Provenance and Tectonic Setting of the Lower Cretaceous Huanhe Formation in the Northwestern Ordos Basin and Its Implications for Uranium Mineralization

The Ordos Basin is an important sandstone-type uranium enrichment region in China, and the Lower Cretaceous Huanhe Formation has attracted significant attention as a newly discovered ore-bearing stratum. To elucidate the provenance, tectonic background, and sedimentary environment constraints on uranium enrichment in the Huanhe Formation sandstone-type uranium deposits, 10 representative sandstone samples from the study area were analyzed by using electron microscopy, X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and electron probe microanalysis. Independent uranium minerals in the Yihewusu area of Hangjin Banner were shown for the first time to be composed mainly of coffinite and titanium-uranium oxide, with trace amounts of pitchblende. The major element diagrams of the sandstone and ratios of Sr/Ba, V/Cr, and U/Th and enrichment factors of Mo and U revealed that the source rocks of the Huanhe Formation sandstone in the study area were intermediate-felsic igneous rocks. The tectonic setting is characterized as an active continental margin, with later deposition in brackish-to-marine water environments. The ore-bearing strata indicate a reducing environment, whereas the nonore-bearing strata indicate a weakly oxidizing environment. With reference to previous studies, the sedimentary material primarily originated from the medium-acidic intrusive rocks exposed in the northern portion of the basin, including the Daqing-Wula Mountains, the Yin Mountains, and middle-acidic intrusions along the eastern margin of the Alxa region in the western part of the basin. The uranium-rich granitic pluton of the source area contributed to the preenrichment of uranium in the target sandstone layer. Under oxidizing aqueous conditions, U6+ migration was activated, whereas under reducing aqueous conditions, U6+ was reduced to U4+, resulting in eventual sedimentation of coffinite as ore.