Magmatic-Hydrothermal Fluids Leaching Older Seafloor Exhalative Rocks To Form The Iocg Deposits Of The Carajas Province, Brazil: Evidence From Boron Isotopes

The notable group of iron oxide-copper-gold (IOCG) deposits at the Carajas Province (Amazonian Craton, Brazil) contain remarkable copper reserves due to a complex Neoarchean metallogenetic evolution. The overlay of diachronic hydrothermal-mineralizing events is revealed by two clusters of boron isotope compositions of hydrothermal tourmaline obtained in IOCG deposits (Salobo, Igarape Bahia, GT-46, Grota Funda and Furnas) with (i) high delta B-11 (+8.2 to +17.2 parts per thousand) and (ii) low delta B-11 (+3.4 to +9.8 parts per thousand) values. The isotopically heavy boron signatures, recorded in tourmaline cores, are inherited from the volcano-sedimentary sequences of the ca. 2.76 Ga Itacaiunas Supergroup, which includes exhalative tourmaline and spilites formed in extensive exhalative systems prior to the IOCG formation. During the main IOCG event at ca. 2.55 Ga, the regional circulation of magmatic-hydrothermal boron-rich fluids originated tourmaline rims and new crystals with lower d11B values. We describe the critical role of an older volcano-sedimentary as a source for boron and likely metals leached by magmatic-hydrothermal fluids to form the IOCG deposits of the Carajas Province.