Origen, transporte y deposición del uranio en los yacimientos en pizarras de la provincia de Salamanca

Abstract

The numerous U deposits occurring in the schist-graywacke complex (CEG) of the Iberian Peninsula, characterized by the mineral association carbonates, pitchblende (coffinite), adularia and Fe sulphides, have both a considerable economic importance and a high metallogenic interest as their origin has not been convincingly explained yet. In fact, since 1959,these mineralizations have been successively attributed lo the concentration of U in fractured and brecciated zones of the schists due to one of these processes: magmatic, by transportation of U in hydrotermal fluids related to the evolution and emplacement of the Hercynian granites; supergenic , by the release of U from the Hercynian granites during the weathering and erosion processes which gave place to the Pliocene peneplain; segregation, by leachin, g of U from plutonic rocks under the effects of late- and /or post-Hercynian tectonic movements; and diffusion, by concentration of U from fertile metasediments by thermal diffusion or hydrothermal flow.
In this paper, taking into account field and laboratory studies carried out recently in the FE mine, so far the most important Spanish U deposit of this kind, the above mentioned hypothesis are discussed and the main metallogenic features of the orebody are given, Among these, the most significant are: the high geochemical U content, 30 to 200 ppm, of the CEG carbonaceous slates prevalent in the area; the nature and alteration processes, chloritization and hematitization, of the host rocks; the radiometrie age of the pitchblende 37 to 57 m. y.; the low temperature mineral association; the peculiar gravitational textures of the ore minerals; the temperature and salinity of the fluid inclusions of the carbonates, ranging from 230º to less than 70º C, and from 0 to 25% NaCl equiv , respectively; and the shallow tectonic activity giving place to the hydraulic fracturing and fault breccias controlling the ore deposition. Finally, he FE orebody is compared with similar U deposits in Spain and elsewhere, and the origin of the mineralization is attributed to seismic pumping of U contained in the CEG carbonaceous slates.
According to this model, the strains developed in the Hercynian basement around shear zones resulting from the Alpine tectonic activity, during Lower to Middle Tertiary times, would have given rise to episodic remobilizations of the U by hydrothermal flu ids. Consequently, when the dilatant zones collapsed, these U bearing solutions, geothermal in character, were expelled towards the surface depositing the U and accompanying minerals in fractures and breccias associated to the wrench faults.