B. Buil

Factors that control the geochemical evolution of hydrothermal systems of alkaline water in granites in Central Pyrenees (Spain)

La evolucion geoquimica de las soluciones de los sistemas hidrotermales de Caldas de Boi, Arties, Tredos y Les, situados en el Pirineo Central (Spain), muestra diferencias reflejadas en la concentracion de solidos disueltos y en la presencia de una situacion de equilibrio o desequilibrio en profundidad de las soluciones respecto a la albita. La datacion (14C) de las aguas pertenecientes a los distintos sistemas hidrotermales ha puesto de manifiesto que el tiempo de residencia no es el factor de control determinante de las diferencias existentes en el grado de evolucion geoquimica alcanzado por las mismas, teniendo que contemplarse otro tipo de factores relacionados con la cinetica de disolucion de los minerales, como responsables de estas diferencias. El analisis de la influencia de los principales factores que controlan la tasa de disolucion de los minerales y, mas en concreto, de los feldespatos, muestra como las diferencias de temperatura de las soluciones en profundidad son capaces de justificar el distinto grado de evolucion geoquimica, incluso en soluciones con tiempos de residencia muy similares.

Mineralogical Control of the REE Distribution in the Fracture Fillings of an Uranium-Ore (Caceres-Spain)

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.

Rare Earth Elements: A Tool for Understanding the Behaviour of Trivalent Actinides in the Geosphere

Rare earth element (REE) concentrations have been determined in groundwaters, granite and fracture fillings in a restored uranium mine. The granitoids normalized REE patterns of groundwaters show heavy rare earth elements (HREE)-enrichment and positive Eu anomalies. This suggests that the REE are fractionated during leaching from the source rocks by groundwaters. Preferential leaching of HREE would be consistent with the greater stability of their aqueous complexes compared to those of the light rare earth elements (LREE), together with the dissolution of certain fracture filling minerals, dissolution/alteration of phyllosilicates and colloidal transport. (authors)