Iron oxide copper gold ore deposits

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.

Some examples include the Olympic Dam, South Australia and La 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, 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.

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