鄂尔多斯盆地东北部砂岩型铀矿伴生矿物组合及其地质意义

Associated Mineral Assemblage of Sandstone-type Uranium Deposit in the Northeastern Ordos Basin and its Geological Significance

特征矿物是地质作用的直接记录,研究铀矿物伴生组合类型和特征为探讨铀矿床成因提供直接信息。本文以鄂尔多斯盆地东北部杭锦旗‒纳岭沟地区含铀岩系中侏罗统直罗组为研究对象,通过岩心观察、显微观察、扫描电镜和电子探针分析等,系统研究了含铀砂岩中铀矿物种类、赋存特征及典型矿物伴生组合类型,在此基础上,结合铀成矿过程中流体作用探讨了铀成矿机理。取得的主要成果和认识如下:(1)研究区含矿目的层最主要的矿物类型为铀石,与铀矿物相关的矿物组合包括:莓球状黄铁矿‒铀石、黑云母‒它形黄铁矿‒铀石、胶状黄铁矿‒铀石、钛铁矿‒锐钛矿‒铀石、高岭石‒铀石、蒙脱石/伊蒙混层‒铀石、方解石‒铀石、有机质‒铀石和石英‒铀石9种;(2)研究区成矿环境经历了成岩期的中性‒弱碱性氧化环境→弱酸性氧化环境→成矿早期的弱酸性还原环境→成矿中‒晚期的弱碱性还原环境→成矿期后的碱性还原环境的转变;(3)铀矿物的富集可以分为5个阶段:①早期预富集的碎屑铀;②石英颗粒边缘富集的沥青铀矿;③莓球状、它形、胶状黄铁矿、高岭石、钛铁矿边缘沉淀富集,与酸性还原流体有关的铀石;④与石英、方解石、蒙脱石/伊蒙混层等伴生,与碱性还原流体有关的铀石;⑤与莓球状黄铁矿、胶状黄铁矿、方解石伴生的受中低温热液改造的铀石。综上所述,研究区主成矿作用受酸性和碱性还原流体共同控制。

The typical diagnostic minerals are direct records of geological processes.The types and characteristics of minerals associated with uranium minerals may provide direct information concerning the genesis of the deposits.Here we report a case study on the uranium-bearing rock series in the Hangjinqi-Nalinggou area.Through core observation,microscopic observation,scanning electron microscopy and electron probe analysis,the uranium mineral types,occurrence characteristics and typical mineral assemblage were systematically investigated,combining with the fluid features,the mechanism of uranium mineralization is discussed.The main conclusions can be summarized as follows:(1)The main mineral type in the study area is coffinite.The mineral assemblages associated with uranium enrichment include 9 types:framboidal pyrite-coffinite,biotite-xenomorphic pyrite-coffinite,colloidal pyrite-coffinite,ilmenite-anatasecoffinite,kaolinite-coffinite,montmorillonite/montmorillonite-illite mixed layer-coffinite,calcite-coffinite,organic mattercoffinite and quartz-coffinite.(2)The ore-forming environment varied from neutral-weakly alkaline oxidation→weakly acidic oxidation→weakly acidic oxidation in the early stage;weakly alkaline reduction environment in the middle and late stage;and alkaline reduction in the post-mineralization stage.(3)Uranium enrichment can be divided into 5 stages:①pre-enrichment of detrital uranium minerals;②pitchblende enrichment at the edge of quartz particles;③coffinite precipitation at the edge of the colloidal pyrite,framboidal pyrite,anatase and kaolinite under acidic reducing environment;④coffinite precipitated from alkaline reducing fluids and associated wiht quartz,calcite and montmorillonite/montmorilloniteillite mixed layer;⑤coffinite modified by medium-low temperature hydrothermal fluids and associated with framboidal pyrite,colloidal pyrite and calcite.In summary,the main mineralization in the study area was controlled by both acidic reducing and alkaline reducing fluids.