The link between hydrothermal nickel mineralization and an iron oxide-copper–gold (IOCG) system: Constraints based on mineral chemistry in the Jatobá deposit, Carajás Province

Abstract

The Jatobá deposit represents a specific portion of an Archean-Paleoproterozoic iron oxide–copper–gold system, notable due to its hydrothermal nickel-rich zones. Neoarchean mafic and acid volcanic rocks of the Itacaiúnas Supergroup host the mineralized zones. Pre-, syn- and late shearing hydrothermal alteration, associated with the Canaã shear zone development, was pervasive and intense. Early distal hydrothermal alteration encompasses potassic-iron-silica (quartz–biotite–magnetite–apatite), sodic (albite, scapolite), and sodic-calcic (ferro-pargasite) alteration. The main syn-tectonic hydrothermal alteration stage resulted in scapolite–hastingsite–biotite controlled by the mylonitic foliation. Syn-deformation massive magnetite bodies represent proximal envelopes of the mineralized zones. They were intercepted by actinolite-apatite-magnetite and potassic-iron (biotite–magnetite) alteration fronts. Late tectonic hydrothermal alteration comprises chlorite–(epidote–quartz–calcite), fibrous scapolite veins, and green biotite–scapolite–F-Cl-apatite veinlets. The copper–(nickel) mineralized zones in the Jatobá deposit were formed in four stages, coeval to ductile and ductile-brittle deformational events. The mineralization stage (I), spatially related to massive magnetite–(apatite–actinolite) bodies, has the highest nickel content. It is characterized by Ni-pyrrhotite, Ni-pyrite, and subordinately, Co-chalcopyrite, Ce-allanite, Co-pentlandite, and Ce-monazite. The mineralization style evolved from replacement fronts controlled by the mylonitic foliation to hydraulic breccia zones. The mineralization stage (II) was synchronous to the development of syn-tectonic potassic-iron alteration and Co-chalcopyrite (±Ni-pyrite ± Ni-pyrrhotite). The mineralization stage (III), controlled by ductile-brittle and brittle structures, enabled the formation of veins with typical open-space filling textures infilled by paler brown or green biotite, Co-chalcopyrite, and siegenite (±Co-pyrite, ±Co-magnetite, ± cassiterite). The late mineralization stage (IV), coeval to widespread chlorite alteration, formed the most important copper mineralization at Jatobá. It is represented by branching veinlets and breccias with chalcopyrite, Co-chalcopyrite, Co-pyrite, sphalerite, molybdenite, uraninite, monazite, W-bearing hematite, and rare earth carbonates (bastnäsite, coskrenite, and sahamalite). The high chlorine contents of halogen-rich minerals (e.g., scapolite, biotite, apatite, and amphibole) imply the participation of highly saline, supersaturated fluids in the system evolution. Metalliferous fluids were highly focused, and mineralized zones represent the hotter hydrothermal centers. The high V and Ti contents in magnetite and its coexistence with hercynite and ilmenite indicate temperatures above 600 °C. These conditions would be similar to that of magnetite formed in the transition from magmatic to deep magmatic-hydrothermal systems, including the IOA deposits, especially on the onset of the early nickel-enriched mineralization stage. Such conditions enabled nickel leaching from mafic and mafic-ultramafic rocks, its transport in Cl-rich hydrothermal fluids, and precipitation related to episodic decompression. Temperature decrease (<380 °C), indicated by chlorite geothermometers, favored the bulk of chalcopyrite precipitation in late mineralization stages. The Jatobá deposit has attributes comparable to those of relatively deep portions of IOCG mineral systems formed from the channeling of overpressured superheated deep-seated fluids of probable magmatic origin.