Abstract. The age of ore deposits constitutes a decisive element in understanding their formation. Deciphering their precise chronology may be a challenge in the absence of mineral phases that can be dated by conventional geochronometers. Fluorite is very common either as the major or accessory mineral in a wide variety of ores and may provide information regarding the origin and timing of mineralizing fluid flows. In this contribution, we explore U–Pb dating on fluorite crystals from the world-class carbonate strata-bound fluorite ore of Pierre-Perthuis in Burgundy (Morvan massif, France). The uranium distribution within fluorite is mapped using induced fission-track and synchrotron radiation X-ray fluorescence nano-imaging, showing that higher U content is measured in an overgrowth of fluorite (Flog) as a discrete band. Preservation of a micrometer-thick zonation in U, associated with other substituted elements such as Sr, Y, Fe and Zr, implies that neither solid-state diffusion nor dissolution–recrystallization occurred. These U-bearing external fluorite overgrowths contain solid inclusions of about 30 µm globular pyrite crystals with a mean δ34S of −23.6 ± 0.4 ‰V-CDT. We propose that the U incorporation in the fluorite lattice results from the development of a redox front during bacterial sulfate reduction. Flog generation sampled and analyzed by laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) on four different crystals provides identical U–Pb ages within the limits of analytical uncertainty. Considered altogether, these four crystals yield an age estimate of 40.0 ± 1.7 Ma, not corrected for matrix-related elemental fractionation. Our results show that fluorite LA-ICP-MS U–Pb geochronology has potential for dating distinct crystal growth stages, although further research should be conducted to evaluate its accuracy.

Abstract. The age of ore deposits constitutes a decisive element in understanding their formation. Deciphering their precise chronology may be a challenge in the absence of mineral phases that can be dated by conventional geochronometers. Fluorite is very common either as the major or accessory mineral in a wide variety of ores and may provide information regarding the origin and timing of mineralizing fluid flows. In this contribution, we explore U-Pb dating on fluorite crystals from the world-class carbonate strata-bound fluorite ore of Pierre-Perthuis in Burgundy (Morvan massif, France). Uranium distribution within fluorite is mapped using induced fission-track and Synchrotron radiation X-Ray Fluorescence nano-imaging, showing that higher uranium content is measured in an overgrowth of fluorite (Flog) as a discrete band. Preservation of a micrometer-thick zonation in U, associated with other substituted elements such as Sr, Y, Fe and Zr implies that neither solid-state diffusion nor dissolution-recrystallization did occur. These U-bearing external fluorite overgrowths contain solid inclusions of about 30 µm globular pyrite crystals with a mean δ34S of −23.6 ± 0.4 ‰ V-CDT. We propose that the U incorporation in the fluorite lattice results from its reduction mediated by H2S release during bacterial sulphate reduction. Flog generation sampled and analyzed by LA-ICP-MS on four different crystals provides identical U-Pb ages within the limits of analytical uncertainty. Considered altogether, these four crystals yield an age estimate of 40.0 ± 1.7 Ma, not corrected for matrix-related elemental fractionation. Our results show that fluorite LA-ICP-MS U-Pb geochronology has potential for dating distinct crystal growth stages, although further research should be conducted to evaluate its accuracy.

All around the Aquitain basin, fluid circulates at the interface between the basement and impermeable shale (Toarcian) within high porosity levels (Hettangian) as evidenced by many mineral deposits including baryte, fluorine and galena. On the Jard-sur-Mer area, these minerals are also found higher in the sedimentary series suggesting that fluid flow through the impermeable layer. Our field observations highlight that 1) fluid circulations are coeval with an episode of brittle deformation that is compatible with an upper-Jurassic N-S direction of extension, 2) fluid where over pressurized at the time of brittle deformation and 3) fluids overpressure resulted from the additional contribution of fluid from the basement ascending all along inherited brittle faults. When compared with the rest of the basin, these new constrains on the northern limit suggests that the Aquitain basin recorded several stages of fluid overpressure before and during the lower Cretaceous rifting phase and Pyrenees building up.

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