ИСТИНА

Mineralization in porphyry Cu deposits is formed by chloride-bearing fluids of a wide Cl concentration range, from a few wt.% to 50 wt.% NaCl eq. In order to quantify the hydrothermal mobility of Cu, we performed an investigation of the speciation of Cu in chloride-bearing fluids using solubility determinations. For this purpose, the coupled solubility of Cu and Ag was determined in acidic chloride solutions at 350 and 450°С and 1000 bar using the autoclave technique with the determination of dissolved metal contents after quenching. The method avoids the control of the most important parameters of the system, f(2), pH, and chloride activity. The fluid composition varied from diluted HCl-bearing (0.1m HCl) to NaCl-bearing (0.01-0.02m HCl + 0.2-15m NaCl). The measured concentration of Cu was 2-2.5 log units higher than the concentration of Ag. The solubility data over the entire Cl concentration range is accurately described by the reaction Cu(cr) + AgCl2- = Ag(cr) + CuCl2-, which implies that CuCl2- is the dominant complex of Cu in all the experimental fluids. The reaction constant, log K(Cu–Ag), decreases with increasing temperature from 2.7 ± 0.4 at 350°С to 2.3 ± 0.2 at 450°С. Therefore, the difference in the stability of CuCl2- and AgCl2- becomes less pronounced with increasing temperature. Combination of the reliable literature values of the AgCl2- formation constant with log K(Cu–Ag) determined in the present study yields the Cu solubility constant Cu(cr) + HCl° + Cl- = CuCl2-+ 02(aq), log K(Cu), which increases from 1.5 ± 0.4 at 350°С to 1.9 ± 0.2 at 450°С. These results imply that the forms of occurrence of Cu and Ag in high temperature hydrothermal fluids are similar. The main aqueous species of these metals are CuCl2- and AgCl2-, their predominance fields extend from diluted Cl-bearing fluids to the NaCl saturation limit.