Texture and Compositional Evidence for Epigenetic Alteration in Mn Carbonate Protore at the Palaeo-proterozoic Nsuta Manganese Deposit, Western Ghana”

Abstract

This communication is the first to report the presence of significant epigenetic alteration in protore manganese carbonate at the Palaeo-proterozoicNsuta manganese deposit in the Birimian of Ghana. The alteration is recognisable macroscopically by strong veining of quartz and/or pink rhodochrosite and, in some cases, in slightly sheared or deformed carbonate rock. Under the ore microscope, the alteration assemblage commonly consists of micro-granular rhodochrosite, occasionally coexisting with one or more of alabandite, mangano-cummingtonite, alleganyite and jacobsite. The compositions of carbonate minerals also demonstrably show an increase in the MnCO3 end-member in the altered compared to the least altered carbonate protore. These textural and compositional features sharply contrast with low regionally metamorphosed protore that shows remarkable preservation of primary sedimentary features including lamination and is, in addition, dominated by micro-concretionary to microcrystalline rhodochrosite ± kutnahorite ± mixed Ca-Mg carbonates ± spessartine garnet. The alteration is not only suggested to be post-metamorphic (low-metamorphosed) and pre-supergene but may also, contrary to the interpretations of some previous workers, have introduced unusual manganese-bearing minerals (e.g. alabandite and mangano-cummingtonite) as well as possibly enriched the protore carbonate.