Petrography and geochemistry of intraclastic manganese-carbonates from the ∼2.2 Ga Nsuta deposit of Ghana: Significance for manganese sedimentation in the Palaeoproterozoic of West Africa
No Thumbnail Available
Date
2008-02
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Journal of African Earth Sciences
Abstract
Intraclastic Mn carbonate rocks occur in the marginal areas of the manganese-carbonate orebody (manganesestone) of the Palaeoproterozoic Nsuta deposit in the Birimian of Ghana. Macroscopically the intraclastic rocks display graded bedding and are typified by a matrix-supported fabric with subangular to subrounded particles less than a millimetre to ∼1.5 × 0.5 cm. Both clasts and matrix consist mainly of varying proportions of microcrystalline and microconcretionary carbonates, quartz, muscovite and subordinate pyrite. Within individual intraclasts, carbonate minerals (including distinctly zoned microconcretions) are essentially Mg kutnahorite and Mg-Ca rhodochrosite, similar to the carbonate minerals in the manganesestone. Whole rock chemistry of the intraclastic carbonates shows significant variability in the amounts of SiO2, Al2O3, MnO, MgO, CaO, Na2O and, to a lesser extent, K2O. Major element contents of the manganesestone similarly vary widely, except that these have, in particular, comparably higher MnO but less SiO2 and Al2O3 than the intraclastic carbonates and host rock Mn phyllite. Rare earth element (REE) concentrations in the intraclastic carbonates are approximately an order of magnitude higher than in the manganesestone. Whereas both rocks exhibit positive Eu anomalies, only the manganesestone shows a discernibly negative Ce anomaly. Petrographic and geochemical features suggest that the intraclasts are fragments of reworked Mn carbonate sediments derived from intraformational erosion and subsequent (mass flow) deposition as carbonate "turbidite" mud. Processes such as submarine slumping, sliding and other sediment gravity flows may have likely interrupted Mn sedimentation and transported partially consolidated manganiferous sediments down slopes into the early Birimian ocean. © 2007 Elsevier Ltd. All rights reserved.
Description
Keywords
Birimian, Intraclasts, Manganese deposits, Nsuta, Palaeoproterozoic, West African craton