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  

Geochemical Characteristics and Water Rock Interaction of the Ancient Interlayer Oxidation Zone of the Zhiluo Formation in the Xunyi−Yijun Area,Southern Ordos Basin

作　　者：张良 胡菲菲 武正乾 王晓鹏 刘坤鹏 毛宁 ZHANG Liang;HU Feifei;WU Zhenqian;WANG Xiaopeng;LIU Kunpeng;MAO Ning(No.203 Research Institute of Nuclear Industry,Xi'an,Shaanxi 710086)

机构地区：[1]核工业二〇三研究所,陕西西安710086

出　　处：《地质与勘探》2023年第6期1250-1259,共10页Geology and Exploration

基　　金：中国核工业地质局铀矿地质项目(编号:202217、202217-2、202115-1、202115-2)资助。

摘　　要：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Fe^(2+)氧化成Fe^(3+)(Fe^(2+)含量最低,Fe^(3+)最高);砂体中的U被氧化为U^(6+),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(SiO_(2)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、ΣS含量最高、Fe^(2+)含量仅低于灰绿色砂体),还原剂(有机质和黄铁矿等)将U^(6+)还原成U^(4+),形成沥青铀矿;U^(4+)与SiO_(4)^(4-)发生反应形成铀石(Th/U比值最低,U含量最大值),SiO_(2)发生沉淀(SiO_(2)含量增高),石英发生次生加大,使得砂体变得致密坚硬。(2)二次还原改造阶段:成矿后下伏的还原性气(流)体沿断层进入目的层砂体,Fe^(3+)被还原成Fe^(2+)(Fe^(2+)含量最高,Fe^(3+)/Fe^(2+)比值最低);黑云母发育绿泥石化,析出K^(+)离子(K_(2)O含量最低),使得砂体呈灰绿色,古层间氧化砂体表现为灰绿色砂岩及包裹着红色残留体的灰绿色砂岩。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 Fe^(2+)in the sand body to oxidize into Fe^(3+)(with the lowest Fe^(2+)content and the highest Fe^(3+)content).The U in the sand body was oxidized to U^(6+),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 SiO_(2)content).The oxidation fluid with a large amount of U migrated to the redox transition zone(with the highest content of Corg andΣS,and only lower Fe^(2+)content than the gray green sand body),and the reducing agent(organic matter and pyrite,etc.)reduced U^(6+)to U^(4+),forming pitchblende.U^(4+)reactd with SiO_(4)^(4-)to form uranium ore(with the lowest Th/U ratio and the highest U content),SiO_(2)precipitated(with an increase in SiO_(2)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 Fe^(3+)was reduced to Fe^(2+)(with the highest Fe^(2+)content and the lowest Fe^(3+)/Fe^(2+)ratio).The biotite developed chloritization and precipitated K^(+)ions(with the lowest K_(2)O 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.

关 键 词：地球化学 水-岩作用 古层间氧化带 直罗组 旬邑-宜君地区 鄂尔多斯盆地 

分 类 号：P592[天文地球—地球化学] P595[天文地球—地质学]