M. Pelayo

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 ( and ) 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 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.

Mineralogical and geochemical evidence of the migration/retention processes of U and Th in fracture fillings from the El Berrocal granitic site (Spain)

Abstract The fracture fillings in the El Berrocal site are the result of several overlapping alteration processes of the granite since the formation of the fractures. They are essentially composed of quartz, feldspars, clay minerals, carbonates and minor pyrite and Fe oxyhydroxides. The U(IV) oxides occur only in some samples, whereas uranyl phosphates and silicates, sometimes with Th and REE, are present in almost all the samples. Adsorption and/or coprecipitation of U onto Fe oxyhydroxides were also observed. In relation to the granite, the fracture fillings are enriched in U by a factor ranging between 60 and 2, and in Th by a factor of 3. δ18O values in clay minerals, quartz and carbonates and δ13C in the latter indicate that quartz, illite and kaolinite were formed at a minimum temperature of 100 °C; the CO32−, of edaphic origin, was formed between 15 °C and at least 60 °C. Smectite and a second generation of kaolinite were formed at ambient temperature. Just under half of the bulk samples of the fracture fillings are in secular equilibrium with respect to U isotopes. The rest have at least one activity ratio less or larger than unity. The existence of uranyl minerals and the UTh enrichment factors observed indicate retention processes mainly by precipitation of both natural radionuclides. The different values of the 234 U 238 U activity ratio observed show that fixation or leaching of U may be currently in progress at various locations in the fracture system. However, some samples in equilibrium for the 234 U 238 U pair are taking up 228Ra (228Th) from the waters, whereas samples with 234 U 238 U ≠ 1 generally function as a close system for the 228 Ra ( 228 Th ) 232 Th pair. This is consistent with the extremely open character of the site studied.

Uranium isotopic distribution in the mineral phases of granitic fracture fillings by a sequential extraction procedure

In order to study the recent rock-water interaction processes in the E1 Berrocal site, a sequential leaching method has been applied to granitic fracture fillings to obtain the U isotopic distribution in the mineral phases of these samples. Based on the mineralogical composition of these materials, six dissolution steps have been chosen to extract U as exchangeable cation, from carbonates, amorphous Fe-oxyhydroxides, labile resistates and highly insoluble resistates. In this way, the processes involved in the rock-water interaction phenomena, mainly dissolution, precipitation, coprecipitation and adsorption can be distinguished and even approximately dated.

Argillization processes at the El Berrocal analogue granitic system (Spain): mineralogy, isotopic study and implications for the performance assessment of radwaste geological disposal

Abstract The El Berrocal granite/U-bearing quartz vein (UQV) system has been studied as a natural analogue of a high-level radioactive waste repository. The main objective was to understand the geochemical behaviour of natural nuclides under different physicochemical conditions. Within this framework, the argillization processes related to fracturing and formation of the uranium–quartz vein were studied from a mineralogical and isotopic standpoint in order to establish their temperatures of formation and thus complete the geothermal history of the system. For this purpose, δ 18 O values were determined for pure mineral from the unaltered granite and quartz from the uranium–quartz vein, as well as for mixture samples from the hydrothermally altered granite (sericitised granite) and clayey samples from fracture fillings, including the clayey walls of the uranium–quartz vein. The isotopic signature of quartz from the uranium–quartz vein and the monophasic nature of its fluid inclusions led us to conclude that the isotopic signature of water in equilibrium with quartz was approximately in the range from −8.3‰ to −5.7‰ V-SMOV, its temperature of formation being around 85–120 °C. The δ 18 O values of pure sericite from the hydrothermally altered granite, calculated by means of the oxygen fraction molar method, indicate that its temperature of formation, in equilibrium with the aforementioned waters, is also in the range from 70 °C to approximately 120 °C. Clays from fracture fillings and clayey walls of the uranium–quartz vein are usually mixtures, in different proportions, of illite, approximately formed between 70 and 125 °C; two generations of kaolinite formed at approximately 90–130 °C and at around 25 °C, respectively; smectite, formed at ≤25 °C; and occasionally palygorskite, formed either between 30 and 45 °C or 19 and 32 °C, depending on the fractionation equation used. These data suggest that sericite from the hydrothermally altered granite, quartz from the uranium–quartz vein, illite and the first generation of kaolinite from the fracture fillings resulted from the same hydrothermal process affecting the El Berrocal granite in relation to fracturing. Under certain physicochemical conditions ( T ≈100 °C, pH≈8 and log [H 4 SiO 4 ] between −4 and −3), illite and kaolinite can be paragenetic. As a result of weathering processes, smectite was formed from hydrothermal illite and inherited albite under alkaline weathering, while the second generation of kaolinite was formed from smectite, under acid conditions and close to the sulphide-rich uranium–quartz vein. Palygorskite is an occasional mineral formed probably either during the thermal tail of the above-described hydrothermal process or during weathering processes. In both cases, palygorskite must have formed from alkaline Si–Mg-rich solutions. Finally, these data and processes are discussed in terms of natural analogue processes, drawing some implications for the performance assessment of a deep geological radwaste repository (DGRR).

U and Th series disequilibrium in unaltered and hydrothermally-altered granites from the El Berrocal site (Spain): Weathering effects

Abstract Within the framework of analysis of the safety assessment of high level radioactive waste repositories, the migration of natural radionuclides in a fissured granitic environment, the El Berrocal site, is being studied in Spain. In order to determine the weathering effects on the natural radionuclides of the site, the top parts of the 238 U and 232 Th decay series have been measured in bulk samples of unaltered and differently altered granites by α-particle spectrometry. The results have established the recent geochemical processed involving 238 U, 234 U, 230 Th, 232 Th and 228 Th such as: 234 U recoil loss, bulk U assimilation or 234 U recoil gain, equilibrium states, bulk U leaching without fractionation, bulk uranium assimilation with activity ratio near unity, and loss or gain of 228 Ra ( 228 Th) as a function of the hydrochemical characteristics of the percolating water. These results have been interpreted in terms of the different degrees of weathering according to literature data.