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Anomalous uranium contents were recognized in the phosphatic beds of the Campanian-Maastrichtian Sudr Chalk of Wadi El-Quseiyib at the East Central Sinai. Microscopic examination,and scanning electron microscopy revealed the presence of carnotite patches disseminated in thestudied Sudr Chalk phosphatic rocks. This mineral is recorded for the first time in the east centralSinai and it may be significant in discussing uranium prospection and the origin of secondary uraniumdeposits. The identification of this mineral directed the attention to study the natural radionuclidescontent which have been measured by gamma-ray spectrometry using a shielded HPGe detector. Thisstudy revealed that 238U and some of its progenies were redistributed after subjection to severalalteration processes.Petrographic study of Wadi El-Quseiyib phosphatic rocks revealed that they are consisted mainlyof collophane pellets, bone and teeth fragments, which were diagenetically cemented by calcite,gypsum and quartz materials. The studied phosphatic microfacies was described texturally aspelphospharudite and pelphospharenite. X-ray diffraction analysis proved the presence of carbonatefluorapatite (francolite) as the dominant phosphatic mineral, while calcite, gypsum and quartz are thedominant non-phosphatic minerals with minor contents of bassanite and jarosite and siderotil.Averages of P2O5 and uranium contents are 16.7 % and 70 ppm in the analyzed phosphatic rocks,respectively. Carnotite characteristics are identified by SEM/EDX in the studied phosphatic rockswhich indicate that the uranium is leached out from the phosphatic materials and re-deposited as finespecks in open pores by circulating meteoric water. The leached uranyl ions together with vanadateions are forming the identified carnotite under the influence of evaporation. The calculated annualeffective dose equivalent values (AEDE) are 0.478 and 0.504 mSv/yr for the lower and upperphosphatic beds, respectively. The calculated radium equivalent activity values of (Raeq) are 2319 and2802 Bq/kg for the lower and upper phosphatic beds, respectively. Accordingly, although theestimated (AEDE) to the public is below the recommended limits, but the equivalent radium activityand the expected coincided radon gas emanation renders the phosphatic exposures in the study areaclassified unsafe for prolonged public activities. |
