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

Abstract

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 (XCO2) 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 XCO2 up to 0.8 at 340 °C and XCO2 up to 0.5 at 800 °C, whereas the “dry” [AuCl]0 species predominates at XCO2 > 0.8 at 340 °C and XCO2 > 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 en...