Ore Deposit Types and Their Primary Expressions

Metals are largely derived from the mantle or crust by partial melting and fluid-related leaching. Ligands can be provided from the same sources, or from the atmosphere, hydrosphere and biosphere. Transport is mainly by mechanical or mass transfer mechanisms and by fluids. On and near the surface, biological processes can also concentrate and transport ore components or remove non-ore components. Hydrothermal fluids are a major transport medium for many ore systems; these fluids are essentially water, with lesser and variable amounts of CO 2 , H 2 S, SO 2 , CH 4 , N 2 , NaCl and other salts, as well as dissolved metal complexes. They are derived from a variety of sources, including i) water-rich silicate melts, ii) circulated sea, connate and meteoric waters, iii) formational, diagenetic and metamorphic fluids. At upper crustal levels, the fluids are typically hotter than the rocks they traverse and in which they deposit their ores; they have variable pH and Eh, and they may be charged with a range of metal complexing agents including Cl and HS. Deposition of ore minerals results from changes to physiochemical parameters, including temperature, pressure, pH, redox state and total concentration of ligands. These changes are associated with such processes as addition of components by contamination, phase separation, cooling across a temperature gradient, pressure decrease, fluid mixing and reaction with host rocks.