Hydrothermal deposits formed at shallow depths below a boiling hot spring system 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.The relationship between hot springs and epithermal veins.Encyclopædia Britannica, Inc.AdvertisementPorphyry depositsAmong the most distinctive hydrothermal deposits is a class known as porphyry copper deposits, so called because they are invariably associated with igneous intrusives that are porphyritic (meaning the rock is a mixture of coarse and fine mineral grains). Porphyry copper deposits (and their close relatives, porphyry molybdenum deposits) contain disseminated mineralization, meaning that a large volume of shattered rock contains a ramifying network of tiny quartz veins, spaced only a few centimetres apart, in which grains of the copper ore minerals chalcopyrite and bornite (or the molybdenum ore mineral molybdenite) occur with pyrite. The shattered rock serves as a permeable medium for the circulation of a hydrothermal solution, and the volume of rock that is altered and mineralized by the solution can be huge: porphyry coppers are among the largest of all hydrothermal deposits, with some giant deposits containing many billions of tons of ore. Although in most deposits the ore averages only between 0.5 and 1.5 percent copper by weight, the tonnages of ore mined are so large that more than 50 percent of all copper produced comes from porphyry coppers.porphyryPorphyry.Encyclopædia Britannica, Inc.AdvertisementPorphyry coppers are often associated with stratovolcanoes. As a result of the volcanism that rings the Pacific Ocean basin, porphyry coppers are conspicuous features of mineralization along the western borders of North and South America and in the Philippines. Among the major deposits are El Teniente, El Salvador, and Chuquicamata in Chile, Cananea in Mexico, and, in the United States, Bingham Canyon in Utah, Ely and Yerington in Nevada, and San Manuel in Arizona.Idealized drawing of a porphyry copper deposit, showing the relationship between the porphyry body, the altered and mineralized rock, and the overlying volcano.Encyclopædia Britannica, Inc.SkarnsWhen a limestone or marble is invaded by a high-temperature hydrothermal solution, the carbonate minerals calcite and dolomitereact strongly with the slightly acid solution to form a class of mineral deposit called a skarn. Because solutions tend to have high temperatures close to a magma chamber, most skarns are found immediately adjacent to intrusive igneous rocks. The solutions introduce silica and iron, which combine with the calcium and magnesium in the parent rock to form silicate minerals such as diopside, tremolite, and andradite. The hydrothermal solutions may also deposit ore minerals of iron, copper, zinc, tungsten, or molybdenum.The mining of magnetite from a skarn deposit at Cornwall, Pennsylvania, U.S., commenced in 1737 and continued for two and a half centuries. Copper skarns are found at many places, including Copper Canyon in Nevada and Mines Gaspé in Quebec, Canada. Tungsten skarns supply much of the world’s tungsten from deposits such as those at Sangdong, Korea; King Island, Tasmania, Australia; and Pine Creek, California, U.S.Volcanogenic massive sulfidesWherever volcanism occurs beneath the sea, the potential exists for seawater to penetrate the volcanic rocks, become heated by a magma chamber, and react with the enclosing rocks—in the process concentrating geochemically scarce metals and so forming a hydrothermal solution. When such a solution forms a hot spring on the seafloor, it can suddenly cool and rapidly deposit its dissolved load. Mineral deposits formed by this process, which are called volcanogenic massive sulfide (VMS) deposits, are known in ancient seafloor rocks of all geologic ages. In addition, deposits forming today as a result of submarine hot-spring activity have been discovered at a number of places along the oceanic ridge (the most volcanically active zone on Earth), and in back-arc basins associated with subduction zones.VMS deposits constitute some of the richest deposits of copper, lead, and zinc known. Some of the most famous, found in Japan and called kuroko deposits, yield ores that contain as much as 20 percent combined copper, lead, and zinc by weight, plus important amounts of gold and silver.
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