Mineralogy and origin of surficial uranium deposits hosted in travertine and calcrete from central Jordan

Secondary uranium encrustations are hosted in thick travertine and calcrete deposits of Pleistocene-Recent age in central Jordan. The central Jordan varicolored marble and travertine are equivalent to the active metamorphic area in Maqarin, north Jordan. More than 100 samples were collected from the outcrops of the varicolored marble, travertine, calcrete, and the yellow uranium encrustations. The secondary yellow encrustations are mainly composed of uranyl vanadate complexes. Tyuyamunite Ca(UO2)(2) V25+O8 . 3(H2O)-strelkinite Na-2(UO2)(2)V2O8 6(H2O) solid solution series are the major components and their composition reflects changes in the Ca/Na ratio in solution. Potentially, new vanadium free calcium uranate phases (restricted to the varicolored marble) were identified with CaO:UO3 ratios different from the known mineral vorlanite (CaU6+)O-4. Carbon and oxygen isotope data from calcite in the varicolored marble are characterized by Rayleightype enrichment in light isotopes associated with release of C-13 and O-18 enriched CO2 by high temperature decarbonation during combustion of the bituminous marl. Stable isotope results from uranium hosted travertine and calcrete varieties exhibit a wide range in isotopic values, between decarbonated and normal sedimentary carbonate rocks. The depleted delta C-13 and delta O-18 values in the travertine are related to the kinetic reaction of atmospheric CO2 with hyperalkaline Ca(OH)(2) water. The gradual enrichment of delta C-13 and delta O-18 values in the calcrete towards equilibrium with the surrounding environment is related to continuous evaporation during seasonal dry periods. Uranium mineralization in central Jordan resulted from the interplay of tectonic, climatic, hydrologic, and depositional events. The large distribution of surficial uranium occurrences hosted in travertine and calcrete deposits is related to the artesian ascending groundwater that formed extensive lakes along NNW-SSE trending depressions. Fresh groundwater moved upward through the highly fractured phosphate, bituminous marl and varicolored marble to form unusual highly alkaline water (hydroxide-sulfate type) enriched with sensitive redox elements among which were U and V. (c) 2014 Elsevier Ltd. All rights reserved.