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Exploration for Iron Oxide Copper Gold Deposits in Zambia and Sweden; Comparison with the Australian Experience
Bruce Nisbet, John Cooke, Michael Richards and Craig Williams, Equinox Resources Limited, Australia.

in - Porter, T.M. (Ed), 2000 - Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide, v. 1, pp 297-308.


   The major Iron Oxide Copper-Gold (IOCG) deposits in Australia (Olympic Dam, Ernest Henry) are 'blind' deposits that were discovered under younger cover. Exploration for this style of mineralisation presents a new set of problems to the explorationist, and involves target definition applying criteria gleaned from work in known areas and extrapolating into new target areas.

Equinox applies a model for IOCG mineralisation principally derived from studies of known mineralisation in the Cloncurry region and the Stuart Shelf/Gawler Craton of South Australia. This model was initially applied in Australia, and was later extrapolated to Zambia in Central Africa and the Norrbotten region in Sweden.
   Key features of the model for IOCG mineralisation used by Equinox in the early-1990's were:
1.  IOCG deposits are essentially iron deposits with variable concentrations of Cu, Au, U, REE and other metals;
2.  The country/host rocks of significant deposits are typically feldspathic rocks such as felsic to intermediate volcanics, granitoids and psammites;
3.  Host rocks are strongly brecciated where mineralised and are commonly intensely altered by iron oxide-potassic alteration proximal to mineralisation and extensive sodic-calcsilicate alteration on a regional scale;
4.  Deposits are associated with radioactively anomalous A-type granitoids, typically fractionated and multi-stage, commonly in the roof zone or carapace of such granites;
5.  Structures active contemporaneously with granite emplacement provide the conduits for the flow of mineralising fluids, which focus into zones of structural dilation; and
6.  Fluids involved have a high magmatic component, and are generally oxidising and highly saline.

   IOCG mineralisation in Zambia is related to the Cambrian age Hook Granitoid suite, a late tectonic A-type suite associated with regional Fe-Cu-Co-U mineralisation. There is evidence that classic Copperbelt mineralisation may also be of this age and genesis. In Norrbotten, IOCG mineralisation is associated with alkalic to A-type granitoids of 1790 to 1895Ma age.
   Experience in these countries and Australia, combined with current field research, suggests IOCG mineralisation can range from Early Proterozoic to Mesozoic in age, with examples recognised worldwide. EquinoxÕs IOCG model has evolved over time, and modifications to the CompanyÕs earlier model for IOCG mineralisation are discussed.

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