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

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 234U/238U 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 234U/238U pair are taking up 228Ra (228Th) from the waters, whereas samples with 234U/238U ≠ 1 generally function as a close system for the 228Ra(228Th)/232Th pair. This is consistent with the extremely open character of the site studied