Ore genesis 3425287 219855128 2008-06-17T04:22:36Z Rolinator 685714 /* Lead zinc silver */ main link The various theories of '''ore genesis''' explain how the various types of mineral deposits form within the Earth's [[Crust (geology)|crust]]. Ore genesis theories are very dependent on the [[mineral]] or commodity. Ore genesis theories generally involve three components: source, transport or conduit, and trap. This also applies to the [[petroleum]] industry, which was first to use this methodology. * Source is required because metal must come from somewhere, and be liberated by some process * Transport is required first to move the metal bearing fluids or solid minerals into the right position, and refers to the act of physically moving the metal, as well as chemical or physical phenomenon which encourage movement * Trapping is required to concentrate the metal via some physical, chemical or geological mechanism into a concentration which forms mineable [[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== Evans (1993) divides ore genesis into the following main categories based on physical process. These are internal processes, hydrothermal processes, metamorphic processes and surficial processes. ===Internal processes=== These processes are integral physical phenomena and chemical reactions internal to magmas, generally in [[igneous rocks|plutonic]] or [[igneous rocks|volcanic]] rocks. These include; * [[Fractional crystallization (geology)|Fractional crystallization]], either creating monominerallic [[cumulate rocks|cumulate]] ores or contributing to the enrichment of ore minerals and metals * Liquation, or ''liquid [[Miscibility|immiscibility]]'' between melts of differing composition, usually sulfide segregations of nickel-copper-platinoid sulfides and silicates. === Hydrothermal processes === These processes are the physico-chemical phenomena and reactions caused by movement of [[hydrothermal]] waters within the crust, often as a consequence of magmatic intrusion or tectonic upheavals. The foundations of hydrothermal processes are the source-transport-trap mechanism. Sources of hydrothermal solutions include seawater, formational [[brine]]s (water trapped within sediments at deposition) and metamorphic fluids created by dehydration of hydrous minerals during [[metamorphism]]. Metal sources may include a plethora of rocks. However most metals of economic importance are carried as trace elements within rock-forming minerals, and so may be liberated by hydrothermal processes. This happens because of * incompatibility of the metal with its host mineral, for example [[zinc]] in [[calcite]], which favours aqueous fluids in contact with the host mineral under [[diagenesis]]. * solubility of the host mineral within nascent hydrothermal solutions in the source rocks, for example mineral salts ([[halite]]), carbonates ([[cerussite]]), phosphates ([[monazite]] and [[thorianite]]) and sulfates ([[barite]]) * elevated temperatures causing decomposition reactions of minerals Transport by hydrothermal solutions usually requires a salt or other soluble species which can form a metal-bearing complex. These metal-bearing complexes facilitate transport of metals within aqueous solutions, generally as hydroxides, but also by processes similar to [[chelation]]. <Br> This process is especially well understood in gold metallogeny where various thiosulfate, chloride and other gold-carrying chemical complexes (notably [[tellurium]]-chloride/sulfate or antimony-chloride/sulfate). The majority of metal deposits formed by hydrothermal processes include [[sulfide]] minerals, indicating sulfur is an important metal-carrying complex. Sulfide deposition:<br> Sulfide deposition within the ''trap'' zone occurs when metal-carrying sulfate, sulfide or other complexes become chemically unstable due to one or more of the following processes; * falling temperature, which renders the complex unstable or metal insoluble * loss of pressure, which has the same effect * reaction with chemically reactive wall rocks, usually of [[reduced]] [[oxidation state]], such as iron bearing rocks, [[mafic]] or [[ultramafic]] rocks or [[carbonate]] rocks * degassing of the hydrothermal fluid into a gas and water system, or boiling, which alters the metal carrying capacity of the solution and even destroys metal-carrying chemical complexes Metal can also become precipitated when temperature and pressure or oxidation state favour different ionic complexes in the water, for instance the change from sulfide to sulfate, oxygen [[fugacity]], exchange of metals between sulfide and chloride complexes, et