松辽盆地南部海力锦铀矿床姚家组下段黏土矿物特征及其铀成矿意义

Characteristics of Clay Minerals in the Lower Member of Yaojia Formation and Its Significance for Hailijin Uranium Deposit in the South of Songliao Basin

黏土矿物是表征盆地砂岩型铀矿含矿建造地球化学环境以及反演其经历的表生流体作用的重要指示性矿物。文章以钱家店凹陷新发现的海力锦铀矿床含矿层姚家组下段为研究对象,通过偏光显微镜、X射线衍射、扫描电镜及能谱分析等方法,系统开展了不同地球化学分带的岩石学、黏土矿物类型、形貌及含量等特征研究。姚家组下段红色和灰色泥岩原生黏土矿物组合为伊利石+伊蒙混层,高岭石含量较低,总体反映了沉积期偏干旱炎热古气候环境。泥岩中不同类型黏土矿物含量具有一定的相关性,推测其在沉积-成岩过程中存在一定的相互转化关系。与泥岩明显不同,各地球化学分带砂岩中的黏土矿物主要为高岭石,其次为伊利石和伊蒙混层,显示具有后生外来酸性流体改造的特点,应发生在区域层间氧化水渗入之前。铀含量与黏土矿物含量间没有明显的相关性,结合镜下铀-黏土矿物空间赋存关系和区内构造-流体演化史显示,矿石中铀沉淀富集受多种吸附还原剂控制,局部与高岭石关系密切,存在一期深部酸性还原流体渗出改造作用,为原生偏弱还原性的姚家组下段砂岩铀成矿提供了有利条件。

Clay minerals are important to the study of geochemical environment of the ore-bearing layer of sandstone-type uranium deposits and the investigation of fluid-rock reaction process.The lower member of Yaojia Formation in Hailijin deposit that was recently discovered in the Qianjiadian sag was studied with microscopy,X-ray diffraction,scanning electron microscopy,and energy spectrum to examine the petrographic characteristics of rocks in various petro-geochemical zones,and the clay mineral combination,content and microscopic morphology.The results showed that the native clay mineral assemblage of red and gray mudstone in the lower member of Yaojia Formation were mainly composed of high content illite-smectite mixed-layer and low levels kaolinite,which was consistent with the arid and warm paleoclimate condition during the deposition period.The content of clay minerals of mudstone is positive or negative correlated indicating that they underwent mutual-transformation during the diagenetic evolution.The clay minerals of the sandstones with different geochemical properties are all predominantly consisted of kaolinite,followed by illite-smectite mixed-layer,which are distinctly different from the mudstones.This was believed the result of post-exogenic exotic acidic fluid seepage alteration that occurred prior to the infiltration of regional interlayer oxidized water.No clear correlation was established between the uranium and clay minerals content.Combining the microscopic evidence of uranium-clay occurrence and the structural-fluid evolution history of the region,uranium in sandstone was suggested to be precipitated and enriched under the comprehensive action of various adsorption reductants.The modification from early-stage acidic and reducing foreign fluid was the key controlling factor for uranium mineralization of the lower member of Yaojia Formation that have primary weakly reductive capacity.