Trapping Processes of Uranium at Low Temperature

English Abstract:

There is a growing interest in uranium geochemistry and mineralogy at low temperature, in order to get an accurate picture of the processes that can act on the mobility of this important element. This may be relevant to the modeling of uranium migration in the environment, in the perspective of remediation strategies, as well as to increasing our knowledge on U-concentration under low-temperature conditions. We will review some recent results concerning the behavior of uranium in epithermal volcanic deposits of the Sierra Peña Blanca (Mexico) as well as in mine tailings from Gunnar (Saskatchewan) and in granitic waste rocks around a former U-mine from Massif Central (France). These examples illustrate the diversity of the trapping processes, encompassing (i) biological activity, indicated by a geochemical signature of biogenic reducing conditions favoring uranium mineralization in an epithermal deposit, (ii) metastable, nanoscale uranate crystals that enlarge the diversity of low-temperature U-phases, (iii) pH-sensitive inner-sphere surface complexes bound to ferrihydrite and associated clay minerals and (iv) precipitation of secondary U-phases, e.g. (nano-) uranyl phosphates, under subsurface conditions. Unique information on the timescale characteristic of uranium mobilization and migration/trapping is provided by environmentally relevant sites such as mine tailings and mine wastes.