蒙其古尔铀矿床灰白色砂岩成因及其成矿作用机制

Origin of Off-white Sand Body and Metallogenic Mechanism of Mengqigur Deposit

蒙其古尔作为典型的砂岩型铀矿床,发育大规模灰白色砂体,且与铀成矿具有密切的空间关系。文章旨在通过研究含矿目的层水西沟群砂体的岩石学特征和地球化学特征,分析成矿流体性质,研究大规模灰白色砂体的主要成因及其与铀成矿作用之间的关系。通过研究表明造成砂体“白色化”的原因主要有:砂岩发生了强烈的以高岭石为主的黏土化蚀变;砂岩中铁离子发生迁移,且高价铁离子发生了还原反应;砂岩中大量有机质被氧化。其中关键原因是以高岭石为主的黏土矿物的“增白反应”。当层间红色氧化砂体遇到酸性还原性流体,在经历长期稳定/动荡交替的水-岩反应后,形成大量以高岭石为主的黏土矿物,砂体呈现“白色化”,此过程说明灰白色砂体的形成和形态主要受控于成矿流体的性质。成矿流体中的无机流体主要为层间含铀含氧水,可将含矿建造中的铀活化出来并迁移;有机流体中有大量来源于含矿建造中析出的煤成气和深部烃源岩沿裂隙逸散的天然气,成矿流体的氧化还原环境和酸碱性的改变,可直接将层间流体中的高价铀还原为低价铀,同时增加流体H+浓度,可形成大量以高岭石为主的黏土矿物,这是砂体“白色化”的主要成因,黏土矿物的对铀吸附作用,可加速铀的富集。

As a typical sandstone-type uranium deposit,Mongqigur deposit developed a large scale off-white sand body,which has close spatial relationship with uranium mineralization.The petrological and geochemical characterstics of the Shuixigou Group sandstone was studied and the relationship between the sandbodies and mineralization was analyzed.The results showed that the“whitening”of the sandstone had undergone strong clay alteration dominated by kaolinite which made the iron in the sandstone migrated out with the reduction of highvalence iron and the oxidation of a large amount of organic matter in sandstone.The key reason is the“whitening reaction”of clay minerals dominated by kaolinite.When the interlayer oxidized red sand body encounters acid reducing fluid and experienced long-term stable and turbulent alternating water-rock reaction,forming a large number of clay minerals will be formed and dominated by kaolinite,and the sand body appeared“whiteness”,indicating that the formation and morphology of off-white sand body are mainly controlled by the nature of oreforming fluid.The inorganic fluid in ore-forming fluids is mainly interlayer water containing uranium and oxygen,which can activate and transport the uranium in sedimentary formations.Acidic reductive substances in organic fluids were derived from coal gas precipitated from ore-bearing formations and gas escaping from deep source rocks along fractures,changed the redox environment and pH value,which can reduced the high-valence uranium to lowvalence uranium in interlayed fluid.At the same time,it increased the concentration of H+in the fluid,resulting in the production of a lot of clay minerals,and accelerate uranium enrichment through adsorption.