These are astounding concentrations of uranium, a very common heavy metal that is easily dissolved and transported at today’s earth surface. Ore deposits of this type comprise pods, veins and disseminations of uranium oxide located at or close to unconformity at the base of uranium-depleted flat-lying strata about 1.7 to 1.8 billion years old. The underlying uranium-rich basement is highly deformed metamorphosed sedimentary and granitoid rock from 1.75 to 2.6 billion years old. The flat-lying strata that hide and host such deposits are mainly fluvial, areally extensive, and pervasively altered red to pale tan conglomerate, sandstone, siltstone and mudstone composed >95% of quartz with minor interstitial clay. They were deposited on paleo-weathered basement with a well developed lateritic profile. The basement complex includes graphitic metapelitic gneiss that preferentially hosts shear zones and many of the uranium deposits. Key factors in localizing the deposits are faults with long reactivation histories that offset the unconformity and are spatially related to basement highs and lows. The deposits form two compositional end-members. Polymetallic lenses are located just above or straddle the unconformity, with variable Ni, Co, As, Pb and traces of Au, Pt, Cu, REE and Fe. Monometallic veins are generally basement-hosted in shear zones. Some 33% of the world’s known conventional uranium resources are hosted by these deposits, mainly in Australia and Canada, with the highest grades and potential being in the Athabasca Basin. For example, the McArthur River deposit has an average grade of 22.3 % in a known conventional resource of 192,000 tonnes uranium metal. Every year since 1968 when Rabbit Lake was recognized for what it was, there has been a significant discovery or advancement of resources in the Athabasca Basin.
Acknowledgments. The review of unconformity-associated deposits is co-authored with D.J. Thomas, S.S. Gandhi, P. Ramaekers, G. Delaney, D. Brisbin, C. Cutts, P. Portella and R.A. Olson. The new uranium project will involve these and numerous other stakeholders across Canada, under the Secure Canadian Energy Supply Program of ESS.
