Modern industrial civilization is based primarily upon raw materials produced from various types of ore deposits. Machines are fabricated from metals, and even agriculture depends increasingly on metallic fuel-burning equipment and chemical fertilizers. Virtually all industrial energy is derived from coal, petroleum, and uranium. An ore deposit is any geological, or nonrenewable, material that can be mined at a profit. Although nonmetallic ores and fuels can be included in this economic definition, only metallic ore deposits and their mineralogy, classification, distribution, genesis, and national importance will be discussed here. Although the size, tenor (see \Tassay of ores\t), shape, depth, and other geological characteristics of the deposit are important, nongeological factors are at least equally important in this economic definition of ore. Nongeological factors include prices, geography, climate, availability of transportation, labor contracts, and governmental policies (especially those dealing with environmental considerations, property rights, and taxation). An ore deposit has a high unit value if the ore minerals are so valuable that transportation costs do not greatly affect the economics of the mining operation. Conversely, if a mineral deposit must be close to a market in order to be economic, that deposit has a high place value. Most metals and fuels have high unit values; most nonmetallic deposits, especially sand and gravel, have high place values. ORE GENESIS Ore deposits are concentrations of the chemical elements well above their average crustal abundance. For aluminum and iron the minimum necessary concentrations are only 2.2 and 3.4 times their crustal average. For many other metals, such as copper, molybdenum, nickel, uranium, and zinc, the concentration factor is between 100 and 1,000; for chromium and rare metals such as tungsten, lead, gold, silver, platinum, and mercury, ore deposits represent concentrations of more than 1,000 times the average crustal abundance. To generate such high concentrations, geological processes locally must have been abnormally efficient, or unusual combinations of processes must have occurred. For example, hypogene, or below-surface, processes may have concentrated the elements into a deposit of less than ore grade, and later genetically unrelated processes may have increased the concentration to ore grade. Most ore minerals were precipitated from waters, gases, or magmas. Only a few kinds of deposits originated by the mechanical accumulation of chemically inert ore minerals (see \Tplacer deposit\t). The most common types of ore deposits are those that precipitate from hot, aqueous (hydrothermal) solutions. Most metals are soluble in hot, saline, sulfidic, and acidic waters; ores may be precipitated as these solutions cool and react with the rocks through which they pass (see \Talteration, mineral\t). Thus hydrothermal ores are most commonly precipitated where former permeable zones intersected reactive rocks. The common rock-forming silicate minerals rarely are ore minerals for two major reasons. First, the amount of commercially important elements in most silicate minerals is low compared to sulfide and oxide minerals; for example, the highest iron content of micas is less than 20% but magnetite is 72% iron. Second, the extraction of elements from silicate minerals is chemically difficult and energy consumptive compared to extraction from the more weakly bonded sulfide, oxide, chloride, and sulfate minerals. CLASSIFICATION Ore deposits are rocks, albeit uncommon rocks; thus they can be classified in the same manner as rocks. Since the Middle Ages, however, ore deposits have been and continue to be classified in a number of ways. Previously they were classified according to shape, such as in a \Tvein deposit\t, lode, or stockworks. In the early part of the 20th century Waldemar LINDGREN was the foremost proponent of classifying ore deposits according to their environment of deposition. At that time most hypogene ore deposits were thought to have been precipitated from hydrothermal solutions derived from cooling granitic magmas. Lindgren classified these deposits as epithermal, mesothermal, and hypothermal, connoting progressively higher temperatures and pressures of formation. By the 1960s, Lindgren's classification was found to be substantially incomplete. Furthermore, heated groundwater and seawater as well as volcanic and metamorphic hydrothermal solutions are now recognized as possible ore-forming solutions. Some classifications of ore deposits rely on the age of the deposit compared to its surrounding rocks. Syngenetic deposits formed at the same time as the rocks were deposited; diagenetic deposits formed after deposition but before the rocks became lithified, or changed to stone; and epigenetic deposits formed after the rocks were lithified. Placer deposits are syngenetic; veins are epigenetic; and many copper, zinc, lead, and uranium deposits in sedimentary rocks appear to be syngenetic to diagenetic. Because a great deal of controversy still exists about the syngenetic, diagenetic, or epigenetic origin of some deposits, such a classification is premature. The present tendency is to classify ore deposits descriptively without regard to origin. This classification uses type examples, such as Kambalda-type nickel deposits, or groups deposits into similar descriptive types. For example, porphyry