Genesis of secondary uranium minerals associated with jasperoid veins, El Erediya area, Eastern Desert, Egypt

Abstract

Uranium mineralization in the El Erediya area, Egyptian Eastern Desert, has been affected by both high temperature and low temperature fluids. Mineralization is structurally controlled and is associated with jasperoid veins that are hosted by a granitic pluton. This granite exhibits extensive alteration, including silicification, argillization, sericitization, chloritization, carbonatization, and hematization. The primary uranium mineral is pitchblende, whereas uranpyrochlore, uranophane, kasolite, and an unidentified hydrated uranium niobate mineral are the most abundant secondary uranium minerals. Uranpyrochlore and the unidentified hydrated uranium niobate mineral are interpreted as alteration products of petscheckite. The chemical formula of the uranpyrochlore based upon the Electron Probe Micro Analyzer (EPMA) is $$ ^{A} {\left( {{\text{U}}_{{1.07}} {\text{Ca}}_{{0.28}} {\text{Pb}}_{{0.03}} {\text{Na}}_{{0.21}} {\text{Mg}}_{{0.02}} } \right)}_{{\Sigma 1.6}} \;^{B} {\left( {{\text{Nb}}_{{0.57}} {\text{Si}}_{{0.62}} {\text{Zr}}_{{0.35}} {\text{P}}_{{0.20}} {\text{Fe}}_{{0.17}} {\text{Al}}_{{0.06}} {\text{Ti}}_{{0.03}} } \right)}_{{\Sigma 2}} $$ . It is characterized by a relatively high Zr content (average ZrO2 = 6.6 wt%). The average composition of the unidentified hydrated uranium niobate mineral is $$ ^{{\text{U}}} {\left( {{\text{U}}_{{1.89}} {\text{Ca}}_{{0.49}} {\text{Pb}}_{{0.13}} {\text{Na}}_{{0.06}} {\text{Mg}}_{{0.02}} } \right)}_{{\Sigma 2.59}} \;^{{{\text{Nb}}}} {\left( {{\text{Nb}}_{{1.31}} {\text{Fe}}_{{0.34}} {\text{Si}}_{{0.14}} {\text{P}}_{{0.10}} {\text{Ti}}_{{0.05}} {\text{Zr}}_{{0.03}} {\text{Al}}_{{0.03}} } \right)}_{{\Sigma 2.0}} $$ , where U and Nb represent the dominant cations in the U and Nb site, respectively. Uranophane is the dominant U6+ silicate phase in oxidized zones of the jasperoid veins. Kasolite is less abundant than uranophane and contains major U, Pb, and Si but only minor Ca, Fe, P, and Zr. A two-stage metallogenetic model is proposed for the alteration processes and uranium mineralization at El Erediya. The primary uranium minerals were formed during the first stage of the hydrothermal activity that formed jasperoid veins in El Eradiya granite (130–160 Ma). This stage is related to the Late Jurassic–Early Cretaceous phase of the final Pan-African tectono-thermal event in Egypt. After initial formation of El Erediya jasperoid veins, a late stage of hydrothermal alteration includes argillization, dissolution of iron-bearing sulfide minerals, formation of iron-oxy hydroxides, and corrosion of primary uranium minerals, resulting in enrichment of U, Ca, Pb, Zr, and Si. During this stage, petscheckite was altered to uranpyrochlore and oxy-petscheckite. Uranium was likely transported as uranyl carbonate and uranyl fluoride complexes. With change of temperature and pH, these complexes became unstable and combined with silica, calcium, and lead to form uranophane and kasolite. Finally, at a later stage of low-temperature supergene alteration, oxy-petscheckite was altered to an unidentified hydrated uranium niobate mineral by removal of Fe.