A hot, fluid residue, usually of magmatic origin, that is chiefly water contains a rich supply of metallic elements. It becomes further enriched through chemical reactions with the surrounding crustal rock. As it cools, this solution many form massive concentrations of minerals bearing gold, copper, and zinc and other metallic elements that may have great commercial value. Some of the world’s richest deposits of these metals were formed under such hydrothermal conditions.
Magmatic fluids are chemically quite complex and, during the cooling process, may develop into a pegmatitic fluid. It is possible, too, that the magma may differentiate into what is often called a hydrothermal fluid, a very hot and exceptionally watery fluid. In a manner of speaking, the pegmatitic fluid is a hydrothermal fluid, too.
But pegmatitic fluid does not usually contain concentrations of the metallic elements, such as zinc, iron, copper, and lead, or such precious metallic elements as gold and silver. These metallic elements, present in the hydrothermal fluid, and carried in it's movement through the Earth's crust, may also interact chemically with already-form minerals as they pass through the rock.
Geoscientists classify hydrothermal fluids essentially in terms of the temperature and pressure conditions under which they begin to crystallize and form minerals. Those hydrothermal fluids that form under fairly high temperature and pressure conditions are referred to as hypothermal mineral deposits. Those forming under more moderate temperature conditions, the mesothermal mineral deposits. And the epithermal mineral assemblage is developed under the relatively coolest, and, therefore, shallowest, portions of Earth's crust.
In the series of three cases that you see here, all the minerals were developed under the hydrothermal environment. Notice how strikingly different are the minerals in the epithermal environment from those formed in the hypothermal environment. The mesothermal minerals are somewhat transitional between those in the hypothermal and epithermal environments.
Notice, also, the striking metallic appearance of most of these minerals. Minerals like chalcopyrite, and pyrite, and cassiterite, are all minerals have a distinctive metallic luster. And these are minerals, also, that are particularly valuable for their metallic mineral content. Economic mineral deposits — that is to say, mineral deposits that are commercially mined and quarried for their metallic element content are deposits that, in one way or another, are developed within the hydrothermal environment.