MINERAL DEPOSIT

aggregate of a mineral in a concentration and size that makes mining feasible. About half of the known chemical elements possess some metallic properties. The term metal, however, is reserved for those chemical elements that possess two or more of the characteristic physical properties of metals (opacity, ductility, malleability, fusibility) and are also good conductors of heat and electricity. Approximately 40 metals are made available through the mining and smelting of the minerals in which they occur. Certain kinds of mineral can be smelted more readily than others; these are commonly referred to as ore minerals. Ore minerals tend to be concentrated in small, localized rock masses that form as a result of special geologic processes, and such local concentrations are called mineral deposits. Mineral deposits are what prospectors seek. The terms ore mineral and mineral deposit were originally applied only to minerals and deposits from which metals are recovered, but present usage includes a few nonmetallic minerals, such as barite and fluorite, that are found in the same kinds of deposit as metallic minerals. No deposit consists entirely of a single ore mineral. There are always admixtures of valueless minerals, collectively called gangue. The more concentrated an ore mineral, the more valuable the mineral deposit. For every mineral deposit there is a set of conditions, such as the level of concentration and the size of the deposit, that must be reached if the deposit is to be worked at a profit. A mineral deposit that is sufficiently rich to be worked at a profit is called an ore deposit, and in an ore deposit the assemblage of ore minerals plus gangue is called the ore. All ore deposits are mineral deposits, but the reverse is not true. Ore deposit is an economic term, while mineral deposit is a geologic term. Whether a given mineral deposit is also an ore deposit depends on many factors other than the level of concentration and the size of the deposit; all factors that affect the mining, processing, and transporting of the ore must be considered as well. Among such factors are the shape of a deposit, its depth below the surface, its geographic remoteness, access to transportation, the political stability of the region, and market factors such as the price of the metal in world trade and the costs of borrowing the money needed to develop a mine. Because market factors change continually, a given mineral deposit may sometimes be an ore deposit, but at other times it may be uneconomic and hence not an ore deposit. In this section, emphasis is placed on the geologic category of mineral deposits. Mineral deposits have been found both in rocks that lie beneath the oceans and in rocks that form the continents, although the only deposits that actually have been mined are in the continental rocks. (The mining of ocean deposits lies in the future.) The continental crust averages 3540 kilometres (2025 miles) in thickness, and below the crust lies the mantle. Mineral deposits may occur in the mantle, but with present technology it is not possible to discover them. Additional reading Two texts for nonspecialists on the subject that also address questions of how the use of mineral resources affects the use of other resources and the environment are Brian J. Skinner, Earth Resources, 3rd ed. (1986); and James R. Craig, David J. Vaughan, and Brian J. Skinner, Resources of the Earth (1988). Three excellent references dealing with the broad topic of the genesis of mineral deposits, but written at a level requiring at least a beginning familiarity with geologic terminology, are Anthony M. Evans, An Introduction to Ore Geology, 2nd ed. (1987); Richard Edwards and Keith Atkinson, Ore Deposit Geology and Its Influence on Mineral Exploration (1986); and John M. Guilbert and Charles F. Park, Jr., The Geology of Ore Deposits (1986). More specialized texts deal with specific classes of mineral deposits or with specific processes that form mineral deposits. In all instances, such texts presume a working knowledge of the technical terms used in geology. Among the best specialized texts are Hubert Lloyd Barnes (ed.), Geochemistry of Hydrothermal Ore Deposits, 2nd ed. (1979); J. Barry Maynard, Geochemistry of Sedimentary Ore Deposits (1983); Anthony J. Naldrett, Magmatic Sulfide Deposits (1989); F.J. Sawkins, Metal Deposits in Relation to Plate Tectonics, 2nd rev. ed. (1990); B.R. Berger and P.M. Bethke (eds.), Geology and Geochemistry of Epithermal Systems (1985); J.A. Whitney and Anthony J. Naldrett (eds.), Ore Deposition Associated with Magmas (1989); and E.R. Force, J.J. Eidel, and J. Barry Maynard (eds.), Sedimentary and Diagenetic Mineral Deposits: A Basin Analysis Approach to Exploration (1991). Mineral Facts and Problems (quinquennial) is a massive review regularly published by the U.S. Bureau of Mines. Brian J. Skinner