Ore

For other uses, see Ore (disambiguation).

Iron ore.

Manganese ore.

Lead ore.

Gold ore.

An ore is a volume of rock containing valuable minerals that occur at sufficiently high concentrations for profitable mining, transportation, milling, and processing. Ore deposits are mineral deposits that, by definition, are economically recoverable. 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 that are 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 which rock is considered valuable, and hence "ore," and which rock is not 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 Genesis

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:
  • Lead-zinc-silver, typified by Red Dog, MacArthur River, Mt. Isa
  • Stratiform tungsten, typified by the Erzgebirge deposits, Czechoslovakia
  • Exhalative spilite-chert hosted gold 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:
  • Phosphorus-tantalite-vermiculite (Phalaborwa/Palabora South Africa)
  • Rare earth elements (Mount Weld, Australia, and Mongolia)

Genesis of common ores

Specific ores are organized here according to the metal commodities.

Iron

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-zinc deposits are generally accompanied by silver, hosted within the mineral galena (lead sulfide) or sphalerite (zinc sulfide).

Lead and zinc deposits are formed by the discharge of deep sedimentary brine onto the seafloor (termed SEDEX deposits), or by replacement of limestone, in skarn deposits, some associated with submarine volcanoes (called volcanic-hosted massive sulfide or VHMS) or in the aureole of subvolcanic intrusions of granite. The vast majority of lead and zinc deposits are Proterozoic in age. 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 are all SEDEX type deposits.

The limestone replacement type of deposit exemplifies the Mississippi Valley Type (MVT). Some of these occur by replacement and degradation of hydrocarbons, which are thought important for transporting lead.

The subvolcanic intrusion type of deposit is renowned for high silver grades, and typifies the deposits of Argentina, Bolivia and Peru. These deposits are essentially Cenozoic in age and are known as the Andean silver belt, the most recent example being San Cristobal with 450 million ounces of silver. These deposits form by discharge of fluids bearing incompatible elements from the cooling granite mass, and have low lead grades but exceptional silver enrichment.

Gold

Gold deposits are formed via a very wide variety of geological processes. Deposits are classified as primary, alluvial or placer deposits, or residual or laterite deposits. Often a deposit will contain a mixture of all three types of ore.

Plate tectonics is the underlying mechanism for generating gold deposits. The majority of primary gold deposits fall into two main categories: lode gold deposits or intrusion-related deposits.

Lode gold deposits are generally high-grade, thin, vein and fault hosted. They are comprised primarily 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 and other plate collision events within 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.

Intrusive related gold (Lang & Baker, 2001) is generally hosted in granites, porphyry or rarely dikes. Intrusive related gold usually also contains copper, and is often associated with tin and tungsten, and rarely molybdenum, antimony and uranium. Intrusive-related gold deposits rely on gold existing in the fluids associated with the 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 sourced from pre-existing gold deposits and are secondary deposits. Placer deposits are formed by alluvial processes within rivers, streams and on beaches. Placer gold deposits form via gravity, with the density of gold causing it to sink into trap sites within the river bed, or where water velocity drops, such as bends in rivers and behind boulders. Often placer deposits are found within sedimentary rocks and can be billions of years old, for instance the Witwatersrand deposits in South Africa. Sedimentary placer deposits are known as 'leads' or 'deep leads'.

Placer deposits are often worked by fossicking, and panning for gold is a popular pastime.

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 reworking by erosion. Some laterite deposits are formed by wind erosion of the bedrock leaving a residuum of native gold metal at surface.

Platinum

Platinum and palladium are precious metals generally found in ultramafic rocks. The source of platinum and palladium deposits is ultramafic rocks which have enough sulfur to form a sulfide mineral while the magma is still liquid. This sulfide mineral (usually pentlandite, pyrite, chalcopyrite or pyrrhotite) gains platinum by mixing with the bulk of the magma because platinum is chalcophile and is concentrated in sulfides. Alternatively, platinum occurs in association with chromite either within the chromite mineral itself or within sulfides associated with it.

Sulfide phases only form in ultramafic magmas when the magma reaches sulfur saturation. This is generally thought to be nearly impossible by pure fractional crystallisation, so other processes are usually required in ore genesis models to explain sulfur saturation. These include contamination of the magma with crustal material, especially sulfur-rich wall-rocks or sediments; magma mixing; volatile gain or loss.

Often platinum is associated with nickel, copper, chromium, and cobalt deposits.

Nickel

Nickel deposits are generally found in two forms, either as sulfide or laterite.

Sulfide type nickel deposits are formed in essentially the same manner as platinum deposits. Nickel is a chalcophile element which prefers sulfides, so an ultramafic or mafic rock which 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.

Komatiitic nikel-copper sulfide deposits are considered to be formed by a mixture of sulfide segregation, immiscibility, and thermal erosion of sulfidic sediments. The sediments are considered to be necessary to promote sulfur saturation.

Some subvolcanic sills in the Thompson Belt of Canada host nickel sulfide deposits formed by deposition of sulfides near the feeder vent. Sulfide was accumulated near the vent due to the loss of magma velocity at the vent interface. The massive Voisey's Bay nickel deposit is considered to have formed via a similar process.

The process of forming nickel laterite deposits is essentially similar to the formation of gold laterite deposits, except that ultramafic or mafic rocks are required. Generally nickel laterites require very large olivine-bearing ultramafic intrusions. Minerals formed in laterite nickel deposits include gibbsite.

