Geochemical characteristics and paleoenvironmental significance of marls from the Anding Formation in the Zhidan area, Ordos basin

Based on the systematic petrological and geochemical analysis of the marls at the top of the Anding Formation in the Zhidan area of Ordos basin, the paleo-salinity, redox environment, paleoclimate and provenance background at the end of the deposition period of the Anding Formation were studied. The results show that the marls of the Anding Formation are mainly composed of calcite and dolomite, of which calcite content is 26.5%~68.5%, with an average of 42.5%, and dolomite content is 10.1%~40.5%, with an average of 25.3%. The rock has a high Sr abundance and Sr / Ba ratio, indicating that the water salinity is high during sedimentation. The range of carbon and oxygen isotopes is not large, δ13CPDB is -3.5‰~2.2‰, with an average of 0.55‰, δ18OPDB is -6‰~-1.2‰, with an average of -3.87‰, which is close to the saltwater carbonate deposits, and the Z value range is 119.53~130.01, with an average of 126.51, indicating that the marl section of the Anding Formation was formed during the lake salinization period. The chemical alteration index CIA and Sr/Cu, SiO2/Al2O3 comprehensively indicate that the chemical weathering of the source area of marl is not strong, and it was deposited in a semi-arid and hot climate. The redox environmental indicators V/Cr, V/(V+Ni) ratio, and the high enrichment of sensitive metal elements U and Mo indicate that the paleowater conditions during the deposition of marl were generally in a reducing environment. La/Yb-∑REE and La/Sc-Co/Th source rock attribute discrimination diagrams show that the main source of marl sedimentary rocks is dominated by felsic rocks, mixed with medium-basic magmatic rocks, which have a certain affinity with Paleoproterozoic metamorphic rock series in the Yinshan area of northern Ordos. The semi-arid and hot climatic conditions at the end of the Anding Formation are conducive to the activation and migration of uranium elements, the formation of oxygen-containing uranium-containing fluids in the source area and the original enrichment and precipitation of uranium in the lake basin, which provides important conditions for large-scale sandstone-type uranium mineralization in the Ordos basin. © 2021, Science Press. All right reserved.