Iron oxide copper gold ore deposits (IOCG) are important and highly valuable concentrations of copper, gold and uraniumores hosted within iron oxide dominant gangue assemblages which share a common genetic origin.[1]
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.[2] 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.[3]
The tremendous size, relatively simple metallurgy and relatively high grade of IOCG deposits can produce extremely profitable mines, although the formation of these deposits is still not fully understood, and the fluid origin of the world class deposits are still being investigated.[4]
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.
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 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 the source intrusion, whereas porphyries are much more proximal to intrusive bodies.
Deposit features
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.[5]
An important feature of these deposits is the depth of formation, which ranges from the deep upper crust at depths of over 10km, to paleosurfaces.[6] 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.[6]
Similar deposit styles
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 the strictest sense.[1]
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 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.[7]
Genesis
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;
Major regional thermal event broadly coeval with IOCG formation, represented by low to medium grade metamorphism, and/or mafic intrusions, and/or I- or A-type granitoids
Host stratigraphy is relatively Fe-enriched (BIF, ironstones), but have relatively little reduced carbon (e.g.; coal, etc.).
Regional-scale alteration systems, operating over tens or hundreds of kilometres, involving admixture of at least two fluids
Large-scale crustal structures which allow extensive hydrothermal circulation of mineralising fluids
IOCG deposits typically occur at 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.[8]
IOCG deposits are usually associated with distal zones of particular large-scale igneous events, for instance 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 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 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.
Ore fluids formation
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 the alteration within these deposits.[9] IOCG deposits have a distinctive set of two fluids vital in their formation:[9]
Highly oxidized fluids such as a meteoric or ground waters
Brines of magmatic hydrothermal fluid or fluids that have reacted with metamorphic rocks, that are deep sourced and high temperature
There is also evidence of other fluids that are volatile rich in the formation of these deposits.[9]
Factors of ore formation
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 the lack of a complete model for the deposits' formation.[5][4]
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.[4] 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 the Olympic Dam deposit, where using fluorites rare-earth element (REE) chemistry, the fluids in the formation of the deposits were identified.[4]
Mineralogy and alteration
Chalcopyrite-rich ore specimen from Olympic DamChalcopyrite in hematised breccia from Prominent Hill
Ore minerals in IOCG deposits are typically copper-iron sulfide chalcopyrite and ganguepyrite, 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 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 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.
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.
Economic mineral occurrence
IOCG ore deposits containing economic quantities (highly profitable) of both copper and gold originate from the Precambrian. Larger deposits with >100 tons of resources occur near Paleoprotozoic and Archean cratons.[6] 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.[6]
The content of gold within these deposits is largely variable, and can be a factor in the economic value of 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.[2]
The occurrence of native gold mineralization. Example from Kalgoorlie Australia.
The contents of gold can appear in three different forms in these deposits:[2]
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.[6]
Exploration
Within the Olympic Domain of the Gawler Craton, exploration for Olympic Dam style IOCG deposits has relied on four main criteria for targeting exploratory drill holes;
A substantial gravity anomaly, taken to be representative of accumulation of iron oxide minerals within the crust, which is seen as being associated with classic Olympic Dam style IOCG mineralisation. Gravity data is often interpreted via a 3D inversion to resolve the density contrast and sub-surface position of a dense body of rock. More qualitatively, the "edges" of a gravity body are considered more prospective as this theoretically represents the mineralised margins of an intrusive body.
High magnetism within the crust, again taken to be representative of accumulation of substantial iron oxide minerals within proximity to the targeted IOCG mineralising events
Proximity to apparent crustal-scale linear features in geophysical data, which are taken to represent the fundamental crustal architectural faults up which mineralising intrusions and fluids would by preference travel
Presence of the prospective Hiltaba Granite Suite, which is dated to 1570 Ma coeval with Olympic Dam and the other known IOCG examples within the province
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 is also likely to yield success.
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 the Gawler Craton, South Australia, which would be recognised as IOCG deposits.
Ore is natural rock or sediment that contains one or more valuable minerals concentrated above background levels, typically containing metals, that can be mined, treated and sold at a profit. The grade of ore 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.
