Hydrogeologic characterization of the abandoned mining site of Castelejo, Portugal by VLF-EM & RMT-R geophysical surveying

Abstract

There are about 175 abandoned mining sites in Portugal, of which about 60 are old uranium workings. The Castelejo mine is an old uranium located about 2 km W of Vila Cortés da Serra, in Guarda district, on the River Mondego basin. In geologic terms, the mine is located in medium-grained two-mica monzonitic granite, with porphyroidal tendency. Exploitation started with two open cast mines between 1979 and 1990 which produced about 132 tonnes U3O8.From 1992, production started to be carried out through the in situ H2SO4 leaching of the mine’s low-grade materials. The superficial and groundwater circulation are structurally conditioned by the intense fracturing in the area. Two exploratory geophysical field campaigns were conducted in order to characterize the fracturing. The first was a VLF survey that allowed us to identify major fault systems that may provide paths for groundwater flow. From the second campaign, hich was an RMT survey conducted at a fault gauge which records the mine natural gradient flow, we obtained the resistivities and thicknesses of the weathered granite material. The profiles obtained show that resistivity declines with increasing depth. This fact may arise from natural causes - the material which fills the fault gauge is more weathered at depth than on the surface - or it may indicate contamination at depth, due to acid mine drainage.