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).

[1]  J. Parneix.  Effects of hydrothermal alteration on radioelement migration from a hypothetical disposal site for high level radioactive waste: example from the Auriat granite, France , 1992 .

[2]  S. P. Dutton,et al.  Cementation of a Pennsylvanian Deltaic Sandstone: Isotopic Data , 1978 .

[3]  S. Savin,et al.  Mineralogy and oxygen isotope geochemistry of the hydrothermally altered rocks of the Ohaki-Broadlands, New Zealand, geothermal area , 1973 .

[4]  G. Choppin,et al.  Current status of radioactive waste disposal , 1996 .

[5]  H. Taylor,et al.  Deuterium and oxygen-18 correlation: Clay minerals and hydroxides in Quaternary soils compared to meteoric waters , 1971 .

[6]  B. Raymahashay.  A geochemical study of rock alteration by hot springs in the Paint Pot Hill area, Yellowstone Park , 1968 .

[7]  A. Delgado,et al.  Carbonatation processes at the El Berrocal natural analogue / granitic system Spain : inferences from mineralogical and stable isotope studies , 1998 .

[8]  A. Delgado,et al.  Mineralogical and geochemical evidence of the migration/retention processes of U and Th in fracture fillings from the El Berrocal granitic site (Spain) , 1997 .

[9]  L. P. D. Villar,et al.  U and Th series disequilibrium in unaltered and hydrothermally-altered granites from the El Berrocal site (Spain): Weathering effects , 1996 .

[10]  K. Bell,et al.  K-Rb relationships in some continental alkalic rocks associated with the East African Rift Valley System , 1971 .

[11]  V. L. Parsegian,et al.  NUCLEAR SCIENCE AND TECHNOLOGY , 1971 .

[12]  B. Boizot,et al.  Radiation-induced defects in dickites from the El Berrocal granitic system (Spain): relation with past occurrence of natural radioelements , 2003 .

[13]  N. Chapman.  The geologist's dilemma: predicting the future behaviour of buried radioactive wastes , 1994 .

[14]  P. Holliger,et al.  Geochemical and Neutronic Characteristics of the Natural Fossil Fission Reactors at Oklo and Bangombé, Gabon , 1998 .

[15]  E. Reyes,et al.  Kaolinite and dickite formation during shale diagenesis: isotopic data , 1998 .

[16]  A. Delgado,et al.  Geochemistry of Spanish sepiolite-palygorskite deposits: Genetic considerations based on trace elements and isotopes☆☆☆ , 1994 .

[17]  R. Clayton,et al.  Oxygen isotope exchange between quartz and water , 1972 .

[18]  S. Savin,et al.  Isotopic studies of phyllosilicates , 1988 .

[19]  F. Gauthier-Lafaye,et al.  Composition isotopique de l'oxygène de palygorskites associées à des calcrètes : conditions de formation , 1993 .

[20]  R. Harmon,et al.  A note regarding CIF3 as an alternative to BrF5 for oxygen isotope analysis , 1982 .

[21]  Fred Karlsson,et al.  Natural analogue studies: present status and performance assessment implications , 1997 .

[22]  T. Vennemann,et al.  The rate and temperature of reaction of CIF3 with silicate minerals, and their relevance to oxygen isotope analysis , 1990 .

[23]  G. Brown,et al.  The X-Ray Identification And Crystal Structures Of Clay Minerals , 1961 .

[24]  Antonio Garralon,et al.  The uranium ore from Mina Fe (Salamanca, Spain) as a natural analogue of processes in a spent fuel repository , 2002 .

[25]  J. Smellie,et al.  Introduction and summary of the workshop , 1986 .

[26]  G. Whitney,et al.  Experimental investigation of the smectite to illite reaction; dual reaction mechanisms and oxygen-isotope systematics , 1988 .