Abstract 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.

Abstract The partitioning of an ionic species between two or more co-existing phases is closely related to chemical speciation, solubility, sorption and solid-solution formation in the geochemical system. The aim of chemical thermodynamic modelling is to find the stable or metastable chemical speciation in such a system; from that, any possible ion partition or distribution coefficient can be retrieved. This chapter aims to highlight some typical features of modelling the equilibrium partitioning of inorganic ionic species between aqueous electrolyte and solid phases, including adsorption and ion exchange on their surfaces. Some guidance is also given on methods and computer codes that can be used in helping us understand and predict ion partitioning in complex scenarios, where several competing solids are involved in a minor element uptake by different mechanisms.