Evolution of the Igarape Bahia Cu-Au deposit, Carajas province (Brazil) : early syngenetic chalcopyrite overprinted by IOCG mineralization

The Igarape Bahia IOCG Cu-Au deposit, located in the Carajas Domain, the northern part of the Carajas Province in the Amazon Craton, is one of the most economically important deposits in the province. The deposit is hosted in the metavolcanosedimentary Igarape Bahia Group and the metasedimentary Aguas Claras Formation. The Igarape Bahia Group encompasses a lower unit with metavolcanic rocks and metagabbros, and metasedimentary rocks (metarhythmites, epiclastic rocks, and banded iron formation) of the upper unit. Epidastic rocks are predominant in the Aguas Claras Formation. Basement xenoliths within the lower unit yielded an U-Pb zircon age of 2935 +/- 36 Ma, suggesting that a sialic crust was present prior to basin installation likely due to rifting. The U-Pb dating of detrital zircons yielded maximum deposition age at 2784 +/- 27 Ma for the upper unit, and 2763 +/- 32 Ma and 2774 +/- 19 Ma for the Aguas Claras Formation. Chalcopyrite nodules and layers are found within metarhythmites, concordant to primary structures, but without hydrothermal alteration halos and iron oxide. This chalcopyrite generation seems to have precipitated synchronously to the deposition of the Igarape Bahia Group. Chalcopyrite nodules and layers show delta(34)S(VCDT )values ranging from + 0.29 to +1.56 parts per thousand. These data indicate that most of its sulfur is likely derived from the metavolcanic rocks of the lower unit. Host rocks and chalcopyrite nodules and layers were overprinted by the IOCG mineralization. The latter formed extensive halos of hydrothermal alteration and was accompanied by ductile deformation and hydrothermal brecciation. These processes resulted in (tourmaline)-carbonate-magnetite, (tourmaline)-carbonate-chlorite and (tourmaline)-(biotite)-chlorite mylonites and breccias. Chalcopyrite from magnetite-rich zones (i.e. IOCG mineralization) displays delta S-34(VCDT) values from + 1.36 to + 5.35 parts per thousand. In addition to magmatic sulfur, seawater-derived sulfate may have been incorporated in sulfides via thermochemical sulfate-reduction reactions. Trace element geochemistry in distinct copper ores (i.e. nodules and layers versus magnetite-rich) also point to different origins of both styles of mineralization. The timing of the IOCG mineralization was constrained at 2559 +/- 34 Ma in the Alemao orebody. These data suggest that an IOCG-type metallogenetic event at ca. 2.5 Ga overprinted an older syngenetic-exhalative type copper mineralization. They also indicate that precipitation of early sulfide minerals within the Itacaiunas Supergroup may have created Cu-rich sequences that could have been remobilized, generating the broad group of the IOCG deposits at Carajas111CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP481969/2013-6; 308365/2014-3; 457689/2014-5não tem2016/13162-