Multistage evolution of the Neoarchean (ca. 2.7 ga) Igarapé Cinzento (GT-46) iron oxide copper-gold deposit, Cinzento shear zone, Carajás province, Brazil.

Abstract

The Igarapé Cinzento (GT-46) iron oxide copper-gold deposit is located in the northwestern part of the Cinzento shear zone in Carajás province, Brazil. Copper mineralization in the deposit is hosted by a hydrothermally altered metavolcanosedimentary sequence that consists of interlayered amphibolites, (actinolite)-biotite schists, almandine-biotite schists, and metamorphosed banded iron formations, intruded by granitoids (tonalite to granite) and mafic dikes. The complex evolution of the deposit involved (1) deposition of a volcanosedimentary sequence (2774 ± 19 Ma U-Pb zircon), (2) a tectonometamorphic event (Dn = upper amphibolite facies), restricted in time between 2.77 and 2.72 Ga, (3) development of the Cinzento shear zone (Dn + 1) at 2.72 Ga, with associated hydrothermal activity and deposit formation, (4) superimposed hydrothermal events (ca. 2.6 and 2.5 Ga), and (5) granitoid emplacement at ca 2.5 Ga in the region. The GT-46 deposit is the result of overprinting hydrothermal stages with at least two distinct mineralization events. Variations in alteration styles are a consequence of changes in the mechanisms of fluid control, first regulated by ductile deformation and later by brittle fracturing. Early hydrothermal alteration is characterized by zones of sodic-calcic and potassic alteration as well as iron metasomatism with associated disseminated mineralization I, controlled by ductile structures (chalcopyrite-bornite-magnetite, 2718 ± 56 Ma, Re-Os molybdenite). Veins and breccias with chalcopyrite-magnetite(-chlorite-calcite-quartz) represent an overprinting episode of mineralization (II) at ca. 2.6 Ga. Late alteration stages (ca. 2.5 Ga) represent fluid remobilization with chloritization zones and crystal growth spatially related to pegmatite intrusions. Re-Os molybdenite ages of 2503 ± 51 and 2449 ± 44 Ma provide evidence for younger episodes of hydrothermal fluid circulation in the deposit. Fluid inclusion study reveals aqueous-saturated, highly saline (32–>50 wt % NaCl equiv, total homogenization temperature [Tht] = 203°–>450°C) and carbonic fluids (CO2 melting temperature [TmCO2] = –56.3° to –54.5°C) for early alteration zones and saline (2.1–27.5 wt % NaCl equiv, Tht = 153.3°–215.5°C) solutions for late alteration zones. These data provide evidence for several alternative fluids involved in the evolution of the deposit: (1) hypersaline fluids derived from hydrothermal-magmatic brines or evaporative brines/evaporite-derived fluids, (2) saline solutions generated by progressive dilution of primary hypersaline fluids with shallower, colder, and less saline fluids (e.g., meteoric, seawater), and (3) carbonic components sourced from magmatic fluids, devolatilization of carbonate units and, partially, metamorphic fluids. Sulfur is interpreted to have been leached from country rocks in the earlier hydrothermal stages (mineralization I, δ34S = 0.01–1.99‰), and to have come from mixed granitic and external oxidized sources for later overprinting events (mineralization II, δ34S = 9.75–11.25‰). These data provide evidence for the recurrence of hydrothermal activity, indicating the existence of different potential sources of metals and sulfur during the evolution of the deposit.