The behavior of actinides in a deeply buried radioactive waste cannot be predicted from evidence of their movement in geological environments in the geological past because these elements do not occur naturally in appreciable concentration. A useful means of reducing the uncertainty is to observe the behavior, both past and present, of chemical analogues in geological environment. In this sense, the rare earth elements (REE) have been used to predict the behaviour of actinide series elements in water-rock systems on account of the similar valence and ionic radii and high similarity in its electronic structure.

This paper describes the factors that control REE signature in the fracture fillings of an ancient uranium mine (dug granite) located in the Central-Iberian Zone of the Hesperian Massif (Caceres, Spain).

The study of REE distribution in the fracture fillings provides interesting information about the REE distribution in the hydrothermal fluids that flow through the system. In order to obtain this information, it is necessary to evaluate the control of different minerals in the total REE content of the fracture fillings.

The fracture fillings display a large mineralogical variety dominated by K-feldspars, quartz, plagioclase, micas, monazites, xenotimes, apatites, uraninites, inherited of the granites, and carbonates, quartz, apatites, phyllosilicates, sulphides and Fe-oxihydroxydes, neoformed in successive hydrothermal stages.

In general, the fracture fillings present different kind of REE patterns, just as positive or negative Eu anomalies. The REE final distribution in the fracture fillings is controlled by the presence of REE-minerals, both inherited and neoformed, and of carbonates, with great capacity to fractionate HREE in the system. On the other hand, the chemical composition of the carbonates and accessory minerals is responsible, as a last resort, for the Eu anomalie (positive or negative) observed in the fracture fillings.