The Carajás Mineral Province in northern Brazil contains a variety of world-class (>100 Mt ore) iron oxide copper-gold (IOCG) deposits, including the only Archean examples of this deposit class (e.g., the Igarapé Bahia/Alemão and Salobo deposits). Tourmaline of schorl-dravite composition, a common gangue mineral in these deposits, precipitated shortly prior to and after the ore assemblage. A boron isotope study of texturally different tourmaline from three IOCG deposits (Igarapé Bahia, Salobo, and Sossego) using secondary ion mass spectrometry (SIMS) provides new evidence in the long-standing debate of magmatic versus non-magmatic sources for the high salinity (up to 50 wt% NaCl equiv.) of ore fluids in these deposits. Values of δ11B from 14‰ to 26.5‰ for the Igarapé Bahia and Salobo deposits confirm marine evaporite–derived brines in the ore fluids, whereas lower δ11B values for the Igarapé Bahia deposit (5.8‰ to 8.8‰) suggest that these fluids may have mixed with an isotopically different hydrothermal fluid, or one that had a mixed boron source. More variable and isotopically lighter boron in tourmaline from the Sossego deposit (−8‰ to 11‰) is attributed to mixed sources, including light boron leached from felsic intrusive and volcanic host rocks, and heavy boron derived from marine evaporites. The boron isotope data indicate that the characteristic high salinity of the ore fluids in the Carajás Mineral Province was acquired by the interaction of hydrothermal fluids with marine evaporites. For IOCG deposits that contain tourmaline as a common gangue mineral, boron isotopes offer a valuable tool to constrain the high-salinity source problem, which is a critical issue in metallogenesis of IOCG deposits worldwide.