Hydrothermal solutions and mineralization
Background (Encyclopedia of Global Resources)
Essential conditions for the formation of hydrothermal mineral deposits include metal-bearing mineralizing solutions, openings in rocks through which the solutions are channeled, sites for deposition, and chemical reaction resulting in deposition. The term “ore” is used for any assemblage of minerals that can be mined for a profit. “Gangue” is the nonvaluable mineral that occurs with the ore.
During the crystallization of igneous rocks, water and other volatile fluids concentrate in the upper part of the magma. These volatiles carry with them varying amounts of the ions from the melt, including high concentrations of ions that are not readily incorporated into silicate rock-forming minerals. If the vapor pressure in the magma exceeds the confining pressure of the enclosing rocks, the fluids are expelled to migrate though surrounding country rock. These solutions travel along natural pathways in the rock such as faults, fissures, or bedding planes in stratified rocks. As the solutions migrate away from their source region, they lose their mineral content through deposition in natural openings in the host rock (forming open space-filling deposits) or by chemical reaction with the host rock (forming metasomatic replacement deposits). A part of these solutions may make it to the surface to form fumaroles (gas emanations) or hot springs. In addition, some hydrothermal solutions may be derived from water trapped in ancient...
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Nature of Open Spaces (Encyclopedia of Global Resources)
Hydrothermal solutions find ready-made escape routes through the surrounding country rock in the form of faults and fissures. Ore and gangue minerals of cavity-filling deposits are found in faults or fissures (veins), in open spaces in fault breccias, in solution openings of soluble rocks, in pore spaces between the grain of sedimentary rocks, in vesicles of buried lava flows, and along permeable bedding planes of sedimentary strata. The shape of the mineral deposit is controlled by the configuration of structures controlling porosity and permeability. Fracture patterns, and therefore veins, may take on a wide variety of geometric patterns, ranging from tabular to rod-shaped or blanketlike deposits.
Some deposits are characterized by ore minerals that are widely disseminated in small amounts throughout a large body of rock such as an igneous stock. These igneous bodies undergo intense fracturing during the late stage of consolidation, and residual fluids permeate the fractured rock to produce massive deposits of low-grade ores. In such deposits, the entire rock is extracted in mining operations. The famous porphyry copper deposits of the southwestern United States—including those of Santa Rita, New Mexico; Morence, Arizona; and Bingham, Utah— are of this type, as are the molybdenum deposits of Climax, Colorado.
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Metasomatic Replacement (Encyclopedia of Global Resources)
Some hydrothermal deposits are emplaced by reaction of the fluids with chemically susceptible rocks such as limestone or dolostone. Metasomatic replacement is defined as simultaneous capillary solution and deposition by which the host is replaced by ore and gangue minerals. These massive deposits or lodes take on the shape and the original textures of the host. Replacement is especially important in deep-seated deposits where open spaces are scarce. Replacement deposits of lead-zinc are common in limestones surrounding the porphyry copper of Santa Rita, New Mexico, and at Pioche, Nevada.
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Classification by Temperature and Depth (Encyclopedia of Global Resources)
Veins are zoned, with higher-temperature minerals deposited near the source and lower-temperature minerals farther away. Hypothermal or high-temperature and high-pressure mineral assemblages include the minerals cassiterite (tin), scheelite and wolframite (tungsten), millerite (nickel), and molybdenite (molybdenum), associated with gangue minerals quartz, tourmaline, topaz, and other silicates. The mineral deposits of Broken Hill, Australia, the tin deposits of Cornwall, England, and Potosí, Bolivia, and the gold of the Homestake Mine, South Dakota, are hypothermal.
Mesothermal, or moderate-temperature and moderate-pressure deposits consist of pyrite (iron sulfide), bornite, chalocite, chalcopyrite and enargite (copper), galena (lead), sphalerite (zinc), and cobaltite or smaltite (cobalt). Gangue minerals include calcite, quartz, siderite, and rhodochrosite. The zinc-lead-silver replacement deposits of Leadville, Park City, and Aspen, Colorado, and the Coeur d’Alene, Idaho, lead veins are mesothermal.
Epithermal or low-temperature, near-surface deposits are often associated with regions of recent volcanism. The ore is characterized by stibnite (antimony), cinnabar (mercury), native silver and silver sulfides, gold telluride, native gold, sphalerite, and galena. Gangue minerals include barite, fluorite, chalcedony, opal, calcite, and aragonite. The extensive silver-gold mineralization of the...
(The entire section is 262 words.)
Further Reading (Encyclopedia of Global Resources)
Barnes, Hubert Lloyd. “Energetics of Hydrothermal Ore Deposition.” In Frontiers in Geochemistry: Organic, Solution, and Ore Deposit Geochemistry, edited by W. G. Ernst. Columbia, Md.: Bellwether for the Geological Society of America, 2002.
_______, ed. Geochemistry of Hydrothermal Ore Deposits. 3d ed. New York: John Wiley & Sons, 1997.
Guilbert, John M., and Charles F. Park, Jr. The Geology of Ore Deposits. Long Grove, Ill.: Waveland Press, 2007.
Pirajno, Franco. Hydrothermal Processes and Mineral Systems. London: Springer/Geological Survey of Western Australia, 2009.
Thompson, J. F. H., ed. Magmas, Fluids, and Ore Deposits. Nepean, Ont.: Mineralogical Association of Canada, 1995.
(The entire section is 94 words.)