Occurrence of Carnotite in the Phosphatic horizon of the Sudr Chalk, Wadi El-Quseiyib, East Central Sinai, Egypt: Paleoenvironmental and Radioactivity Implications

Abstract

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 the studied Sudr Chalk phosphatic rocks. This mineral is recorded for the first time in the east central Sinai and it may be significant in discussing uranium prospection and the origin of secondary uranium deposits. The identification of this mineral directed the attention to study the natural radionuclides content which have been measured by gamma-ray spectrometry using a shielded HPGe detector. This study revealed that 238 U and some of its progenies were redistributed after subjection to several alteration processes. Petrographic study of Wadi El-Quseiyib phosphatic rocks revealed that they are consisted mainly of collophane pellets, bone and teeth fragments, which were diagenetically cemented by calcite, gypsum and quartz materials. The studied phosphatic microfacies was described texturally as pelphospharudite and pelphospharenite. X-ray diffraction analysis proved the presence of carbonate fluorapatite (francolite) as the dominant phosphatic mineral, while calcite, gypsum and quartz are the dominant non-phosphatic minerals with minor contents of bassanite and jarosite and siderotil. Averages of P 2 O 5 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 rocks which indicate that the uranium is leached out from the phosphatic materials and re-deposited as fine specks in open pores by circulating meteoric water. The leached uranyl ions together with vanadate ions are forming the identified carnotite under the influence of evaporation. The calculated annual effective dose equivalent values (AEDE) are 0.478 and 0.504 mSv/yr for the lower and upper phosphatic beds, respectively. The calculated radium equivalent activity values of (Ra eq) are 2319 and 2802 Bq/kg for the lower and upper phosphatic beds, respectively. Accordingly, although the estimated (AEDE) to the public is below the recommended limits, but the equivalent radium activity and the expected coincided radon gas emanation renders the phosphatic exposures in the study area classified unsafe for prolonged public activities.