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Evidence of a Magmatic Fluid Source for Iron Oxide-Cu-Au Mineralisation
Peter J Pollard, School of Earth Sciences, James Cook University, Townsville, Australia

in - Porter, T.M. (Ed), 2000 - Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide, v. 1, pp 27-41.


   Fe-oxide-Cu-Au deposits typically formed in continental arc and intracratonic tectonic settings, predominantly in the Proterozoic, but also during the Phanerozoic. The lack of reported evaporitic sequences in several major Fe-oxide-Cu-Au districts including the Gawler Craton and Stuart Shelf, Tennant Creek, Great Bear Magmatic Zone, and Carajas districts suggests that the presence of evaporites is not a prerequisite for the formation of these deposits.
   Fluid inclusion studies of Fe-oxide-Cu-Au deposits typically indicate the presence of coexisting hypersaline and CO
2-rich fluid inclusions that may have originated by unmixing of an original H2O-CO2-salts fluid. At the Lightning Creek prospect in the Cloncurry district, these fluid types were generated during crystallization of granitic sills that are associated with a major magnetite-rich vein system. Stable isotope data from Lightning Creek and a number of Cu-Au deposits in the Cloncurry district and elsewhere are compatible with formation of these deposits principally from magmatic-hydrothermal fluids. Syn- to post-mineralization Na-Ca-rich fluids are present in many deposits, and may represent meteoric and/or connate fluids that mixed with the hypersaline magmatic fluids during or after mineralisation.
   Fe-oxide-Cu-Au deposits form part of the spectrum of intrusion-related Cu-Au deposits that also encompasses porphyry Cu-Au deposits. The intrusive rocks associated with both types of mineralization have several features in common, including a magnetite-series, high-K and mildly alkaline character and a range in composition from diorite to granite. In intracratonic environments, magmas are typically formed by high temperature partial melting of older igneous rocks triggered by intrusion of mantle-derived magmas into the crust. This promotes enrichment of the magmas in components such as U, F, Mo and REE, which may be enhanced by subsequent fractionation and ultimately transferred to the Fe-oxide-Cu-Au deposits that occur in these environments.
   The incorporation of components derived from host rocks and/or non-magmatic fluids is a common feature of Fe-oxide-Cu-Au and porphyry Cu-Au deposits. Elements such as Fe, K, and Cu can be mobilized during sodic-calcic alteration caused by magmatic and non-magmatic fluids and may be contributed to the deposits. However, Fe-oxide-Cu-Au deposits, like porphyry Cu-Au deposits, are formed dominantly via magmatic-hydrothermal processes and contain an imprint of the nature of the parental intrusions and magmatic-hydrothermal fluids in their alteration and metal enrichment characteristics.

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