Hydrothermal solutions and mineralization
Hydrothermal solutions and mineralization refer to the processes through which mineral deposits form from heated water and volatile substances found in the Earth's crust, particularly during the crystallization of igneous rocks. Essential factors for the creation of hydrothermal mineral deposits include the presence of metal-bearing solutions, pathways in rocks for these solutions to travel, and sites for mineral deposition through chemical reactions. The term "ore" describes mineral assemblages that can be profitably mined, whereas "gangue" denotes the nonvaluable minerals associated with ore.
As hydrothermal solutions migrate through the rock, they can deposit minerals by filling natural openings or reacting with the surrounding rock, creating various deposit types such as veins, cavity-filling deposits, and metasomatic replacement deposits. The characteristics of these mineral deposits vary based on the temperature and depth of formation, ranging from hypothermal (high temperature and pressure) to epithermal (low temperature, often near the surface). Commonly extracted metals include copper, lead, zinc, and gold, with quartz frequently serving as the main gangue mineral. Understanding the conditions and processes involved in hydrothermal mineralization is critical for mining and resource exploration.
Hydrothermal solutions and mineralization
Hydrothermal solutions are “hot-water” solutions rich in base metals and other ions that create deposits of minerals. Most hydrothermal solutions are exhalations from magmas, but some hydrothermal deposits have no identifiable magma source. Hydrothermal processes are responsible for the major part of the world’s base metals upon which modern society is so dependent. They have given rise to many of the great mining districts of the world.
Background
Essential conditions for the formation of 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 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 rock-forming minerals. If the vapor pressure in the 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 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 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 sediments or by dehydration of water-bearing minerals during metamorphism.
The observed volatiles from magmas, as seen during volcanic eruptions and at fumaroles, are 80 percent water. Carbon dioxide, hydrogen sulfide, sulfur, and sulfur dioxide are also abundant. Nitrogen, chlorine, fluorine, boron, and other elements are present in smaller amounts. In addition, metal ions are carried in this residual fluid. Especially abundant are the base metals—iron, tungsten, copper, lead, zinc, molybdenum, silver, and gold. Quartz is the most common nonore, or gangue, mineral deposited. Calcite, fluorite, and barite are also common as gangue minerals. Base metals combined with sulfur as sulfide minerals, with arsenic as arsenides, or with tellurium as tellurides form the most common minerals. Gold often occurs as a native mineral.
Nature of Open Spaces
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 rocks, in vesicles of buried 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 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.
Metasomatic Replacement
Some hydrothermal deposits are emplaced by reaction of the fluids with chemically susceptible rocks, such as 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.
Classification by Temperature and Depth
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 San Juan Mountains of Colorado, including Cripple Creek, Ouray, and Creede, are epithermal deposits.
Telethermal deposits are formed by hydrothermal solutions that have cooled to approximately the same temperature as the near-surface rocks. These solutions may originate as mobilized connate and deeply circulating meteoric waters rather than fluids expelled from magma. The principal ore minerals are sphalerite and galena, with gangue minerals marcasite, fluorite, calcite, and chalcopyrite. The Mississippi Valley-type deposits of the tristate district of Missouri, Kansas, and Oklahoma exemplify this low-temperature mineralization.
Bibliography
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Guilbert, John M., and Charles F. Park, Jr. The Geology of Ore Deposits. Long Grove, Ill.: Waveland Press, 2007.
Perkins, Dexter. "Hydrothermal Ore Deposits." LibreTexts, 28 Aug. 2022, geo.libretexts.org/Bookshelves/Geology/Mineralogy‗(Perkins‗et‗al.)/09%3A‗Ore‗Deposits‗and‗Economic‗Minerals/9.03%3A‗Types‗of‗Ore‗Deposits/9.3.02%3A‗Hydrothermal‗Ore‗Deposits. Accessed 27 Dec. 2024.
Pirajno, Franco. Hydrothermal Processes and Mineral Systems. London: Springer/Geological Survey of Western Australia, 2009.
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