copper deposit connotes various descriptive characteristics about copper deposits associated with granitic intrusions. DISTRIBUTION Ore deposits are irregularly distributed within the Earth's crust. Their distribution depends primarily on the types of rocks present, the age of these rocks, the level of erosion with regard to these rocks and, of course, nongeological factors. Processes that operate below the Earth's crust, such as subduction and processes within the mantle, may also be important. Each type of ore deposit is restricted to certain types of rocks. If erosion has removed the ore-bearing portion of these rocks, the area will be devoid of that particular type of ore deposit. Just as organic evolution restricted certain fossils to rocks of specific ages, evolution of the Earth's crustal lithologies, organisms, and atmosphere limited the time during which certain types of rocks and ore deposits formed. Lake Superior-type banded-iron formations and Witwatersrand-type conglomerates became extinct when the atmosphere became oxygen-rich about 2.2 billion years ago. Metallogenic provinces are portions of the Earth's crust that have unusual concentrations of one or more kinds of ore deposits. Examples are the porphyry-copper province of the southwestern United States and adjacent Mexico, the tin province of southeast Asia, and the iron provinces of Kiruna, Sweden, and the Lake Superior region of the United States and Canada. The origin of metallogenic provinces is controversial, but rock types, the ages of the rocks, and the present level of erosion clearly are important factors. ORE RESERVES AND MINERAL RESOURCES The distinction between ore reserves and mineral resources is of prime economic and political importance. Reserves include all known deposits that are economical to extract under existing laws. To be identified as reserves, the material must have been actually sampled or be an inferred extension of sampled reserves. Resources are all identified mineral deposits that are uneconomic or illegal to mine plus estimates of undiscovered ores in known or undiscovered areas. The estimation of reserves and identified resources is done by physically determining their size and assaying the deposits well enough to determine if they are ore or not. Conversely, unknown resources are estimated by various statistical methods, such as tabulating the amount of ore discovered per meter of drilling during the past year, the amount and rate of discoveries in past years, and the relative sizes of known deposits, and comparing the known or inferred geology of unprospected areas with the production and reserves of geologically similar known areas. Not surprisingly, the estimation of both reserves and resources is sensitive to changes in the price of the mineral commodity. A significant price increase, as in the case of uranium in the mid-1970s, may so encourage exploration that significant new reserves or resources are found. Because the estimation of resources involves an estimation of the unknown, resources can never be known with certainty, and estimates of the same resources by different methods or by different experts may vary widely. Nonetheless, the estimation of resources has become increasingly important in determining the policies of nations whose economies depend heavily on the importation or exportation of minerals and fuels. In such cases, prudence would dictate basing such political decisions only on reserves and identified resources. RESEARCH Industry, government, and universities constantly conduct research in an effort to increase reserves and resources. A new mining machine, drilling technique, or explosive may change previously uneconomic rock into ore. Much research is currently being conducted on new mining methods, especially the feasibility of leaching ores in place instead of physically mining them. New techniques being developed for extracting ores from currently uneconomic rocks include grinding the rocks to ever finer particles to liberate the ore minerals, converting metals such as nickel in silicate minerals into synthetic sulfide minerals that can be routinely treated, and reducing iron ores to metallic iron in order to eliminate conventional smelting-stage techniques. New or improved geophysical techniques are constantly being developed to detect hidden ore deposits. Simultaneously, exploration geologists look for known types of deposits that might have been uneconomic to mine in the past and for totally new types of deposits in geological environments that might not have been considered a few years earlier. The most fundamental kind of research in mineral exploration is geological mapping of existing and prospective ore deposits. Mapping the distribution of rock types, alteration-mineral assemblages, mineralization, and geological structures such as faults, fractures, and folds of known deposits commonly leads to the discovery of previously unknown extensions of ore bodies. A thorough geological understanding of an existing ore deposit generally is the basis for discovering similar deposits in similar geological environments elsewhere. Geochemical prospecting has become increasingly important since the 1960s because of the development of rapid and inexpensive instrumental techniques, especially atomic absorption, for determining trace amounts of several metals in large numbers of samples. Trace amounts of various metals in