• Barou, Fabrice
• Cenki, Bénédicte
• Cugerone, Alexandre
• Oliot, Emilien
• Munoz, Manuel
• Motto-Ros, Vincent

Abstract

<jats:title>Abstract</jats:title><jats:p>Sphalerite (ZnS) is a sulfide found in a large variety of ore deposits and is frequently hosted in metamorphic terranes that have undergone deformation and related recrystallization. However, deformation mechanisms of sphalerite are still poorly understood because recrystallization evidence is barely visible under the optical microscope and may reflect complex and frequently multistage mechanisms. Furthermore, sphalerite may host up to a few thousands ppm of critical metals such as gallium (Ga), germanium (Ge), and indium (In). Metamorphic conditions and dynamic recrystallization may have induced local or total redistribution of these elements. Modern techniques such as electron backscattered diffraction analyses (EBSD) and laser induced breakdown spectroscopy (LIBS) applied on sphalerite allow to detect grain boundaries, crystal-plastic deformation and internal chemical diffusion, which classically reflect active deformation mechanisms. In this study, a microstructural and in-situ chemical comparison between four sphalerite types (type i, ii, iii, iv) is made for the first time. The four sphalerite types present different deformation imprints although they are hosted in a similar geological setting: the Pyrenean Axial Zone and the Montagne Noire Variscan massifs (France). Based on EBSD and LIBS mapping, we describe two regional sphalerite growth stages composed of dark red crystals with polygonal shape (type i; Bentaillou-Liat deposit) and light to dark-brown euhedral crystals (type iii; Saint Salvy deposit). New investigation at microscale on sphalerite grains from the Saint-Salvy deposit shows late Cu-Ge-Ga enrichment not only in specific sector zonings, but also in grain boundaries, growing crystal edges and in low angle misorientation or twin boundaries. Following a deformation event that probably occurred during the Pyrenean-Alpine orogeny, these two sphalerite mineralizations have both endured plastic deformation in a dislocation creep regime and dynamically recovered by subgrain rotation (SGR) mechanism. Two mechanisms of Cu-Ga-Ge spatial redistribution are observed and are key processes for the crystallization of Cu-Ga-Ge-rich minerals in sphalerite veins. The first mechanism involved the in-situ redistribution of Cu-Ga-Ge contents from a pre-existing concentration in the sphalerite lattice (type iii, Arre deposit), creating Ge-sulfides (briartite), probably during Pyrenean-Alpine orogeny. Formation of this type of Ge-mineral may be related to solid-state diffusion processes. The second mechanism is associated with the circulation of a Cu-Ga-Ge-rich fluid in surrounding rocks. In the pre-existing polygonal sphalerite from Late-Variscan veins (type ii, Pale Bidau deposit), millimeter size bands of small (&amp;lt;50 µm), recrystallized sphalerite grains are locally observed. Thos domains contain inclusions of Cu (chalcopyrite), Ga and Ge minerals (brunogeierite, carboirite). Fluid-induced diffusion in the polygonal sphalerite aggregates may occur with superimposed dynamic recrystallization, such as the Late-Variscan veins (type ii, Pale Bidau-type). During post-Variscan time, this fluid enriched in Cu-Ga-Ge largely circulated in the upper-crust of this Variscan terrane. This study highlights the key importance of coupled textural (EBSD analyses) and in-situ chemical analyses (LIBS mapping) of diverse sphalerite types at regional scale in order to indirectly unravel the origin of vein mineralization, and their related critical metal distribution.</jats:p>

Topics

• impedance spectroscopy
• mineral
• polymer
• grain
• inclusion
• mobility
• dislocation
• electron backscatter diffraction
• deformation mechanism
• recrystallization
• crystallization
• creep
• trace element
• Gallium
• Indium
• Germanium