Natural attenuation of uranium and formation of autunite at the expense of apatite within an oxidizing environment, south Eastern Desert of Egypt

X-ray diffraction (XRD), back scattered electron imaging (BSE), wavelength-dispersion spectral scan (WDS), X-ray compositional mapping and quantitative electron probe micro analyses (EPMA) have been used to examine a natural attenuation of U during low temperature alteration of the Sela granite, south Eastern Desert of Egypt. The data confirmed that a pre-existing hydroxyapatite was transformed to autunite through an unidentified intermediate phase. The boundaries between these three phases are not sharp and are generally interfering indicative of the replacement of Ca by U. The hydroxyapatite, intermediate phase and autunite show similar chondrite normalized rare earth elements (REE) patterns suggesting a genetic relationship. Alteration processes have enriched the three phases with heavy rare earth elements (HREE) and Eu and caused Ce, Dy and Yb negative anomalies. Based on the pH of the aqueous solutions, two mechanisms may explain the conversion of hydroxyapatite to autunite: (1) the dissolution of hydroxyapatite and precipitation of autunite which would happen when the uranyl bearing solutions were acidic enough (pH = 3–6.8) to be able to dissolve the pre-existing hydroxyapatite and (2) sorption of the uranyl ion on the surface of hydroxyapatite followed by substitution of (UO₂)²⁺ at the expense of Ca²⁺. The latter mechanism would have happened if the pH of the aqueous solutions were near neutral and at low dissolved concentrations of uranyl ion. The genesis of uranyl mineralization in the Sela area supports the use of apatite-based technologies for U remediation in an oxidizing environment.