Iron oxide copper gold ore deposits (IOCG) are important and highly valuable concentrations of copper, gold and uranium ores hosted within iron oxide dominant gangue assemblages which share a common genetic origin.
These ore bodies range from around 10 million tonnes of contained ore, to 4,000 million tonnes or more, and have a grade of between 0.2% and 5% copper, with gold contents ranging from 0.1 to >3 grams per tonne (parts per million). 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.
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, though not named as IOCG, by discovery and study of the supergiant Olympic Dam copper-gold-uranium deposit, 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.
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.
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.
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 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 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.
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.
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.
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;
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.
Gawler Craton IOCG province, South Australia
Punta del Cobre IOCG province, Chile
Cloncurry district, Queensland, Australia:
Para State IOCG province, Brazil
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.