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.
Important ore minerals
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.
- Argentite: silver sulfide (Ag2S)
- Barite: barium sulfate (BaSO4)
- Bauxite: mixture of aluminum oxides and hydroxides, used for producing aluminum
- Beryl: beryllium aluminum cyclosilicate (Be3Al2(SiO3)6)
- Bornite: a sulfide of copper and iron (Cu5FeS4)
- Cassiterite: an oxide of tin (SnO2)
- Chalcocite: copper(I) sulfide (Cu2S), for production of copper
- Chalcopyrite (or "peacock pyrite"): copper iron sulfide (CuFeS2)
- Chromite: iron magnesium chromium oxide ((Fe,Mg)Cr2O4), for production of chromium
- Cinnabar: red mercury(II) sulfide (HgS), for production of mercury
- Cobaltite: cobalt, iron, arsenic sulfide ((Co,Fe)AsS)
- Columbite-Tantalite or Coltan: oxide mixture containing iron, manganese, niobium, and tantalum ((Fe,Mn)(Nb,Ta)2O6)
- Galena: lead sulfide (PbS)
- Gold: The metal gold (Au) is typically associated with quartz or is found as placer deposits
- Hematite: iron(III) oxide (Fe2O3)
- Ilmenite: a crystalline form of iron titanium oxide (FeTiO3)
- Magnetite: iron(II,III) oxide (Fe3O4), a ferrimagnetic mineral
- Molybdenite: molybdenum disulfide (MoS2)
- Pentlandite: a sulfide of iron and nickel ((Fe,Ni)9S8)
- Pyrolusite: manganese dioxide (MnO2)
- Scheelite: calcium tungstate (CaWO4)
- Sphalerite: zinc sulfide (ZnS), with variable amounts of iron
- Uraninite (pitchblende): mainly uranium dioxide (UO2), used for production of metallic uranium
- Wolframite: a tungstate of iron and manganese ((Fe,Mn)WO4)
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.
- Source: The "source" indicates where the metal comes from and by what process it is liberated.
- Transport: The metal-bearing fluids or solid minerals need to move into the right position. Thus the term "transport" refers to the physical movement of the metal and includes the physical and chemical processes that encourage this movement.
- Trap: "Trapping" is the process of concentrating the metal by physical, chemical, and geological mechanisms to form the ore.
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 processes
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).
- Internal processes: These are the physical and chemical processes that take place within magmas (molten rock beneath the Earth's surface) and lava flows (molten rock ejected by volcanic activity).
- Hydrothermal processes: These are the physical and chemical phenomena and reactions that occur during the movement of hydrothermal (hot-water) solutions within the crust.
- Metamorphic processes: Metamorphic (rock-transforming) reactions occur during geological shearing. These processes may liberate minerals from deforming rocks, focusing them into zones of reduced pressure or dilation such as geological faults. Metamorphic processes also control many physical processes that are the source of hydrothermal fluids.
- Surficial processes: These are the physical and chemical processes that occur on the Earth's surface, generally by the action of the environment. Examples of these processes are erosion and sedimentation. They concentrate ore material within the regolith (loose material covering solid rock).
Classification of ore deposits
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.
Common classification groupings
- IOCG (iron oxide, copper, gold) deposits: typified by the supergiant Olympic Dam deposit
- Mesothermal lode gold deposits: typified by the Golden Mile, Kalgoorlie
- Archaean conglomerate hosted gold-uranium deposit: sole example is Witwatersrand
- Carlin-type gold deposits: includes the dolomite-hosted jasperoid replacement subtype
- Epithermal stockwork vein deposits
- Porphyry copper gold
- Intrusive-related copper-gold +/- (tin-tungsten): typified by the deposits of Tombstone, Alaska
- Broken Hill-type lead-zinc-silver
- SEDEX (sedimentary exhalative) deposits:
- Mississippi Valley-type (MVT) zinc-lead deposits
- Andean-type silver-lead-zinc deposits
- Magmatic nickel-copper-iron PGE deposits, including:
- Cumulate vanadium- or platinum-bearing magnetite or chromite
- Cumulate hard-rock titanium (ilmenite) deposits
- Komatiite-hosted nickel-copper-PGE deposits
- Subvolcanic feeder subtype, typified by Noril'sk-Talnakh and the Thompson Belt, Canada
- Intrusive-related nickel-copper-PGE deposits: typified by Sudbury Basin, Ontario, and Jinchuan, China
- Laterite nickel
- Volcanic hosted massive sulfide (VHMS) copper-lead-zinc, including:
- Besshi type
- Kuroko type
- Podiform serpentinite-hosted paramagmatic iron oxide-chromite deposits: typified by Savage River iron ore, Tasmania, Coobina chromite deposit
- Banded iron formation iron ore deposits: such as channel iron or pisolite type
- Carbonatite, alkaline igneous-related deposits, including:
Genesis of common ores
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, zinc, silver
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.
