Tl, Mo and U isotopes in U-ore deposits record Earth's fundamental redox processes

• Published in: Chemical geology Vol. 661; p. 122154
• Main Authors: Voinot, A., Kyser, T.K., Chipley, D., Valentino, M., Uvarova, Y., Layton-Matthews, D., Leybourne, M.I.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05.09.2024
• Subjects: Isotope geochemistry; Molybdenum isotopes; Thallium isotopes; U-ore deposits; Uranium isotopes; Molybdenum isotopes; Thallium isotopes; U-ore deposits; Isotope geochemistry; Uranium isotopes
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2024.122154

Understanding changes in redox conditions in the Earth's geological record is fundamental to unravelling some of the major events of our planet's history, such as the Great Oxidation Event (G.O.E.), biological changes through time related to mass extinctions, or the formation of ore deposits for primordial metal resources. Rocks from uranium deposits are ideal candidates to examine the coupled isotopic fractionation of Tl, Mo and U, as they are fundamentally caused by redox processes, occur in a wide variety of geological settings throughout much of geological history (from about 3 Ga), and result from the interplay between fluids and rocks of variable redox states. Herein we measure δ97Mo (−10.2 to +6.1 ‰), ε205Tl (−9.5 to +5.0 ε-units), and δ238U (−0.72 to 0.30 ‰) for 34 samples from granite-related, intrusive, metamorphic, and Proterozoic unconformity related U deposits. Collectively, these results suggest: 1) in metamorphic and unconformity-related contexts, samples with decreasing ε205Tl values also have decreasing δ238U values, indicative of change in redox conditions, 2) ε205Tl values in the sulfide-rich samples likely reflect the affinity of 205Tl for these minerals, 3) Tl undergoes significant isotopic fractionation during the reduction of U-bearing fluids to precipitate uraninite, and 4) Tl isotopes do not fractionate during secondary remobilization of elements within these systems, and thus retain a record of the conditions of formation, whereas Mo isotopes are heavily fractionated by secondary remobilization. We suggest that the study of U, Mo, and Tl isotopes in U-ore deposits conveys new information related to building blocks of life on Earth and shows great potential for the study of past oxygenation events, thus potentially allowing us to reconstruct the geological and chemical history of our planet. •Uranium and thallium isotopes show potential as proxies for redox reactions in U-ore deposits.•Thallium isotopes do not seem to be affected by alteration and secondary remobilization of uranium.•Molybdenum isotopes in U-ore deposit samples are strongly affected by secondary processes.