Mn-carbonates in carbonate-shale succession in Palaeoproterozoic Lastoursville sub-basin, Gabon

Abstract

The aim of this study was to describe the petrography, mineralogical and geochemical composition of Mn-bearing carbonates in the 139 m long LST12 core succession from Palaeoproterozoic Francevillian Lastoursville sub-basin in Gabon. Using the bulk geochemical and mineralogical data combined with trace element enrichment and total organic carbon (TOC) trends, it was possible to interpret the changes in redox conditions during the deposition of Mn-rich carbonates and black shales of the LST12 core. The petrography, microstructure of the carbonate phases and distribution of major elements in different Units were described using optical and scanning electron microscope (SEM). Unit I-III are mainly composed of dolostones with occasional shaly dolomarl interlayers. Primary carbonate phase represented by stoichiometric dolomite with low TOC concentrations and minor trace element enrichment factors of these Units suggest shallow oxic depositional environment during the deposition of Unit I-III. Precipitation of thin Fe-Mn-rich rims on stoichiometric dolomite core formed due to the pore fluid saturation of Fe(Mn)-dolomite, possibly caused by the reductive dissolution of Fe-Mn-oxyhydroxides. Unit IV is composed of pyrite-rich black shales with interlayering of dolomarls, whereas Unit V of dolo-rhytmites where Mn-carbonates start to appear. Carbonates of these Units are characterized by dolomite cores with well-defined and wide rims, where the precipitation occurred at high carbonate saturation levels and in Fe-Mn-enriched fluid possibly changing in composition over the time supported by several distinct Fe-Mn-rich rims. These Units possibly formed at fluctuating redoxcline, where Mn-carbonates precipitated through diagenetic (microbial) reduction of Mn-oxyhydroxides. The composition and sedimentological characteristics suggest that pyrite-rich Unit VI shales formed in deeper reduced environment. Primary dominant carbonate phases in Unit VI are characterized by Mn-carbonate cores close to kutnohorite, that were probably precipitated in carbonate saturated anoxic sediment/mud enriched in dissolved Mn2+, with further porewater saturation changing towards Fe-Mn-dolomite, causing the precipitation of Fe-Mn rims on Mncarbonate nucleus. Microlaminae features observed in Unit VI black shales suggest bacterial processes during the deposition of Mn-carbonate rich lens-like interlayers. The Mn-carbonates of Unit VI co-occur with abundant pyrite framboids, possibly formed by microbial sulphate reduction. Values of high TOC concentrations and trace element enrichment factors reflect the alternation of euxinic setting and environment beneath perennial OMZ for Units IV and VI. The chemical and mineralogical composition of Mn carbonate minerals, sediment facies and presence of large framboidal pyrite aggregates in LST12 succession support the oxic model as a deepening basin, where the oxic surface water and deep anoxic water were separated by shallow redoxcline, influenced by variable influx of Mn2+.