The genesis of uranium in manganese and phosphorite assemblages, Timna Basin, Israel

Abstract

AbstractUranium enrichments (up to 4000 ppm) occur in the manganese and phosphorite assemblages of the Lower Cambrian clastic marine sedimentary sequence, Timna Basin, Israel. Two types of mineralization assemblages can be defined. Sedimentary stratabound assemblages consist of uranium-enriched stratiform manganese and phosphatic laminae, diagenetic (type A) manganese nodules composed of pyrolusite and hollandite laminae and phosphorite lenses. Fission-track maps show that the uranium is homogeneously distributed within host manganese and phosphatic minerals of these assemblages. Epigenetic assemblages are mainly composed of manganese- and phosphorite-bearing veins and secondary (type B) manganese nodules with a coronadite dominant mineralogy. Uranium is depleted in these assemblages, relative to the sedimentary stratabound assemblages.The distribution of manganese and phosphorite assemblages has a marked bimodal character. Alternation between manganese and phosphatic laminae in the stratiform deposits reflects cycles of oxidizing and reducing conditions brought about by mixing and stratification of the waters in the Timna semi-closed depositional basins. Compaction of wet sediments led to remobilization and the formation of uranium-enriched manganese nodules at the aerated sediment–water interface, and uranium-enriched phosphorite lenses below the interface in reducing conditions. Epigenesis occurred through the passage of solution fronts which recrystallized the manganese and phosphatic minerals and remobilized metallic elements, particularly uranium which was leached away and is still being remobilized today.The mechanism of uranium uptake in manganese phases is shown most probably to involve adsorbtion of [(UO2)3. (OH)5]+ complexes on precipitating minerals. Uranium is enriched in both the pyrolusite and hollandite laminae of type A nodules, but is particularly concentrated in the former (4000–10000 ppm). Thermodynamic calculations of the relative stabilities of pyrolusite and hollandite suggest that the pH conditions of hollandite formation were close enough to the pH limit of efficient uranium adsorption to inhibit its uptake relative to pyrolusite.