SOLUBILITY OF SPHALERITE IN AQUEOUS SULFIDE SOLUTIONS AT TEMPERATURES BETWEEN 25-DEGREES AND 240-DEGREES-C

In order to assess the role of zinc sulfide complexes in ore-forming solutions, the solubility of sphalerite was measured in NaOHH2S aqueous solutions of 0.0 to 3.0 m NaHS concentration at temperatures of 25 to 240°C. Solubilities vary with temperature, activities of H2S(aq) and HS-, total reduced sulfur concentration (∑S), and pH. From the solubility data, the main reactions that form zinc sulfide complexes were determined as follows: ZnS(s) + H2S(aq) = Zn(HS)20, ZnS(s) + H2S(aq) + HS- = Zn(HS)-3, ZnS(s) + H2S(aq) + 2HS- = Zn(HS)42-, ZnS(s) + H2O(1) + HS- = Zn(OH)(HS)-2, and ZnS(s) + H2O(1) + 2HS- = Zn(OH)(HS)2-3. Their equilibrium constants (log K) are 25°C: -5.3, -3.3, -3.4, -4.4, -4.9; 100°C: -5.2, -3.5, -3.2, -4.1, -5.0; 150°C: -4.7, -3.8, -3.1, -4.7, -5.2; 200°C: -5.1, -3.4, -3.1, -4.6; 240°C: -4.9, -3.3, -3.1, -4.9, respectively. Zn(OHXHS)2-3 is not stable at temperatures higher than 200°C. Zinc sulfide complexes predominate over chloride complexes in relatively low temperature hydrothermal solutions which have high ∑S, low ∑C1-, and high pH values. In these solutions, ZnS is precipitated in response to changes of temperature, pH, and ∑S. Among them, decrease of ∑S is more effective than that of temperature and pH. Zinc sulfide complexes do not transport significant zinc in those ore-forming solutions responsible for economic zinc sulfide deposits. However, they become predominant zinc species in certain geothermal solutions and ore-forming solutions responsible for some epithermal precious metal deposits. © 1990.