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 Pa laeozoic basement of the Albigeois in southwestern French Massif Central, T he massive fluorite is hosted within large E-W striking fractures, crosscut ting Cambro-Ordovician clastics, associated with large zones of hypersilici fied tectonic breccia which form the wall of the mined deposits. Fluid incl usion data for pre-fluorite and fluorite stage fluids have salinities betwe en 20-26 wt% NaCl equiv., with homogenisation temperatures between 85-170 d egrees C. Furthermore, low first ice melting temperatures (around -50 degre es C) indicates the presence of significant CaCl2 and possibly MgCl2 togeth er with NaCl. Calculated fluid delta(18)O for pre-fluorite quartz ranges fr om -9.1 parts per thousand to -5.2 parts per thousand, with delta D between -55 parts per thousand to -64 parts per thousand, placing the data directl y on the present day meteoric water line. Fluorite stage fluids have delta( 18)O between +0.1 parts per thousand to +3.2 parts per thousand, and delta D ranging from -53 parts per thousand to -75 parts per thousand, indicating an interacted meteoric fluid origin. Combining the fluid inclusion and sta ble isotope data illustrates that the main fluorite depositing fluid has ch aracteristics 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 ev ents 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, contine ntal, evaporite-bearing basins coincident with, and controlled by the E-W f ractures. (C) 1999 Elsevier Science Ltd. All rights reserved.