The interactions between the meteoric agents and the lithosphere transform “fresh” rocks (“protores”) into weathered rocks (“alterites”). Among many factors influencing the weathering of rocks, climate carries the water (fluids) input and determines the kinetics of the chemical reactions, whereas geodynamics allows rocks to be exposed to meteoric agents. The development of weathering profiles in the supergene zone of manganiferous protores produces Mn oxides and increases the grade of several metals within the ore. The detailed mineralogical, geochemical and petrological characterization of these supergene ores is therefore of paramount importance to evaluate their economic potential. The occurrence of K-bearing Mn oxides, belonging to the hollandite supergroup, within this oxidation zone, enables the dating of the supergene processes by using the K-Ar and 40Ar/39Ar radiometric methods. Nowadays, the interpretation of these ages is often attributed to favorable climatic periods, probably diminishing the contribution of an essential geodynamics component. Therefore, this thesis has three main objectives: (1) determining the genesis and metallogenesis of the supergene Mn ores in these complex systems; (2) identifying the ore-forming parameters in order to understand the origin of these deposits; and (3) positioning the K-Mn oxides within the crystallization sequences in order to date them. Four sampling areas have been selected to address these issues: (1) the weathering zone of Mn veins in the Vosges (France); (2) the weathering crust of Mn-rich Ordovician slates in the Stavelot massif (Ardenne, Belgium); (3) the karst-hosted deposits in the Imini-Tasdremt district (High Atlas, Morocco); and (4) the mixed hydrothermal-supergene Fe-Mn deposit of Tamra (Tunisia). Vein type deposits produce limited K-Mn oxides, which is probably due to the presence of barite in the protore that favors the formation of hollandite sensu stricto. The K-rich phyllosilicates in the weathering crust of sedimentary protores easily recombine with the released Mn to precipitate cryptomelane. The precipitation of Mn in karst-hosted deposits in the Imini-Tasdremt area involves a joint increase of the pH and the Eh, favoring the formation of coronadite, hollandite and cryptomelane, which are probably formed beyond meteoric temperatures. Given the K-Ar and 40Ar/39Ar dating of these K-Mn oxides, karst-hosted deposits in the Imini area (Morocco), which are relatively isolated from the climatic influence, favor the expression of the geodynamics component. Such correlation is particularly well illustrated between the ages and the first two steps of the Atlas deformation. These phases, Late Eocene on the one hand and Early Miocene on the other hand, are accompanied by massive pyrolusite mineralization, providing to the Imini ore a high grade. The other weathering products make the geodynamic interpretation on the age record difficult, given the pervasive influence of the climatic component. However, such correlation is suggested in the Stavelot Massif, where the oldest ages obtained in the saprolite correspond to the late Oligocene doming in the Ardenne. The erosion and the weak vertical extension of the weathering zone in the Vosges deposits make this connection unclear. In addition, the hydrothermal contribution on the formation of the Fe-Mn Tamra deposit makes any supergene contribution difficult to recognize. Consequently, karst-hosted Mn mineralization and/or Mn accumulations located in slightly uplifted zones (doming, foreland basins) seem to be the most suitable sites to investigate the geodynamic effect on supergene processes.
|la date de réponse||29 août 2019|
|Superviseur||Johan Yans (Promoteur), Vincent Hallet (Président), Jocelyn Barbarand (Jury), Paulo Vasconcelos (Jury), Mohammed Bouabdellah (Jury), Alain Bernard (Jury) & Simon Philippo (Jury)|