Ore fluids of orogenic gold deposits of the Gurupi Belt, Brazil: a review of the physico-chemical properties, sources, and mechanisms of Au transport and deposition
Evandro L. Klein, 2014. "Ore fluids of orogenic gold deposits of the Gurupi Belt, Brazil: a review of the physico-chemical properties, sources, and mechanisms of Au transport and deposition", Gold-Transporting Hydrothermal Fluids in the Earth’s Crust, P. S. Garofalo, J. R. Ridley
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The Neoproterozoic Gurupi Belt in northern Brazil developed at the southwestern margin of the Palaeoproterozoic São Luís-West Africa Craton. Orogenic gold deposits of this belt are hosted in Palaeoproterozoic (2160–2147 Ma) metavolcano-sedimentary and calc-alkaline granitoid rocks formed in arc and/or back-arc settings during a protracted Rhyacian orogeny (2240–2080 Ma). These host rock assemblages were tectonically and isotopically reworked during the Neoproterozoic and represent the reworked margin of the craton, that is, the external domain of the Neoproterozoic (Brasiliano-Pan African) orogen. The location of the gold deposits is controlled by the Tentugal shear zone, which represents the tectonic boundary between craton and the Gurupi Belt, and its subsidiary structures. Gold occurs in veins and in association with pyrite, and subordinately arsenopyrite and chalcopyrite, in strongly altered and variable deformed host rocks. Geological characteristics, petrographic, fluid inclusion, and isotopic evidence indicate near-neutral, reduced aqueous-carbonic metamorphic fluids, with local contributions from host rocks at the deposit site. Ore deposition occurred at about 300–370 °C and up to 3 kbars in response to fluid immiscibility and fluid-rock reactions (sulphidation, desulphidation, carbonatization, CO2 removal) and local fluid mixing and oxidation.
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Gold-Transporting Hydrothermal Fluids in the Earth’s Crust
Hydrothermal ore deposits that are exploited for gold include both gold-only deposits, such as orogenic deposits, and gold-bearing examples of the common hydrothermal deposits types that are formed around upper-crustal magmatic centres, in particular porphyry and epithermal deposits. Fluid-inclusion data have shown that ore fluids of gold-only deposits are compositionally distinct compared to fluids of other deposit types. This Special Publication includes an up-to-date summary of thermodynamic parameters of aqueous Au species at high temperatures and pressures; a dataset of fluid inclusion properties and compositions from orogenic deposits of different parts of the world; several comprehensive case studies of different types of gold deposits and their fluids from USA, Brazil, Egypt, Slovakia and Bulgaria; and numerical modelling aimed to define key parameters that affect fluid flow and gold deposition at a range of scales.