Revisiting the IOCG geology and age of Alemao in the Igarape Bahia camp, Carajas province, Brazil

The Fe oxide-Cu-Au (IOCG) mineralization at Alema tilde o in the Caraj acute accent as province of the Amazon craton occurs in a volcanosedimentary sequence, part of the Igarape acute accent Bahia Group, at the gradational contact between a lower, volcanic-dominated sequence and an upper, predominantly siliciclastic unit. The deposit is the concealed, hy-pogene root of the Acampamento Norte body of the larger Igarape acute accent 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 Alema tilde o contain bornite in addition to chalcopyrite, as well as molybdenite, a feature common to all the mineralized centers in the Igarape acute accent 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 Alema tilde o. Geologic relationships and similarities with the rest of the hypogene Cu-Au miner-alization in the area imply that this date is also valid for the entire Igarape acute accent Bahia camp. The age confirms the epigenetic nature of the mineralization, its emplacement during or following inversion of the Caraj acute accent as basin, and the protracted recurrence of IOCG mineralization throughout the Caraj acute accent as province. A genetic relationship be-tween the Cu-Au mineralization and the widespread district-scale mafic intrusive magmatism is speculated.