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 phases on geological processes requires knowledge of mineral solubility and metal speciation and partitioning among the different fluid phases. The degree of this knowledge also significantly varies across this TP range, as roughly illustrated by the three fields identified in Figure 1. In the domain of moderate-temperature aqueous solutions (light-gray field, labeled 1) a large amount of data exists about the nature and stability of major metal complexes, and thermodynamic models have been developed over~ 50 years for predicting ore mineral solubilities. By contrast, it is only recently that new insights were obtained into the speciation and transport of economic metals in low-density vapor and fluid phases typical of hydrothermal-magmatic deposits (white field, labeled 2 in Fig. 1), but a number of questions still remain concerning the role of the vapor phase in metal transfers. Similarly, our current knowledge is still insufficient in the domain of high-pressure fluids typical of subduction zones (dark-gray field, labeled 3 in Fig. 1), for which very little is known about the effect of the major ligands like chloride, sulfur, carbon or silica on metal mobility. These lacks hamper our understanding of the geological impact of all these fluids in the continuity of metal and volatile transfers through Earth’s entire crust.

Because the density of these aqueous phases varies over several orders of magnitude across the TP range of terrestrial processes, it is convenient to consider the different types of aqueous fluid phases in terms of this parameter. In this article, we use the term “vapor” in a wider sense than it is usually employed by chemists; here it refers to a volatile fluid phase which is produced by magma degassing and/or fluid boiling over a wide T range (~ 100-1000 C) and whose density is lower that the critical density of the given fluid composition. The term “liquid” is used for an aqueous, commonly salt-bearing, solution whose density is greater than its critical density.