The clayey fissural fillings associated with N100º-110ºE fractures at the El Berrocal uranium mine (Sierra de Gredos, Spain): characterization, genesis and retention capacity of radioactive and other elements

Abstract

The clayey fissural fillings (< 60 μm and < 2 μm fractions) associated with the N100-110 ºE fractures, at the El Berrocal U mine, have been studied in relation to the natural radionuclide migration/retention processes in a fissured granitic environment. The fracture filled with pyrite, chalcopyrite, sphalerite, galena-bearing quartz, later mineralized by pyrite, pitchblende, carbonate and barite also belongs to that fracture seto According to the data obtained by X-ray diffraction, thermal and thermogravimetric analyses, EDX coupled to both scanning and transmission electron microscopes and cation exchange methods, the clayey fissural fillings are essentially composed of quartz, sericite, illite, Ca/Mg beidellite, K-feldspar, albite, apatite and occasional kaolinite. Traces of monazite, torbernite, autunite, other unidentified Ca, Y, Ce, La and REE phosphates, ferric sulfates, jarosite, gypsum, barite, calcite, Pb silicates, Fe-oxyhydroxides, gibbsite, Al gels and REE silicate complex gels have been also detected in the < 2 μm fraction. From a geochemical point ofview, the < 2 μm fractions, in relation to the < 60 μm fractions, are very positively anomalous in U, Th, Y, Cu, Zn, Sn, Ni, As, Ba, Ca and organic C. Furthermore, Cl is present, in important amounts, in almost all of the essential and accessory minerals and mineraloids in the < 2 μm fractions. These data suggest that the clayey materials analyzed were mainly produced by hydrothermal alteration of the granitic gouges during the N100-110 ºE fracturation phase. Based on the δ ¹⁸O data obtained from the quartz vein and the < 2 μm fractions, the argillitization process was probably produced by two phases. The first, of sericitic-illitic nature, was caused by the interaction between acid, KCI-rich hydrothermal solutions and the granitic fault gouges, at a temperature range between 70-120° C. The second, of smectitic nature, was probbly produced during ancient and/or present weathering processes by strong K leaching of the pre-existing sericite-illite, at room temperature. During the weathering phases, illite and probbly smectite were transformed to kaolinite in those fractures with sulfide mineralizations. Jarosite, ferric sulfates, gypsum, torbernite, autunite, gibbsite and Al gels were also probbly formed. The new formed accessory minerals and mineraloids play an important role, either by adsorption or precipitation, in the retention of the radioactive elements, U and Th, and other analogous elements, Y, Ce, La, REE, released during the hydrothermal alteration of the granitic fault gouges and weathering of the uraniferous mineralization. Th is mainly heId in inherited monazite. The U sequential leaching indicates that Fe oxyhydroxides and carbonates are important mineral phases for U retention. On the contrary, the clay minerals do not seem to play an important role in the U retention by adsorption, but they do work as a physico-chemical barrier for the UO₂⁺⁺ phosphate precipitation. The high organic C content in the clay fractions could be due to present biological activity in the clayey fissural filling or to organic acids transported from the topographic surface by the percolating water. In this last case, organie-clay complexes would probbly be formed.