Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Characterization and (U-Th-Sm)/He dating of Moroccan supergene ore deposits : New insights to unravel the periods and potential causes of weathering processes.

Auteur : Verhaert Michele

Université de soutenance : University of Namur

Grade : Doctor of Sciences 2020

Résumé partiel
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.

Présentation

Version intégrale (21,1 Mb)