Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. Ore grade refers to the concentration of the desired material it contains. The value of the metals or minerals a rock contains must be weighed against the cost of extraction to determine whether it is of sufficiently high grade to be worth mining and is therefore considered an ore. A complex ore is one containing more than one valuable mineral. Minerals of interest are generally oxides, sulfides, silicates, or native metals such as copper or gold. Ore bodies are formed by a variety of geological processes generally referred to as ore genesis and can be classified based on their deposit type. Ore is extracted from the earth through mining and treated or refined, often via smelting, to extract the valuable metals or minerals. Some ores, depending on their composition, may pose threats to health or surrounding ecosystems. The word ore is of Anglo-Saxon origin, meaning "lump of metal".
Gangue and tailings In most cases, an ore does not consist entirely of a single mineral, but is mixed with other valuable minerals and with unwanted or valueless rocks and minerals. The part of an ore that is not economically desirable and that cannot be avoided in mining is known as gangue. The valuable ore minerals are separated from the gangue minerals by froth flotation, gravity concentration, electric or magnetic methods, and other operations known collectively as mineral processing or ore dressing. Mineral processing consists of first liberation, to free the ore from the gangue, and concentration to separate the desired mineral(s) from it. Once processed, the gangue is known as tailings, which are useless but potentially harmful materials produced in great quantity, especially from lower grade deposits.
Ore deposits
An ore deposit is an economically significant accumulation of minerals within a host rock. This is distinct from a mineral resource in that it is a mineral deposit occurring in high enough concentration to be economically viable. An ore deposit is one occurrence of a particular ore type. Most ore deposits are named according to their location, or after a discoverer (e.g. the Kambalda nickel shoots are named after drillers), or after some whimsy, a historical figure, a prominent person, a city or town from which the owner came, something from mythology (such as the name of a god or goddess) or the code name of the resource company which found it (e.g. MKD-5 was the in-house name for the Mount Keith nickel sulphide deposit).
Classification
Ore deposits are classified according to various criteria developed via the study of economic geology, or ore genesis. The following is a general categorization of the main ore deposit types:
Magmatic deposits Magmatic deposits are ones which originate directly from magma Pegmatites are very coarse grained, igneous rocks. They crystallize slowly at great depth beneath the surface, leading to their very large crystal sizes. Most are of granitic composition. They are a large source of industrial minerals such as quartz, feldspar, spodumene, petalite, and rare lithophile elements. Carbonatites are an igneous rock whose volume is made up of over 50% carbonate minerals. They are produced from mantle derived magmas, typically at continental rift zones. They contain more rare earth elements than any other igneous rock, and as such are a major source of light rare earth elements. Magmatic Sulfide Deposits form from mantle melts which rise upwards, and gain sulfur through interaction with the crust. This causes the sulfide minerals present to be immiscible, precipitating out when the melt crystallizes. Magmatic sulfide deposits can be subdivided into two groups by their dominant ore element: Ni-Cu, found in komatiites, anorthosite complexes, and flood basalts. This also includes the Sudbury Nickel Basin, the only known astrobleme source of such ore. Platinum Group Elements (PGE) from large mafic intrusions and tholeiitic rock. Stratiform Chromites are strongly linked to PGE magmatic sulfide deposits. These highly mafic intrusions are a source of chromite, the only chromium ore. They are so named due to their strata-like shape and formation via layered magmatic injection into the host rock. Chromium is usually located within the bottom of the intrusion. They are typically found within intrusions in continental cratons, the most famous example being the Bushveld Complex in South Africa. Podiform Chromitites are found in ultramafic oceanic rocks resulting from complex magma mixing. They are hosted in serpentine and dunite rich layers and are another source of chromite. Kimberlites are a primary source for diamonds. They originate from depths of 150 km in the mantle and are mostly composed of crustal xenocrysts, high amounts of magnesium, other trace elements, gases, and in some cases diamond.
Metamorphic deposits These are ore deposits which form as a direct result of metamorphism.
Skarns occur in numerous geologic settings worldwide. They are silicates derived from the recrystallization of carbonates like limestone through contact or regional metamorphism, or fluid related metasomatic events. Not all are economic, but those with potential value are classified depending on the dominant element such as Ca, Fe, Mg, or Mn among many others. They are one of the most diverse and abundant mineral deposits. As such they are classified solely by their common mineralogy, mainly garnets and pyroxenes. Greisens, like skarns, are a metamorphosed silicate, quartz-mica mineral deposit. Formed from a granitic protolith due to alteration by intruding magmas, they are large ore sources of tin and tungsten in the form of wolframite, cassiterite, stannite and scheelite.
Porphyry copper deposits These are the leading source of copper ore. Porphyry copper deposits form along convergent boundaries and are thought to originate from the partial melting of subducted oceanic plates and subsequent concentration of Cu, driven by oxidation. These are large, round, disseminated deposits containing on average 0.8% copper by weight.
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