Critical assessment of geochronological data from the Carajás Mineral Province, Brazil: Implications for metallogeny and tectonic evolution

Abstract

The Carajás Domain in the Amazonian Craton, Brazil, displays remarkable metallogenetic diversity, containing iron-oxide-copper–gold (IOCG), Cu-Au, Mn, Fe, Ni-Co, PGE-Cr and Au-PGE mineral systems. It consists of Mesoarchean (2.83–3.08 Ga) basement granitoids and gneisses overlain by the ca. 2.73–2.76 Ga rift related meta-volcanosedimentary units of the Neoarchean Carajás Basin. Coeval ca. 2.76 mafic–ultramafic complexes host nickel–cobalt laterite (Vermelho, Puma–Onça) and magmatic PGE-Cr (Luanga) deposits. Unconformably overlying this is the Águas Claras Formation, a package of fluvial to shallow marine sedimentary rocks of controversial but probable Archean age, based on 2.6–2.7 Ga cross cutting mafic dikes. This hosts manganese deposits (Azul), small polymetallic Cu-Au deposits (Breves) and the Serra Pelada Au-Pt-Pd deposit. IOCG deposits are located along WNW regional shear zones hosted within the Itacaiúnas Supergroup, Mesoarchean basement, and bimodal 2.76–2.73 Ga Neoarchean granitoids and gabbro.Geochronology supports the existence of multiple, superimposed and episodic hydrothermal and mineral systems. Multiple mineralization ages are common within single deposits and establishing the age of primary mineralization, as opposed to overprinting hydrothermal events, is challenging. Copper mineralization formed as early as ca. 2.76 Ga, coeval with basin formation, while structurally controlled IOCG deposits formed, or were substantially modified, during multiple hydrothermal episodes: ca. 2.72–2.68 Ga, 2.6–2.45 Ga and 1.88 Ga.For the northern IOCG deposits (e.g. Salobo and Igarapé Bahia/Alemão), a temporal overlap between 2.57 Ga mineralization and the 2.57 Ga alkaline Old Salobo Granite is commonly used to infer a genetic link. Yet this geochronology’s reliability is debatable; younger ages exist for the Old Salobo Granite (2.55–2.53 Ga), zircons are metamict, and older mineralization ages have been reported, suggesting a more complex history. The occurrence of ages ca. 2.60–2.45 Ga is spatially restricted to the north and coincides with an important, yet poorly understood, tectono-thermal event involving reactivation of the northern Cinzento Shear Zone and amphibolite facies metamorphism. Whether IOCG deposits formed during this tectonic event or represent substantially modified or metamorphosed older mineralization (or both), is contentious.Older mineralization ca. 2.71 Ga at the Igarapé Cinzento IOCG deposit in the north is coeval with the southern IOCG deposits, formed ca. 2.72–2.68 Ga (Sequeirinho, Bacaba and Bacuri). This does not convincingly overlap with the intrusion of widespread 2.76–2.73 Ga “syntectonic” granitoids so a genetic link with magmatic fluids is speculative and basinal brines may play a role. Due to controversy regarding the timing of compression and basin closure, the tectonic setting is debated; mineralization either formed during rifting or synchronous with basin inversion, but the latter is more supported by geochronology and ore textures.In the south, Paleoproterozoic ca. 1.88 Ga IOCGs (the Sossego-Curral orebodies of the Sossego deposit and Alvo 118), or alternatively overprinting Cu-Au systems, formed coeval with craton wide, A-type, 1.88 Ga granite magmatism. This magmatic-hydrothermal system is responsible for a large part of the metallogenetic diversity of Carajás Domain. Small (generally < 50Mt) Cu-Au deposits (e.g. Breves and Estrela) formed 1.88 Ga and show close spatial relationships to 1.88 Ga granites and alteration styles consistent with granitic influence (e.g. greisenization, enrichment in granitophile elements Bi-W-Sn). These granites are locally Sn-W mineralized in the Rio Maria Domain (e.g. Velho Guilherme Intrusive Suite). The enigmatic Serra Pelada hydrothermal Au-Pt-Pd deposit also formed ca.1.88 Ga and magmatic fluids have been implicated in the hypogene enrichment of the giant Serra Norte iron ore deposits.