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THE IGARAPÉ BAHIA GOLD MINE, SERRA DOS CARAJÁS, PARÁ: GEOLOGY AND GEOCHEMISTRY OF THE PRIMARY Cu-Au SULFIDE ORE Chalcopyrite and pyrite are the primary sulfides in the southernmost part of the Bahia gold Mine. They occur either in veins or'disseminatedin all rock types. Gold was found solely associated with a few chalcopyrite crystals. The sulfide host rocks comprise conformable metasandstones and siltstones, metabasalts and a granophyric gabbro, whose contacts are delimitated by a mineralyzed hydraulic breccia. There is an intense hydrothermal alteration associated with the ore. Chlorite (Fe-chlinoclore) is the most important alteration mineral, occurring associated to albite, calcite, quartz and turmaline. Mass balance calculations on granophyric gabbro indicate that the hydrothermal alteration took place with small (2%) volume increase. Due to this, SiO 2 (+2.88 to +7.65%), MgO (+0.87 to +1.16%), U (+7.91 to +9.13%) were gained and CaO (-6.39 to -6.60%), Al 2 O 3 (-1.88 to -3.52%), K 2 O (-0.8%) and Na 2 O (-0.54 to -0.93%) were lost by the rocks . The sulfide host rocks are enriched in LREE, U, Kb, F and Cl, together with Cu, Au, Ag, Mo. They also show good correlation between Mo-Pb (0.913), Ag-Mo (0.794) and Ag-Pb (0.782), strongly suggesting that highly saline (up to 40% wt. eq. NaCl) crustal fluids, probably derived from granitic rocks, played an important role during mineralization. Similar influence was described in Salobo Cu (Au-Mo) and Pojuca Cu (Zn) deposits, that are correlated to hydrothermal systems related to the emplacement of Mesoproterozoic anorogenic granites. The hydrothermal system underwent decreasing temperature from 500 to 100°C, according to the homogenization temperatures (HT) determined in multiphasic fluid inclusions in quartz. The widespread chloritization occurred at 270°C, estimated by the chlorite geothermometer, as well as by the fluid inclusions data. Keywords: Hydrothermal alteration, fluid inclusions, granite-related fluids.RESUMO No extremo sul do corpo mineralizado da Mina de Au do Igarapé Bahia, a mineralização sulfetada primária, disseminada e venular, composta por calcopirita e pinta, apresenta o Au associado à calcopirita. As rochas hospedeiras da mineralização compreendem um pacote de meta arenitos e meta ritmitos, metabasaltos toleíticos e uma intrusão gábrica granofírica. Esta última, colocada entre as rochas metassedimentares e os metabasaltos, é balizada por brechas hidráulicas. A mineralização distribui-se tanto nas rochas sedimentares quanto ígneas, embora o sulfeto venular, mais importante, ocorra principalmente em zonas de uma brecha hidráulica. A mineralização está associada a uma intensa alteração hidrotermal. As assembléias minerais, predominantes em todos os tipos de rochas da área, consistem de clorita (Fe-clinocloro), quartzo, albita e carbonato (siderita e calcita). As rochas encaixantes da mineralização estão enriquecidas em ETRL, U, Rb, F, Cl, Cu, Au, Ag, Mo, e mostram boa correlação entre Mo-Pb-Ag (Mo-Pb = 0,913, Ag-Mo = 0,794 e Ag-Pb = 0,782)...