Calcrete-hosted surficial uranium systems in Western Australia: Prospectivity modeling and quantitative estimates of resources. Part 1 – Origin of calcrete uranium deposits in surficial environments: A review

This three-part paper reports the results of geochemical modeling, prospectivity modeling and quantitative resources assessments of calcrete-hosted surficial uranium deposits in the palaeochannels of geologically, physiographically, and climatologically permissive part of Western Australia. In this Part 1, geochemical dynamics of uranium mobilization and precipitation in near-surface oxidized groundwater systems are reviewed in order to understand the processes responsible for precipitation of uranyl vanadate minerals in valley/lacustrine calcrete within palaeochannels in arid and semi-arid desertic regions. The review indicates that uranium precipitation is essentially a function of concurrent changes in three mutually interdependent parameters of groundwaters, namely, ∑CO₂ (i.e. activities of all the carbonate bearing species), Eh-pH, and the activities of other ionic species. Geochemical modeling of groundwater data from northern part of the Yilgarn craton in Western Australia indicates that opening of the groundwater aquifer system to the atmosphere and consequent evaporation is likely the key process leading to precipitation of uranyl vanadate minerals in valley calcrete and playa lake sediments. Fluid mixing could also induce the precipitation below the water table, provided that the mixing groundwaters have contrasting geochemistry. These processes may operate in diverse geomorphic traps in palaeochannels. A generalized conceptual model of calcrete-hosted uranium systems is presented and regional-scale targeting criteria and their spatial proxies are identified, which, in turn, are used in Part 2 to develop a targeting model for calcrete-hosted uranium deposits in Western Australia.