Fragmental rocks of the Igarapé Bahia CU-AU deposit, Carajas Mineral Province, Brazil

Abstract

The Igarapé Bahia primary Cu-Au deposit in the Carajás Mineral Province, northern Brazil, is hosted by hydrothermally altered fragmental rocks that occur at the contact between the volcanic and the sedimentary units of the Igarapé Bahia Group. This group is considered as a low greenschist facies unit of the major Archean (~2.75 Ga) metavolcano-sedimentary Itacaiúnas Supergroup, which hosts the most important Cu-Au and iron ore deposits of the Carajás Province. The fragmental rocks constitute an extensive, approximately concordant layer 2 km long and 30-250 m thick. They are heterolithic, usually matrix-supported rocks, made up mainly of coarse, angular to rounded, basalt, BIF and chert clasts set in a matrix of fine magnetite, siderite, Fe-chlorite and disseminated to massive chalcopyrite. Amphibole, quartz, gold and a series of accessory minerals containing elements such as Co, Mo, W, F, P, U, Mn, Pb, Sn, Ag, B, Cl, and LREE are also present in the matrix. The footwall metavolcanic rocks to the fragmental unit are flow basalts, autobreccia and in situ hyaloclastite, lacking evidence of explosive volcanism. The hanging wall metasedimentary sequence contains mainly coarse and fine-grained metaturbidites showing abundant slump and disruption features indicative of a deep, tectonically active marine setting. The fragmental rocks were probably deposited after a period of stability and volcanic quiescence – as evidenced by the presence of BIF and chert clasts - and define a sudden extensional event related to syndepositional faulting that resulted in deepening of the basin. According to this model, the fragmental layer represents a debris flow deposit made up of collapsed material that accumulated in a submarine depression adjacent to the fault zone.