From the roots to the roof: An integrated model for the Neoarchean Carajás IOCG System, Brazil

Abstract

The Carajás Mineral Province, located in northern Brazil, comprises the oldest iron-oxide copper–gold mineralizing system on Earth and is one of the world’s most significant cluster of large-tonnage IOCG deposits. Based on a critical assessment of the literature a new integrated model for the Carajás Neoarchean IOCG deposits. The model considers the hydrothermal alteration footprint and the rock textures, the geometry of the mineralization and the ore assemblage, and the isotopic and geochronological signature. The majority of the deposits form in the hypozonal domain. Pervasive and well-developed sodic (-magnesian) and calcic-iron hydrothermal alteration halos form through magmatic-derived hydrothermal fluids. The occurrence of hydrothermal orthopyroxene(-marialite) in the Ni-rich GT-34 and Salobo deposits extend further the P-T boundaries to temperatures of up to 700 °C and > 5 kbars, thus co is proposedrresponding to the root zone of the system. Epizonal deposits, conversely, are characterized by low-temperature potassic (biotite/phlogopite), propylitic and chloritic-hydrolytic hydrothermal assemblages, towards shallower crustal levels. Although a mantle contribution to the hydrothermal fluids is recognized, the O-B stable isotopic signature reveals the participation of surface-derived, basinal brines, in the ore-forming process. The occurrence of polymetallic ooids in the Alemão orebody (Igarapé-Bahia deposit), hosting identical hydrothermal alteration mineralogy to the chalcopyrite breccias of other IOCG deposits in the Carajás Mineral Province suggests that the mineralizing fluids may have reached the paleo-surface of the Carajás basin in the Neoarchean, representing the roof of the IOCG system. Conversely, the ore mineralogy is consistent throughout the vertical scheme. Magnetite is the only iron oxide formed and precipitate from iron-rich hydrothermal alteration accompanying the calcic alteration. Similarly, chalcopyrite represents the Cu sulfide. There is no transition to more oxidized assemblages towards shallower zones, supporting that the overall system was relatively reduced since more oxidizing phases (hematite, bornite) are not observed as part of the Neoarchean IOCG system. Thus, the Neoarchean Carajás IOCG System represents a crustal-scale continuum, of a vertical paleo-hydrothermal system formed coevally to the voluminous, bimodal magmatism and the formation of the world-class, banded iron formation hosted iron ore.