The Origin of Fluorite-Barite Mineralization at the Interface Between the Paris Basin and Its Variscan Basement: Insights from Fluid Inclusion Chemistry and Isotopic (O, H, Cl) Composition

We provide new constraints for the fluid flow system at the origin of two F-Ba deposits located at the unconformity between the south of the Paris Basin and the northern edge of the French Massif Central. We used microthermometry and bulk crush-leach analyses to determine isotope ratios of mineralizing fluids (delta 18O, delta D, delta 37Cl), together with cation and anion composition of fluid inclusions hosted by fluorite. Chlorinity and Cl/Br molar ratios (212-521) indicate the involvement of a brine, whose origin likely corresponds to Triassic evaporated seawater compatible with supratidal dolomitic facies preserved nearby. Microthermometry reveals high Ca/Na ratios, suggesting that the brine composition evolved from hydrothermal alteration of the Variscan basement and partial dissolution and replacement of the host sedimentary rocks. delta 37Cl values are lower than the expected value of evaporated seawater, suggesting Cl isotope fractionation by ion filtration in clay-rich horizons. Fluorite crystallized at minimum temperatures of 70 to 110 degrees C, 10-40 degrees C warmer than the host Triassic sedimentary rocks. Ascending brines were expelled during the Early Cretaceous and experienced a drop in pressure and temperature, together with possible mixing with the SO4-rich pore water of the sedimentary rocks, causing precipitation of silica, followed by fluorite and barite, forming a stratabound deposit similar to those found in many areas in Western Europe.