Abstract:The ancient interlayer oxidation zone of the Zhiluo Formation in the Xunyi - Yijun area of the southern Ordos Basin has obvious zoning characteristics. By comparing the characteristics of major, trace, rare earth elements, and environmentally sensitive parameters of sandstone in different zones, the water- rock interaction of the ancient interlayer oxidation zone in the Xunyi- Yijun area was divided into two stages. (1) The formation of ancient interlayer oxidation zone+large-scale mineralization stage: Oxygenated water entered the target layer sand body, causing Fe2+ in the sand body to oxidize into Fe3+ (with the lowest Fe2+ content and the highest Fe3+ content). The U in the sand body was oxidized to U+, forming a uranyl humic acid salt complex for migration (with the lowest U, Corg, S content, and the highest Th/U ratio). Feldspar kaolinization causes Si loss (decrease in SiO2 content). The oxidation fluid with a large amount of U migrated to the redox transition zone (with the highest content of Corg and Z S, and only lower Fe2+ content than the gray green sand body), and the reducing agent (organic matter and pyrite, etc.) reduced U6+ to U4+, forming pitchblende. U4+ reactd with SiO24- to form uranium ore (with the lowest Th/U ratio and the highest U content), SiO2 precipitated (with an increase in SiO2 content), and quartz underwent secondary enlargement, making the sand body dense and hard. (2) Secondary reduction and transformation stage. After mineralization, the underlying reducing gas (flow) body entered the target sand body along the fault, and Fe3+ was reduced to Fe2+ (with the highest Fe2+ content and the lowest Fe3+/Fe2+ ratio). The biotite developed chloritization and precipitated K+ ions (with the lowest K2O content), resulting in grayish green color of the sand body. The ancient interlayer oxidized sand body was characterized by grayish green sandstone and grayish green sandstone surrounded by red residual bodies.
