Hydrothermal metamorphism, also called metasomatism, may take place across wide regions of rock, thereby constituting a variant of regional metamorphism. It may alternately may take place in a limited, localized area and constitute a variant of local metamorphism.
Hydrothermal metamorphism takes place when hot, volatile solutions percolate into and react with the protolith, or the original rock. The heat of the intrusive igneous body and the hot volatile fluids serves to catalyze metamorphic reactions in the host rock. The incident fluids enhance ion mobility in the system and are highly reactive. Both the intrusive magma and the associated volatile fluids may introduce elements which were not present in the protolith into the reaction process. The incident volatile fluids may also dissolve and remove elements originally abundant in the host rock. The chemical composition of the host rock may be extensively altered during the process of hydrothermal metamorphism. Hydrothermal metamorphism is therefore a type of allochemical metamorphism.
The hot, volatile fluids responsible for hydrothermal metamorphism may be derived from several sources. Such fluids may consist of volatiles escaping directly from the intrusive magma; they may also be composed of meteoric groundwater heated by the igneous intrusion or by the geothermal gradient. High temperatures and pressures can expel water from hydrated minerals such as gypsum, talc, chalk, and clay. These sources of fluid are typically associated with local metamorphism. When an oceanic plate is subsumed at a boundary between two plates, water may be transported along with it and may subsequently take part in any metamorphic processes which occur. Seawater may also seep into the crust as two oceanic plates move apart. Such sources of volatile fluids are associated with regional metamorphism and the movement of crustal plates.
Much hydrothermal metamorphism occurs at the boundaries of oceanic plates. Plates which are moving apart allow seawater to percolate through the oceanic crust. As the seawater migrates, it heats and reacts with the host rock. Large quantities of metals such as iron, cobalt, nickel, silver, gold, and copper are dissolved from the crust. When the hot, metal-laden fluid later comes into contact with cold seawater, sulfide and carbonate minerals precipitate to form deposits of metal ores. The copper ore which has been mined on Cyprus for several thousand years is thought to have been deposited in this way.
The presence of previously unavailable elements and the catalyzation provided by heat and volatile fluids enables a wide variety of mineral species to form during hydrothermal metamorphism. Many of these species contain elements not originally present in the host rock. The process of hydrothermal metamorphism thus produces a greater variety of minerals than may be formed by thermal metamorphism alone. Silicates which are found in hydrothermal metamorphic deposits include quartz, garnet, wollastonite, olivine, topaz, and tourmaline. The halide fluorite may be present. Sulfide ores such as pyrite, chalcopyrite, sphalerite, and molybdenite and oxides such as magnetite, hematite, spinel, and corundum may also occur.
Local hydrothermal metamorphic deposits may grade into hydrothermal vein deposits. A vein forms when hot, mineral-bearing fluids deposit new materials along the walls of a preexisting fissure or crack in the host rock. In the case where minerals merely exsolute from the aqueous solution to crystallize along the walls of the fissure, leaving the wall rock intact and unchanged, no metamorphism is involved. However, the minerals of the fissure walls often react with the passing volatile fluids. Some may be dissolved and transported away; some may recrystallize or crystallize as new mineral species. The formation of this type of vein deposit, called a hydrothermal replacement deposit, does constitute hydrothermal metamorphism.