The El Berrocal project: Geological characterization and radionuclide migration studies in a fractured granitic environment

El Berrocal is an abandoned uranium mine in a mineralized quartz vein hosted by a Hyercynian granite in central Spain. This mine is the focus of an international project to characterize and model natural elemental migration in a fractured-rock environment as an aid to understanding and predicting processes that may occur in a geological repository for radioactive wastes. Uranium in the mineralized quartz vein has been shown to have originated from the orthomagmatic uraninite in the granite with the elemental removal and migration occurring predominantly by hydrothermal fluids. Mobilization of uranium from the mineralized quartz vein and from granite adjacent to hydraulically-active fractures away from the vein occurred over the geologically-recent past and in the present-day. The most recent mobilization is evidenced by dissolution features seen in SEM photomicrographs; mineral growth and sorption signatures identified by enhanced uranium concentrations on the surfaces of preexisting minerals; and measured disequilibrium in the uranium series for whole rock close to fracture walls. Present-day groundwaters in the studied area are young meteoric waters. They are generally calcium-sulfate enriched, oxidizing and mildly acidic near the surface, becoming more bicarbonate-rich with near neutral pH in the deeper zones, except around the mineralized vein where the waters are acid (pH around 3) due to oxidation of the sulfide minerals. No deep, chemically-reducing groundwaters have yet been identified in the El Berrocal boreholes