This contribution provides an overview of available experimental, thermodynamic, and molecular data on Au aqueous speciation, solubility, and partitioning in major types of geological fluids in the Earth's crust, from low-temperature aqueous solution to supercritical hydrothermal-magmatic fluids, vapours, and silicate melts. Critical revisions of these data allow generation of a set of thermodynamic properties of the AuOH, AuCl ⁻ ₂ , AuHS, and Au(HS) ⁻ ₂ complexes dominant in aqueous hydrothermal solutions; however, other complexes involving different sulphur forms, chloride, and alkali metals may operate in high-temperature sulphur-rich fluids, vapours, and melts. The large affinity of Au for reduced sulphur is responsible for Au enrichment in S-rich vapours and sulphide melts, which are important gold sources for hydrothermal deposits. Thermodynamic, speciation, and partitioning data, and their comparison with Au and S contents in natural fluid inclusions from magmatic-hydrothermal gold deposits, provide new constraints on the major physical-chemical parameters (temperature, pressure, salinity, acidity, redox) and ubiquitous fluid components (sulphur, carbon dioxide, arsenic) affecting Au concentration, transport, precipitation, and fractionation from other metals in the crust. The availability and speciation of sulphur and their changes with the fluid and melt evolution are the key factors controlling gold behaviour in most geological situations.

Instead of sulfide or sulfate, the trisulfur ion S ₃ ^– is stable at high temperatures.

Geological aqueous fluids operate in a wide range of temperatures (from 0 to 1000 °C) and depths (from Earth surface to ~10s km), over which the physical-chemical properties of water and water-salt-gas systems and, consequently, their capacities to dissolve minerals and to transport chemical elements are very different. The principal types of geological fluids are illustrated in Figure 1, showing the domains of the liquid, vapor, and supercritical fluid phases in the water phase diagram as a function of temperatures (T) and pressures (P) typical of Earth’s crust. Understanding the impact of these different fluid...