Copper

Copper is found in association with many other metals and deposit styles. 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 the copper is generally considered to be the lower crust or mantle where the granite melt forms. The copper is enriched by processes during crystallisation of the granite and forms as chalcopyrite — a sulfide mineral, which is carried up with the granite.

Sometimes granites erupt to suface as volcanoes, and copper mineralisation forms during this phase when the granite and volcanic rocks cool via hydrothermal circulation.

Sedimentary copper forms within ocean basins in sedimentary rocks. Generally this forms by brine from deeply buried sediments discharging into the deep sea, and precipitating copper and often lead and zinc sulfides directly onto the sea floor. This is then buried by further sediment.

Often copper is associated with gold, lead, zinc and nickel deposits.

Uranium

Uranium deposits are usually sourced 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 neutralised. Generally this occurs in certain carbon-bearing sediments, within an unconformity in sedimentary strata. The majority of the world's nuclear power is sourced from uranium in such deposits.

Uranium is also found in nearly all coal at several parts per million, and in all granites. Radon is a common problem during mining of uranium as it is a radioactive gas.

Uranium is also found associated with certain igenous rocks, such as granite and porphyry. The Olympic Dam deposit in Australia is an example of this type of uranium deposit. It contains 70% of Australia's share of 40% of the global low-cost recoverable uranium inventory.

Titanium

Titanium ore is formed as placer deposits - literally 'mineral sands' - or as layers within ultramafic layered intrusions. Titanium within layered intrusions forms as ilmenite, a titanium oxide mineral, via the process of crystallisation as the intrusion cools. Sufficiently thick ilmenite layers will form ore. These layers can form considerable tonnages and lengths. This type of ore is known as 'hard rock titanium'. Hard rock titanium mineralisation may contain vanadium as a second ore metal, as a contaminant within the ilmenite.

Mineral sands

Mineral sands are the predominant type of titanium, zirconium and thorium deposit. They are formed by accumulation of such heavy minerals within beach systems, and are a type of placer deposits. The minerals which contain titanium are ilmenite and leucoxene, zirconium is contained within zircon, and thorium is generally contained within monazite. These minerals are sourced from primarily granite bedrock by erosion and transported to the sea by rivers where they accumulate within beach sands. Rarely, but importantly, gold, tin and platinum deposits can form in beach placer deposits.

Tin, tungsten, and molybdenum

These three metals generally form in a certain type of granite, via a similar mechanism to intrusive-related gold and copper. They are considered together because the process of forming these deposits is essentially the same. Skarn type mineralisation related to these granites is a very important type of tin, tungsten and molybdenum deposit. Skarn deposits form by reaction of mineralised fluids from the granite reacting with wall rocks such as limestone. Skarn mineralisation is also important in lead, zinc, copper, gold and occasionally uranium mineralisation.

Greisen granite is another related tin-molybdenum and topaz mineralisation style.

Rare earth elements, niobium, tantalum, lithium

The overwhelming majority of rare earth elements, 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.

Carbonatite intrusions are an important source of these elements. Ore minerals are essentially part of the unusual carbonatite mineralogy.

Phosphate

Phosphate is used in fertilisers. Immense quantities of phosphate rock occur in older sedimentary basin, generally formed in the Proterozoic. Phosphate deposits are thought to be sourced from the skeletons of dead sea creatures which 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 within the geological past.

Phosphate deposits are also formed from alkaline igneous rocks such as nepheline syenites, carbonatites and associated rock types. The phosphate is, in this case, contained within magmatic apatite, monazite or other rare-earth phosphates.

See also

• Mineral
• Rock (geology)

References

ISBN links support NWE through referral fees

• Arne, D.C.; Bierlein, F.P.; Morgan, J.W. & Stein, H.J., 2001. "Re-Os Dating of Sulfides Associated With Gold Mineralisation in Central Victoria, Australia." Economic Geology, 96, pp1455-1459.

• Elder, D. & Cashman, S., 1992. "Tectonic Control and Fluid Evolution in the Quartz Hill, California, Lode-gold Deposits." Economic Geology, 87, pp1795-1812.

• Evans, A.M., 1993. Ore Geology and Industrial Minerals, An Introduction, Blackwell Science, ISBN 0-632-02953-6.

• Groves, D.I. 1993. "The Crustal Continuum Model for late-Archaean lode-gold deposits of the Yilgran Block, Western Australia." Mineralium Deposita, 28, pp366-374.

• Lang, J.R. & Baker, T., 2001. "Intrusion-related gold systems: the present level of understanding." Mineralium Deposita, 36, pp477-489.

• Lindberg, W., 1922. "A suggestion for the terminology of certain mineral deposits." Economic Geology, 17, pp. 292-294.

• Lowenstern, J.B., 2001. "Carbon dioxide in magmas and implications for hydrothermal systems." Mineralium Deposita, 36, pp490-502.

• Pettke, T; Frei, R.; Kramers J.D. & Villa, I. M. 1997. "Isotope systematics in vein gold from Brusson, Val d'Ayas (NW Italy); (U+Th)/He and K/Ar in native Au and its flid inclusions." Chemical Geology, 135, pp173-187.

• White, A.J.R, 2001. "Water, restite and granite mineralisation." Australian Journal of Earth Sciences, 48, pp551-555.

External links

• Ore textures
• Victoria, Australia, mineral endowment, Victorian Government geoscience portal.
• Geoscience Australia Uranium Infosheet

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