Origin of red beds in the Paleoproterozoic Franceville Basin, Gabon, and implications for sandstone-hosted uranium mineralization
|Author(s)||Bankole Olabode M.1, El Albani Abderrazak1, Meunier Alain1, Rouxel Olivier2, Gauthier-Lafaye Francois3, Bekker Andrey4|
|Affiliation(s)||1 : Univ Poitiers, CNRS, UMR 7285, Hydrasa IC2MP, Batiment B35,5 Ave Albert Turpain, F-86073 Poitiers, France.
2 : IFREMER, Ctr Brest, F-29280 Plouzane, France.
3 : CNRS, UMR 7517, Lab Hydrol & Geochim Strasbourg, F-67084 Strasbourg, France.
4 : Univ Calif Riverside, Dept Earth Sci, Riverside, CA 92521 USA.
|Source||American Journal Of Science (0002-9599) (Amer Journal Science), 2016-11 , Vol. 316 , N. 9 , P. 839-872|
|WOS© Times Cited||20|
|Keyword(s)||Red beds, atmospheric oxygenation, diagenesis, iron isotopes, Franceville Basin, oldest sandstone-type U deposit|
|Abstract||Red beds are extensively used as evidence for an oxygenated atmosphere in the Paleoproterozoic. Red beds in the unmetamorphosed, ca. 2.15 Ga FA Formation of the Franceville Basin, Gabon were geochemically, petrographically, and mineralogically characterized to constrain the process of their formation and their relationship to atmospheric oxygenation in the early Paleoproterozoic. Petrographic observations indicate that ferric oxides are dispersed in clay filling intergranular pores and along platy cleavage in altered phyllosilicates. Grain-coating hematite is generally rare to absent in most samples suggesting hematite precipitation after sediment deposition and during burial diagenesis. Textures and geochemical data suggest that iron was likely sourced internally by alteration of iron-bearing minerals such as biotite and chlorite in sediments and redistributed in rocks during late diagenesis. Positive correlation between Fe/Mg ratio and δ56Fe values of bulk samples suggests mixing relationship with end members being authigenic hematite and iron-bearing silicates (biotite and chlorite). The lack of relationship between Fe3+/FeT ratios and iron isotope compositions suggest that the isotopically heavy iron oxide was already present in the sediments during early diagenesis, and was incorporated into green (reduced) facies that likely replaced red facies during diagenesis and burial. Large range of δ56Fe values extending towards positive values, up to +1 permil, is similar to that observed in Archean and Paleoproterozoic iron formations or modern groundwater-derived Fe-oxyhydroxides, suggesting partial oxidation of Fe under mildly oxidizing conditions during early diagenesis. In addition, positive correlation between Cr/Fe ratios and iron isotope values, especially in the fine-grained sandstones and mudstones, is evidence for authigenic Cr enrichment under locally mildly-reducing conditions in a fluvial setting and strongly oxidizing conditions during weathering and riverine transport. Uranium released from the lower, fluvial oxidized sandstones and added to the reduced sandstones and silty mudstones in the upper tidal-deltaic sediments potentially resulted in a uranium mineralization of a sandstone-type in the FA Formation of the Franceville Basin.|