Hydrothermal circulation in its most general sense is the circulation of hot water; 'hydros' in the Greek meaning water and 'thermos' meaning heat. Hydrothermal circulation occurs most often in the vicinity of sources of heat within the Earth's crust. This generally occurs near volcanic activity, but can occur in the deep crust related to the intrusion of granite, or as the result of orogeny or metamorphism.
Seafloor hydrothermal circulation 
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.
Hydrothermal circulation is not limited to ocean ridge environments. The source water for hydrothermal explosions, geysers and hot springs is heated groundwater convecting below and lateral to the hot water vent. Hydrothermal circulating convection cells exist any place an anomalous source of heat, such as an intruding magma or volcanic vent, comes into contact with the groundwater system.
Deep crust 
Hydrothermal also refers to the transport and circulation of water within the deep crust, generally from areas of hot rocks to areas of cooler rocks. The causes for this convection can be:
- Intrusion of magma into the crust
- Radioactive heat generated by cooled masses of granite
- Heat from the mantle
- Hydraulic head from mountain ranges, for example, the Great Artesian Basin
- Dewatering of metamorphic rocks which liberates water
- Dewatering of deeply buried sediments
Hydrothermal ore deposits 
During the early 1900s various geologists worked to classify hydrothermal ore deposits which were assumed to have formed from upward flowing aqueous solutions. Waldemar Lindgren developed a classification based on interpreted decreasing temperature and pressure conditions of the depositing fluid. His terms: hypothermal, mesothermal, epithermal and teleothermal were based on decreasing temperature and increasing distance from a deep source. Only the epithermal has been used in recent works. John Guilbert's 1985 redo 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 - moderate pressure
- Cordilleran veins, intermediate to shallow depths
- Epithermal, shallow to intermediate, 50 - 300 °C, low pressure
- Circulating heated meteoric solutions
- Circulating heated seawater
- Oceanic ridge deposits, 25 - 300 °C, low pressure
See also 
- W. Lindgren, 1933, Mineral Deposits, McGraw Hill, 4th ed.
- Guilbert, John M. and Charles F. Park, Jr., 1986, The Geology of Ore Deposits, Freeman, p. 302 ISBN 0-7167-1456-6