(1) magmatic water, which separates from magmatic melts in the
process of solidification and formation of igneous rock;
(2) metamorphic water, which is freed in the deep zones of the
earth’s crust from water-containing minerals during their
(3) water buried in the pores of marine sedimentary rock, which
begins to move as a result of disturbances in the earth’s crust or under
the influence of heat from within the earth; and
(4) meteoric water, which penetrates into the depths of the earth
through water-permeable strata. The mineral substance found in the
solution whose deposition forms hydrothermal deposits can be separated out
by congealing magma or can be mobilized from the rocks through which
subterranean waters are filtered.
—from the surface of the earth down to more than 10 km; the optimal
conditions for their formation occur at a depth of several hundred meters
to 5 km.
The initial temperature of this process can be 700°-600° C, gradually
decreasing to 50°-25° C; the most abundant formation of hydrothermal ore
takes place in the range of 400°-100° C.
In the early stage, the water existed as steam, which condensed during
gradual cooling and passed into the liquid state.
This was a true ionic solution of complex compounds of various elements,
which precipitated out upon changes in pressure, temperature, and
acid-alkali and oxidation-reduction characteristics.
The deposition of these elements could occur in open cavities and as a
result of the replacement of rock through which hydro-thermal solutions
flowed; in the first case mineral veins appeared, and in the second case,
the mineral deposits took the form of metasomatic bodies. The most
widespread form of hydrothermal bodies are veins, stockworks, and
stratified and irregularly shaped deposits. They reach a length of several
kilometers and a width of several centimeters to dozens of meters.
Hydrothermal deposits are flanked by a halo of diffusion of its component
elements (primary diffusion halos), whereas the adjoining rocks are
hydrothermally transformed. The most common processes of hydrothermal
transformation are silicification and alkali transformation, in which the
introduction of potassium leads to the development of muscovite, sericite,
and clay minerals and the action of sodium leads to the formation of
(1) sulfide ores, which form deposits of copper, zinc, lead,
molybdenum, bismuth, nickel, cobalt, antimony, and mercury;
(2) oxide ores, which are typical for deposits of iron, tungsten,
tantalum, niobium, tin, and uranium;
(3) carbonaceous ores, which are found in certain deposits of iron
(4) native ores, which are characteristic of gold and silver; and
(5) silicate ores, which create deposits of nonmetallic minerals
(asbestos and mica) and some deposits of rare metals (beryllium, lithium,
thorium, and rare earths).
Hydrothermal ores are distinguished by their large number of component
They are usually unevenly distributed in the contours of ore bodies,
forming alternating zones of high and low concentration that determine the
primary mineral and geochemical zonality of the deposits.
There are several variants of genetic classifications.
In 1907 the American geologist W. Lindgren proposed a division into three
classes, taking account of depth and temperature of formation (hypothermal,
mesothermal, and epithermal).
In 1940 another American geologist, A. Bateman, noted two classes of
(1)those laid down in cavities and
(2) those formed by replacement.
In 1941, the Swiss geologist P. Niggli divided these deposits according to
their characteristics in relation to magmatic rocks and their temperature
The Soviet geologist M. A. Usov (1931) and the German geologist H.
Schneiderhöhn (1950) distributed hydrothermal deposits according to the
level of solidification of the ore-bearing magmas.
The Soviet geologists S. S. Smirnov (1937) and Iu. A. Bilibin (1950)
grouped hydrothermal deposits according to their connection with
tectonomagmatic complex igneous rocks.
V. I. Smirnov (1965) proposed a grouping of hydrothermal deposits
according to the natural associations of their component mineral
complexes, which would reflect their genesis.
Hydrothermal deposits are of enormous significance in the extraction of
many important minerals. They are essential for the production of
nonferrous, rare, noble, and radioactive metals. In addition, hydrothermal
deposits serve as the source of asbestos, magnesite, fluorspar, barite,
crystal, Iceland spar, graphite, and several precious stones (tourmaline,
topaz, and beryllium).
Veins are mineral deposits which form when a preexisting fracture or
fissure within a host rock is filled with new mineral material. The
deposition of minerals is typically performed by circulating aqueous
solutions. Many ore deposits of economic importance occur in veins.
Vein deposits are believed to form when aqueous solutions carrying various
elements migrate through fissures in rock and deposit their burden onto
the fissure walls. Hot, rising water escaping from cooling igneous plutons
may deposit minerals as it ascends through the crust. As heated magmatic
waters rise, the temperature and pressure of their environment drop and
minerals exsolute and crystallize. Meteoric ground water may also
percolate down through the earth's crust, dissolving surface minerals and
gaining heat from the geothermal gradient or from nearby igneous
intrusions. At greater depths the dissolved substances may precipitate and
crystallize along the walls of the fissures and cavities through which the
Most vein deposits are formed as new mineral species are precipitated onto
rock walls which themselves remain unaltered. In such cases mineral
deposits fill the original crack or fissure in the host rock but do not
extend into the host rock itself. The boundary betwen host rock wall and
deposited vein minerals therefore remains clearly delineated. Vein
deposits of this nature are a type of hydrothermal deposit because
the mineral species which compose the veins were precipitated by hot
However, sometimes the preexisting rock wall which contains the vein
undergoes alteration. Portions of the host rock may either dissolve and be
transported away or else react chemically with the circulating volatile
fluids or the newly formed mineral species. In this case the boundary
between vein deposit and original rock wall will be unclear. If most of
the mineralization process occurs within the space once occupied by
unaltered wall rock then the vein is termed ahydrothermal replacement
deposit. A hydrothermal replacement deposit occurs when hot circulating
aqueous solutions replace the original rock with new mineral species. This
typically occurs in more soluble rocks such as limestone.
