Weathering profiles do form when rocks and primary (hypogene) ore bodies are brought to the Earth’s surface and exposed to atmospheric conditions. The oxidation of (mostly) sulfides, the dissolution of host rocks, the transport and the (re)concentration of elements enable the precipitation of a wide assortment of secondary (supergene) phases, increase the grade of some metals within the ore, and lead to the formation of supergene ore deposits. The detailed mineralogical, petrological and geochemical characterization of supergene ores, motivated by the growing demand for metals and rare elements that are necessary for the new technologies, targets the identification of eventual economical grades of these elements, in the weathered part of deposits. The ubiquity of iron (oxyhydr-)oxides such as goethite and hematite within supergene environments enables the dating of weathering processes by using the (U-Th-Sm)/He method, while the local occurrence of alunite allows the obtention of crystallization ages through the 40K-39Ar dating method. This thesis follows three objectives: 1) defining the paragenetic sequences of supergene base metal deposits and positioning the datable phases within the sequence, 2) dating iron (oxyhydr-)oxides in order to integrate weathering processes in a reliable temporal frame, 3) determining the relative importance of parameters influencing the development of weathering profiles, i.e. climate, which provides water and determines the kinetic of chemical reactions, and geodynamics, which triggers the exhumation of rocks and generates gradients for the progression of fluids. Four sampling areas have been selected to address these issues: 1) three sites of the Eastern High Atlas of Morocco, the Cu site of Jbel Klakh, the Pb-Zn site of Jbel Haouanit, and the Fe-Mn-Cu site of Jbel Rhals, 2) the Mn deposit of Imini, in the Central High Atlas of Morocco, 3) three deposits and several gossans of the Occidental Anti-Atlas of Morocco, the Cu deposits of Tazalaght, Agoujgal and Tizert, and 4) the polymetallic district of Nefza-Sejnane in Northern Tunisia. The vertical distribution of weathering zones, composed from base to top of the hypogene zone (sulfides such as chalcopyrite, pyrite, tennantite, galena, sphalerite), cementation zone (sulfides such as bornite, digenite, chalcocite, djurleite, covellite), oxidized zone (i.e. malachite, azurite, brochantite, olivenite, chenevixite), and leached zone or gossan (iron (oxyhydr-)oxides, mottramite, quartz, calcite) is particularly emphasized in the Cu deposits, but less obvious in the Pb-Zn-Fe-Mn deposits of Jbel Haouanit, Jbel Rhals and Agoujgal. Hand samples of weathered tennantite from Tazalaght and Agoujgal deposits, presenting boxwork textures, also record these transitions at the scale of the sample, reflecting the evolution of fluid-rock interactions with time. The first-order influence of the host rock nature on supergene processes is highlighted in the carbonate-hosted deposits that exhibit a broad panel of oxidized mineral phases of which the precipitation was enabled by an efficient neutralization of the acidic fluids by host rock dissolution. The testing and dating of iron (oxyhydr-)oxides emphasized that the careful selection of samples and their thorough characterization are of the major importance for the acquisition of accurate and meaningful ages. As such, the purest botryoidal goethite samples are considered as the excellent material for (U-Th-Sm)/He dating, while microcrystalline samples are viewed as a second choice. The dating of iron (oxyhydr-)oxides pseudomorphosing pyrite should be avoided. In the High Atlas and Anti-Atlas of Morocco, a correlation between iron (oxyhydr-)oxides and alunite crystallization ages with deformation events of the Late Cretaceous and Cenozoic, related to the Alpine orogeny, suggests that the exhumation of preexisting mineralization and/or of host rocks is a key process required for the formation of supergene ore deposits and gossans. Similar interpretations are propounded in the Tunisian Nefza-Sejnane polymetallic district, where supergene goethite ages indicate the circulation of meteoric fluids in relation with geodynamic events recorded in the area. Therefore, geodynamics is regarded, in the Moroccan Atlas and in Northern Tunisia, as the first-order control for supergene mineralization that generates weathering gradients necessary for the circulation of meteoric fluids. The lack of fit between supergene mineralization ages and Cenozoic (sub)tropical climates in North Africa suggests that climatic parameters only play a secondary role, even if precipitations are required for the circulation of meteoric fluids.
|Date of Award||9 Sep 2020|
|Sponsors||Fund for Research Training in Industry and Agriculture (FRIA)|
|Supervisor||Johan YANS (Supervisor), Alain Bernard (Co-Supervisor), Cécile Gautheron (Co-Supervisor), Vincent HALLET (President), Jocelyn Barbarand (Jury), Eric Pirard (Jury), Vinciane Debaille (Jury) & Lhou Maacha (Jury)|