Decoupling ore formation, ductile fabric and hydrothermal upgrading in the giant Salobo IOCG deposit, Carajás Province, Brazil

Eric Bernard Gonçalves Prado, Felipe Mattos Tavares , José Carlos Sicoli Seoane, Rudolph Allard Johannes Trouw, Julio Cezar Mendes, Junny Kyley Mastop Oliveira

The Salobo deposit is the largest Cu-Au deposit of the Carajás Mineral Province, northern Brazil, and is hosted by altered volcanosedimentary schists of the Itacaiúnas Supergroup along the Cinzento Lineament. Multiscale structural analysis, mineral chemistry and published geochronology define four structural families: Pre-Dn, Dn, Dn+1 and Dn+2. The Pre-Dn event comprises high-temperature K, K-Fe and Na-Ca hydrothermal alteration, Fe-rich clinopyroxene, and chalcopyrite-magnetite mineralization associated with Old Salobo-type quartz-feldspathic bodies and the ca. 2.57-2.55 Ga hydrothermal system. Fe-rich clinopyroxene supports the high-temperature hydrothermal character of this assemblage. Dn formed the penetrative NW-SE Sn schistosity, parallelizing and deforming pre-existing ore, with localized mylonitic top-to-SW transport along the southern wall-rock contact. Dn+1 produced oblique Sn+1 foliation and shear bands with top-to-NNW transport. Dn+2 comprises brittle, fluid-dominated structures with bornite, chalcocite and minor late chalcopyrite in undeformed chlorite-altered veins. Chlorite geothermometry, anchored on syn-Dn+2 vein chlorite, indicates ca. 250-290 °C for this event, and chlorite chemistry supports involvement of a felsic fluid component. Together with the spatial relation to Young Salobo granitic dykes, these data support correlation with the ca. 1.88 Ga granite-related system. Regional correlation favors linking Dn with the Transamazonian collision and Dn+1 with the Sereno event. Salobo is therefore interpreted as a Neoarchean IOCG system that was subsequently metamorphosed, structurally transposed, and locally upgraded during a Paleoproterozoic granite-related hydrothermal overprint.