On the use of geophysical methods for roll-front uranium deposits characterization.

Roll-front uranium deposits type, extracted by the in situ recovering method (ISR), are strategic ore deposits for mining major companies. They are classically identified with drill­holes, carried out in a dichotomous way between the oxidised area localised upstream the ore deposit and the reduced part downstream. The interface between the two areas is the main uranium mineralization location. This front is highlighted by a differential mineralogy, in particular with a high content in Fe (III) minerals in the oxidised area and a high content in sulfides in the reduced area. This thesis intends to assess some geophysical methods which could help to detect this mineralogical zonation from the ground surface, in the context of sedimentary deposits with a low electrical resistivity. The first step consisted of implementing magnetometry, self-potential and spectral induced polarization methods in a first Mongolian roll-front. The main results evidenced that magnetic and self-potential methods are essentially related to the tectonic directions of the investigated area. The induced polarization measurements, while impacted by large electromagnetic coupling effects, underline a polarization of the investigated area close to zero, meaning that the roll-front cannot be detected by this method. A laboratory study, performed on a thousand of rock samples from a second Mongolian roll-front, showed that measured polarization is mostly linked to the clay content, and is not correlated with the redox facies. These geophysical methods are unlikely suitable to detect roll-front deposits in such a geological context. However, their application at a small scale could be usefull for the environmental monitoring of the exploitation by lixiviation or the remediation phase.