An ore (or ore deposit) is a volume of rock containing valuable minerals that occur at sufficiently high concentrations for profitable mining, transportation, milling, and processing. If the body of mineralization is of too low a grade or tonnage, or the desired mineral is technically too difficult to extract, then the deposit is not called an ore.
The value of the deposit is generally considered in purely economic terms. At times, however, the cultural, social, or strategic goals of various peoples may render a deposit valuable for extraction in non-economic terms. Examples are deposits of ochre, some clays, and ornamental stones of religious, cultural, or sentimental value. In addition, rare samples of ore, such as nuggets or special formations of gold or copper, may command a value well beyond any utilitarian value of their mineral content.
Fluctuations in commodity prices may determine whether a rock is considered valuable enough to be called "ore," or not sufficiently valuable, and hence "waste." Likewise, extraction costs may fluctuate, for example with fuel costs, so that mining an ore may become unprofitable, turning it into waste.
The grade of an ore is based on the concentration of the desired mineral and its form of occurrence—factors that directly affect the costs associated with mining the ore. A "cut-off grade" is used to define what is ore and what is waste.
Ore minerals are generally oxides, sulfides, and silicates. In addition, they may be "native" metals (such as copper) that are not commonly concentrated in the Earth's crust, or "noble" metals (not usually forming compounds) such as gold. The ores must be processed to extract the metals of interest from the deposit.
Ore bodies are formed by a variety of geological processes. The process of ore formation is called ore genesis.
Various theories of ore genesis explain how the different types of mineral deposits in the Earth's crust have been formed. These theories vary according to the mineral or commodity, but each theory generally has three components: source, transport or conduit, and trap.
The biggest deposits are formed when the source is large, the transport mechanism is efficient, and the trap is active and ready at the right time.
Ore genesis may be divided into several categories, based on the processes involved. These categories are: internal processes, hydrothermal processes, metamorphic processes, and surficial processes (Evans 1993).
Ore deposits are usually classified by ore formation processes and geological settings. For example, SEDEX (sedimentary exhalative) deposits, are a class of sedimentary deposits formed on the seafloor by the "exhalation" of brines into seawater. In other words, when brines (waters with dissolved minerals) mix with seawater and cool, the ore minerals precipitate out.
Yet, ore deposits rarely fit snugly into the boxes in which geologists attempt to place them. Many are formed by more than one of the basic genesis processes noted above, leading to ambiguous classifications and much argument and conjecture. Ore deposits are often classified based on examples of their type, such as Broken Hill-type lead-zinc-silver deposits, or Carlin-type gold deposits.
Hydrothermal ore deposits are also classified according to the temperature of formation, which roughly correlates with particular mineralizing fluids, mineral associations, and structural styles. Lindgren (1933) proposed a scheme that classifies hydrothermal deposits as hypothermal, mesothermal, epithermal, and telethermal.
Specific ores are organized here according to the metal commodities.
Iron ores are overwhelmingly derived from ancient sediments known as banded iron formations (BIFs). These sediments are composed of iron oxide minerals deposited on the seafloor. Particular environmental conditions were needed to transport enough iron in seawater to form these deposits, such as acidic and oxygen-poor atmospheres in the Proterozoic Era.
In addition, weathering during the Tertiary or Eocene periods converted the usual magnetite minerals into hematite, which is more easily processed. Some iron deposits in the Pilbara of West Australia are placer deposits, formed by the accumulation of hematite gravels called pisolites. They are less expensive to mine.
Lead and zinc deposits are formed by the discharge of deep sedimentary brine onto the seafloor (termed SEDEX deposits), or by the replacement of limestone in skarn deposits, or by subvolcanic intrusions of granite. The vast majority of lead and zinc deposits are Proterozoic in age.
They are classified as (a) primary deposits, (b) alluvial or placer deposits, and (c) residual or laterite deposits. A deposit may contain a mixture of all three types of ore.
