Sphalerite‐galena replacement in sodium chloride solution: A thermodynamic approach

Considering the material balances of the constituents including solid phases, replacement reaction of the sphalerite‐galena pair in chloride solution is examined quantitatively under equilibrium conditions of 250°C, water saturation vapor pressure, and initial Cl concentration of 1 mol/L. NaCl+PbCl2 solution with solid sphalerite, dissolves and releases both total Zn and total S of 1.26 × 10−5 mol/L into the solution under without or lower PbCl2 concentration. If the PbCl2 concentration is higher than 1.32 × 10−6 mol/L, precipitation of galena as replacement occurs, suggesting that sphalerite has an ability to trap a lower concentration of Pb. If PbCl2 concentration of the solution is higher than 1.32 × 10−6 mol/L, the majority of Pb deposited as galena with using sulfur originated from solid sphalerite dissolved, and the amount of Zn from sphalerite equivalent to the amount of galena deposited releases into the solution. On the other hand, NaCl+ZnCl2 solution with solid galena under the same environmental conditions, dissolves and releases both total Pb and total S of 6.43 × 10−6 mol/L into the solution under without or lower ZnCl2 concentration. Over the ZnCl2 concentration of 6.40 × 10−5 mol/L in the solution, precipitation of sphalerite occurs, indicating that galena cannot trap a low concentration of Zn. Zinc would drain away from the hydrothermal depositional environment under the presence of only galena. These relationships are controlled mainly by the reaction of predominant metal chloride or metal hydroxide species in the solution. Sphalerite is a good scavenger for Pb, but galena is not for Zn.