Tourmaline B-isotopes fingerprint marine evaporites as the source of high-salinity ore fluids in iron oxide copper-gold deposits, Carajas Mineral Province (Brazil)

The Carajas 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 Igarape Bahia/Alemao and Salobo deposits). Tourmaline of schorldravite 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 (Igarape 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 (tip to 50 wt% NaCl equiv.) of ore fluids in these deposits. Values of delta B-11 front 14 parts per thousand to 26.5 parts per thousand for the Igarape Bahia and Salobo deposits confirm marine evaporite-derived brines in the ore fluids, whereas lower delta B-11 values for the 1garap6 Bahia deposit (5.8 parts per thousand to 8.8 parts per thousand) suggest that these fluids may have mixed with an isotopically different hydrothermall fluid, or one that had a mixed boron source. More variable and isotopically lighter boron in tourmaline front the Sossego deposit (-8 parts per thousand to 11 parts per thousand) 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 Carajiis Mineral Province was acquired by the interaction of hydrothermall 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.