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 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 × 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 gold 8,9 .Models for the formation of hydrothermal ore deposits imply either protracted fluid flow with relatively low metal concentration 10 , or more discrete pulses of extremely metal-rich fluids 8,9 . The dynamics of metal input in the ore systems has profound consequences for the duration of mineralizing events, a crucial issue to ore genesis 8-10 . If there is no possibility of acquiring sufficiently precise geochronological data, as in the present case 1 , the duration of the mineralizing events is first-order approximated by (1) metal concentration in mineralizing fluids, (2) fluid-flow rates and (3) rates of precipitation of the metal-bearing minerals. Therefore, to understand massive metal transport and deposition in the Earth's crust, both analytical and experimental studies on deep fluids are needed 8-14 .Unconformity-related U deposits from the Proterozoic Athabasca (Canada) and Kombolgie (Australia) basins show spectacular grades and tonnages (up to 200 kt U and up to 20% U on average) when compared with the average U content of ∼1 ppm in the continental crust 15 . They document the remarkable efficiency of the processes of U concentration from the source rocks to the UO 2 deposits 1 . In these basins, the basal sandstones underwent large-scale circulation of evaporated seawater-derived oxidizing brines at a temperature of 120-200 • C (refs 16-18). These