| Abstract: | Elements and natural radionuclides in the contact zone of two granites with different ages would migrate from one to the other because of the difference in their chemical contents and later water-rock interactions. This migration could serve as an analogue for the near-field process of radwastes in a high-level radwaste deep geological disposal repository.In the contact between the Indosinian granite (whole-rock Rb-Sr isochron age at 214±3 Ma) and Hercynian granite (zircon U-Pb isochron age at 296±31 Ma) located in Ziyuan County, Guangxi, the O and Pb isotope characteristics and the activity ratios of 234U/238U, 230Th/238U, 230Th /234U and 226Ra/230Th show that, based on the whole-rock chemical contents, both of the two granites have maintained a relatively open chemical system in their evolution processes. However, as there is no obvious open fault, the migration of major elements, trace elements and natural U-series nuclides takes place within only 1- 2 m in the contact zone, and water-rock interactions in the weak contact plane only affect the redistribution of chemical constituents at a distance of 30 cm.Fractures are the pathways for fluid flow. In term of uranium-series disequilibrium and the radionuclide data (the abnormally high value of 234U/238U=4.60 in the fracture sample), significant water-rock interactions could cause the bordering rocks to be altered and leach some radionuclides out. U and Ra migrated selectively at the time scale of 106 a and 104 a, respectively, but the redistribution of granite is only within 10 cm from the contact, and Th seems to be immobile. On the other hand, clay-enriched fillings in the fractures can adsorb most of the leached radionuclides. The adsorption could significantly retard radionuclide migration in the fractures, but its influence in distance is less than 10 cm from the fracture plane. The present data could be viewed as evidence for safety confidence for the high-level radwaste disposal repositories hosted by granitic rocks in China. |
