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. [...]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 XCO2. The first-shell coordination of Au(I) complexes remains constant over wide ranges of XCO2, whereas first-row divalent transition metal complexes undergo entropy-driven geometric changes with a decreasing water activity