Rare metal concentration associated to recrystallization: example from the Pyrenean Axial Zone
Résumé
Germanium is often found in trace concentration in non-deformed sphalerite (ZnS). However, locally in sphalerite from the Variscan Pyrenean Axial Zone, the presence of Ge-minerals (oxides, sulphides or chloritoids with up to 70% wt Ge) is remarkable. The rise question is thus what are the chemical and/or mechanical processes that control rare element contents of sulphides which have undergone deformation and metamorphism? In order to answer this question, we document microstructural and chemical heterogeneities in sphalerite, performed with EBSD (electron backscatter diffraction) coupled to LA-ICPMS in situ analysis. Deformation could induce recrystallization of primary sphalerite. Recrystallized domains contain low Ge contents (1-50 ppm Ge) whereas porphyroclastic sphalerite grains present higher Ge concentrations (up to 550 ppm Ge). Geminerals (up to 70 wt% Ge) are exclusively hosted in the Ge-poor recrystallized domains. We propose that Germanium was removed from the primary sphalerite crystal lattice during sulphide recrystallization and subsequently concentrated in Ge-minerals, leaving behind a Ge-depleted fine-grained recrystallized sphalerite matrix. Numerous sulphide ores enriched in rare elements similar to Pyrenean deposits may present recrystallization features like deformed Mississippi-Valley-Type, Kipushi-type or Volcanic-Hosted-Massive-Sulphide deposits. Consequently, the potential of this type of deposits shall be evaluated and we suggest that exploration methods must integrate precise textural and microstructural information coupled with trace element in situ and mapping techniques.
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