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55-533: These ore bodies range from around 10 million to >4,000 million tonnes of contained ore, and have a grade of between 0.2% and 5% copper, with gold contents ranging from 0.1 to 1.41 grams per tonne. These ore bodies tend to express as cone-like, blanket-like breccia sheets within granitic margins, or as long ribbon-like breccia or massive iron oxide deposits within faults or shears. The tremendous size, relatively simple metallurgy and relatively high grade of IOCG deposits can produce extremely profitable mines, although

110-467: A skarn ) and is typically accompanied by a substantial enrichment in iron oxide minerals ( hematite , magnetite ). IOCG deposits tend to accumulate within iron-rich rocks such as banded iron formations , iron schists, etcetera, although iron enrichment of siliciclastic rocks by metasomatism is also recognised within some areas. Although not exclusively Proterozoic , within Australia and South America

165-625: A kilometer in length. Within the volcanic conduits of explosive volcanoes the volcanic breccia environment merges into the intrusive breccia environment. There the upwelling lava tends to solidify during quiescent intervals only to be shattered by ensuing eruptions. This produces an alloclastic volcanic breccia. Clastic rocks are also commonly found in shallow subvolcanic intrusions such as porphyry stocks, granites and kimberlite pipes, where they are transitional with volcanic breccias. Intrusive rocks can become brecciated in appearance by multiple stages of intrusion, especially if fresh magma

220-578: A known impact crater, and/or an association with other products of impact cratering such as shatter cones , impact glass, shocked minerals , and chemical and isotopic evidence of contamination with extraterrestrial material (e.g., iridium and osmium anomalies). An example of an impact breccia is the Neugrund breccia , which was formed in the Neugrund impact . Hydrothermal breccias usually form at shallow crustal levels (<1 km) between 150 and 350 °C, when seismic or volcanic activity causes

275-551: A majority of IOCG deposits are recognised to be within Neoproterozoic to Mesoproterozoic basement. Worldwide, ages of recognised IOCG deposits range from 1.8 Ga to 15 Ma, however, the majority are within the 1.6 Ga to 850 Ma range. One of the biggest factors in the formation of IOCG deposits is the presence of ore fluids. The driving factor for the fluids movement in the upper crust is the present paleogeothermal gradients, as well as regional hydrothermal systems responsible for

330-418: A particular Suite or Supersuite of granites, intermediate mafic intrusives of a particular age. Often the mineralising intrusive event becomes a diagnostic association for expressions of IOCG mineralisation within a given province. IOCG mineralisation may accumulate within metasomatised wall rocks, within brecciated maar or caldera structures, faults or shears, or the aureole of an intrusive event (possibly as

385-436: A section. The samples are analyzed to determine their detrital remanent magnetism (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. For sedimentary rocks this is possible because, as they fall through the water column, very fine-grained magnetic minerals (< 17  μm ) behave like tiny compasses , orienting themselves with Earth's magnetic field . Upon burial, that orientation

440-404: A void to open along a fault deep underground. The void draws in hot water, and as pressure in the cavity drops, the water violently boils. In addition, the sudden opening of a cavity causes rock at the sides of the fault to destabilise and implode inwards, and the broken rock gets caught up in a churning mixture of rock, steam and boiling water. Rock fragments collide with each other and the sides of

495-416: Is a branch of geology concerned with the study of rock layers ( strata ) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks . Stratigraphy has three related subfields: lithostratigraphy (lithologic stratigraphy), biostratigraphy (biologic stratigraphy), and chronostratigraphy (stratigraphy by age). Catholic priest Nicholas Steno established

550-504: Is a mixture of sodic-calcic ( albite - epidote ) to potassic (K- feldspar ) in style, and may vary from province to province based on host rocks and mineralising processes. Typically for large-scale hydrothermal systems, fluid types within IOCG systems show a mixed provenance of magmatic, metamorphic and often meteoric waters. Deposits may be vertically zoned from deeper albite-magnetite assemblages trending toward silica-K-feldspar- sericite in

605-431: Is also commonly used to delineate the nature and extent of hydrocarbon -bearing reservoir rocks, seals, and traps of petroleum geology . Chronostratigraphy is the branch of stratigraphy that places an absolute age, rather than a relative age on rock strata . The branch is concerned with deriving geochronological data for rock units, both directly and inferentially, so that a sequence of time-relative events that created

