Hydrothermal alteration systems as analogues of nuclear waste repositories in granitic rocks. An example: the Langenberg hydrothermal system

The results of this partial study on element distribution of a hydrothermally altered granite in the Langenberg site lead to conclude on: - probable mobility for most part of major elements except SiO2 and Al2O3; - a pronounced mobility for light REE (La, Ce) ; - a weak mobility for U ; - no or weak mobility for heavy REE. Comparison with chemical analogues for long-lived nuclides present in high-level radioactive waste are discussed. Element mobilities are strongly controlled by intensive chemical parameters, e.g., pH or fCO2, that destroy element-bearing minerals or allow crystallization of scavenger minerals. According to the now admitted temperature for radwaste disposal of about 1000C for a far field, and data on stability of alteration paragenesis, it is possible to deduce that the alteration grade reached in granite, by induced hydrothermal system, will be clay facies. (author)