Breccia is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
Chalcopyrite ( KAL-kə-PY-ryte, -koh-) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green-tinged black.
Skarns or tactites are coarse-grained metamorphic rocks that form by replacement of carbonate-bearing rocks during regional or contact metamorphism and metasomatism. Skarns may form by metamorphic recrystallization of impure carbonate protoliths, bimetasomatic reaction of different lithologies, and infiltration metasomatism by magmatic-hydrothermal fluids. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate minerals. These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a granitic pluton found in and around faults or shear zones that commonly intrude into a carbonate layer composed of either dolomite or limestone. Skarns can form by regional or contact metamorphism and therefore form in relatively high temperature environments. The hydrothermal fluids associated with the metasomatic processes can originate from a variety of sources; magmatic, metamorphic, meteoric, marine, or even a mix of these. The resulting skarn may consist of a variety of different minerals which are highly dependent on both the original composition of the hydrothermal fluid and the original composition of the protolith.
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is traditionally defined as metamorphism which involves a change in the chemical composition, excluding volatile components. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.
Volcanogenic massive sulfide ore deposits, also known as VMS ore deposits, are a type of metal sulfide ore deposit, mainly copper-zinc which are associated with and created by volcanic-associated hydrothermal events in submarine environments.
Porphyry copper deposits are copper ore bodies that are formed from hydrothermal fluids that originate from a voluminous magma chamber several kilometers below the deposit itself. Predating or associated with those fluids are vertical dikes of porphyritic intrusive rocks from which this deposit type derives its name. In later stages, circulating meteoric fluids may interact with the magmatic fluids. Successive envelopes of hydrothermal alteration typically enclose a core of disseminated ore minerals in often stockwork-forming hairline fractures and veins. Because of their large volume, porphyry orebodies can be economic from copper concentrations as low as 0.15% copper and can have economic amounts of by-products such as molybdenum, silver, and gold. In some mines, those metals are the main product.
Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.
A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around 100 km2 (39 sq mi) to over 50,000 km2 (19,000 sq mi) and several hundred metres to over one kilometre (3,300 ft) in thickness. While most layered intrusions are Archean to Proterozoic in age, they may be any age such as the Cenozoic Skaergaard intrusion of east Greenland or the Rum layered intrusion in Scotland. Although most are ultramafic to mafic in composition, the Ilimaussaq intrusive complex of Greenland is an alkalic intrusion.
A polymetallic replacement deposit, also known as carbonate replacement deposit or high-temperature carbonate-hosted Ag-Pb-Zn deposit, is an orebody of metallic minerals formed by the replacement of sedimentary, usually carbonate rock, by metal-bearing solutions in the vicinity of igneous intrusions. When the ore forms a blanketlike body along the bedding plane of the rock, it is commonly called a manto ore deposit. Other ore geometries are chimneys and veins. Polymetallic replacements/mantos are often stratiform wall-rock replacement orebodies distal to porphyry copper deposits, or porphyry molybdenum deposits. The term manto is from the Spanish word for mantle, or cloak, although the geologic manto is more like a mantle roll than a sheetlike structure.
Seafloor massive sulfide deposits or SMS deposits, are modern equivalents of ancient volcanogenic massive sulfide ore deposits or VMS deposits. The term has been coined by mineral explorers to differentiate the modern deposit from the ancient.
The Broken Hill Ore Deposit is located underneath Broken Hill in western New South Wales, Australia, and is the namesake for the town. It is arguably the world's richest and largest zinc-lead ore deposit.
Uranium ore deposits are economically recoverable concentrations of uranium within the Earth's crust. Uranium is one of the most common elements in the Earth's crust, being 40 times more common than silver and 500 times more common than gold. It can be found almost everywhere in rock, soil, rivers, and oceans. The challenge for commercial uranium extraction is to find those areas where the concentrations are adequate to form an economically viable deposit. The primary use for uranium obtained from mining is in fuel for nuclear reactors.
