鄂尔多斯盆地纳岭沟铀矿床板状矿体形成机制:来自含矿层黏土矿物研究的制约

Formation mechanism of tabular orebody in Nalinggou uranium deposit, Ordos Basin: Constraints on study of clay minerals from ore-bearing sandstone

纳岭沟铀矿床是近年来在鄂尔多斯盆地北部发现的一个大型铀矿床。与典型层间氧化带砂岩型铀矿不同,纳岭沟铀矿床具有矿体呈板状、似层状产出与铀矿物以铀石为主等特点。文章通过扫描电镜与X衍射等方法对含矿层不同地球化学分带砂岩黏土矿物组合类型、相对含量的变化开展了系统的研究,为该区铀成矿模式与板状矿体的形成机制提供依据。研究结果表明,含矿层砂岩中黏土矿物以呈蜂窝状、片絮状产出的蒙脱石为主,其次为呈片状、书页状及蠕虫状产出的高岭石与呈叶片状、花朵状集合体产出的绿泥石,局部可见伊利石与蒙脱石向绿泥石转化而成的绿蒙混层;从二次还原绿色带→矿带与原生带,呈现蒙脱石、绿泥石明显减少,高岭石含量与w(TOC)、CH4明显增加的趋势,说明二次还原绿色带与矿带之间存在Eh-pH突变界面;含矿层经历了弱碱性大气降水(含铀含氧)、酸性流体(有机酸与煤成气)、中-低温碱性热液油气与富Na+、Ca2+的弱碱性流体(盆地卤水)的共同作用,其中弱碱性大气降水与中-低温碱性热液混合而成的碱性-氧化成矿热液与下伏延安组煤系地层演化产生的酸性-还原流体(有机酸与煤成气)在相互接触面上因Eh-pH突变而造成铀石的沉淀,并在Eh-pH突变界面上形成板状矿体。

The Nalinggou uranium deposit is another large uranium deposit found in recent years in northern of Ordos Basin.It has the characteristics of tabular orebody and coffinite,which is different from features of the typical interlayer oxidation zone sandstone type uranium deposit.The authors carried out a systematic study of the variation of the composition type and relative content of the clay mineral assemblage in different geochemical zones of the ore-bearing layer by scanning electron microscopy and X-diffraction method,and studied the components and sources of fluid in the ore-bearing layer in recent years.Then,the authors investigated the formation mechanism of tabular orebody in the Nalinggou uranium deposit.The results show that the clay minerals in ore-bearing sandstone are composed mainly of montmorillonite with honeycomb and flocculation forms,subordinately of kaolinite with flake,book-page,vermicular forms and chlorite exhibiting blade and flower-shaped aggregates.It can be seen that illite and montmorillonite have been transformed into chlorite-montmorillonite.From the secondary reduction green zone to the ore zone and primary zone,montmorillonite and chlorite decrease significantly and kaolinite and TOC,CH4 obviously increase,suggesting that the Eh-pH interface exists between the ore zone and secondary reduction green zone.The results of the comparison of the relative values of clay minerals in different geochemical zones show that ore-bearing sandstone was affected by multiple fluids which contained weakly alkaline meteoric waters(water with oxygen and uranium),organic acidic fluid(organic acids and coal-derived gas),oil gas,medium and low temperature alkaline hydrothermal solution,and the basin brine was weakly alkaline and rich in Na+、Ca2+.Coffinite and tabular orebody were formed in the Eh-pH interface which was composed of alkaline-oxidized metallogenic hydrothermal fluid mixed with weakly alkaline meteoric waters and medium and low temperature alkaline hydrothermal solution as well as acid-reduction fluid(organic acid and coal gas)produced by the evolution of Yanan formation.