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| Fig. 1: Diagram of a Proterozoic unconformity deposit at Cigar Lake, Canada. (Image Source: S. Erickson, after Jefferson et al. [2] |
Nuclear reactors use Uranium as their fuel source. We examine the natural abundance of Uranium on Earth, since many are interested in the available supply of this resource.
Uranium is found in trace amounts around the globe, with a concentration of ~2 parts per million (ppm) in the crust and 3 × 10-3 ppm in seawater (see Table 1). This means that in every 1 million kg of seawater, on average there is 0.003 kg of Uranium. For now, this resource remains untapped, since over geological time, more concentrated deposits have formed. There are 15 main types of Uranium deposits, as classified by the IAEA. Within this classification scheme, there are 3 types that are responsible for over 75% of known resources: proterozoic unconformity deposits, iron oxide-copper-gold deposits, and sandstone-hosted deposits [1]. The richest deposits known are Proterozoic unconformity deposits [2], which are found primarily in Canada and Australia [1].
Proterozoic unconformity deposits form where there is a discontinuity between older basement rock and younger overlying rock. Near the unconformity, Uranium is deposited in an ore called uraninite. [1,2] (See Fig. 1 for an idea of how the configuration looks.) The highest quality Uranium deposits are of this type, and are found in the Athabasca Basin in Canada. Within this basin, two deposits in particular, at Cigar Lake and McArthur River, are estimated to contain 131,400 tons and 192,085 tons of Uranium, respectively. [2]
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| Table 1: Types of Uranium resources and their concentrations. [4] |
Measuring trace amounts of Uranium is no small feat. The value for seawater concentration in Table 1 is from a 1986 study on Uranium concentration in hydrothermal fluids. [3] In this study, divers collected samples of seawater from areas near hydrothermal vents. In addition, one sample of ambient seawater was also collected for comparison. Samples were brought back to be analyzed by the "Lunatic Asylum" mass spectrometer at Caltech. The ambient seawater sample was found to have a U concentration of 3.22 ng/g, which translations to 3.22 × 10-3 ppm.
Uranium is distributed around the globe in trace amounts. Uranium has also formed rich deposits, like those of the Athabasca Basin, which are mined for nuclear fuel. Scientists have spent much effort to measure the abundance of Uranium on Earth, but our current understanding is not complete, as there are almost certainly more deposits that remain undiscovered.
© Sydney Erickson. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] P. Bruneton, and M. Cuney, "Geology of Uranium Deposits," in Uranium for Nuclear Power (Woodhead Publishing, 2016), p. 11.
[2] C. W. Jefferson et al., "Unconformity-Associated Uranium Deposits of the Athabasca Basin, Saskatchwan and Alberta," Bull. Geol. Surv. Can. 588, 23 (2007).
[3] J. H. Chen et al., "The U-Th-Pb Systematics in Hot Springs on the East Pacific Rise at 21°N and Guaymas Basin," Geochim. Cosmochim. Acta 50, 2467 (1986).
[4] R. M. Hazen, R. J. Ewing, and D. A Sverjensky, "The Evolution of Uranium and Thorium Minerals," Am. Min. 94, 1293 (2009).