Character and genesis of Proterozoic shear zone-hosted gold deposits in Borborema Province, northeast Brazil

Gold mineralization in the Borborema Province of northeast Brazil is hosted by Proterozoic schists and gneisses, which were subjected first to amphibo lite-facies metamorphism and subsequently to retrogressive greenschist-faci es metamorphism. These units were later intruded by granitic rocks during t he Brasiliano Orogeny (750-500 m.y.). The mineralization occurs in veins th at are related to major shear zones. The vein mineralogy is dominated by qu artz and pyrite in association with Fe-Ti oxides, base-metal sulphides and rarer phases that contain Ag, Bi, Cd, Mo, Te and Se. Gold occurs as native Au, electrum and maldonite, as well as in solid solution in pyrite. The pre sence of tourmaline and K-feldspar in the veins suggests the influence of g ranites in the hydrothermal system. Fluid inclusion studies have revealed the presence of a dominant fluid phas e with a low salinity (ca 6 equiv. wt% NaCl) and variably enriched in CO2 ( up to 35 mole %). Temperatures of homogenization are in the range 250-350 d egrees C. The data are compatible with immiscibility of H2O-CO2-NaCl fluids , at a pressure of ca 2 kbar. Oxygen and carbon isotopic analysis of quartz and fluid inclusions indicates that at some localities meteoric waters wer e incorporated into a metamorphic or magmatic fluid-dominated hydrothermal system. Lead isotopic analysis of vein sulphides and host rocks supports th e proposal that a major phase of metamorphism and Ph mobilization occurred at ca 750 m.y. and that most of the mineralization occurred after this meta morphic event. The gold mineralization in the Borborema Province exhibits many features th at are similar to those of the classic Archaean lode-gold deposits. It is p roposed, however, that granite-related hydrothermal activity and the incurs ion of meteoric waters both played an important role in the genesis of thes e deposits.