Uranium incorporation in fluorite and exploration of U–Pb dating

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

doi:10.5194/gchron-3-199-2021

Abstract

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.

Highlights

Sedimentary rocks lying unconformably on a crystalline basement may host large concentrations of authigenic F, Ba, Pb, Zn and U minerals that are of economic interest (Boiron et al, 2002; Gigon et al, 2020; Gigoux et al, 2015; Leach et al, 2005; Sizaret, 2006)
Petrographic observations coupled to induced fission tracks and synchrotron radiation X-ray fluorescence (SR-XRF) mapping show that U is incorporated in the fluorite crystal lattice by elemental substitution
The incorporation of U is related to bacterial sulfate reduction, evidenced by globular pyrite inclusions

Summary

Introduction

Sedimentary rocks lying unconformably on a crystalline basement may host large concentrations of authigenic F, Ba, Pb, Zn and U minerals that are of economic interest (Boiron et al, 2002; Gigon et al, 2020; Gigoux et al, 2015; Leach et al, 2005; Sizaret, 2006). These unconformity-related deposits are usually the result of multiple episodes of fluid flow, inducing mineral dissolution–recrystallization, remobilization and precipitation (Chi et al, 2018; Walter et al, 2018). Fluorite Sm–Nd geochronology (Chesley et al, 1991; Galindo et al, 1994; Dill et al, 2011) might help to reconstruct the geological scenario that led to the mineral deposition, together with other direct radiometric dating of authigenic phases, either by bulk dissolution, e.g., Rb–Sr on sphalerite (Nakai et al, 1993), Re–Os on molybdenite (Markey et al, 1998; Stein et al, 2001) and Re–Os on pyrite (Cardon, 2007; Mathur et al, 2000), or by in situ sampling and analysis, e.g., Ar–Ar on adularia (Cathelineau et al, 2012; Mark et al, 2005), Ar–Ar on Mn oxides (Deng and Li, 2017) or U–Pb on uraninite (Alexandre et al, 2009; Martz et al, 2019).

Objectives

Methods

Results

Discussion

Conclusion