Hydrothermal mineral deposits are the products of crustal geochemical processes that extract metals from source regions and concentrate them at depositional sites. These processes and the regions in which they occur constitute mineral systems. Processes that occur within source regions define the characteristics (e.g. temperature, pressure, pH, salinity, redox, sulphur content) of hydrothermal fluids, which, in turn, determine the metal carrying capacity of the fluid. Mineral deposits that contain Cu, Zn, Pb, Ag and/or Au can be classified into three general groupings based on metal assemblages: (1) Zn-Pb-Ag±Au , (2) Cu±Au , and (3) Au±Ag. These groupings are predicted from fluid characteristics and the solubility of metals in the fluids. The hydrothermal geochemistry of Sn, W, U and Mo is not considered in this discussion. Mineral deposit diversity results from the complex interplay of processes in source regions and depositional sites. First-order processes that occur in source regions include devolatilisation and degassing, circulation of surficial fluids, and expulsion of connate fluids. These and second-order processes such as SO2 disproportionation result in hydrothermal fluids with diverse temperatures, acidities, salinities, redox conditions, and, hence, metal-carrying capacities. Many of the processes in the source region are directly or indirectly linked to geological characteristics of the source region, such as granitoid type and basinal lithology. Three first-order depositional processes cause metal deposition: phase separation, reaction with host rocks , and mixing with ambient fluids. The efficiency of metal deposition depends critically on the steepness of chemical gradients in the depositional environment. The interplay of source and depositional processes produces the observed diversity of mineral deposits, and placing these processes in a mineral system framework provides a powerful tool for understanding and, potentially, for discovering mineral deposits.
