Giant uranium deposits formed from exceptionally uranium-rich acidic brines

Giant uranium deposits were formed during the Mesoproterozoic era, 1.6-1.0Gyr ago, in both Canada and Australia. The deposits are thought to have formed from large-scale circulation of brines at temperatures of 120-200 degrees C that percolated between sedimentary basins and underlying crystalline basement rocks(1-3). However, the precise conditions for transport of the uranium in these brines are poorly understood(4-7). 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 x 10(-6)-2.8 x 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 degrees 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 gold(8,9).