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 severa l 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 sa mples, Adsorption and/or coprecipitation of U onto Fe oxyhydroxides we re also observed, In relation to the granite, the fracture fillings ar e enriched in U by a factor ranging between 60 and 2, and in Th by a f actor of 3. delta(18)O values in clay minerals, quartz and carbonates and delta(13)C in the latter indicate that quartz, illite and kaolinit e were formed at a minimum temperature of 100 degrees C; the CO32-, of edaphic origin, was formed between 15 degrees C and at least 60 degre es C. Smectite and a second generation of kaolinite were formed at amb ient temperature. Just under half of the bulk samples of the fracture fillings are in secular equilibrium with respect to U isotopes. The re st have at least one activity ratio less or larger than unity. The exi stence of uranyl minerals and the U-Th enrichment factors observed ind icate retention processes mainly by precipitation of both natural radi onuclides. The different values of the U-234/U-238 activity ratio obse rved 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 U-234/U-238 pair are taking up Ra-228 (Th-228) fro m the waters, whereas samples with U-234/U-238 not equal 1 generally f unction as a close system for the Ra-228(Th-228)/Th-232 pair. This is consistent with the extremely open character of the site studied. (C) 1997 Elsevier Science B.V.