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660-564: Is also likely to yield success. Gawler Craton IOCG province, South Australia Cloncurry district , Queensland, Australia: Punta del Cobre IOCG province, Chile Para State IOCG province, Brazil Marcona IOCG district in Southern Peru Some authors (e.g., Skirrow et al. 2004) consider the iron ore deposits of Kiruna, Sweden as being IOCG deposits. Similar styles of fault-hosted magnetite-hematite breccias with minor copper-gold mineralisation and skarns are recognised within

715-414: Is composed of coarse rock fragments held together by cement or a fine-grained matrix. Like conglomerate , breccia contains at least 30 percent of gravel -sized particles (particles over 2mm in size), but it is distinguished from conglomerate because the rock fragments have sharp edges that have not been worn down. These indicate that the gravel was deposited very close to its source area, since otherwise

770-534: Is due to physical contrasts in rock type ( lithology ). This variation can occur vertically as layering (bedding), or laterally, and reflects changes in environments of deposition (known as facies change). These variations provide a lithostratigraphy or lithologic stratigraphy of the rock unit. Key concepts in stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries imply about their original depositional environment. The basic concept in stratigraphy, called

825-489: Is intruded into partly consolidated or solidified magma. This may be seen in many granite intrusions where later aplite veins form a late-stage stockwork through earlier phases of the granite mass. When particularly intense, the rock may appear as a chaotic breccia. Clastic rocks in mafic and ultramafic intrusions have been found and form via several processes: Impact breccias are thought to be diagnostic of an impact event such as an asteroid or comet striking

880-406: Is less common in the mesothermal regime, as the formational event is brief. If boiling occurs, methane and hydrogen sulfide may be lost to the steam phase, and ore may precipitate. Mesothermal deposits are often mined for gold. For thousands of years, the striking visual appearance of breccias has made them a popular sculptural and architectural material. Breccia was used for column bases in

935-411: Is preserved. For volcanic rocks, magnetic minerals, which form in the melt, orient themselves with the ambient magnetic field, and are fixed in place upon crystallization of the lava. Oriented paleomagnetic core samples are collected in the field; mudstones , siltstones , and very fine-grained sandstones are the preferred lithologies because the magnetic grains are finer and more likely to orient with

990-466: Is uniform in rock type and chemical composition. Caldera collapse leads to the formation of megabreccias, which are sometimes mistaken for outcrops of the caldera floor. These are instead blocks of precaldera rock, often coming from the unstable oversteepened rim of the caldera. They are distinguished from mesobreccias whose clasts are less than a meter in size and which form layers in the caldera floor. Some clasts of caldera megabreccias can be over

1045-466: The Gawler Craton , exploration for Olympic Dam style IOCG deposits has relied on four main criteria for targeting exploratory drill holes; This exploration model is applicable to the most basic of exploration criteria for identifying prospective areas likely to form IOCG deposits. In better exposed terranes, prospecting for alteration assemblages and skarns, in concert with geochemical exploration

1100-619: The Minoan palace of Knossos on Crete in about 1800 BC . Breccia was used on a limited scale by the ancient Egyptians ; one of the best-known examples is the statue of the goddess Tawaret in the British Museum. Breccia was regarded by the Romans as an especially precious stone and was often used in high-profile public buildings. Many types of marble are brecciated, such as Breccia Oniciata. Stratigraphy Stratigraphy

1155-703: The Olympic Dam deposit, where using fluorites rare-earth element ( REE ) chemistry, the fluids in the formation of the deposits were identified. Ore minerals in IOCG deposits are typically copper-iron sulfide chalcopyrite and gangue pyrite , forming 10–15% of the rock mass. Supergene profiles can be developed above weathered examples of IOCG deposits, as exemplified by the Sossego deposit, Para State, Brazil , where typical oxidised copper minerals are present, e.g.; malachite , cuprite , native copper and minor amounts of digenite and chalcocite . Alteration

