鄂尔多斯盆地纳岭沟铀矿床铀矿物特征与形成机理

Characteristic of Uranium Minerals in Nalinggou Uranium Deposit of Ordos Basin and Their Formation Mechanism

纳岭沟铀矿床位于鄂尔多斯盆地北东部,是我国近年发现的一个特大型砂岩铀矿床,铀矿体赋存于中侏罗统直罗组下段河流相砂体中,受古层间氧化带控制,呈板状。铀矿物主要为铀石、沥青铀矿和钛铀矿,其中铀石是最主要的铀矿物。沥青铀矿中w(CaO)较高;铀石中w(UO_2)偏低、w(SiO_2)偏高,w(UO_2)/w(SiO_2)达到1:1.29,远低于正常值;钛铀矿则呈现U低Ti高的特点。根据各铀矿物的形成、富集机理,认为纳岭沟铀矿床在古层间氧化发育阶段赋矿砂体属于酸性环境,SiO_2活度较低,形成铀矿物主要为沥青铀矿。始新世晚期及以后,氧化作用不发育,赋矿砂体被二次还原,岩石地球化学环境由酸性转变为弱碱性,SiO_2活度增大,使早期阶段形成的沥青铀矿转变为铀石。钛铀矿主要是在氧化—还原作用下由含铀溶液中的铀(UO_2^(2+))交代重矿物—钛铁矿中的Fe2+而形成。

Nalinggou uranium deposit, located in northeastern of Ordos Basin, is one of the in situ leachable sandstone type uranium deposit in China. Its uranium body hosted in braided river sand body of the Zhiluo Formation, the Midde Jurassic Series, is controlled by paleo interlayer oxidation zone, occurring as tabula shape. Coffinite is the main uranium mineral with uraninite and brannerite. The CaO content of uraninite is relatively high; the content of UO2 in coffinite is relatively low; SiO2 is relatively high, and w（UO2）/w（SiO2） （1：1.29） is far lower than normal value; brannerite are characterized by high UO2 and low TiO2. According to uranium minerals＇ formation and acceleration mechanism, we obtained that Nalinggou uranium deposit belongs to acid environment at the paleo interlayer oxidation zone developmental stage, the activity of SiO2 is relatively low, and mainly generated uraninite. After the Late Eocene, oxidationwas poorly developed and the ore beating sand body was reduced secondly. The petrogeochemical environment turned to weak alkaline, so the activity of SiO2 increased, and prompted the pre-forming uraninite transformed into coffinite. The brannerite formed with metasomatism of UO2 in uranium-bearing solution and FeE＋ in ilmenite with the action of oxidization-reduction.