Revisiting the IOCG geology and age of Alemão in the Igarapé Bahia camp, Carajás province, Brazil

Abstract

The Fe oxide–Cu–Au (IOCG) mineralization at Alemão in the Carajás province of the Amazon craton occurs in a volcanosedimentary sequence, part of the Igarapé Bahia Group, at the gradational contact between a lower, volcanic-dominated sequence and an upper, predominantly siliciclastic unit. The deposit is the concealed, hypogene root of the Acampamento Norte body of the larger Igarapé Bahia camp, in which chalcopyrite-dominant associations are intimately associated with steeply-dipping, massive to semi-massive magnetite bodies that are broadly subparallel to the host strata. Chlorite, amphibole, tourmaline, quartz, biotite, albite, K-feldspar, and carbonates constitute the main alteration products accompanying mineralization, although chlorite is dominant. The broadly tabular, massive and semi-massive bodies account for the highest-grade parts of the deposit and are surrounded by voluminous halos of intensely to moderately veined host rocks, with the same accompanying opaque and silicate assemblages that, although lower grade, contribute to the large-scale Cu–Au mineralization. Locally, the central lodes at Alemão contain bornite in addition to chalcopyrite, as well as molybdenite, a feature common to all the mineralized centers in the Igarapé Bahia camp.Molybdenite from a central body of massive magnetite–chalcopyrite, and demonstrably paragenetically related to the Cu-bearing sulfides, returned a Re–Os age of 2627 ± 11 Ma, which defines the timing of the IOCG mineralization at Alemão. Geologic relationships and similarities with the rest of the hypogene Cu–Au mineralization in the area imply that this date is also valid for the entire Igarapé Bahia camp. The age confirms the epigenetic nature of the mineralization, its emplacement during or following inversion of the Carajás basin, and the protracted recurrence of IOCG mineralization throughout the Carajás province. A genetic relationship between the Cu–Au mineralization and the widespread district-scale mafic intrusive magmatism is speculated.