Genesis of chemical sediments in Birimian greenstone belts: evidence from gondites and related manganese-bearing rocks from Northern Ghana

Abstract

AbstractEarly Proterozoic chemical sediments of the Birimian Supergroup in northern Ghana host several types of metamorphosed manganese-bearing rocks. Differences in the mineralogy and geochemistry can be attributed to facies changes in a mixed volcanic-volcaniclastic depositional environment. Manganese oxide-bearing phyllite, which is enriched in transition metals (Cu, Ni, Co, Zn), formed on the flanks of submarine volcanic edifices above an oxidation boundary. Towards the deeper basin, manganese silicate-rich gondites occur. These consist either of spessartine + quartz + ilmenite, or of spessartine + quartz + Mn amphiboles ± rhodonite ± hyalophane ± Mn stilpnomelane ± ilmenite. The Mn amphiboles are identified as manganoan actinolite, tirodite, and dannemorite. Sulphides are widespread as premetamorphic inclusions in Mn garnet grains. In the basin centre, chlorite schist containing garnet with 50–60 mol.% spessartine represents the most distal manganese-bearing rock which is highly diluted by volcanogenic background sedimentation. The origin of Mn-rich rocks is explained by heat-driven seawater convection systems active in submarine volcanic centres that provided hydrothermal solutions. Precipitation of different minerals depended on the geochemical conditions prevailing on the seafloor. During the Eburnean event (around 2000 Ma), the chemical sediments were metamorphosed to upper greenschist or lower amphibolite facies. Mineral assemblages in the gondites point to metamorphic conditions in the range of 450–500°C at 2–3 kbar.