新疆伊犁盆地511砂岩型铀矿床成矿作用机理研究

Geological characteristics and metallogenic mechanism of No. 511 sandstone-type uranium ore deposit in Yili basin,Xinjiang

为探讨511铀矿床的形成机理,通过野外岩心观察和系统样品采集,结合室内化学分析、扫描电镜、电子探针等方法,详细研究了赋存于Ⅴ旋回砂体的铀矿地质特征,对砂岩铀的成矿作用展开了初步讨论。认为Ⅴ旋回层间氧化带可划分为氧化带、氧化_还原过渡带(包括褪色亚带和铀矿石亚带)和还原带;铀矿赋存在过渡带砂岩中,空间展布严格受层间氧化带控制;铀主要以显微浸染状铀矿物(沥青铀矿+铀石)形式散布于炭屑木质细胞腔内壁和砂岩碎屑颗粒的填隙物中,部分以吸附形式赋存于粘土矿物和炭屑;层间氧化带发育是含氧层间承压水与砂岩之间水岩作用的结果。伴随砂岩中有机质、S2-和Fe2+被氧化,形成的有机酸类、SO2-4和Fe3+等组分部分迁移至过渡带沉淀,也是砂岩中铀元素活化迁移、沉淀富集,再活化、再富集的循环过程;层间氧化带及其砂岩铀成矿作用发生于表生环境,始于喜马拉雅早期(66Ma),具有长期性和滚动式向前发展的特点;铀成矿可能与微生物细菌活动及其还原作用密切相关。

The ore bodies of the No. 511 uranium deposit on the southern edge of Yili basin are mainly hosted in Ⅴ- cycle sandstone of Lower to Middle Jurassic Shuixigou Group. The deposit is one of the recently-discovered large-size ISL-amenable sandstone-type uranium deposits in China. A detailed study of ore body geological characteristics and a tentative discussion on mineralization were conducted in this paper based on field work as well as such indoor analytical methods as chemical analysis, scanning electron microanalyzer （SEM） analysis and electron microprobe analysis. It is shown that the V-cycle interlayer oxidation zone can be divided into oxidized zone, redox zone and unoxidized zone. Ore bodies are hosted in sandstone of the redox zone, and their spatial distribution is strictly controlled by the interlayer oxidation zone. Uranium mainly occurs in pitchblende and coffinite which are mainly dispersed in the fossil wood cells and pelitic cement of sandstone detrital grains, and partly adsorbed by clay minerals or carbonaceous debris. The interlayer oxidation zone was formed in the process of water-rock reaction between oxygen-bearing interlayer water and sandstone. Uranium was activated, trans-ferred, deposited and concentrated in the redox zone by the ore-forming fluids which contained oxidization components such as organic acids, SO4^2- and Fe^3＋ . The development of the interlayer oxidation zone and the oreforming process started in early Himalayan period（66 Ma）in a surficial environment. The ore-forming process was characterized by rolling and protracted development. Mineralization seems to have been closely related to the reducing function of microbial bacteria.