Role of argillic alteration in uranophane precipitation along shear zones of the Gattar granites, Eastern Desert, Egypt

Abstract EN

To study uranophane precipitation in the shear zones of the Gattar granites, 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) were performed on uranophane, kaolinite and illite separated from these zones.

Secondary uranium mineralization is controlled by shear zones in which the degree of fluid-rock interaction was very high and argillic alteration is abundant.

The argillic alteration, represented mainly by kaolinite and illite, played an important role in uranophane precipitation within the altered parts of the Gattar granites.

Presence of calcite as void-filling in association with uranophane within the shear zones of Gattar granites may indicate that uranium was probably transported as uranyl-carbonate complexes.

The sorption of U(VI) in the Gattar clay minerals should be low due to the presence of dissolved carbonate ions and to the slightly alkaline conditions under which U(VI) occurs as the weakly sorbed UO2(CO3)3-4 U could be removed from solution through dissolution of feldspar, formation of clay minerals and subsequent precipitation of uranophane.

Dissolution of the feldspars resulted in a diffusional gradient of Ca and SiO4 ions away from its surface.

Formation of uranophane would likely form near the clay minerals surface, where there was increased probability of successful interaction between U and both Ca and SiO4.

As the uranium loading increases, surface-precipitation of uranophane on clay mineral surfaces can occur.