Iron Oxide Cu-Au Systems
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% to 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, ore 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.
The Olympic Dam ore deposit lies within the outer margins of the Palaoeproterozoic Gawler Craton in northern South Australia.
Mineralisation is hosted by the Olympic Dam Breccia Complex (ODBC) which is developed within the Mesoproterozoic (1600-1585 Ma) Roxby Downs Granite. The Roxby Downs Granite is a pink to red coloured, undeformed, unmetamorphosed, coarse to medium grained, quartz-poor syenogranite.
The complex has a funnel-shaped, barren, hematite-quartz breccia "core" surrounded by an irregular array of variably mineralised and broadly zoned hematite-granite breccia bodies. The strike length of more hematite altered breccias within the complex is greater than 5 km in a NW-SE direction, and it is up to 3 km across and is known to extend to a depth of at least 1400 m.
The outer margins of the complex are diffuse, grading outwards from the "core" to heterolithic breccia, through granite breccia, to crackle breccia to fractured granite.
The highest grade mineralisation occurs where an up to 40 m thick, shallowly inward dipping, irregularly developed, chalcocite-bornite 'zone', cuts hematite rich breccia zones.
The principal copper-bearing minerals are chalcopyrite, bornite, chalcocite (djurleite-digenite). The main uranium mineral is uraninite (pitchblende), with lesser coffinite and brannerite. Minor gold and silver is intimately associated with the copper sulphides. The main REE-bearing mineral is bastnaesite. Copper ore minerals occur as disseminated grains, veinlets and fragments within the breccia zones. Massive ore is rare.
Total resources amounted to: 7738 Mt @ 0.87% Cu, 0.3 g/t Au, 1.61 g/t Ag, 0.29 kg/tonne U3O8. The separate non-sulphide gold resource 117 Mt @ 1.19 g/t Au