The hydrosulphide sulphide complexes of copper(I): Experimental determination of stoichiometry and stability at 22 degrees C and reassessment of hightemperature data

The solubility of chalcocite has been measured over the pH range 4 - 11.5 i n aqueous sulphide solutions in order to determine the stoichiometry and st ability 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 degrees C for I = 0.0: Cu+ + 2HS(-) = Cu(HS)(2)(-) log beta(122) = +17.18 +/- 0.13 2Cu(+) + 3HS(-) = Cu2S(HS)(2)(2-) + H+ log beta(232) = +29.87 +/- 0.14. The stability of a third complex expected in the low pH region has been est imated: Cu+ + HS- = CuHS0 log beta(111) approximate to + 13. The Cu(HS), complex will predominate in the near-neutral region at intermed iate to high sulphide concentrations (>0.001 mol kg(-1)) while Cu2S(HS)(2)( 2-) will only be important at basic pH values and high sulphur concentratio ns. At lower sulphur concentrations (<0.001 mol kg(-1)), CuHS0 is the domin ant hydrosulphide complex. In natural anoxic bottom waters and porewaters, sulphide concentrations fall in the region where both Cu(HS)(2)(-) and CuHS 0 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) we re refit using CuHS0 + Cu(HS)(2)(-). The data show an excellent fit with th is model and the following equations for the temperature dependence (25 les s than or equal to T less than or equal to 350 degrees C) of the cumulative stability constants were derived: log beta(111) = 3.798 + 2752/T log beta(122) = -614.3 + 6.702 X 10(4)/T - 5 .920 X 10(6)/T-2 + 83.06 ln T where T is temperature in Kelvin. Speciation calculations show that for a h ydrothermal fluid at 300 degrees C with sulphur concentration buffered at p yrite-pyrrhotite-magnetite, pH = 4-6, the dominant hydrosulphide complex wi ll be either CuHS0 or Cu(HS)(2)(-) depending on the pH. In lower pH solutio ns, CuHS0 is expected to be the dominant hydrosulphide complex at most geol ogical sulphur concentrations. Comparison with Cu(I) chloride complexes sho ws that, at 300 degrees C, CuCl2- will predominate when chloride concentrat ions exceed 0.1 to 1.0 mol kg(-1) at pH values buffered at potassium feldsp ar-muscovite-quartz. As temperature decreases, the stability of the chlorid e complexes declines and therefore hydrosulphide complexes predominate over an increasingly wider range of chloride concentration. In hydrothermal sol utions, copper transport as hydrosulphide complexes reaches mineralizing le vels only at high total sulphur concentrations and basic pH values. Under m ore acidic conditions and lower total sulphur, chloride complexing is requi red for the transport of sufficient copper to form economic mineralization. Copyright (C) 1999 Elsevier Science Ltd.