EXPERIMENTAL STUDY OF COPPER(I) CHLORIDE COMPLEXING IN HYDROTHERMAL SOLUTIONS AT 40 TO 300°C AND SATURATED WATER VAPOR PRESSURE - APPLICATION TO THE DETERMINATION OF EQUILIBRIUM CONSTANTS FROM SOLUBILITY DATA

Abstract

The solubility of Cu phases was measured in vapor-saturated aqueous HCl/NaCl solutions at temperatures ranging from 40 to 300°C, total chloride concentration from 0.01 to 1 m, and pH from 0 to 3.5. For temperatures up to and including 150°C, CuCl(s) was used as the solid reactant. At higher temperatures, foils of metallic Cu and Ag were used. Silver was added as a redox sensor, as the equilibrium constants describing dissolution of this metal as chloride complexes are already known to high precision. Copper was found to dissolve primarily as CuCl(aq), CuCl2-, and CuCl32-. Data collected from the experiments were regressed to determine the following equilibrium constants as functions of temperature (K): Cu(s)+1/4O2(g)+H++Cl-=CuCl(aq)+1/2H2O(l), log K1=0.1316*(1000/T)2+2.865*(1000/T)+4.4243,R2=0.9958; Cu(s)+1/4O2(g)+H++2Cl-=CuCl2-+1/2H2O(l), log K2=1.0981*(1000/T)2-2.2961*(1000/T)+12.916,R2=0.9896; Cu(s)+1/4O2(g)+H++3Cl-=CuCl32-+1/2H2O(l), log K3=2.2704*(1000/T)2-8.7646*(1000/T)+20.643,R2=0.9941. These equations can be used to calculate equilibrium constants at temperatures up to 350°C and vapor saturated pressure. %Our results at T < 150°C agree well with those published by other researchers, but the agreement is variable for results at T > 150°C. At higher temperatures, our data for CuCl2- are in accord with those of Var'yash (1992), whereas our data for CuCl(aq) deviate significantly from the results of Crerar and Barnes (1976). The agreement with published theoretical estimates of the formation constants for Cu(I) chloride complexes (Helgeson, 1969; Ruaya, 1988; Sverjensky et al., 1997) is not good, especially at T > 200°C.The solubility of chalcopyrite was calculated for a variety of conditions. For unit activity of Cl-, pH between 3 and 5, and oxygen and sulfur fugacity buffered by the assemblage pyrite-pyrrhotite-magnetite, Cu is transported mainly as CuCl2-, and has a solubility of 212 moles/kg (13.5 ppm) at 350°C and pH = 3. Chalcopyrite will deposit in response to an increase in pH, or decreases in aCl-, temperature, and oxygen fugacity. Calculations of the solubility of chalcopyrite in seafloor hydrothermal systems show that the Cu-rich zones in volcanogenic massive sulfide deposits form at temperatures > 250°C and that cooling and/or pH increase are the most likely depositional controls. Below 250°C, chloride brines are incapable of transporting significant quantities of Cu unless conditions are unusually oxidized.