Epithermal, Carlin, and orogenic Au deposits form in diverse geologic settings and over a wide range of depths, where Au precipitates from hydrothermal fluids in response to various physical and chemical processes. The compositions of Au-bearing sulfidic hydrothermal solutions across all three deposit types, however, are broadly similar. In most cases, they comprise low-salinity waters, which are reduced, have a near-neutral pH, and CO₂ concentrations that range from <4 to >10 wt %. Experimental studies show that the main factor controlling the concentration of Au in hydrothermal solutions is the concentration of reduced S, and in the absence...