite in equilibrium with orpiment have strong dependence on pH. Solubility calculations in combination with field observations provide insight into the formation of hydrothermal thallium mineralization, especially independent thallium deposits, and therefore may provide valuable guidance for exploration for thallium mineralization. The most favorable combination conditions for thallium mineralization are (1) leaching thallium from source rocks by acidic hydrothermal fluids at higher temperatures (up to 250 °C) at the early stage, and (2) subsequent neutralization of the acidic hydrothermal fluids by host rocks such as carbonate rocks at the late stage, resulting in the formation of thallium mineralization at low temperatures (around 100 °C).