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

Abstract

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, U 6+ migration was activated, whereas under reducing aqueous conditions, U 6+ was reduced to U 4+ , resulting in eventual sedimentation of coffinite as ore.

Sedimentary Environment, Tectonic Setting, and Uranium Mineralization Implications of the Yimin Formation, Kelulun Depression, Hailar Basin, China

“…Based on the major elements, trace elements, and REEs, four major tectonic setti are distinguished for sandstones [47,48]: oceanic island arc (OIA), continental island a (CIA), active continental margin (ACM), and passive continental margin (PCM). Accor ing to the SiO2/Al2O3-K2O/Na2O diagram (Figure 9a), Al2O3/(Na2O+CaO)-(Fe2O3 T + Mg diagram (Figure 9b), K2O/Na2O-(Fe2O3 T + MgO) diagram (Figure 9c), and SiO2/Al2O K2O/(Na2O + CaO) diagram (Figure 9d), the samples fall mainly within the PCM regio or on the border between the PCM and ACM regions.…”

“…Based on the major elements, trace elements, and REEs, four major tectonic setting are distinguished for sandstones [47,48]: oceanic island arc (OIA), continental island arc (CIA), active continental margin (ACM), and passive continental margin (PCM). According to the SiO 9d), the samples fall mainly within the PCM regions or on the border between the PCM and ACM regions.…”