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1210-720: The Olympic Dam, South Australia , and Candelaria, Chile deposits. Iron oxide copper gold (IOCG) deposits are considered to be metasomatic expressions of large crustal-scale alteration events driven by intrusive activity. The deposit type was first recognised by discovery and study of the supergiant Olympic Dam copper-gold-uranium deposit ( Olympic Dam mine ), and South American examples. IOCG deposits are classified as separate to other large intrusive related copper deposits such as porphyry copper deposits and other porphyry metal deposits primarily by their substantial accumulations of iron oxide minerals, association with felsic-intermediate type intrusives (Na-Ca rich granitoids), and lack of

1265-431: The law of superposition , states: in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence. Chemostratigraphy studies the changes in the relative proportions of trace elements and isotopes within and between lithologic units. Carbon and oxygen isotope ratios vary with time, and researchers can use those to map subtle changes that occurred in the paleoenvironment. This has led to

1320-564: The natural remanent magnetization (NRM) to reveal the DRM. Following statistical analysis, the results are used to generate a local magnetostratigraphic column that can then be compared against the Global Magnetic Polarity Time Scale. This technique is used to date sequences that generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling means that it is also a powerful technique for

1375-490: The Earth and are normally found at impact craters . Impact breccia, a type of impactite , forms during the process of impact cratering when large meteorites or comets impact with the Earth or other rocky planets or asteroids . Breccia of this type may be present on or beneath the floor of the crater, in the rim, or in the ejecta expelled beyond the crater. Impact breccia may be identified by its occurrence in or around

1430-501: The Gawler Craton, South Australia, which would be recognised as IOCG deposits. Breccia Breccia ( / ˈ b r ɛ tʃ i ə / BRETCH -ee-ə or / ˈ b r ɛ ʃ i ə / BRESH -ee-ə , Italian: [ˈbrettʃa] ; Italian for 'breach') is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix . The word has its origins in

1485-506: The Italian language, in which it means "rubble". A breccia may have a variety of different origins, as indicated by the named types including sedimentary breccia, fault or tectonic breccia, igneous breccia, impact breccia, and hydrothermal breccia. A megabreccia is a breccia composed of very large rock fragments, sometimes kilometers across, which can be formed by landslides , impact events , or caldera collapse. Breccia

1540-474: The Precambrian. Larger deposits with >100 tons of resources occur near Paleoprotozoic and Archean cratons. These large deposits formed by mantle underplating impacts to metasomatized lithospheric mantle, and smaller deposits form by tectonic settings replication of this process in more recent times. The content of gold within these deposits is largely variable, and can be a factor in the economic value of

1595-535: The alteration within these deposits. IOCG deposits have a distinctive set of two fluids vital in their formation: There is also evidence of other fluids that are volatile rich in the formation of these deposits. There is controversy in regards to the factors that control the formation of the ore in these deposits, as they display a lot of variety between deposits in regards to the ore grades, alteration styles, fluid inclusion characteristics, and their links to their tectonic settings, and nearby intrusions. This has led to

1650-615: The ambient field during deposition. If the ancient magnetic field were oriented similar to today's field ( North Magnetic Pole near the North Rotational Pole ), the strata would retain a normal polarity. If the data indicate that the North Magnetic Pole were near the South Rotational Pole , the strata would exhibit reversed polarity. Results of the individual samples are analyzed by removing

1705-451: The clasts are so large that the brecciated nature of the rock is not obvious. Megabreccias can be formed by landslides , impact events , or caldera collapse. Breccias are further classified by their mechanism of formation. Sedimentary breccia is breccia formed by sedimentary processes. For example, scree deposited at the base of a cliff may become cemented to form a talus breccia without ever experiencing transport that might round

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1760-412: The complex zonation in alteration mineral assemblies commonly associated with porphyry deposits. The relatively simple copper-gold +/- uranium ore assemblage is also distinct from the wide spectrum of Cu-Au-Ag-Mo-W-Bi porphyry deposits, and there is often no metal zonation within recognised examples of IOCG deposits. IOCG deposits tend to also accumulate within faults as epigenetic mineralisation distal to

1815-591: The crust (e.g. Prominent Hill , some Mount Isa examples, Brazilian examples). What is common in IOCGs is their genesis within magmatic-driven crustal-scale hydrothermal systems. Iron oxide copper gold deposits typically form within 'provinces' where several deposits of similar style, timing and similar genesis form within similar geologic settings. The genesis and provenance of IOCG deposits, their alteration assemblages and gangue mineralogy may vary between provinces, but all are related to; IOCG deposits typically occur at

