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.
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 1.8Ga to 15 Ma, however the majority are within the 1.6Ga to 850Ma range.
Read more about this topic: Iron Oxide Copper Gold Ore Deposits
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