Hydrothermal Deposits

a large group of mineral deposits formed from the sediments of hot aqueous solutions that circulate deep inside the earth. Four groups of sources of the water of hydrothermal solutions are distinguished: (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 re-crystallization; (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.

Hydrothermal deposits were formed over a wide range of depths—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 albite.

The following main types of hydrothermal ores are distinguished according to the predominant minerals: (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 and manganese; (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 minerals. 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 deposits: those laid down in cavities and 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 of formation. 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).