Hydrothermal replacement deposits are a form of hydrothermal
metamorphism or metasomatism.
Some mineral species crystallize mainly at preferred temperatures and
pressures. Because the temperatures and pressures are different for each
type of hydrothermal deposit, each has a different, characteristic set of
Hypothermal deposits are formed at great depths and high pressures
and temperatures. Temperatures may range from 300° to 500° Celsius during
the formation of such deposits. Casseterite, wolframite and molybdenum
veins; gold-quartz veins; copper-tourmaline veins; and lead-tourmaline
veins provide examples of mineral associations which may occur in
hypothermal deposits. Minerals which are found in hypothermal veins
include quartz, fluorite, tourmaline, and topaz. Ore minerals found may
include native gold (Au); the sulfides galena (PbS), chalcopyrite (CuFeS2),
pyrite (FeS2), molybdenite (MoS2), bismuthinite (Bi2S3),
and arsenopyrite (FeAsS); the oxides uraninite (UO2),
cassiterite (SnO2), and magnetite (Fe3O4);
and the tungstates wolframite ((Fe,Mn)WO4) and scheelite (CaWO4).
Metals which may be extracted from hypothermal deposits consist of copper
(Cu), molybdenum (Mo), tin (Sn), tungsten (W), gold (Au), and lead (Pb).
Mesothermal deposits form at intermediate depths, temperatures, and
pressures. Temperatures may range from 200° to 300° Celsius during the
formation of such deposits. Quartz and carbonate minerals such as calcite
(CaCO3), ankerite (CaFe(CO3)2), siderite
(FeCO3), dolomite (CaMg(CO3)2), and
rhodocrosite (MnCO3) occur in mesothermal deposits. Ore
minerals which may be found include native gold (Au) and the sulfides
galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2), pyrite
(FeS2), bornite (Cu5FeS4), arsenopyrite
(FeAsS), and tetrahedrite ((Cu,Ag)12Sb4S13).
Metals which are mined consist of copper (Cu), zinc (Zn), silver (Ag),
gold (Au), and lead (Pb).
Epithermal deposits form at shallow depths under relatively low
temperatures and pressures. Temperatures during formation may range from
50° to 200° Celsius. Minerals found include quartz, opal, and chalcedony
(SiO2); calcite (CaCO3), aragonite (CaCO3),
and dolomite (CaMg(CO3)2); the halides fluorite (CaF2)
and chlorargyrite (AgCl); the sulfate barite (BaSO4); native
gold (Au); and the sulfides realgar (AsS), cinnabar (HgS), acanthite (Ag2S),
pyrite (FeS2), orpiment (As2S3), stibnite
(Sb2S3), proustite (Ag3AsS3),
and pyrargyrite (Ag3SbS3). Metals which are mined
from epithermal deposits include silver (Ag), gold (Au), and mercury (Hg).
The association of gold mineralization with volcanic and geothermal hot
spring activity has long been recognized by prospectors and geologists.
We now know that this association is a consequence of the hot magmas which
not only produce volcanic eruptions and volcanic rocks but also are the
source of the hot fluids that transport gold and other metals and may in
fact be the source of gold itself. Fluids emanating from a molten magma
are extremely hot and under high pressure deep below the surface.
As these fluids rise, they mix with surface waters and change the
composition of the rocks with which they come into contact. This process
is known as alteration.
Eventually the fluids breach the surface and form either acidic lakes
known as fumaroles common in the craters of volcanoes or dilute, neutral
hot springs like those at Yellowstone or the Geysers in California. These
two different surface manifestations – acidic lakes or neutral hot springs
– reflect two different fluid types that each result from the two
different paths taken by the magma as it rises to the surface. Both form
gold deposits and are known respectively as low- and high-sulphidation
gold deposits. In both subtypes gold will largely be precipitated from 2.5
kilometers depth to surface.
Recognizing that gold precipitates near the surface in these systems, the
great American geologist Waldemar Lindgren coined the term epithermal in
1933, epi meaning shallow and thermal referring to the heated fluid. The
chemist Werner Giggenbach further subdivided epithermal gold deposits into
low and high sulphidation types (illustrated right1). Low and high do not
refer to each type’s relative amount of sulphide minerals (metal complexes
of sulfur with metals). Rather the distinction is based on the different
sulfur to metal ratio within the sulphide minerals of each subtype. While
this discussion deals with high-sulphidation epithermal systems, it is
worth mentioning that low-sulphidation systems also form economic gold
deposits although they develop under vastly different chemical conditions.