Fluid migration regimes during the formation of the unconformity-related uranium deposits of the Alligator Rivers Uranium Field, Australia

Abstract

In world U production, the unconformity-related deposits currently yield ~24%. The main unconformity- related deposits were found in the Athabasca U-bearing province in the southwestern part of the Canadian Shield and Alligator Rivers Uranium Field of the North Australian Shield. The deposits are localized close to the surfaces of structural-stratigraphic unconformity in the bottom of sedimentary basins: Athabasca in Canada and Kombolgie in Australia. According to a series of geological–structural parameters of formation, the Canadian and Australian unconformity-related deposits significantly differ allowing their classification as subtypes. In the Canadian deposits, the ore bodies occur both above and beneath the unconformity surface of the Athabasca basin, whereas the ores of the Australian deposits are localized exclusively beneath the unconformity surface in rocks of the basement of the Kombolgie basin. This paper is devoted to the reconstruction of paleohydrodynamic formation conditions of the Australian unconformity-related deposits. The computer models of fluid migration of following three scenarios were successively considered during the analysis of fluid mass transfer processes: (i) thermal fluid convection in a fault zone with periodic upward and downward free thermal fluid convection, (ii) forced convective fluid migration at subcritical permeability and therefore the absence of free thermal convection in the fault zone, and (iii) mixed convection with upward and downward fluid movement along the fault zone. We concluded that the processes of periodic thermal convective fluid circulation in the fault zone contradict the idea of infiltration mechanism of the formation of the Australian unconformity-related deposits in fault zones in the basement of the Kombolgie basin. We, therefore, considered possible influence of facies zoning of a combined aquifer in the basement of clastic sediments of the Kombolgie Supergroup on paleohydrodynamics of the ore-bearing system, which played the role of a main migration pass for U-transporting fluids. The analysis of zonal distribution of primary sedimentation environments and later diagenetic transformations of rocks of aquifers and aquitards of this basin allowed us to substantiate an idea on leading influence of zonal decrease in permeability of rocks of the combined aquifer on a circulation structure of U-transporting fluids with the change in their lateral migration in the basement of sedimentary deposits of the Kombolgie Supergroup on the downward infiltration along a transverse zone of the ore-controlling fault. This circulation of the structure of U-transporting fluids was accepted as a hypothesis of paleohydrodynamic formation conditions of Australian unconformityrelated deposits exclusively in rocks of the basement of the Kombolgie basin. The additional computer calculations, however, showed that a trend of the directed change in the permeability of rocks along the lateral movement pass of U-transporting fluids is a trigger condition, which can be responsible both for the upward and downward fluid movement along the ore-bearing fault zone depending on the direction of the trend. In the Kombolgie basin in Australia, the direction of this trend depended on the change of facies conditions on the regional migration pass of diagenetic fluids, whereas the local topographic features of the unconformity surface could affect the Athabasca basin in Canada. At an alternative trend of variation in permeability along the lateral migration of U-transporting fluids, the proposed transport mechanism of the formation of the Australian unconformity-related deposits could probably contribute also to the formation both of infiltration and exfiltration Canadian unconformity-related deposits, which are described by a paleohydrodynamic interfault geothermal convection model.