Continental basinal origin of ore fluids from southwestern Massif central fluorite veins (Albigeois, France): evidence from fluid inclusion and stable isotope analyses

The most important fluorspar mining district in France is located in the Palaeozoic basement of the Albigeois in southwestern French Massif Central. The massive fluorite is hosted within large E–W striking fractures, crosscutting Cambro–Ordovician clastics, associated with large zones of hypersilicified tectonic breccia which form the wall of the mined deposits. Fluid inclusion data for pre-fluorite and fluorite stage fluids have salinities between 20–26 wt% NaCl equiv., with homogenisation temperatures between 85–170°C. Furthermore, low first ice melting temperatures (around −50°C) indicates the presence of significant CaCl2 and possibly MgCl2 together with NaCl. Calculated fluid δ18O for pre-fluorite quartz ranges from −9.1‰ to −5.2‰, with δD between −55‰ to −64‰, placing the data directly on the present day meteoric water line. Fluorite stage fluids have δ18O between +0.1‰ to +3.2‰, and δD ranging from −53‰ to −75‰, indicating an interacted meteoric fluid origin. Combining the fluid inclusion and stable isotope data illustrates that the main fluorite depositing fluid has characteristics typical of a basinal brine. The authors have no evidence that a magmatic system was involved in the deposit genesis. The proposed model highlights that mineralisation was related to major Mesozoic extensional events coinciding with the gradual opening of the Atlantic and Tethys oceans. In order to account for the chemistry of the fluids, and the siting of the deposits, the authors postulate a genetic relationship with local, continental, evaporite-bearing basins coincident with, and controlled by the E–W fractures.