Hydrothermal mineral deposits are those in which hot water serves as a
concentrating, transporting, and depositing agent. They are the most numerous of all classes
of deposit.
Hydrothermal deposits are never formed from pure water, because pure water is a poor
solvent of most ore minerals. Rather, they are formed by hot brines, making it more
appropriate to refer to them as products of hydrothermal solutions. Brines, and especially
sodium-calcium chloride brines, are effective solvents of many sulfide and oxide ore minerals,
and they are even capable of dissolving and transporting native metals such as gold and silver.
The water in a hydrothermal solution can come from any of several sources. It may be
released by a crystallizing magma; it can be expelled from a mass of rock undergoing
metamorphism; or it may originate at the Earth's surface as rainwater or seawater and then
trickle down to great depths through fractures and porous rocks, where it will be heated, react
with adjacent rocks, and become a hydrothermal solution. Regardless of the origin and initial
composition of the water, the final compositions of all hydrothermal solutions tend to
converge, owing to reactions between solutions and the rocks they encounter.
Hydrothermal solutions are sodium-calcium chloride brines with additions of magnesium and
potassium salts, plus small amounts of many other chemical elements. The solutions range in
concentration from a few percent to as much as 50 percent dissolved solids by weight. Existing
hydrothermal solutions can be studied at hot springs, in subsurface brine reservoirs such as
those in the Imperial Valley of California or the Cheleken Peninsula on the eastern edge of the
Caspian Sea in Turkmenistan, and in oil-field brines. Fossil hydrothermal solutions can be
studied in fluid inclusions, which are tiny samples of solution trapped in crystal imperfections
by a growing mineral.
Because hydrothermal solutions form as a result of many processes, they are quite common
within the Earth's crust. Hydrothermal mineral deposits, on the other hand, are neither
common nor very large compared to other geologic features. It is apparent from this that most
solutions eventually mix in with the rest of the hydrosphere and leave few obvious traces of
their former presence. Those solutions that do form mineral deposits (and thereby leave
obvious evidence of their former presence) do so because some process causes them to
deposit their dissolved loads in a restricted space or small volume of porous rock. It is most
convenient, therefore, to discuss hydrothermal mineral deposits in the context of their
settings.
Veins
The simplest hydrothermal deposit to visualize is a vein, which forms when a hydrothermal
solution flows through an open fissure and deposits its dissolved load. A great many veins
occur close to bodies of intrusive igneous rocks because the igneous rocks serve as heat
sources that create convectively driven flows in hydrothermal solutions. Precipitation of the
minerals is usually caused by cooling of the hydrothermal solution, by boiling, or by chemical
reactions between the solution and rocks lining the fissure. Some famous deposits are the tincopper-lead-zinc veins of Cornwall, Eng.; the gold-quartz veins of Kalgoorlie, W. Aus.,
Australia, and Kirkland Lake, Ont., Can.; the tin-silver veins of Llallagua and Potosí, Bol.; and
the silver-nickel-uranium veins of the Erzgebirge, Ger., which were first described by Georgius
Agricola in his book De re metallica (1556).
Hydrothermal deposits formed at shallow depths below a boiling hot spring system (see figure)
are commonly referred to as epithermal, a term retained from an old system of classifying
hydrothermal deposits based on the presumed temperature and depth of deposition.
Epithermal veins tend not to have great vertical continuity, but many are exceedingly rich and
deserving of the term bonanza. Many of the famous silver and gold deposits of the western
United States, such as Comstock in Nevada and Cripple Creek in Colorado, are epithermal
bonanzas.