Uranium in animals, vegetables and minerals: landscape geochemical and biogeochemical expressions of the Four Mile West sedimentary uranium deposit, South Australia

Abstract

The sediment-hosted uranium deposit at Four Mile West is buried by 130–150 m of sediments yet has a landscape as well as a surficial geochemical and biogeochemical expression of the mineral system. The landscape expression includes exposure of stratigraphy with now-oxidised redox overprinting (including ‘roll-fronts’) equivalent to those in the buried mineralisation. The landscape also includes morpho-tectonic expressions of faults that have been responsible for the development and preservation of mineralisation. Most importantly, this faulting has produced an embayment in the range front corresponding to a down-faulted area on the plains that has hosted throughflow of surface and groundwater and preservation of underlying, chemically reduced, ore-hosting Mesozoic and Cenozoic sediments. Landscape geochemical and biogeochemical expressions of buried mineralisation are best represented in U2/Th and selected trace elements, rather than simply elevated U contents in isolation. These associations are especially important for distinguishing between the U-rich expressions of buried, sediment-hosted mineralisation and laterally transported U-rich detritus from the adjacent ranges. The surficial geochemical and biogeochemical expression of the underlying mineralised substrate, with higher U2/Th, is best expressed in regolith carbonates, river red gums and potentially ants and macropod droppings. The mechanism proposed for this vertical geochemical transfer is in part tree-root depth penetration (or burrowing in the case of ants) but most importantly upward groundwater flux. Important groundwater fluctuations are connected to low-frequency–high-intensity rainfall periods that drive groundwater recharge and significantly raises the water-table towards the land surface, especially along faults and fractures near the range front and the immediately adjacent plains. During seasons with high rainfall, this groundwater rise may even express itself as water discharge at springs along the range front. Other media, such as stream sediments, soils and inland tea tree are more closely associated with expressing lateral physical dispersion and re-accumulation of U within the landscape. This study shows that surficial mapping, geochemistry and biogeochemistry within a landscape processes/landscape system context can be invaluable for the detection of, and exploration for, buried sediment-hosted uranium mineral systems, and most especially understanding more about the behaviour of chemical elements in the Australian landscape.