- SEDEX type deposits: Examples are the immense Broken Hill, Century Zinc, Lady Loretta, and Mt. Isa deposits in Australia; the Sullivan, Red Dog, and Jason deposits of North America; and the Hindustan zinc belt in India.
- Limestone replacement-type deposits: They are exemplified by the Mississippi Valley-type (MVT) deposit. Some of these occur by replacement and degradation of hydrocarbons, which are thought important for transporting lead.
- Subvolcanic intrusion-type of deposits: Renowned for high silver grades, they are typified by deposits in Argentina, Bolivia, and Peru. These deposits are essentially Cenozoic in age and are known as the Andean silver belt. The most recent example is San Cristobal, with 450 million ounces of silver. These deposits were formed by the discharge of fluids bearing incompatible elements from the cooling granite mass, and have low lead grades but exceptional silver enrichment.
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.
- The majority of primary gold deposits fall into two main categories: lode gold deposits and intrusion-related deposits.
- Lode gold deposits: They consist chiefly of quartz veins, also known as lodes or reefs, which contain either native gold or gold sulfides and tellurides. Lode gold deposits are usually hosted in basalt or in sediments known as turbidite, although when in faults, they may occupy intrusive igenous rocks such as granite. Lode-gold deposits are intimately associated with orogeny (mountain-forming processes) and other plate collision events in geologic history. Most lode gold deposits sourced from metamorphic rocks because it is thought that the majority are formed by dehydration of basalt during metamorphism. The gold is transported up faults by hydrothermal waters and deposited when the water cools too much to retain gold in solution.
- Intrusion-related gold deposits (Lang & Baker 2001): Generally hosted in granites and porphyry, this gold usually contains copper and is often associated with tin and tungsten. Intrusion-related gold deposits rely on gold existing in the fluids associated with magma (White 2001), and the inevitable discharge of these hydrothermal fluids into the wall-rocks (Lowenstern 2001). Skarn deposits are another manifestation of intrusive-related deposits.
- Placer deposits are secondary deposits, derived from pre-existing gold deposits. They are formed by alluvial processes in rivers and streams and on beaches. Placer gold deposits form by gravity, when the density of gold causes it to sink into trap sites in the river bed, or when water velocity drops, such as at bends in rivers and behind boulders. Placer deposits are often found in sedimentary rocks and can be billions of years old, such as the Witwatersrand deposits in South Africa. Sedimentary placer deposits are known as 'leads' or 'deep leads'.
- Laterite gold deposits are formed from pre-existing gold deposits (including some placer deposits) during prolonged weathering of the bedrock. Gold is deposited within iron oxides in the weathered rock or regolith, and may be further enriched by erosion. Some laterite deposits are formed by wind erosion of the bedrock, leaving a residue of native gold metal at the surface.
Platinum and palladium
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.
- Sulfide-type nickel deposits are formed in essentially the same manner as platinum deposits. Nickel has an affinity for sulfur, so an ultramafic or mafic rock that has a sulfide phase in the magma may form nickel deposits. The best nickel deposits are formed where sulfide accumulates, much like in a placer gold deposit, in the base of lava tubes or volcanic flows—especially komatiite lavas.
- Nickel laterite deposits are formed by a process essentially similar to the formation of gold laterite deposits, except that ultramafic or mafic rocks are required. Generally, nickel laterites require large, olivine-bearing ultramafic intrusions. Minerals formed in laterite nickel deposits include gibbsite.
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
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.
Rare earth elements, niobium, tantalum, lithium
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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