Evolution of the Igarapé Bahia Cu-Au deposit, Carajás Province (Brazil) : early syngenetic chalcopyrite overprinted by IOCG mineralization.

Abstract

The Igarapé Bahia IOCG Cu–Au deposit, located in the Carajás Domain, the northern part of the Carajás Province in the Amazon Craton, is one of the most economically important deposits in the province. The deposit is hosted in the metavolcanosedimentary Igarapé Bahia Group and the metasedimentary Águas Claras Formation. The Igarapé 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. Epiclastic rocks are predominant in the Águas 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 Águas 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 Igarapé Bahia Group. Chalcopyrite nodules and layers show δ34SVCDT values ranging from +0.29 to +1.56‰. 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 δ34SVCDT values from +1.36 to +5.35‰. 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 Alemão 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 Itacaiúnas Supergroup may have created Cu-rich sequences that could have been remobilized, generating the broad group of the IOCG deposits at Carajás.