Compositional variations in manganese carbonate micronodules from the Lower Proterozoic Nsuta deposit, Ghana: Product of authigenic precipitation or post-formational diagenesis?

Detailed petrographic and microprobe studies of micronodules identified in the manganese carbonate orebody and adjacent Mn-rich rocks at the ∼ 2.2 Ga Nsuta deposit, Ghana, indicate distinct lithologic control on nodule structure and chemical composition. Nodules in the Mn-rich rocks adjacent to the manganese carbonate orebody conform with the well-banded nature of the rock and are distinctly zoned into alternating bands compositionally dominated by Mg kutnahorite and Mg-Ca rhodochrosite. Individual nodules are partially or completely cemented by thin dark silica and carbonates of Ca-Mg±Mn solid solution. Nodules in the orebody, on the contrary, are generally massive, homogeneous, consist predominantly of rhodochrosite and are only partially cemented by dark silica. Based on the stratigraphic relationship of the nodule-bearing rocks to the manganese carbonate orebody and the variations in texture and composition observed in the nodules, it would appear that the Mn carbonate nodules precipitated diagenetically in response to changing pore water chemistry and/or metal (Mn and Ca) supply. The overall mineralogical and geochemical features indicate, in addition, that the nodules may have preserved original carbonate compositions, both metastable and stable, during burial diagenesis and low-grade regional metamorphism.