Formation of large carbonate concretions in black cherts in the Gufeng Formation (Guadalupian) at Enshi, South China

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.

Response of Paleogene Fine-Grained Clastic Rock Deposits in the South Qiangtang Basin to Environments and Thermal Events on the Qinghai-Tibet Plateau

The Chamdo Basin is a secondary basin in the eastern part of Tibet China and is one of the most promising of petroliferous basins for new petroleum exploration. The Qamdo Basin records a complex burial history from the Mesozoic to the Cenozoic; however, the poorly constrained sedimentology of Cenozoic strata in this basin has severely obscured the overall profile and impeded further explorations of oil and gas resources. Here, we conduct whole-rock geochemical analyses of major, trace, and rare earth elements in fine-grained clastic rocks of the Paleocene Gongjue Formation, Qamdo Basin to reveal depositional environments, provenance, and tectonic setting. Petrologically, the Gongjue Formation is dominated by red fine-grained sandy mudstones/siltstones with ripple marks. The high values of the chemical index of alteration (avg. of 78.93), chemical index of weathering (avg. of 90.10), and index of compositional variability (avg. of 2.5) suggest that the basin has undergone heavy weathering. Cross-plots of La vs Th, Th vs Sc vs Zr/10, and Th vs Co vs Zr/10 reveal a continental arc tectonic setting. Paleosalinity (Sr/Ba), paleoclimate (Sr/Cu), and redox proxies (V/Cr, U/Th, and enrichment factors of Mo and U) indicate brackish to saline and oxidizing paleowater masses during deposition of the Gongjue Formation. Provenance analyses via elements and petrology reveal that sediments in the Gongjue Formation are mainly derived from intermediate-acidic rocks of the upper crust. We conclude that the first and third members are more arid climate and heavily chemically weathered than the second member. In combination with previous studies of the structural evolution of the Qamdo Basin since the Paleogene, a model is built to describe the sedimentary environment and evolution of the Qamdo Basin during transition to the Paleocene. The first and third members, i.e., the Eg¹ and Eg³ members of the Gongjue Formation, are dominated by an oxidizing environment of seawater-saltwater, and the climate ranges from warm and humid to arid and hot, with relatively stable environmental changes. The Eg² member of the Gongjue Formation is dominated by an oxidizing environment of seawater-saltwater, and the climate ranges from warm and humid to arid and hot, with more frequent environmental evolution. Our model aids in better understanding of the Paleocene climate evolution of the eastern Tibetan Plateau.