Résumé étendu du congrès SGA

à télécharger : 2013_brigaud_SGA

Stratiform fluorite deposits in the south-eastern part of the Paris basin (Morvan): paragenetic sequence, trace elements and Sr-Nd isotopes

Abstract: In France, stratiform fluorite deposits occurred at the base of Mesozoic sedimentary units from the Paris Basin around the Morvan massif, and reserves are estimated to about 5Mt. Detailed paragenetic sequence fluid nature and the timing of the mineralization are still poorly constrained. In this study we examine the petrographical properties, trace elements composition and isotopic signatures of the fluorite deposits. These investigations provide insights into the parameters controlling the mineralization stages. The petrographical observations revealed a distinct growth zonation of fluorite crystals and a mineral paragenesis composed of
multiple fluorite stages associated with various minerals (barite, sphalerite, galena and others). Significant variations of U, Th, Sr, Ba, and especially U/Th ratios are recorded from all fluorite stages. Concerning Rare Earth Element (REE), a bell-shaped pattern is recognized for all fluorite stages. Sr isotopic data of fluorite (87Sr/86Sr=0.7119 to 0.7134) are incompatible with a seawater signature. Available microthermometric data and the thermal history of the basin indicate hydrothermal fluid flow. A preliminary isochron data suggest a Lower Cretaceous age for the fluorite mineralization at the Pierre-Perthuis deposits.