Metallogenesis of the Paleoproterozoic Piaba orogenic gold deposit, São Luís cratonic fragment, Brazil

Piaba is an orogenic gold deposit (~3.5 Moz) of the São Luís cratonic fragment in north-northeastern Brazil. The deposit is epizonal-mesozonal and associated with the development of a subvertical strike-slip fault that cut across Rhyacian metavolcano-sedimentary rocks and granitoids. The metavolcano-sedimentary sequence comprises carbon-bearing schists, andesite/dacite, ultramafic rocks and felsic tuffs formed at 2240±5Ma to 2227 ± 33 Ma that were intruded at 2214±3Ma by fine-grained, subvolcanic granophyric granodiorite. Whole-rock geochemistry indicates that the hosting metavolcano-sedimentary sequence formed in a subduction-related (arc, back-arc?) setting, which has previously been interpreted as the early-arc stage of an accretionary orogen (2240-2214Ma) that was followed by voluminous subduction-related calc-alkaline arc magmatism (2168-2145Ma) and by a collisional phase (2100±15Ma) that produced several bodies of peraluminous granites. The hydrothermal alteration of the host rocks produced early, distal carbonization and hematitization and proximal, ore-related chlorite-sericite-carbonate-sulfide alteration. Two styles of mineralization, occurring in association, are present: (1) a network of thin quartz±sulfide veinlets and subordinate narrow breccia veins with refractory- to free-milling gold dissemination in hydrothermally-altered host rocks, and (2) subordinate low-sulfide Au-quartz veins with free-milling gold. Fluid inclusions and stable isotopes characterize the mineralizing solution as a relatively reduced (log fO₂=-30 to -35), neutral to slightly alkaline (pH5 to 6.2), low-salinity (average 5% NaCl), aqueous-carbonic (XCO₂=5 to 59mol%, XH₂O=40 to 93mol%, XN₂ and XCH₄ <1mol%) metamorphic fluid (δ¹⁸O_H2O=+5.5 to +8.9‰, δD_H2O=-8 to -69‰), and suggest that gold mineralization occurred at 250° to 330°C and 1.25 to 2.8kbar in response to phase separation and fluid-rock interactions that produced sulfidation and minor carbonatization. Sulfur (δ³⁴S=-2.7 to -3.8‰) is from an undetermined source, probably magmatic (oxidized) or sedimentary. Lead isotope compositions of gold, ore-related pyrite and K-feldspar from regional rocks suggest that collision-related granitoids of ca. 2100±15Ma are the most likely sources for Pb in the ore, although mantle and lower crustal sources are also observed. ⁴⁰Ar/³⁹Ar in hydrothermal sericite yielded a plateau age of 1980 ± 13 Ma, which is similar to the Pb-Pb model age of gold of 1959Ma. These are considered the minimum age for gold mineralization, whereas the maximum age of 2009±11Ma is given by a small gold-hosting intrusion in the same region.