The early Proterozoic Birimian Supergroup of Ghana and some aspects of its associated gold mineralization

摘要: Abstract Rocks of the Birimian Supergroup early Proterozoic age (Sm-Nd isochron = 2166 ± 66 Ma) form part West African craton. Ghana contains eastern portion terrane. Five parallel, evenly spaced, several-hundred-kilometre-long volcanic belts, consisting mainly low-grade metamorphic, tholeiitic lavas (MORB chemistry), characterize Ghana. The belts are separated by basins containing isoclinally folded dacitic volcaniclastics, wackes and argillitic sediments as well granitoids. Lavas sedimentary rocks were deposited contemporaneously lateral facies equivalents. Within basins, various lithofacies distinguished. In transition zone between belt volcanics basin a chemical occurs. This is defined cherts, manganiferous carbon-rich sediments, Fe-Ca-Mg carbonates, sulphide mineral disseminations. After their emplacement, in course Eburnean tectono-thermal event, intruded types granitoids, uplifted eroded. erosion products Tarkwaian Group long, narrow intramontane grabens which formed due to rifting preferentially central portions all five belts. deformed gravity tectonics. Two major granitoids characterized follows: (= Cape Coast type) occur synorogenic foliated batholiths chiefly basins; they peraluminous generally granodioritic composition. Belt Dixcove mostly late-orogenic unfoliated intrusions They possess metaluminous character commonly tonalitic Bongo post-Tarkwaian K-rich. contrast these granitoid have strong mantle affinities, Winneba type had an Archean sialic precursor. A model involving small-scale, equidimensional, operating convection cells upper caused highly attenuated protocrust linear eruptions magmas proposed. Recent Sm-Nd, Pb-Pb Rb-Sr data plutonic incorporated. Most Ghanaian gold occurrences mines concentrated ‘corridors’ 10–15 km width regionally extensive shear zones at volcanics-sediment interface. present two types: (1) disseminated controlled, i.e. controlled lesser extent selvages gold-quartz veins; (2) quartz vein exclusively structure controlled. Data on geochemical underground traverses across both mineralization briefly summarized.