Uranium incorporation in fluorite and exploration of U–Pb dating

Lenoir, Louise; Blaise, Thomas; Somogyi, Andréa; Brigaud, Benjamin; Barbarand, Jocelyn; Boukari, Claire; Nouet, Julius; Brézard-Oudot, Aurore; Pagel, Maurice

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 (Flinline-formulaog) 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 inline-formulaµm globular pyrite crystals with a mean inline-formulaδ34S of inline-formula−23.6 inline-formula± 0.4 ‰inline-formulaV-CDT. We propose that the U incorporation in the fluorite lattice results from the development of a redox front during bacterial sulfate reduction. Flinline-formulaog 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 inline-formula± 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.

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Lenoir, Louise / Blaise, Thomas / Somogyi, Andréa / et al: Uranium incorporation in fluorite and exploration of U–Pb dating. 2021. Copernicus Publications.

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