Evolution of brines and CO2-rich fluids and hydrothermal overprinting in the genesis of the Borrachudo copper deposit, Carajás Province

Abstract

The Carajás Province is one of the world’s major domains of iron oxide-copper-gold (IOCG) deposits, which formed during the Neoarchean (ca. 2.70–2.68 Ga and ca. 2.57 Ga). The province also hosts other copper-gold deposits of Paleoproterozoic age (ca. 1.88 Ga). The Borrachudo copper deposit is located about 6 km southwest of the world-class Cristalino IOCG deposit. Porphyritic rhyolite and amygdaloidal andesite of the ca. 2.76–2.74 Ga Grão Pará Group, formed in a volcano-plutonic event during the Neoarchean, are the main host to the Borrachudo deposit. Copper mineralization was related to two distinct hydrothermal events. The first, controlled by the development of ductile structures, was likely linked to the Carajás ca. 2.70 Ga IOCG system. It evolved from sodic alteration, silicification, and iron metasomatism through to calcic and potassic alteration. Chalcopyrite strings along foliation, breccias, and massive ore represent early copper mineralization. During the early hydrothermal event, aqueous (165° to >400 °C; 0.23 to >38.32 wt% NaCl equiv.), aqueous-carbonic and carbonic fluid inclusions (TmCO2 = −60.1 to −59.4 °C; CO2 density of 0.82 to 0.97 g/cm3) were trapped in quartz formed during the silicification. Immiscibility of a magmatic fluid or heterogeneous trapping of brines and CO2-bearing fluids, followed by mixing with diluted colder fluids, are suggested. The late event, controlled by brittle structures, records recurrence of hydrothermal alteration stages, such as silicification, sodic alteration, and potassic alteration, as well as chlorite and carbonate alteration. The copper mineralization is represented by chalcopyrite in branched breccia pipes and veins with minor magnetite, pyrite, siegenite, cobaltite, galena, anglesite, uraninite, and thorite. Titanite in veins of albite-chalcopyrite-apatite yielded an LA-ICP-MS U-Pb titanite age of 2011 ± 6.8 Ma. During the late hydrothermal event, aqueous fluids (140 to >320 °C; 6.7 to 40 wt% NaCl equiv.) reflected a more advanced mixing process involving diluted external fluids. The Borrachudo deposit points to the recurrence of multiple hydrothermal events responsible for the copper metallogenesis, which were intrinsically controlled by the magmatic and tectonic evolution of the Carajás Province.