oa Unconformity-related uranium systems: Downunder and over the top

The essentials of the unconformity-related U (± precious and base metals, REE...) association are illustrated by the Mesoproterozoic systems of the Athabasca Basin (Canada) and the Alligator Rivers (Australia) regions, sites of major production and high grade resources of U. Significant elements of such systems include

• i)   a regional redox interface corresponding broadly to the unconformity separating an oxidised sedimentary pile from C and/or Fe²⁺ rich reductants in or near the basement;
• ii)   sources of oxidised (high SO4^2-/H2S, evaporite- interacted, moderate to high salinity) fluids in/above the sedimentary basin succession. Such fluids may transport numerous components and their progressive interaction with reductants may give rise to characteristic geochemical and mineralogical patterns at deposit and broader scales;
• iii)   linked fault systems operative during sedimentary basin inversion or subsequent compression;
• iv)   the mechanical contrasts of the basement with the sedimentary cover package;
• v)   and probably, uranium-enriched rocks in the basement and/or in cover sediments and volcanics.

Other well endowed uranium systems, for example the Proterozoic Westmoreland district (Queensland and the Northern Territory), exhibit the key elements of the unconformity-related association, but the main deposits are localised around redox interfaces the faults away from the basal unconformity. A clear picture of the Westmoreland regional mineralised system is provided by integrated airborne geophysical and geological information, providing a model for systems elsewhere.

The prospectivity of the unconformity-related association is not restricted to the Mesoproterozoic. Examples of systems which display the essentials of the association include the late Palaeozoic-aged Ben Lomond and Maureen U-Mo deposits (Queensland) and others in Central Asia, plus Palaeoproterozoic U, precious and base metal deposits in Canada and elsewhere.

More effective exploration and regional targeting models will result from the application of the model outlined above.