What is an hydrothermalism in geology ?

Hydrothermalism : definition

Hydrothermalism refers to the circulation of hot water in the earth's crust, in connection with volcanic and plutonic systems, and with metamorphic events. Hydrothermal fluids are usually hot (from 50°C to over 350°C), salty, corrosive, and loaded with dissolved substances acquired in magma or during percolation through rocks. They transport, concentrate and redeposit these dissolved salts in the form of minerals in certain particular sites : hydrothermal deposits (veins, sulfidic masses, skarns, etc...). Hot springs, geysers, fumaroles, acid lakes... are current manifestations of hydrothermalism.

These hydrothermal deposits frequently constitute ore deposits (Pb, Zn, F, Ba, Au, Ag, etc...), widely exploited on the planet. The minerals formed by deposits of substances dissolved in these fluids are called hydrothermal agents (barite, fluorite, numerous sulphides, native sulphur, etc...).

Beyond the critical temperature of water (374°C), one passes from the hydrothermal domain to the pneumatolytic domain.

The term "hydrothermal" in gemology

Attention, the name "hydrothermal" or sometimes just "hydro" after the name of a gemstone means that this one is synthetic and that it was produced in an autoclave. Autoclaves are kinds of pressure cookers that allow an aqueous chemical solution to rise in pressure and temperature. This process mainly makes it possible to synthesize emeralds and quartz. It is an expensive synthesis method because it is very slow (3 mm of growth per week for the emerald). Cut emeralds from this process are still relatively expensive to buy, although much more affordable than their natural counterparts.