Relating unconformity-type uranium mineralization of the Alligator Rivers Uranium Field (Northern Territory, Australia) to the regional Proterozoic tectono-thermal activity: An illite K-Ar dating approach

Abstract Illite particles of Comparison with ages obtained before by U–Pb, 207 Pb/ 206 Pb, Sm–Nd and Rb–Sr on UO 2 , and Rb–Sr and 40 Ar/ 39 Ar on illite and other silicates of deposits comparable to those studied here, suggests that illitization was not always linked to U precipitation. However, significant amounts of illite crystallized between ∼1680 and 1520 Ma (K–Ar individual ages) at the time U was deposited. This indicates that the unconformity-type U deposits formed probably during three major regional events between 1680 and 1520 Ma (at 1656, 1600 and 1554 Ma). Only detected away from U concentrations between ∼1457 and 1318 Ma, the younger illitization episodes resulted probably from U-devoid hydrothermal fluids, although probably of basinal origin, during minor tectono-thermal events. Further U–Pb ages from 1300 to 500 Ma obtained previously on UO 2 are also considered as isotopic resets. Not linked to illitization, they were probably not connected to tectono-thermal activity and large-scale brine circulations. These late events appear to have played only a minor role in the U endowment of the region. This study demonstrates also that only specific tectono-thermal events of the 1680–1520 Ma period in the ARUF region induced regional mineralizing brine circulations and subsequent U deposition, while younger illitization episodes until ∼1300 Ma caused restricted fluid circulations and illitization episodes without U supply, as records of the further history of this U deposition. At a broader scale, this study confirms also the critical importance of the geodynamic and subsequent tectono-thermal conditions during this 140-Ma long period during which a variety of world-class metal deposits (U, but also Cu, Pb and Zn) formed in the Paleo-Proterozoic sedimentary basins of the North-Australian Craton.