鄂尔多斯盆地南部旬邑-宜君地区直罗组古层间氧化带地球化学特征及水-岩作用探讨

鄂尔多斯盆地南部旬邑-宜君地区直罗组古层间氧化带具有明显分带特征，通过对不同分带砂岩主量、微量、稀土元素及环境敏感参数特征对比研究，将旬邑-宜君地区古层间氧化带发育的水-岩作用过程划分成两个阶段：古层间氧化带形成和大规模成矿阶段、二次还原改造阶段。(1)古层间氧化带形成和大规模成矿阶段：含氧水进入目的层砂体，使砂体中Fe²⁺氧化成Fe³⁺(Fe²⁺含量最低，Fe³⁺最高)；砂体中的U被氧化为U⁶⁺，形成铀酰腐殖酸盐络合物发生迁移(U、Corg、S含量最低、Th/U比值最高)；长石高岭土化使Si发生流失(SiO₂含量降低)；携带大量U的氧化流体运移至氧化还原过渡带(Corg、∑S含量最高、Fe²⁺含量仅低于灰绿色砂体)，还原剂(有机质和黄铁矿等)将U⁶⁺还原成U⁴⁺，形成沥青铀矿；U⁴⁺与SiO₄^4-发生反应形成铀石(Th/U比...

Geochemical characteristics and water rock interaction of the ancient interlayer oxidation zone of the Zhiluo Formation in the Xunyi-Yijun area, southern Ordos Basin

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.