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.

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...

The basaltic basement of the large igneous province formed by the Kerguelen Plateau and Broken Ridge in the southeastern Indian Ocean has been sampled by three Ocean Drilling Program cruises (Legs 119, 120 and 183). Although the Cretaceous parts of this plateau formed in the embryonic Indian Ocean basin, presumably by melting associated with the Kerguelen plume, trace element abundances and isotopic ratios of Sr, Nd and Pb of Cretaceous basalt from several drill sites indicate that continental lithosphere was involved in their petrogenesis. On the basis of relative depletions in Nb, Ta and Th, and isotopic characteristics similar to those of EMI ocean island basalt, lavas from Leg 120 Site 747 in the Central Kerguelen Plateau contain a component derived from lower continental crust. On the basis of relative abundances of Sr and Eu and EMI-like Pb isotopic ratios, the source of basalt from Leg 120 Site 750 in the northeastern part of the Southern Kerguelen Plateau also contained a component derived from lower continental crust; in this case, the crustal component formed as a plagioclase-rich, clinopyroxene-bearing cumulate. Basalts from Leg 120 Site 749 define two distinct isotopic (Sr, Nd and Pb) groups which differ from the isotopic fields for Site 747 and 750 basalts. Among Site 749 lavas, there is subtle evidence for a continental component, broadly similar (i.e. moderate ²⁰⁶Pb/²⁰⁴Pb ∼18·0) to that expressed more obviously in basalt from Leg 119, Site 738 on the southern edge of the Southern Kerguelen Plateau and Leg 183 Site 1137 on Elan Bank. The continental components in the Kerguelen Plateau basalts may have resided in a heterogeneous mantle plume that was formed, in part, by deep recycling of crust. It is more likely, however, that slivers of Gondwana lithosphere reside within the lithosphere and asthenosphere of the Indian Ocean mantle where they contaminate both plume-derived and mid-ocean ridge basaltic magmas.

Significance: Gold resources on Earth result from an exceptional concentration phenomenon yielding metal contents in ore a thousand to a million times higher than those in common rocks. We show that this process is controlled by sulfur radical ions ( S 3 − ), which strongly bind Au in aqueous solution at elevated temperatures and pressures and allow very efficient extraction, transport, and deposition of gold by geological fluids. Thus, the most inert metal of the periodic table may be very mobile, which explains key features of known gold deposits and offers new possibilities for resource prospecting. Furthermore, the high capacity of the radical ions to solubilize gold may be used for its selective extraction from ores and hydrothermal synthesis of Au-based nanomaterials.