cetera. === Metamorphic processes === Lateral secretion:<br> Ore deposits formed by lateral secretion are formed by metamorphic reactions during [[Shear (geology)|shearing]], which liberate mineral constituents such as quartz, sulfides, gold, carbonates and oxides from deforming rocks and focus these constituents into zones of reduced pressure or dilation such as [[Fault (geology)|faults]]. This may occur without much hydrothermal fluid flow, and this is typical of podiform chromite deposits. Metamorphic processes also control many physical processes which form the source of hydrothermal fluids, outlined above. === Surficial processes === Surficial processes are the physical and chemical phenomena which cause concentration of ore material within the [[regolith]], generally by the action of the environment. This includes [[placer mining|placer]] deposits, [[laterite]] deposits and residual or [[Eluvium (geology)|eluvial]] deposits. The physical processes of ore deposit formation in the surficial realm include; * [[erosion]] * deposition by sedimentary processes, including [[winnowing]], density separation (eg; gold placers) * [[weathering]] via oxidation or chemical attack of a rock, either liberating rock fragments or creating chemically deposited clays, laterites or [[manto ore deposits]] * Deposition in low-energy environments in [[beach]] environments ==Classification of ore deposits== Ore deposits are usually classified by ore formation processes and geological setting. For example, [[SEDEX]] deposits, literally meaning "sedimentary exhalative" are a class of ore deposit formed on the sea floor (sedimentary) by exhalation of brines into seawater (exhalative), causing chemical precipitation of ore minerals when the brine cools, mixes with sea water and loses its metal carrying capacity. Ore deposits rarely fit snugly into the boxes in which geologists wish to place them. Many may be formed by one or more of the basic genesis processes above, creating ambiguous classifications and much argument and conjecture. Often ore deposits are classified after examples of their type, for instance [[Broken Hill|Broken Hill Type]] lead-zinc-silver deposits or [[Carlin, Nevada|Carlin]]-type Gold deposits. Classification of hydrothermal ore deposits is also achieved by classifying according to the temperature of formation, which roughly also correlates with particular mineralising fluids, mineral associations and structural styles. This scheme, proposed by [[Waldemar Lindgren]] (1933) classified hydrothermal deposits as ''hypothermal'', ''mesothermal'', ''epithermal'' and ''telethermal''. ==Genesis of common ores== This page has been organised by [[metal]] [[commodity]]; it is also possible to organise theories according to [[geological]] criteria of formation, as well as by metal association. Often ores of the same metal can be formed by multiple processes, and this is described by commodity. === Iron === {{main|Iron ore}} Iron ores are overwhelmingly derived from ancient [[sediment]]s known as ''[[banded iron formation]]s'' (BIFs). These sediments are composed of [[iron oxide]] minerals deposited on the sea floor. Particular environmental conditions are needed to transport enough iron in sea water to form these deposits, such as acidic and oxygen-poor atmospheres within the [[Proterozoic]] Era. Often, more recent [[weathering]] during the [[Tertiary]] or [[Eocene]] is required to convert the usual [[magnetite]] minerals into more easily processed [[hematite]]. Some iron deposits within the [[Pilbara]] of [[West Australia]] are ''placer deposits'', formed by accumulation of hematite gravels called ''pisolites'' which form [[channel-iron deposits]]. These are preferred because they are cheap to mine. ===Lead zinc silver=== {{main|Sedimentary exhalative deposits}} {{main|Carbonate hosted lead zinc ore deposits}} {{main|Volcanogenic massive sulfide ore deposit}} [[Lead]]-[[zinc]] deposits are generally accompanied by [[silver]], hosted within the lead sulfide mineral [[galena]] or within the zinc sulfide mineral [[sphalerite]]. Lead and zinc deposits are formed by discharge of deep sedimentary [[brine]] onto the sea floor (termed ''sedimentary exhalative'' or [[sedimentary exhalative deposits|SEDEX]]), or by replacement of [[limestone]], in [[skarn]] deposits, some associated with submarine volcanoes (called [[volcanogenic massive sulfide ore deposit]]s or VMS) or in the [[aureole]] of subvolcanic intrusions of granite. The vast majority of SEDEX lead and zinc deposits are [[Proterozoic]] in age, although there are significant Jurassic examples in Canada and Alaska. The