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Impact of recrystallization and metamorphism on the mobility of germanium and related elements in orogenic Pb-Zn deposits : example of the Pyrenean Axial Zone mineralizations (France-Spain)

Abstract : Rare metals are essential to the development of the green technologies that are at the core of emerging low-carbon societies. Germanium is a rare element considered critical by the European Union and the U.S. Geological Survey due to its several uses in optical and electronics devices. Non-deformed sphalerite crystals (ZnS) commonly contain Ge and other critical elements (In, Ga) which may grade up to a few thousands of ppm. Only few studies have described specific minerals with Ge contents above wt.%, because these are apparently rare in Pb-Zn(-Cu) deposits. These Ge-minerals appear to be more abundant in deformed and metamorphosed Pb-Zn(-Cu) deposits. This raises the question of the impact of deformation and metamorphism on Ge and other rare metals mobility. This type of orogenic deposits constitutes the largest known zinc concentrations on Earth and their potential for rare metal is yet to be assessed.The Pb-Zn deposits in the Pyrenean Axial Zone are an ideal target to study the impact of deformation and metamorphism on rare metals mobility as sphalerite is only locally recrystallized. A structural field study allows to discriminate four Pb-Zn mineralization types. Type 1 is a minor syngenetic and stratiform disseminated mineralization. Type 2a is an epigenetic stratabound mineralization, concordant to the S1 Variscan foliation. Type 2b ore are parallel to the S2 cleavage. Sphalerite ore is highly deformed by a late cleavage sub-parallel to S2 probably Pyrenean-Alpine in age. Fluid inclusions study shows the presence of two Type 2b fluids with a low salinity (<20 wt.% NaCl eq.) and high temperatures (200-350 °C) typical of Late-Variscan fluids whereas Mesozoic Type 2b ore exhibits high salinity (15-35 wt.% NaCl eq.) and low temperature (< 200 °C).Germanium and related elements such as Cu, In and Ga are present in the deformed Pyrenean vein mineralizations and are heterogeneously distributed. Electron Backscattered diffraction (EBSD) analyses and chemical investigations allow to define distinct sphalerite textures with specific chemical contents: i) Dark-brown patchy or stripped zonations in coarse parent grains exhibit high Ge-content (up to 600 ppm Ge) in the lattice. ii) Light-brown zonations in coarse parent grains contain low Ge-contents mostly below 100 ppm Ge. iii) Light-brown small recrystallized daughter grains (below ~ 100 µm) present systematically low to very low Ge-contents (~ 20 ppm Ge). Copper contents (up to 1265 ppm) are highly correlated to Ge in sphalerite and Ga only occur in coarse sphalerite crystals (below ~100 ppm). Ge-minerals, such as brunogeierite, carboirite, briartite and argutite (up to ~ 70 wt. % Ge) are mainly hosted in recrystallized sphalerite domains or close to twin boundaries in coarse grains. These observations demonstrate that recrystallization of sphalerite has led to the redistribution of Ge from the sphalerite lattice into Ge-minerals. We suggest that the interactions between intra-granular diffusion and fluid assisted processes are responsible for the formation of patchy-stripped zonations and the crystallization of Ge-oxides, sulfides or chloritoids. A large variability of sphalerite chemistry and texture is frequently reported from other orogenic world-class deposits: these may have been affected by similar recrystallization and redistribution processes. The redistribution of rare metals in sulfide environments must have induced the concentration of rare metal in accessory minerals. These tiny minerals may be missed by punctual chemical analyses without prior detailed textural investigation. Understanding how rare metals concentrate through metamorphism and syntectonic recrystallization at mineral scale is essential to emphasize their spatial redistribution and localization at deposit scale. This study highlights the importance of coupling in situ and mapping chemical analyzes with macro- and microtextural characterization when targeting rare metals in deformed ore.
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Submitted on : Thursday, May 14, 2020 - 7:04:09 PM
Last modification on : Monday, June 8, 2020 - 4:26:06 PM


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Alexandre Cugerone. Impact of recrystallization and metamorphism on the mobility of germanium and related elements in orogenic Pb-Zn deposits : example of the Pyrenean Axial Zone mineralizations (France-Spain). Earth Sciences. Université Montpellier, 2019. English. ⟨NNT : 2019MONTG050⟩. ⟨tel-02578708⟩



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