Rocks of the Birimian Supergroup of early Proterozoic age (Sm-Nd isochron age = 2166 ± 66 Ma) form part of the West African craton. Ghana contains the eastern portion of the Birimian terrane. Five parallel, evenly spaced, several-hundred-kilometre-long volcanic belts, consisting mainly of low-grade metamorphic, tholeiitic lavas (MORB chemistry), characterize the Birimian Supergroup of Ghana. The belts are separated by basins containing isoclinally folded dacitic volcaniclastics, wackes and argillitic sediments as well as granitoids. Lavas and sedimentary rocks were deposited contemporaneously as lateral facies equivalents. Within the sedimentary basins, various lithofacies are distinguished. In the transition zone between belt volcanics and basin sediments a chemical facies occurs. This is defined by cherts, manganiferous and carbon-rich sediments, Fe-Ca-Mg carbonates, and sulphide mineral disseminations. After their emplacement, Birimian lavas and sediments were folded in the course of the Eburnean tectono-thermal event, intruded by various types of granitoids, uplifted and eroded. The erosion products were deposited as sediments of the Tarkwaian Group in long, narrow intramontane grabens which formed due to rifting preferentially in the central portions of all five Birimian volcanic belts. Tarkwaian sediments were deformed by gravity tectonics. Two major types of Eburnean granitoids in Ghana are characterized as follows: basin granitoids (= Cape Coast type) occur as synorogenic foliated batholiths chiefly in the central portions of Birimian sedimentary basins; they are peraluminous and generally granodioritic in composition. Belt granitoids (= Dixcove type) mostly form late-orogenic unfoliated intrusions in volcanic belts. They possess a metaluminous character and commonly are of tonalitic composition. Bongo granitoids are of post-Tarkwaian age and K-rich. In contrast to these granitoid types which have strong mantle affinities, the Winneba type had an Archean sialic precursor. A model involving small-scale, equidimensional, parallel, and contemporaneously operating convection cells in the upper mantle which caused rifting of a highly attenuated protocrust as well as linear eruptions of tholeiitic magmas is proposed. Recent Sm-Nd, Pb-Pb and Rb-Sr data of volcanic and plutonic rocks are incorporated. Most Ghanaian Birimian gold occurrences and mines are concentrated in narrow 'corridors' of 10-15 km width in the transition zone between volcanic belts and sedimentary basins, as are the chemical facies and regionally extensive shear zones at the volcanics-sediment interface. Birimian gold in Ghana is present as two major types: (1) the disseminated sulphide type which is generally lithofacies controlled, i.e. controlled by chemical sediments, and to a lesser extent by selvages of gold-quartz veins; and (2) the quartz vein type which is exclusively structure controlled. Data on geochemical underground traverses across both types of gold mineralization are briefly summarized. © 1990.
