Description du sujet de thèse

The unconformity-type deposits of the Athabasca Basin (Canada) are the highest-grade uranium deposits known on Earth. They were formed following massive circulations of hydrothermal fluids at the interface between the sedimentary basin and the underlying crystalline basement. These circulations were specifically controlled by the permeability of the geological environments and the structural networks, the most visible expression of which today is the presence of faults in the basin and the basement. These hydrothermal circulations led, in addition to the precipitation of uranium oxides, to significant alterations of the host rocks. These alterations are notably characterized by the precipitation of several clayey phases as well as significant mobility of silica (dequartzification, silicification).
Industrial works on the various exploration sites have made possible to define for each site explored an individual spatial distribution of these different markers (such as the presence of uranium and clays, the nature of the structures, the presence or absence of quartz, etc...). These differences and the variation in their location with respect to the unconformity have led to the definition of different subtypes of deposits and genetic models used today by mining companies. Comparison of the distribution of these markers between sites, as well as known geochronological data on mineralization and alteration minerals, indicate that these deposits are the result of multiple hydrothermal episodes that have been active in this environment over the last 1.7 billion years. The probable variations in the number, sequencing and intensity of successive hydrothermal activities in the different parts of the basin are at the origin of the variability in their location with respect to the unconformity, as well as their specific characteristics. These include variabilities in the spatial distribution and content of uranium, variations in the nature of clay minerals (illites, kaolinites, chlorites, tourmalines, APS), specific distributions of quartz and expressions of variable redox states (color changes).
However, the lack of mineralogical, physicochemical, structural data and direct temporal constraints specific to each hydrothermal episode limits our understanding of the role played by each of the different episodes on the current characteristics of these deposits. The objective of this doctoral project is to better define the nature and conditions of successive fluid-rock interactions in these deposits, their timing of formation, their consequences on the properties of the environment (mineralogical, petrophysical, physicochemical) and on the conditions of mobility and precipitation of uranium. For this, the PhD student will focus on the study of marker minerals of the P-T-t-x conditions of hydrothermal circulations associated with alteration deposits: clay minerals, quartz and accompanying minerals (phosphates, carbonates, titanium-bearing minerals).

Three actions are proposed within the framework of this thesis project:

- Identification, qualification, and study of the spatial distribution of various key markers at the scale of an exploration site, by combining field data systematically acquired from boreholes by exploration geologists with additional data acquired by the doctoral student in the field using portable tools. The key markers will be established in conjunction with data previously acquired or to be acquired in the laboratory (actions 2 and 3), allowing the identification of the different hydrothermal episodes. This work will establish the geometry of the associated successive alterations, supporting the numerical models proposed for the characterization of hydrothermalism at the basin-basement interface. The objective is to apply this approach: i) to different uranium-mineralized and non-mineralized sections per site, ii) to sites with different grades/tonnages, and iii) between subregions of the Athabasca Basin. This work will be carried out in close collaboration with exploration geologists from Orano Canada.

- Identification of different hydrothermal processes and distinction with potential regional diagenetic episodes. Defining the marker mineral assemblages of each episode, along with their spatial distribution (in connection with Action 1), will help define the P-T-t-x conditions specific to each hydrothermal episode. This work will be carried out in the laboratory and with national and international collaborations.

- Re-evaluation and development of new 1D numerical geochemical models integrating the phase variations and geochemical conditions known for each hydrothermal episode. The data acquired during the two previous actions will be used to establish these reactive models to understand the fluid/rock interaction processes and their evolution, and to define the critical parameters controlling the mineralogical evolution specific to each event.

All of these results will make it possible to propose a new model for the formation and evolution of the uranium systems of the Athabasca Basin.

Contexte de travail

The Centre National de la Recherche Scientifique (CNRS) is one of the world's leading research institutions. To address the major challenges of the present and future, its scientists explore life, matter, the universe, and the functioning of human societies. Internationally recognized for the excellence of its scientific work, the CNRS is a benchmark both in the world of research and development and for the general public.

The GeoRessources laboratory is a Research Unit (UMR7359) of the Universite de Lorraine and CNRS. The laboratory is divided into four thematic geographical sites; the worksite for this thesis project is located on the campus of the Faculté des Sciences et Technologies in Vandoeuvre-lès-Nancy. GeoRessources is a laboratory of approximately 150 people whose research focuses on subsurface resources. The laboratory is composed of six research teams, divided into three thematic areas. GeoRessources is integrated into the Earth and Environment Observatory in Lorraine (OTELo) which coordinates the research of the laboratories in Geosciences and Environment located in Lorraine whose scientific themes range from the functioning of the planet, to the management of mineral and energy resources and to the knowledge and management of continental environments.

The doctoral PhD project is part of a project co-financed by CNRS and ORANO (a major player in nuclear energy) within the framework of their joint laboratory (Labcom) CREGU (Center for Research and Study of Uranium Deposits). The project includes field missions in Athabasca Basin, analyses at the GeoRessources laboratory and in partner laboratories in France and abroad, and participation in conferences and seminars. The doctoral PhD student will join the team Géologie des Ressources Minérales (GEM) and will work in conjunction with the CREGU Labcom. He or she will actively participate in meetings and seminars organized in partnership with ORANO. This thesis project will be based on the acquisition of data in the field and in the laboratory. The field work will be carried out with the industrial partner, on drill cores provided by the partner.

This work will be based on the use of varied field tools (pSWIR, pDRX, pLIBS). The work in the laboratory will be based on the different analytical platforms available at GeoRessources: scanning electron microscopy, electron microprobe, µXRF, LA-ICP-MS, K-Ar clay separation and dating platform. The models will be carried out with the partner teams of the GeoRessources laboratory. Analyses by TEM, Mössbauer and/or synchrotron will be carried out in partner laboratories. The thesis will be supervised by Julien Mercadier (CNRS Research Director at GeoRessources), Delphine Charpentier (Associate Professor, Chrono-environnement Besançon) and Gaétan Milesi (Associate Professor at GeoRessources) with the strong involvement of Marie Gerardin (CNRS Research Engineer at GeoRessources) . The student will be supervised from an industrial point of view by Pierre Martz (ORANO Engineer).

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