Hydrothermal Processes

photography — Hydrothermal Black Smoker Deposits. Public Domain.

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 as 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 dissolved. They are precipitated, in other words become solid again and are deposited. 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 generally also caused the intrusion of the hot magma, the heat source. 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 gangue or seams. 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.

Sometimes the cells reach the surface, the hot water either flows out on land forming a hot spring, or under the sea forming a black smoker. Hot springs also form deposits, typically limestone and quartz is deposited, often forming rimstone dams. The material differs from the surrounding rocks. often it is mined as marble. Metals are normally oxidized and eroded, so this kind of hot spring does rarely form orebodies. That's different for hot springs under the sea. The seawater cools the mineral rich water very fast, and the precipitated minerals form small particles which make the water appear like black smoke. That's why they are called black smokers. This "smoke" then slowly falls to the ground forming a layer of mineral rich mud. If they are located in a depression and if there are no water currents they may form thick layers of mineral rich deposits.