Description du sujet de thèse

Tungsten (W) is a very hard, dense and refractory metal, used mainly in the form of carbide and special steels. Today, this metal is essential for many European industrial applications, notably in the aerospace (turbine blades for aircraft engines), energy (machine tools for power plants and generators) and armament (armor) sectors. Since 2011, the European Commission has regularly ranked W among the critical raw materials that present a risk both in terms of supply and vulnerability for the high-tech industry. France has several W-Sn deposits that have been mined in the past, most of them in the Massif Central.

The W-Sn district of the southern Massif Central is characterized by an original combination of “classic” perigranitic mineralization, such as the Montredon-Labessonnié deposit (Tarn), with wolframite ([Fe,Mn]WO4) and cassiterite (SnO2) quartz veins and atypical scheelite (CaWO4) and fluorite (CaF2) mineralization (called stratabounb) consistent with the regional metamorphic foliation, such as the Hom-Haut and Auriole deposits around the Montredon-Labessonnié dome and showings on the southeastern edge of the Montagne Noire anatectic dome. While there is consensus on the mainly magmatic-hydrothermal origin and late Variscan age of vein mineralization, there is no satisfactory model to explain the origin of W-Sn-F stratabound mineralization. On a regional scale, stratabound mineralization occurs in the Neoproterozoic to Cambrian metasedimentary terrains surrounding the Montredon-Labessonnié and Montagne Noire gneiss domes. On the southeastern edge of the Montagne Noire, scheelite mineralization occurs discontinuously over a distance of 20-km along the axial zone. Locally, scheelite mineralization occurs in thin carbonate layers (less than 10 m thick) interbedded with micaschists and paragneisses, and is consistent with the regional foliation. Preliminary paragenetic studies indicate that the scheelite is associated with a metasomatic paragenesis formed in retrograde conditions, probably linked to the exhumation of gneiss domes, corresponding to a peri-anatectic model. However, the age of the mineralization, the origin of the fluids and metals and the link with the magmato-tectono-metamorphic history of the domes remain unknown.

Based on this, the proposed project has 4 main objectives: (1) To determine with precision the age and duration of W-Sn mineralizing and/or deformation events, and to place them in the context of late Variscan tectonic evolution; (2) Determine the nature and source of the fluids responsible for the formation of W-Sn-F stratabound mineralization. (3) Evaluate the role of lithological and structural controls (from micrometer to kilometer scale) on hydrothermal circulations during mineralization formation and deformation. (4) Integrate the data into a geodynamic model linking Variscan magmato-tectono-metamorphic events to fluid circulation and W-Sn-F mineralization. The expected results will also provide a better understanding of the source of the W.

To achieve these objectives, the candidate will carry out a field study on the Montredon-Labessonnié dome and the southern edge of the Montagne Noire dome, south of the Massif Central. Emphasis will be placed on the analysis of lithologies with stratabound mineralization and the link with deformation on a local and regional scale. In the laboratory, the mineralogy, textures and chemical composition of mineral parageneses will be studied in detail to establish a robust petrographic framework. The candidate will then carry out in-situ U-Pb isotope analyses on the mineralization-bearing phases, i.e. scheelite (W), cassiterite (Sn), in an attempt to determine the age of precipitation of these minerals. The composition of mineralizing fluids and deposition conditions will be studied based on the analysis of fluid inclusions trapped in the 3 metal carriers mentioned above. Fluid composition (liquid and gas) will be determined by microthermometry and Raman spectroscopy. These data will be integrated into a metallogenic model of W-Sn (F) mineralization related to the emplacement of metamorphic domes in the southern Massif Central at the end of the Variscan orogeny. This model will be compared with data from other European Variscan massifs (Iberia, Bohemia, etc.) to establish a spatio-temporal framework for W stratabound mineralization that can serve as a guide for the exploration of new W-Sn resources.

Contexte de travail

This thesis is part of the “Massif Central” targeted project (PC13) of the PEPR “Sous-sol, bien commun” whose aim is to update existing scientific data and produce new interdisciplinary knowledge in order to: (i) understand the social and geological conditions under which the French Massif Central can contribute to today's energy and digital transitions, and (ii) obtain a shared knowledge of the subsoil at regional and local levels. The results will serve as a basis for reflection on collectively desirable and sustainable scenarios for the French Massif Central's subsoil. The student will take part in annual meetings with all PC13 participants.
The management team includes an associate professor from the Geosciences Environnement Toulouse laboratory (Guillaume Estrade), an associate professor from the Chrono-environnement laboratory in Besançon (Pierre Trap) and an engineer-researcher from BRGM in Orléans (Matthieu Harlaux). The PhD student will depend on the SDU2E “Sciences de l'Univers, de l'Environnement et de l'Espace” doctoral school (https://sdu2e.obs-mip.fr/). The PhD student will be assigned to the Géosciences Environnement Toulouse laboratory (https://www.get.omp.eu/), a joint research unit of the CNRS, IRD, Université Toulouse 3 and CNES, and a multi-disciplinary Earth and Environmental Sciences research laboratory attached to the Observatoire Midi-Pyrénées (https://www.omp.eu/), which brings together laboratories specializing in the sciences of the universe, the planet and the environment in the Midi-Pyrénées region. The PhD student will join the Georesources: genesis and sustainable management team, and will be able to interact with researchers in the Terrestrial Evolution and Dynamics division. He/she will carry out numerous field missions in the southern Massif Central and travel to the chrono-environment laboratory in Besançon and BRGM in Orléans. The position is located in an area covered by the protection of scientific and technical potential (PPST), and therefore requires, in accordance with regulations, that your arrival be authorized by the competent authority of the Ministry of Higher Education and Research.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

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