stream sediments are used in reconnaissance exploration just as previous prospectors used the gold pan to trace placer gold to the mother lode. Soils, plants, waters, and rocks also are analyzed for trace amounts of metals in hopes of finding concealed ore deposits. After the airborne magnetometer was developed to detect submarines in World War II, various airborne geophysical techniques have been developed for mineral exploration. Geological, geochemical, or airborne geophysical anomalies commonly are checked by ground geophysical techniques. In most cases a combination of geological, geochemical, and geophysical techniques is necessary to verify and to evaluate the discovery of a deposit. Once a potential ore deposit has been discovered, its size (tonnage) and grade (tenor) must be determined to decide whether it is economic to mine. Ultimately this involves extensive sampling of the deposit by drill holes, pits and trenches, exploratory shafts, and underground workings. If the ore reserves appear to be sufficient, additional engineering, marketing, and other feasibility studies are conducted on nongeological factors to determine whether the deposit is economic to mine. FACTORS PECULIAR TO THE MINING INDUSTRY The mining industry (see \Tmining and quarrying\t) differs from agricultural and manufacturing industries in a number of important ways. First, mineral deposits are nonrenewable; that is, once mined they are gone forever. This "one-crop" nature of mineral deposits is partially offset by the recycling or scrap return of most metals and some nonmetals. Second, the uneven distribution of ore deposits in the Earth's crust has historically promoted political problems between have and have-not nations and between the mining industry and political groups wishing to preserve some areas for uses such as wildernesses. Finally, the capital investment required to find and develop ore deposits and bring them into profitable production must be undertaken over a time period longer than that needed for agricultural or other industrial enterprises; 3 to 7 years is not unusual. POLITICAL AND ECONOMIC IMPORTANCE Within the past century national power has become virtually synonymous with industrial power, and the growth of industrial power has almost always begun with the national possession of mineral resources. A generalized model of industrial development based on the exploitation of mineral resources will show a correlation between the opening and development of mines, the building of smelters, the production of metal, and the economic growth of a region. During the initial period of mine development, ore deposits are discovered and mines built. During the next period output increases from fewer but larger mines, and expropriation by revolution, decree, or taxation may take place. In the third period the abundant output of metals encourages a burgeoning of industry. The cost of raw materials decreases due to plentiful domestic supplies, and foreign minerals begin to be imported to meet the growing demand. As internal and foreign markets expand, the average standard of living rises. Growing dependence on foreign mineral resources and markets increasingly involve the country in international affairs. The depletion of a cheap supply of domestic raw materials (the fourth period) usually signals the economic and political decline of a nation (fifth period). Dependence on foreign materials leads to increased manufacturing costs, which ultimately generate social problems and political unrest. The country that started with abundant, inexpensively minable resources has now become a have-not, or at least a "have-less," nation. Such a nation may improve its status by a variety of methods. In the past the most common were the acquisition of mineral resources by political alliance, economic domination, military conquest, or some combination of the three. Great Britain has passed through all five periods and has now lost the position it held in the late 19th century as the world's leading mining, industrial, and military power. The United States lags one period behind Britain; it is still at the stage where it possesses mineral resources, but in diminishing quantities. Japan continues to be a major industrial power despite its inadequate domestic deposits and unsuccessful attempt to obtain foreign deposits in World War II. A partial explanation of this is that Japan is the only major industrial power whose constitution forbids the maintenance of significant military forces. The money thus saved has been used to acquire foreign ore deposits, to perfect its merchant marine, and to modernize its industries. Eric S. Cheney Bibliography: Barnes, J.W., Ores and Minerals: Introducing Economic Geology (1988); Baumann, Ludwig, Introduction to Ore Deposits (1976); Garrels, R. M., and Christ, C. L., Solutions, Minerals, and Equilibria (1965; repr. 1982); Guilbert, J. M., The Geology of Ore Deposits (1985); Jain, S.K., Ore Processing (1987); Jensen, M. L., and Bateman, A. M., Economic Mineral Deposits, 3d rev. ed. (1981); Netschert, Bruce C., and Landsberg, Hans H., The Future Supply of Major Metals (1961; repr. 1978); Park, C.F., and Guilbert, J.M., The Geology of Ore Deposits (1985); Plotnikov, N.I., and Roginets, I.I., Hydrogeology of Ore Deposits (1989); Society for Mining, Metallurgy and Exploration, Ore Deposits of the United States (1968). See also: \Toceanic mineral resources\t.