1870-623: The deep upper crust at depths of over 10km, to paleosurfaces. This main feature sets apart IOCG type deposits from porphyry skarn Cu-Au deposits which are from shallow depths of formation (<5km depth). The formation at deeper depths has implications such as ore fluids from a deep source. IOCG deposits are still relatively loosely defined and as such, some large and small deposits of various types may or may not fit within this deposit classification. IOCG deposits may have skarn -like affinities (e.g.; Wilcherry Hill , Cairn Hill ), although they are not strictly skarns in that they are not metasomatites in

1925-479: The deposit. The gold contents of all deposits averages 0.41 g/t Au, with the majority of worldwide deposits averaging less than 1 g/t Au. The contents of gold can appear in three different forms in these deposits: World-class IOCG deposits contain consistent Cu grades, between 0.7–1.5% Cu, higher copper grades than that of most world class gold-rich porphyry copper deposits. Within the Olympic Domain of

1980-410: The edges would have been rounded during transport. Most of the rounding of rock fragments takes place within the first few kilometers of transport, though complete rounding of pebbles of very hard rock may take up to 300 kilometers (190 mi) of river transport. A megabreccia is a breccia containing very large rock fragments, from at least a meter in size to greater than 400 meters. In some cases,

2035-404: The formation of these deposits is still not fully understood, and the fluid origin of the world class deposits are still being investigated. Iron oxide copper-gold deposits are also often associated with other valuable trace elements such as uranium, bismuth and rare-earth metals, although these accessories are typically subordinate to copper and gold in economic terms. Some examples include

2090-512: The gap may be due to removal by erosion, in which case it may be called a stratigraphic vacuity. It is called a hiatus because deposition was on hold for a period of time. A physical gap may represent both a period of non-deposition and a period of erosion. A geologic fault may cause the appearance of a hiatus. Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout

2145-423: The grinding action of two fault blocks as they slide past each other. Subsequent cementation of these broken fragments may occur by means of the introduction of mineral matter in groundwater . Igneous clastic rocks can be divided into two classes: Volcanic pyroclastic rocks are formed by explosive eruption of lava and any rocks which are entrained within the eruptive column. This may include rocks plucked off

2200-522: The lack of a complete model for the deposits' formation. A variety of models have been made to try and model the formation of these deposits, such as IOCG deposits as the lower root portion of iron oxide-apatite formation, or models of complex interactions between more than two fluids of magmatic, surficial, sedimentary, or metamorphic origin. There is still controversy to these origins, but using tracing of fluid sources has opened exploration possibilities in recent years to large deposits in Australia, such as

2255-432: The margins of large igneous bodies which intrude into sedimentary strata. As such, IOCG deposits form pipe-like, mantle-like or extensive breccia-vein sheets within the host stratigraphy. Morphology is often not an important criterion of the ore body itself, and is determined by the host stratigraphy and structures. IOCG deposits are usually associated with distal zones of particular large-scale igneous events, for instance

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2310-403: The mesothermal regime, at much greater depths, fluids under lithostatic pressure can be released during seismic activity associated with mountain building. The pressurised fluids ascend towards shallower crustal levels that are under lower hydrostatic pressure. On their journey, high-pressure fluids crack rock by hydrofracturing , forming an angular in situ breccia. Rounding of rock fragments

2365-428: The overpressure of pore fluid within sedimentary basins . Hydrothermal breccias are usually formed by hydrofracturing of rocks by highly pressured hydrothermal fluids. They are typical of the epithermal ore environment and are intimately associated with intrusive-related ore deposits such as skarns , greisens and porphyry -related mineralisation. Epithermal deposits are mined for copper, silver and gold. In

2420-523: The rock fragments. Thick sequences of sedimentary ( colluvial ) breccia are generally formed next to fault scarps in grabens . Sedimentary breccia may be formed by submarine debris flows . Turbidites occur as fine-grained peripheral deposits to sedimentary breccia flows. In a karst terrain , a collapse breccia may form due to collapse of rock into a sinkhole or in cave development. Collapse breccias also form by dissolution of underlying evaporite beds. Fault or tectonic breccia results from

