Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, water, and θέρμη, heat but can occur in the shallow to mid crust along deeply penetrating fault irregularities or in the deep crust related to the intrusion of granite, or as the result of orogeny or metamorphism. Hydrothermal circulation often results in hydrothermal mineral deposits.
Seafloor hydrothermal circulation
Hydrothermal circulation in the oceans is the passage of the water through mid-oceanic ridge systems.
The term includes both the circulation of the well-known, high-temperature vent waters near the ridge crests, and the much-lower-temperature, diffuse flow of water through sediments and buried basalts further from the ridge crests. The former circulation type is sometimes termed "active", and the latter "passive". In both cases, the principle is the same: Cold, dense seawater sinks into the basalt of the seafloor and is heated at depth whereupon it rises back to the rock-ocean water interface due to its lesser density. The heat source for the active vents is the newly formed basalt, and, for the highest temperature vents, the underlying magma chamber. The heat source for the passive vents is the still-cooling older basalts. Heat flow studies of the seafloor suggest that basalts within the oceanic crust take millions of years to completely cool as they continue to support passive hydrothermal circulation systems.
Hydrothermal vents are locations on the seafloor where hydrothermal fluids mix into the overlying ocean. Perhaps the best-known vent forms are the naturally occurring chimneys referred to as black smokers. This convection can manifest as hydrothermal explosions, geysers, and hot springs, although this is not always the case. Recent studies retain only the epithermal label. John Guilbert's 1985 revision of Lindgren's system for hydrothermal deposits includes the following:
- Ascending hydrothermal fluids, magmatic or meteoric water
- Porphyry copper and other deposits, 200–800 °C, moderate pressure
- Igneous metamorphic, 300–800 °C, low to moderate pressure
- Cordilleran veins, intermediate to shallow depths
- Epithermal, shallow to intermediate, 50–300 °C, low pressure
- Circulating heated meteoric solutions
- Mississippi Valley-type deposits, 25–200 °C, low pressure
- Western US uranium, 25–75 °C, low pressure
- Circulating heated seawater
- Oceanic ridge deposits, 25–300 °C, low pressure
See also
- Volcanogenic massive sulfide ore deposit
- Geothermal gradient
- Hydrothermal synthesis