Platinum and palladium are precious metals generally found in ultramafic rocks (igneous rocks rich in minerals of magnesium and iron). The source of platinum and palladium deposits is ultramafic rocks that have enough sulfur to form a sulfide mineral in molten magma. The sulfide mineral gains platinum by mixing with the bulk of the magma because platinum has an affinity for sulfur and is concentrated in sulfides. Platinum may also occur in association with chromite, either in the chromite mineral itself or in sulfides associated with it. Platinum is often associated with nickel, copper, chromium, and cobalt deposits.
Nickel deposits are generally found in two forms: sulfide and laterite.
Copper is found in association with many other metals and deposit styles, including deposits of gold, lead, zinc, and nickel. Commonly, copper is either formed within sedimentary rocks or associated with igneous rocks.
The world's major copper deposits are formed within the granitic porphyry copper style. The source of copper is generally thought to be the Earth's lower crust or mantle, where the granite melt forms. The copper is enriched by processes during crystallization of the granite and forms as chalcopyrite, a sulfide mineral, is carried up with the granite. Granites sometimes move to the suface with volcanic eruptions, and copper mineralization occurs during this phase, when the granite and volcanic rocks cool via hydrothermal circulation.
Sedimentary copper forms within ocean basins in sedimentary rocks. Generally, this occurs when brines from deeply buried sediments discharge into the deep sea, precipitating copper (and often lead and zinc) sulfides directly onto the seafloor. This is then buried by further sediment.
Uranium deposits are usually derived from radioactive granites, where certain minerals such as monazite are leached during hydrothermal activity, or during circulation of groundwater. The uranium is brought into solution by acidic conditions and is deposited when this acidity is neutralized. Generally, this occurs in certain carbon-bearing sediments, in what is called an "unconformity" in sedimentary strata. The majority of the world's nuclear power is sourced from uranium in such deposits.
Uranium is also found associated with certain igneous rocks, such as granite and porphyry. The Olympic Dam deposit in Australia is an example of this type of uranium deposit. It contains 70 percent of Australia's share of 40 percent of the global, low-cost, recoverable uranium inventory.
Titanium ore is formed as placer deposits (mineral sands, noted below) or within ultramafic layered intrusions. In the latter case, titanium takes the form of layers of ilmenite, a titanium oxide mineral, through the process of crystallization as the intrusion cools. These layers can be considerably heavy and long, and this type of ore is known as "hard rock titanium." In addition, the ore may contain vanadium as a second metal within the ilmenite.
Mineral sands, a type of "placer deposits," are the predominant type of titanium, zirconium, and thorium deposits. They are formed by the accumulation of heavy minerals within beach systems. The minerals that contain titanium are ilmenite and leucoxene; zirconium is contained within zircon; and thorium is generally contained within monazite. These minerals are sourced primarily from granite bedrock by erosion and transported to the seashore by rivers, where they accumulate in beach sands. On rare but important occasions, gold, tin, and platinum deposits also form in beach placer deposits.
Tin, tungsten, and molybdenum generally form in a certain type of granite, by a mechanism similar to that for intrusion-related gold and copper. They are considered together because the process of forming these deposits is essentially the same. Minerals of these three metals are found in an important deposit formed by a process known as skarn-type mineralization. Skarn deposits are formed by the reaction of mineralized fluids from the granite reacting with wall rocks such as limestone. Skarn mineralization is also important in the formation of ores of lead, zinc, copper, and gold, and sometimes uranium as well.
The overwhelming majority of rare earth elements (lanthanoids), niobium, tantalum, and lithium are found within pegmatite. Ore genesis theories for these ores are wide and varied, but most involve metamorphism and igneous activity. Lithium is present as spodumene or lepidolite within pegmatite. In addition, carbonatite intrusions are an important source of these elements.
Immense quantities of "phosphate rock" occur in older sedimentary basins, generally formed in the Proterozoic. Phosphate deposits are thought to be sourced from the skeletons of dead sea creatures that accumulated on the seafloor. Similar to iron ore deposits and oil, particular conditions in the ocean and environment are thought to have contributed to these deposits in the geological past.
Phosphate deposits are also formed from alkaline igneous rocks such as nepheline syenites, carbonatites, and associated rock types. In this case, the phosphate is contained within magmatic apatite, monazite, or other rare-earth minerals.
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