2475-437: The rock layers. Strata from widespread locations containing the same fossil fauna and flora are said to be correlatable in time. Biologic stratigraphy was based on William Smith's principle of faunal succession , which predated, and was one of the first and most powerful lines of evidence for, biological evolution . It provides strong evidence for the formation ( speciation ) and extinction of species . The geologic time scale

2530-432: The rocks formation can be derived. The ultimate aim of chronostratigraphy is to place dates on the sequence of deposition of all rocks within a geological region, and then to every region, and by extension to provide an entire geologic record of the Earth. A gap or missing strata in the geological record of an area is called a stratigraphic hiatus. This may be the result of a halt in the deposition of sediment. Alternatively,

2585-405: The significance of strata or rock layering and the importance of fossil markers for correlating strata; he created the first geologic map of England. Other influential applications of stratigraphy in the early 19th century were by Georges Cuvier and Alexandre Brongniart , who studied the geology of the region around Paris. Variation in rock units, most obviously displayed as visible layering,

2640-403: The source intrusion, whereas porphyries are much more proximal to intrusive bodies. A feature of IOCG ore deposits is the large variability between deposits regarding the ore grades , alteration styles, and fluid inclusion characteristics that leads to the lack of a complete model for the deposits formation. An important feature of these deposits is the depth of formation, which ranges from

2695-406: The specialized field of isotopic stratigraphy. Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals (particularly carbonates ), grain size, thickness of sediment layers ( varves ) and fossil diversity with time, related to seasonal or longer term changes in palaeoclimates . Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in

2750-439: The strictest sense. IOCG deposits can express a wide variety of deposit morphologies and alteration types dependent on their host stratigraphy, the tectonic processes operating at the time (e.g., some provinces show a preference for development within shears and structural zones), and so on. IOCG deposits have been recognised within epithermal regimes ( caldera and maar styles) through to brittle-ductile regimes deeper within

2805-429: The theoretical basis for stratigraphy when he introduced the law of superposition , the principle of original horizontality and the principle of lateral continuity in a 1669 work on the fossilization of organic remains in layers of sediment. The first practical large-scale application of stratigraphy was by William Smith in the 1790s and early 19th century. Known as the "Father of English geology", Smith recognized

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2860-1056: The upper portions of the deposits. Gangue minerals are typically some form of iron oxide mineral, classically hematite , but also magnetite within some other examples such as Ernest Henry and some Argentinian examples. This is typically associated with gangue sulfides of pyrite, with subordinate pyrrhotite and other base metal sulfides. Silicate gangue minerals include actinolite , pyroxene , tourmaline , epidote and chlorite , with apatite , allanite and other phosphate minerals common in some IOCG provinces (e.g.; North American examples), with carbonate - barite assemblages also reported. Where present, rare-earth metals tend to associate with phosphate minerals. When iron oxide species trend towards magnetite or crystalline massive hematite, IOCG deposits may be economic based on their iron oxide contents alone. Several examples of IOCG deposits (Wilcherry Hill, Cairn Hill, Kiruna) are iron ore deposits. IOCG ore deposits containing economic quantities (highly profitable) of both copper and gold originate from

2915-581: The void, and the angular fragments become more rounded. Volatile gases are lost to the steam phase as boiling continues, in particular carbon dioxide . As a result, the chemistry of the fluids changes and ore minerals rapidly precipitate . Breccia-hosted ore deposits are quite common. The morphology of breccias associated with ore deposits varies from tabular sheeted veins and clastic dikes associated with overpressured sedimentary strata, to large-scale intrusive diatreme breccias ( breccia pipes ), or even some synsedimentary diatremes formed solely by

2970-421: The wall of the magma conduit, or physically picked up by the ensuing pyroclastic surge . Lavas, especially rhyolite and dacite flows, tend to form clastic volcanic rocks by a process known as autobrecciation . This occurs when the thick, nearly solid lava breaks up into blocks and these blocks are then reincorporated into the lava flow again and mixed in with the remaining liquid magma. The resulting breccia

3025-529: Was developed during the 19th century, based on the evidence of biologic stratigraphy and faunal succession. This timescale remained a relative scale until the development of radiometric dating , which was based on an absolute time framework, leading to the development of chronostratigraphy. One important development is the Vail curve , which attempts to define a global historical sea-level curve according to inferences from worldwide stratigraphic patterns. Stratigraphy

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