U-series in Fe-U-rich fracture fillings from the oxidised cap of the “Mina Fe” uranium deposit (Spain): implications for processes in a radwaste repository

Within the framework of the ENRESA (Spain) natural analogue programme, the U-ore deposit of “Mina Fe” is being studied as a natural analogue of radioactive spent fuel behaviour. In this context, the knowledge of the role played by fracture minerals as scavengers of certain analogue elements, mainly U, and the establishment of the time scale of the rock-water interaction processes controlling the uptakes or losses of U in the system are two relevant objectives. Fracture-infill materials from the site have first been mineralogically characterised, then the upper part of the U-series determined in both bulk samples and U-rich leachates obtained by sequential leaching. Uranium-series of the bulk samples indicate that most of the fractures remained as closed systems in the last 1.6 Ma, while in other fractures water/rock interaction processes affecting the upper part of the U-series have been identified. These processes indicate recent or rapid U accumulation or losses (<102 ka), old 234U accumulation (> 102 ka) or 234U+230Th recoil gain. The apparently random distribution in depth of fractures where these processes occurred corroborates the different hydraulic behaviour of fractures, as a result of their varying degree of sealing. Uranium concentrations and 234U/238U ARs of the leachates obtained with Morganʹs solution and 6N HCl indicate that minerals dissolved with these reagents (U(IV/VI) oxyhydroxides and goethite+clays, respectively) are responsible for the retention of almost all of the U in the bulk samples. Furthermore, the 234U lost by the U minerals dissolved with Morganʹs solution seems to be fixed onto goethite–clay mixtures, the intersticial water being the vehicle for the isotopic transfer, which in turn is a recent or recent-past process.