Germanium and related elements in sulphide minerals: crystal chemistry, incorporation and isotope fractionation

Abstract : Germanium is a critical metalloid in many high-tech industries, especially for the energy transition and the communication sector. Being distinctly siderophile, lithophile, chalcophile and organophile, Ge can be a particularly useful geochemical tracer. This thesis aims at understanding the Ge geochemistry and the factors controlling its concentration in Ge-bearing minerals and ore deposits. Three contrasted Ge-bearing deposits were studied, the Saint-Salvy Zn vein-type deposit, French Massif Central, the Barrigão Cu vein-type deposit, Iberian pyrite belt, Portugal, and the Kipushi Zn–Cu carbonate-hosted deposit, Central African copper-belt, D.R. Congo. The most important Ge-bearing minerals are sphalerite (up to 2580 ppm Ge), chalcopyrite (up to 5750 ppm Ge), and renierite (5.0–9.1 wt.% Ge). The results show a first order relation between the Ge content and the deposition temperature. Synchrotron-based XANES spectroscopy showed that Ge4+ occur in tetrahedral sites in the studied sulphides. Element correlations suggest that Ge is chiefly incorporated in sphalerite and chalcopyrite through coupled substitutions, e.g., 3Zn2+ <-> Ge4+ + 2(Cu,Ag)+ and 3Fe3+ <-> 2Ge4+ + Cu+, respectively, or via the creation of lattice vacancies, e.g., 2Zn2+ <-> Ge4+ + ?. The positive [delta]74Ge–Ge content correlation observed in sphalerite from Saint-Salvy could indicate that Ge partition coefficient (KdGe) increases with temperature. Ge isotopes in sulphides yield [delta]74Ge values spanning from –5.72‰ to +3.67‰. The light [delta]74Ge compositions of Saint-Salvy and Barrigão ores appear to be related to variations in low to moderate fluid temperatures during Ge uptake in open system (e.g., fluid cooling), while the trend towards heavy [delta]74Ge compositions observed at Kipushi likely translates a Rayleigh fractionation effect during ore formation in closed system, associated with significant fluid modification.
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Rémi Belissont. Germanium and related elements in sulphide minerals: crystal chemistry, incorporation and isotope fractionation. Other. Université de Lorraine, 2016. English. ⟨NNT : 2016LORR0049⟩. ⟨tel-01754681⟩

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