The genetic significance of carbon and oxygen isotopic variations in Mn-bearing carbonates from the Palaeo-Proterozoic (∼2.2 GA) Nsuta deposit in the Birimian of Ghana

Abstract

Carbon and oxygen isotope analyses of manganese-bearing carbonates from the Palaeo-Proterozoic Nsuta manganese deposit, Ghana, suggest important relationships between isotopic composition of the Mn-bearing carbonates and lithology, locality and carbonate microfacies. Lithologically, δ13C and δ18O PDB values obtained from manganese carbonate ore exhibit greater variation than similar values from host Mn bearing phyllite. In terms of locality, quite a restricted range in isotope values is observed in carbonates from various parts of the deposit both stratigraphically and along strike. The greatest spread, however, occurs in samples from an economically important portion of the deposit where δ13C and δ 18O PDB vary from -1.5 to -20.9 and -10.9 to -18.2‰, respectively. With respect to carbonate microfacies, coarser grained carbonates vary greatly in isotopic distribution than carbonates of microcrystalline and microconcretionary texture. Mineralogically, carbonate rocks or ore consisting dominantly of rhodochrosite vary greatly in 13C and 18O than dolomite- and/or complex type carbonates such as kutnahorite, dolomite and solid solutions of magnesite-rhodochrosite. Together with detailed petrographic observations, isotope values obtained from fine-grained Mn carbonate ore and immediate host Mn bearing phyllite are interpreted to be primary, having possibly been only slightly affected by the pervasive but low-grade greenschist facies metamorphism that affected Birimian rocks. On the contrary, isotope values in coarse grained carbonates typical of restricted but economically the most important part of the deposit are interpreted to be secondary in origin, most likely the result of post-metamorphic (hydrothermal?) overprinting on primary carbonate signatures. These genetic interpretations seem to contradict, or are rather opposite to, interpretations of the carbon and oxygen isotopic composition of carbonates reported by earlier workers on the deposit.