Carbonatation processes at the El Berrocal natural analogue granitic system (Spain): inferences from mineralogical and stable isotope studies

The El Berrocal granite/U-bearing quartz vein system has been studied as a natural analogue of a high-level radioactive waste repository. The main objective is to understand the geochemical behaviour of natural radionuclides occurring under natural conditions. In this framework, the carbonatation processes have been studied from a mineralogical and isotopic (δ13C and δ18O) point of view, since carbonate anions are powerful complexing agents for U(VI) under both low-temperature hydrothermal and environmental conditions. The carbonatation processes in the system are identified by the presence of secondary ankerite, with minor calcite, scattered in the hydrothermally altered granite, and Mn calcite in fracture filling materials. The isotopic signatures of these carbonates lead us to conclude that ankerite and calcite from the former were formed at the end of the same hydrothermal process that altered the granite, at a temperature range of between 72° and 61°C for ankerite, and between 52° and 35°C for calcite. The effect of edaphic CO2 on both carbonates, greater on calcite than on ankerite, is demonstrated. Calcites from fracture fillings are, at least, binary mixtures, in different proportions, of hydrothermal calcite, formed between 25° and <100°C, and supergenic calcite, formed at ≤25°C. According to their δ13C signatures, the effect of edaphic CO2 in both calcites is also evident. It is assumed that: (i) hydrothermal calcite from fracture fillings and ankerite from the hydrothermally altered granite are the result of the same hydrothermal process, their chemical differences being due to the intensity of the water/rock interaction which was stronger in the altered granite than in the fractures; and (ii) all of these carbonatation processes are responsible for ancient and recent migration/retention of uranium observed in the hydrothermally altered granite and fracture fillings.