THE HYDROSULPHIDE/SULPHIDE COMPLEXES OF COPPER(I): EXPERIMENTAL DETERMINATION OF STOICHIOMETRY AND STABILITY AT 22°C AND REASSESSMENT OF HIGH TEMPERATURE DATA - FURTHER EVIDENCE FOR CLUSTER COMPLEXES?

Abstract

The solubility of chalcocite has been measured over the pH range 4 - 11.5 in aqueous sulphide solutions in order to determine the stoichiometry and stability of the Cu(I) hydrosulphide/sulphide complexes at room temperature. A flow-through column was used as an alternative method for the measurement of the solubilities. Non-linear least squares fitting of the results gave the following stoichiometries and stability constants at 22°C for I = 0.0: Cu++2HS-=Cu(HS)2- log β122=+17.18+/-0.13 2Cu++3HS-=Cu2S(HS)22-+H+ log β232=+29.87+/-0.14. The stability of a third complex expected in the low pH region has been estimated: Cu++HS-=CuHS0log β111~+13. The Cu(HS)2- complex will predominate in the near-neutral region at intermediate to high sulphide concentrations (>0.001 mol kg-1) while Cu2S(HS)22- will only be important at basic pH values and high sulphur concentrations. At lower sulphur concentrations (<0.001 mol kg-1), CuHS° is the dominant hydrosulphide complex. In natural anoxic bottom waters and porewaters, sulphide concentrations fall in the region where both Cu(HS)2- and CuHS° may contribute significantly to total copper solubility.In order to test the applicability of the low temperature speciation model at elevated temperature, the solubility data of Crerar and Barnes (1976) were refit using CuHS° + Cu(HS)2-. The data show an excellent fit with this model and the following equations for the temperature dependence (25 =< T =< 350°C) of the cumulative stability constants were derived: log β111=3.798+2752T log β122=-614.3+6.702x104T-5.920x106T2+83.06 ln T where T is temperature in Kelvin. Speciation calculations show that for a hydrothermal fluid at 300°C with sulphur concentration buffered at pyrite-pyrrhotite-magnetite, pH = 4-6, the dominant hydrosulphide complex will be either CuHS° or Cu(HS)2- depending on the pH. In lower pH solutions, CuHS° is expected to be the dominant hydrosulphide complex at most geological sulphur concentrations. Comparison with Cu(I) chloride complexes shows that, at 300°C, CuCl2- will predominate when chloride concentrations exceed 0.1 to 1.0 mol kg-1 at pH values buffered at potassium feldspar-muscovite-quartz. As temperature decreases, the stability of the chloride complexes declines and therefore hydrosulphide complexes predominate over an increasingly wider range of chloride concentration. In hydrothermal solutions, copper transport as hydrosulphide complexes reaches mineralizing levels only at high total sulphur concentrations and basic pH values. Under more acidic conditions and lower total sulphur, chloride complexing is required for the transport of sufficient copper to form economic mineralization.