Abstract
El Atshan area, situated in the central Eastern Desert of Egypt, is a good location for studying the influence of high- and low-T alterations on the formation of Zr- and U-bearing minerals within a trachyte sill. These minerals are mainly represented by an unidentified secondary Zr-rich silicate mineral, betafite, and liandratite. The unidentified Zr-rich silicate mineral is considered to be an alteration product of the precursor zirconolite during high-T alteration. This is indicated by the changes in the composition marked by an increase in the amount of hydration (H2O), Si, and U and a decrease in the amount of Ca, Ti, Nb, and Fe. The morphology of this unidentified mineral is similar to that of zirconolite. For seven oxygen atoms, the calculated formula of the unidentified Zr-rich silicate mineral is (Si1.45U0.18Ca0.32Pb0.01Nb0.06Zr1.18Hf0.01Fe0.06Al0.08Ti0.11P0.09Y0.05REE0.09)(Sigma 3.7)O-7. With more extensive alteration, it was found that the unidentified secondary Zr-rich silicate mineral was unstable and it underwent re-equilibration with U-rich fluid, which led to alterations in betafite. The two possible mechanisms responsible for the alteration of the unidentified Zr-rich silicate mineral to betafite are as follows: (1) the dissolution of the unidentified Zr-rich silicate mineral and precipitation of betafite and (2) the ion-exchange between partially to fully amorphized zones and the U-rich fluid. Such alteration is indicated by a marked increase in the amount of U, Ti, and Nb and a decrease in the amount of Zr, Si, Ca, P, Y, and Sigma REE. The single substitution of U4+ <-> Zr4+ and the coupled substitution of (U4+ + Ti4+ + Nb5+) <-> (Zr4+ + Si4++ Ca2+ + P5+ + Y3+ + Sigma REE3+) appear to be the main causes for the formation of betafite. For 2.00 B-site cations, the calculated formula of the betafite is (A)(U0.44Ca0.25REE0.05Y0.03Pb0.02)(Sigma 0.79)(B)(Si0.79Zr0.69Ti0.23Nb0.12Al0.06Fe0.05P0.03V0.02Hf0.01)Sigma O-2.0(7). Betafite was altered to liandratite in the late low-T alteration stage. For 2.00 Nb-site cations, the calculated formula of the liandratite is (U)(U1.35Ca0.41Pb0.04REE0.04Y0.01)(Sigma 1.86)(Nb)(Ti(0.67)Si(0.46)Nb(0.40)Zr(0.26)e(0.09)V(0.08)Al(0.03)Ta(0.01))Sigma O-2.0(8). Among different uranium complexes, the influence of fluoride on the solubility and mobility of uranium is confirmed.