As the name says, hydrothermal processes are based on water (hydros=water, Greek) and temperature (thermos=heat, Greek). The temperature, or better the heat source, is the engine which powers the process. The water is working a transport medium, transporting energy and minerals. Hydrothermal processes are based on convection cells. Water which is heated expands, becomes lighter and thus moves upwards. When it reaches a greater height where the temperature is lower it cools down. Now it is heavier again and descends, just to be heated again. Temperature differences always result in the formation of convection cells.
Water is a very good solvent, even more when it is hot. If it cools down it may not be able to hold all the minerals any more, it has previously solved. As a result the minerals are always transported from the hot end of the cell to the cool end.
Obviously the amount of water flowing through the rock depends on the permeability of the rock. Cracks and fissures are very useful. They are generally formed by tectonic forces which also caused the intrusion of the hot material. When the minerals are deposited at the cold end of the cells, the cracks are filled by them.
Mineral resources are generally the accumulation of a certain element or substance in a certain location. Hydrothermal processes are accumulating substances in clefts, which are called dykes. The most common mineral in those clefts is quartz (SiO2), which is not of economic interest. But often the clefts are also filled with any imaginable metal. There is gold, silver, lead, tin, copper, and much more. This is the reason why hydrothermal dyke deposits are often called polymetallic deposits.
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