TERRESTRIAL HYDROTHERMAL PROCESSES: A REVIEW

Hydrothermal processes on Earth have played an important role in the evolution of our planet. These processes link the lithosphere, hydrosphere and biosphere in a continuously evolving dynamic system. Terrestrial hydrothermal processes have been active since water condensed to form the hydrosphere, most probably from about 4.4 Ga. The circulation of hot aqueous solutions (hydrothermal systems) at and below the Earth's surface, is driven by magmatic heat. Hydrothermal systems form beneath the oceans (e. g. spreading centres, oceanic plateaux), in lakes, intracontinental rifts, continental margins and magmatic arcs. Hydrothermal fluids can be juvenile-magmatic or derived from seawater, metamorphic, meteoric, connate waters or a mix of two or more of these. The interaction of hydrothermal fluids with wallrocks and/or the hydrosphere and changes in their composition through time and space, contribute to the formation of a wide range of mineral deposit types and associated wallrock alteration. On Earth, sites of hydrothermal activity support, both at surface and in the subsurface, varied ecosystems based on a range of chemotrophic microorganisms. The evolution of hydrothermal processes through geological time is ultimately related to planetary cooling and the mode of heat transfer from the mantle towards the surface (e. g. mantle plume events, associated tectonism, continental volcanism, oceanic magmatism). Thus, some hydrothermal deposits are strictly time-dependent, for example the late Archaean Au-U of the Witwatersrand type and the huge accumulation of banded iron-formations. The latter reached a peak during the Palaeoproterozoic and were directly associated with the delivery of large quantities of ferrous Fe from hydrothermal systems at oceanic spreading centres. It is possible that hydrothermal processes also operate on other terrestrial planets, such as Mars, where liquid water is, or was formerly, present.