carbonate replacement type deposit is exemplified by the [[Carbonate hosted lead zinc ore deposits|Mississippi valley type (MVT)]] ore deposits. MVT and similar styles occur by replacement and degradation of carbonate sequences by [[hydrocarbon]]s, which are thought important for transporting lead. ===Gold === Gold deposits are formed via a very wide variety of [[geological]] processes. Deposits are classified as primary, alluvial or [[Placer mining|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 [[telluride]]s. Lode gold deposits are usually hosted in [[basalt]] or in sediments known as [[turbidite]], although when in [[Fault (geology)|faults]], they may occupy intrusive igneous 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 (geology)|porphyry]] or rarely [[Dike (geology)|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 oxide]]s 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=== {{main|Kambalda type komatiitic nickel ore deposits}} {{main|Lateritic nickel ore deposits}} 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 sulfides. The best nickel deposits are formed where sulfide accumulates in the base of [[lava tube]]s or [[volcanic]] flows &mdash; especially [[komatiite]] lavas. Komatiitic nickel-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 === {{main|Porphyry copper}} {{main|Manto ore deposits}} 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. Copper is enriched by processes during crystallisation of the granite and forms as [[chalcopyrite]] &mdash; 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 === {{main|Uranium ore deposits}} 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 (geology)|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 known global low-cost recoverable uranium inventory. ===Titanium and zirconium=== {{main|Heavy mineral sands ore deposits}} Mineral sands are the predominant type of [[titanium]], [[zirconium]] and [[thorium]] deposit. They are formed by accumulation of such [[heavy mineral sands ore deposits|heavy minerals]] within [[beach]] systems, and are a type of ''[[placer deposit]]s''. The minerals which contain titanium are ilmenite, rutile 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 mineralogy of carbonatite. === Phosphate === [[Phosphate]] is used in fertilisers. Immense quantities of ''phosphate rock'' or [[phosphorite]] occur in sedimentary shelf deposits, ranging in age from the [[Proterozoic]] to currently forming environments.<ref>Guilbert, John M. and Charles F. Park, ''The Geology of Ore Deposites'', 1986, Freeman, pp. 715-720, ISBN 0-7167-1456-6 </ref> 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 syenite]]s, [[carbonatite]]s and associated rock types. The phosphate is, in this case, contained within magmatic [[apatite]], [[monazite]] or other rare-earth phosphates. == See also == * [[Mineral exploration]] * [[Copper extraction]] * [[Hydrothermal circulation]] * [[Economic geology]] * [[Mineral redox buffer]] * [[Metasomatism]] * [[Igneous differentiation]] ==References== {{reflist}} *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, 2001. *Elder, D. & Cashman, S. ''Tectonic Control and Fluid Evolution in the Quartz Hill, California, Lode-gold Deposits.'' Economic Geology, 87, pp1795-1812, 1992. *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, 1993. *Lang, J.R. & Baker, T., 2001. ''Intrusion-related gold systems: the present level of understanding.'' Mineralium Deposita, 36, pp477-489, 2001. *Lindberg, W., 1922. ''A suggestion for the terminology of certain mineral deposits.'' Economic Geology, '''17'', pp. 292-294. *Lindgren, Waldemar, 1933. ''Mineral Deposits,'' 4th ed., McGraw-Hill *Lowenstern, J.B., 2001. ''Carbon dioxide in magmas and implications for hydrothermal systems.'' Mineralium Deposita, 36, pp490-502, 2001. *Pettke, T; Frei, R.; Kramers J.D. & Villa, I. M. 1997. ''[[Isotope geochemistry|Isotope]] systematics in vein gold from Brusson, Val d'Ayas (NW Italy); (U+Th)/He and K/Ar in native Au and its fluid inclusions.'' Chemical Geology, 135, pp173-187, 1997. *White, A.J.R, 2001. ''Water, [[restite]] and granite mineralisation.'' Australian Journal of Earth Sciences, 48, pp551-555 2001. ==External links== *[http://franklin-sterlinghill.com/dunn/ch12/special_features.stm Ore textures] *[http://www.dpi.vic.gov.au/dpi/nrenmp.nsf/LinkView/F6BD35916DB5F66ACA256C72000E4CD053FBB40CF848643C4A256DEA001E4B5C Victoria, Australia, mineral endowment, Victorian Government geoscience portal.] [[Category:Economic geology]] [[Category:Geological processes]] [[Category:Resource extraction]] [[Category:geochemistry]] [[Category:mining]]