The Kiruna mine is the largest and most modern underground iron ore mine in the world. The mine is located in Kiruna in Norrbotten County, Lapland, Sweden. The mine is owned by Luossavaara-Kiirunavaara AB (LKAB), a large Swedish mining company. In 2018 the mine produced 26.9 million tonnes of iron ore. The Kiruna mine has an ore body which is 4 km (2.5 mi) long, 80 metres (260 ft) to 120 metres (390 ft) thick and reaching a depth of up to 2 km (1.2 mi). Since mining began at the site in 1898, the mine has produced over 950 million tonnes of ore. As of 2020 the main haulage level is 1365 m below the ore outcrop at Kiirunavaara that existed prior to mining.
The Atacama Fault Zone (AFZ) is an extensive system of faults cutting across the Chilean Coastal Cordillera in Northern Chile between the Andean Mountain range and the Pacific Ocean. The fault system is North-South striking and runs for more than 1100 km North and up to 50 km in width through the Andean forearc region. The zone is a direct result of the ongoing subduction of the Eastward moving Nazca Plate beneath the South American Plate and is believed to have formed in the Early Jurassic during the beginnings of the Andean orogeny. The zone can be split into 3 regions: the North, Central and South.
El Laco is a volcanic complex in the Antofagasta Region of Chile. It is directly south of the Cordón de Puntas Negras volcanic chain. Part of the Central Volcanic Zone of the Andes, it is a group of seven stratovolcanoes and a caldera. It is about two million years old. The main summit of the volcano is a lava dome called Pico Laco, which is variously reported to be 5,325 metres (17,470 ft) or 5,472 metres (17,953 ft) high. The edifice has been affected by glaciation, and some reports indicate that it is still fumarolically active.
Farallon Negro is a volcano in the Catamarca province of Argentina. Active between about 9-8 million years ago, it was formerly a stratovolcano or a multi vent volcano. Eventually, erosion removed most of the volcano and exposed the underlying structure including subvolcanic intrusions.
The Azerbaijan is a country with very favorable natural conditions and rich natural resources. Snowy peaks, high mountains, foothill fertile soils, wide plains, Lowest Land Points Below Ocean Level are the main landscape forms of the republic. This complex landscape structure has resulted in a variety of natural conditions, climate, soil-vegetation, and water resources. This, in turn, led to the uneven distribution of population and farms on the territory, and the specialization of production on different types.
Hydrothermal mineral deposits are accumulations of valuable minerals which formed from hot waters circulating in Earth's crust through fractures. They eventually create metallic-rich fluids concentrated in a selected volume of rock, which become supersaturated and then precipitate ore minerals. In some occurrences, minerals can be extracted for a profit by mining. Discovery of mineral deposits consumes considerable time and resources and only about one in every one thousand prospects explored by companies are eventually developed into a mine. A mineral deposit is any geologically significant concentration of an economically useful rock or mineral present in a specified area. The presence of a known but unexploited mineral deposit implies a lack of evidence for profitable extraction.
The Chilean Iron Belt is a geological province rich in iron ore deposits in northern Chile. It extends as a north-south beld along the western part of the Chilean regions of Coquimbo and Atacama, chiefly between the cities of La Serena and Taltal. The belt follows much of the Atacama Fault System and is about 600 km long and 25 km broad.
Skirrow, R., 2004. Iron oxide Cu-Au deposits: An Australian perspective on their unifying characteristics. In: McPhie, J. and McGoldrick, P. (editors), 2004. Dynamic Earth: Past, Present and Future. Abstracts of the 17th Australian Geological Convention, Hobart, Tasmania. February 8–13, Geological Society of Australia, Abstracts No. 73, p.121
Porter, T. M. editor, Hydrothermal iron oxide copper-gold and related deposits: a global perspective, PGC Publishing a division of Porter GeoConsultancy (2002), 349 pages, ISBN0-9580574-0-0
Porter, T. M. editor, Hydrothermal iron oxide copper-gold and related deposits: a global perspective, Volume 2, PGC Publishing a division of Porter GeoConsultancy (2002) 377 pages, ISBN0-9580574-1-9
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