Thermodynamic modeling of hydrothermal ore deposit formation

• Introducing thermodynamics of fluid-mineral interactions for beginners. • Describing the physical–chemical properties of geological fluids. • Overviewing aqueous speciation and thermodynamic models. • Highlighting case examples of numerical modeling for hydrothermal gold deposits. • Outlining perspectives and links with other approaches. This contribution provides an introductory overview of the chemical thermodynamic modeling approach used in the field of hydrothermal mineral deposit research. An outline of our current knowledge of the physicochemical properties of geological fluids as a function of temperature, pressure, and density is presented. Currently, solubilities of major minerals and metal speciation can be predicted relatively accurately by thermodynamic calculations within the liquid-like density range of hydrothermal fluids. Metal-ligand chemical affinities and stabilities of aqueous complexes in such fluids are reviewed for main groups of metals. The fundamentals of the approach are first introduced in terms of chemical reactions of mineral solubility and dominant aqueous metal complexes. Using gold as an example, it is shown how this first-order approach can place valuable constraints on gold solubility and precipitation mechanisms, greatly aiding the interpretation of natural observations. The use of thermodynamic databases and computer codes for multicomponent systems is introduced, followed by a discussion of caveats and current limitations. Two more detailed examples of case studies of hydrothermal gold ore-forming systems, in porphyry and sedimentary contexts, highlight the value of complementary information that modeling can bring to naturalistic approaches. An overview of key challenges and emerging perspectives in fluid-rock interaction modeling concludes this article.