Hydration Is the Key for Gold Transport in CO2–HCl–H2O Vapor

Carbon dioxide (CO2) is a major component of volcanic gases and ore-forming hydrothermal fluids. However, CO2 has contrasting effects on the speciation of different metal complexes and ore mineral solubility, but a molecular understanding of its effects is lacking. To address this deficiency, we conducted ab initio molecular dynamics (MD) simulations of the behavior of AuCl(aq) in the CO2–H2O system at 340 °C and 118–152 bar and 800 °C and 265–291 bar for CO2 mole fractions (X CO2 ) of 0.1–0.9. The MD simulations indicate that the linear [H2O–Au–Cl]0 structure of gold chloride is not affected by CO2 at X CO2 up to 0.8 at 340 °C and X CO2 up to 0.5 at 800 °C, whereas the “dry” [AuCl]0 species predominates at X CO2 > 0.8 at 340 °C and X CO2 > 0.5 at 800 °C. The number of water molecules hydrating the AuCl(aq) complex decreases systematically with an increasing CO2 mole fraction and increasing temperature. Results of Au solubility experiments at 340 °C in CO2–H2O solutions show that the addition of CO2 does not enhance Au solubility. We conclude that hydrated chloride species with linear geometry are the main means for transporting gold in CO2–H2O–HCl fluids and that Au solubility decreases in CO2-bearing hydrothermal fluids as a result of the decrease in hydration of the Au complexes. This contrasts with the behavior of divalent transition metals (e.g., Fe, Co, Ni, and Zn). We propose that the different solubility behaviors of Au and base metals are due to the changes in translational entropy as a result of the changes in coordination geometry (and associated hydration) of the complexes with increasing X CO2 . The first-shell coordination of Au­(I) complexes remains constant over wide ranges of X CO2 , whereas first-row divalent transition metal complexes undergo entropy-driven geometric changes with a decreasing water activity.