The Salobo and Igarapé Bahia iron oxide copper-gold (IOCG) deposits stand out as economically important deposits in the Carajás Province. Whereas granitoids and gneisses host the Neoarchean Salobo deposit, the Igarapé Bahia deposit occurs in volcanic rocks and associated sedimentary sequences. Paragenetic evolution of the IOCG alteration-mineralization evidences higher temperature conditions at Salobo (almandine-grunerite-biotite-magnetite) than at Igarapé Bahia (tourmaline-carbonate-magnetite-chlorite). At Salobo, iron enrichment at 565° ± 50°C was accompanied by hydrothermal fluids with magmatic or metamorphic compositions, as evidenced by grunerite (δ18OH2O = 7.20–8.50‰, δDH2O = −25.33 to −16.01‰), garnet (δ18OH2O = 7.10–9.70‰), and tourmaline (δ18OH2O = 5.07–7.37‰, δDH2O = −32.13 to +11.60‰). Fluids associated with potassic alteration at 565° ± 50°C also have a typical magmatic/metamorphic composition, indicated by biotite (δ18OH2O = 7.23–8.03‰, δDH2O = −40.94 to −25.94‰) and quartz (δ18OH2O = 7.52‰). Sulfur isotope signatures for chalcopyrite (0.81–1.28‰) and bornite (−0.37 to + 1.63‰) suggest a magmatic sulfur source at Salobo. For the Igarapé Bahia deposit, fluids associated with tourmaline display magmatic or metamorphic signatures (δ18OH2O = 5.07–7.37‰, δDH2O = −34.02 to −19.74‰, at 330° ± 50°C), but those associated with calcite (δ18OH2O = 1.68–3.10‰, 330° ± 50°C) and chlorite (δ18OH2O = 1.92–3.20‰, δDH2O = −57.36 to –21.34‰, 255° ± 50°C) evidence the input of 18O-depleted fluids. δ13CCO2 values (−9.32 to −4.93‰) for fluids in equilibrium with calcite also imply magmatic sources. Our data indicate that the main source of the ore-forming fluids for Salobo was derived from exsolved magmatic brines associated with crystallization of the coeval ca. 2.5 Ga granites. In contrast, though the early fluids at Igarapé Bahia were also magmatic-hydrothermal, the influx of 18O-depleted formation water was prominent during the later stages of ore genesis.