Ore solution chemistry; V, Solubilities of chalcopyrite and chalcocite assemblages in hydrothermal solution at 200 degrees to 350 degrees C

Abstract

Solubilities of the assemblages chalcopyrite (CuFeS 2 ) + pyrite (FeS 2 ) + bornite (Cu 5 FeS 4 ) and copper + chalcocite (Cu 2 S) have been measured in subcritical aqueous solutions of NaCl and NaHS + H 2 S under the following conditions: 200 degrees to 350 degrees C; a range of pH within 2.5 of neutral in sulfide solutions, and 1.5 of neutral in chloride solutions; up to 10.4 m NaCl and 2.85 m HS (super -) ; and from 15 to 256 atm total pressure. Sulfur fugacity was fixed by univariant sulfide assemblages; oxygen fugacity and pH were controlled in NaCl solutions by the graphite-CO 2 buffer. The pale yellow color of high temperature solutions, plus optical absorption spectra at 25 degrees C, indicate that cuprous complexes predominate with this assemblage at temperatures to above 350 degrees C. The important solubility-controlling reactions and logs of their equilibrium constants are:200 degrees C 250 degrees C 300 degrees C 350 degrees C1/4 Cu 5 FeS 4 + HS (super -) + 1/2 H 2 S(g) = 1/4 CuFeS 2 + Cu(HS) (super -) 2 -2.3 + or - 0.7 -2.3 + or - 0.7 -2.3 + or - 0.7 -2.3 + or - 0.71/4 Cu 5 FeS 4 + HS (super -) + 3/2 H 2 S(g) = 1/4 CuFeS 2 + Cu(HS) 2 (H 2 S) (super -) \-- -|3.8 + or - 0.7 -3.3 + or - + or - 0.7 -3.1 + or - 0.75CuFeS 2 + 2H (super +) + 1/2 CO 2 (g) = Cu 5 FeS 4 + 3FeS 2 + 1/2 C (graph.)+ H 2 O(l) + Fe (super 2+) 3.1 + or - 0.5 3.6 + or - 0.5 4.7 + or - 0.5 5.0 + or - 0.5Fe (super 2+) + Cl (super -) = FeCl (super +) 0.67 + or - 1.0 1.0 + or - 1.0 1.2 + or - 1.0 1.8 + or - 1.0CuFeS 2 + H (super +) + 1/4 CO 2 (g) + Cl (super -) = FeS 2 + 1/4 C (graph.)+ 1/2 H 2 O(l) + CuCl(aq) -1.2 + or - 0.5 -0.14 + or - 0.5 0.79 + or - 0.5 1.9 + or - 0.5Cu(c) + H (super +) + 1/4 Co 2 (g)= 1/4 C (graph.)+ 1/2 H 2 O(l) + Cu (super +) -1.6 + or - 0.3 -1.2 + or - 0.3 -0.29 + or - 0.3 0.46 + or - 0.3Cu (super +) + Cl (super -) = CuCl(aq) 2.2 + or - 0.6 2.4 + or - 0.6 2.2 + or - 0.6 2.7 + or -0.6 Iron bisulfide complexes are apparently unstable under the above conditions and could not be detected at the 0.5 to 10 ppm level. At neutral pH and Cl (super -) activity = 0.1, cuprous bisulfide complexes predominate for sulfur activities Sigma S > or = 1.0 (at 350 degrees ) and Sigma S > or = 0.03 (at 250 degrees C); the species CuCl predominates for lower Sigma S. At 350 degrees C with Sigma S < 0.1, under the near-neutral to weakly acidic conditions probably characteristic of porphyry copper ore-forming fluids, approximately 1,000 ppm Fe and Cu dissolve in 1.0 m NaCl as the species CuCl, Fe (super 2+) , and FeCl (super +) . Under similar conditions at 250 degrees C, solubilities fall to roughly 100 ppm Fe and 10 ppm Cu. Simultaneous transport of Fe and of Cu as bisulfide complexes above 200 degrees C is geologically unlikely. Where chloride complexed, Cu-Fe sulfides may be deposited by dilution of hydrothermal solutions, by increasing pH or Sigma S, and by decreasing temperature or oxygen fugacity.