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 <0.4, 1-2 and <2 mu m size were separated from Kombolgie sandstones and basement rocks of the Alligator Rivers Uranium Field (Northern Territory, Australia) at the Jabiluka and Nabarlek deposits and at nearby exploration sites, on the basis of a previous X-ray diffraction and scanning electron microscope study. Dated by the conventional K-Ar method to constrain the regional tectono-thermal evolution, illite crystallized during successive Proterozoic tectono-thermal episodes at 1774 +/- 40 Ma for one separate of a basement rock, and at similar to 1656, 1600, 1554, 1457, 1425, 1371, 1355 and 1318 Ma (mean-age peaks). Most of the studied host-rocks underwent hydrothermal processes, mainly from similar to 1650 to 1300 Ma, that monitored large-scale basin-derived brine flows initiated by contemporary tectonic pulses. Comparison with ages obtained before by U-Pb, Pb-207/Pb-206, Sm-Nd and Rb-Sr on UO2, and Rb-Sr and Ar-40/Ar-39 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 similar to 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 similar to 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 UO2 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 similar to 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.
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Precambrian Research, Elsevier, 2015, 269, pp.107-121. 〈10.1016/j.precamres.2015.08.007〉
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Norbert Clauer, Julien Mercadier, Patricia Patrier Mas, Emmanuel Laverret, Patrice Bruneton. 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. Precambrian Research, Elsevier, 2015, 269, pp.107-121. 〈10.1016/j.precamres.2015.08.007〉. 〈hal-01276855〉

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