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Giant uranium deposits formed from exceptionally uranium-rich acidic brines

Nature Geoscience volume 5, pages 142–146 (2012)Cite this article

Abstract

Giant uranium deposits were formed during the Mesoproterozoic era, 1.6–1.0 Gyr ago, in both Canada and Australia. The deposits are thought to have formed from large-scale circulation of brines at temperatures of 120–200 °C that percolated between sedimentary basins and underlying crystalline basement rocks1,2,3. However, the precise conditions for transport of the uranium in these brines are poorly understood4,5,6,7. Here we use mass spectrometry to analyse the uranium content of brines preserved in naturally occurring fluid inclusions in ore deposits from the Athabasca Basin, Canada. We measure concentrations of uranium in the range 1.0×10−6–2.8×10−3 mol l−1. These concentrations are three orders of magnitude above any other common crustal fluids. Experimentally, we measure the solubility of uranium as a function of NaCl content and pH, in mixtures that are analogous to ore-forming brines at 155 °C. To account for the high uranium content observed in the Athabasca deposits, we find that the brines must have been acidic, with a pH between 2.5 and 4.5. Our results strongly suggest that the world’s richest uranium deposits formed from highly concentrated uranium-bearing acidic brines. We conclude that these conditions are a necessary requirement for the formation of giant uranium deposits in relatively short periods of time of about 0.1–1 Myr, similar to other world-class deposits of lead–zinc and gold8,9.

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Figure 1: Quartz-vein and fluid-inclusion petrography.
Figure 2: LA–ICP–MS determination of U concentration in fluid inclusions.
Figure 3: U(VI) solubility in H2O–NaCl mixtures at 155 °C and Psat as a function of pH155 °C and Na concentration.
Figure 4: Comparison of U concentrations in Athabasca U ore fluids (this study) with U concentration in various crustal fluids.

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Acknowledgements

We thank Centre national de la recherche scientifique and Areva NC for financial support; Areva NC and Cameco for sampling and scientific collaboration; J. Cortot for ICP–atomic emission spectroscopy analyses; J. Grausem for the access to the Raman spectrometer; J. Dubessy, J. L. Vigneresse and B. W. D. Yardley for discussions and T. Pettke for help on early LA–ICP–MS work.

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Author notes

  1. Julien Mercadier

    Present address: Present address: Laboratory of Inorganic Chemistry, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland,

Authors and Affiliations

  1. G2R, Nancy-Université, CNRS, CREGU, Boulevard des Aiguillettes B.P. 239, F-54506, Vandoeuvre-lès-Nancy, France

    Antonin Richard, Christophe Rozsypal, Julien Mercadier, Michel Cuney, Marie-Christine Boiron & Michel Cathelineau

  2. School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK

    David A. Banks

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Contributions

M. Cuney, M. Cathelineau and M-C.B. conceived the project. J.M., M. Cathelineau and M. Cuney collected samples used in this study. A.R., M-C.B. and J.M. did the petrographic and microthermometric analyses of fluid inclusions. D.A.B., A.R. and M-C.B. carried out the LA–ICP–MS analyses of fluid inclusions. C.R. did the experimental U solubility measurements and analyses. A.R. and C.R. wrote the manuscript with contributions from all co-authors. All co-authors contributed to the discussion and interpretation of the data.

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Correspondence to Antonin Richard or Julien Mercadier.

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Richard, A., Rozsypal, C., Mercadier, J. et al. Giant uranium deposits formed from exceptionally uranium-rich acidic brines. Nature Geosci 5, 142–146 (2012). https://doi.org/10.1038/ngeo1338

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