松辽盆地西南部DL矿床青山口组砂-泥岩协同成岩作用及其铀成矿效应

Diagenesis of Sand-Mudstone from Qingshankou Formation and Their Uranium Mineralization in DL Deposit,Southwestern Songliao Basin

为查明松辽盆地西南部DL铀矿床青山口组红杂色层砂-泥岩协同成岩作用及其铀成矿效应,系统开展了不同颜色岩石的岩石、矿物、元素及同位素特征研究.结果表明,砂、泥岩具有相似的碎屑物组成、黏土矿物组合以及主微量元素含量或比值,反映了统一的物源和构造-沉积环境;地层正常的最大埋藏演化达到了中成岩A阶段的早期.砂岩经历了酸碱交替成岩成矿作用演化,沉积-埋藏成岩期普遍发育方解石化、白云石化、菱铁矿化、伊蒙混层化、黄(白)铁矿化、高岭石化及局部(褐)赤铁矿化和铀矿化;古近纪期间深部基性岩浆侵入衍生的碱性热流体使得砂岩发生了(高锰)铁白云石化、绿泥石化、硅化、砷铅矿化以及早期铀矿的叠加改造;表生成岩阶段发育了强烈的(褐)赤铁矿化及其伴生的铀矿体.其中,红色砂岩既有原生同沉积型,也存在后生氧化成因;灰色砂岩为原生弱还原性砂岩叠加了深部流体次生还原后的氧化残留体.所有泥岩表现为碱性成岩作用,黏土矿物以伊蒙混层和伊利石为主;主要发育(褐)赤铁矿化、方解石化、白云石化、伊蒙混层化,且灰色泥岩局部发育黄铁矿化和铀矿化.同一钻孔相邻砂、泥岩在黏土矿物、Ca、Mg、Fe、Mo、U等活动性元素及碳同位素组成方面存在相似的变化规律,指示了原生不同地球化学性质岩石之间在沉积-埋藏成岩阶段短距离发生了一定的物质迁移和流体交换.目的层总体具有青山口-嫩江期同沉积-成岩预富集和潜水氧化初始成矿以及嫩江末以来多元流体耦合叠加层间氧化主成矿的两阶段板状铀成矿过程.

The synergistic diagenesis of the sand-mudstone from the Qingshankou Formation and their uranium metallogenic significance were systematically researched through investigating the petro-mineralogy,major and trace elements,carbon and oxygen isotopes of various rocks with different colors.Results show that both sandstone and mudstone have similar clastic mineral composition,clay mineral assemblage and the major and trace element contents,reflecting the unified provenance and the same tectonic-sedimentary environment.The maximum burial evolution of the target strata probably reached the early stage-A of the middle diagenesis.The sandstone has undergone the acid-alkaline alternating diagenetic evolution.During burial diagenesis stage,it has widely developed the calcitization,dolomitization,sideritization,semctite-illitization,pyritization,kaolinization and local limonitization,hematitization and uranium mineralization.Also,the alkaline hydrothermal fluid derived from the intrusion of deep basic magma in Paleogene period,led to the occurrence of high manganese ankerite,chloritization,silicification,mimetite and the superimposed reformation of the early uranium ores.Then,the sand body developed strong limonitization,hematitization and associated uranium mineralization in the epigenetic stage.Thus,these conclusions could be draw,i.e.the red sandstone mainly formed by epigenetic oxidation,and a few from primary syn-sedimentary.Present residual grey sandstone,characterized by intense reduced capability,was secondary reduced by the deep fluid.The mudstone is characterized by alkaline diagenesis with the respects of the much aboundance of smectite mixed layer and illite,and the extensive development of the limonitization,hematitization,calcitization,dolomitization and semctite-illitization.Moreover,little pyritization and uranium mineralization were locally occurred within grey mudstone.The similar changing rules in terms of clay minerals,the movable element of Ca,Mg,Fe,Mo,U and the carbon isotopic compositions exist both in the sandstone and adjacent mudstone from the same borehole,which indicates that aboundant materials and fluids migrated and exchanged between various rocks with different geochemical properties in sedimentary-burial diagenetic stage.The two-stage ore-forming process of the tabular-shaped uranium deposit was proposed for the target bed of the Qingshankou Formation,i.e.(1)synsedimentary-diagenetic preconcentration and phreatic oxidation uranium initial mineralization from the Qingshankou to the Nenjiang period,(2)main coupling interlayer oxidation uranium mineralization superimposed by deep fluid since the end of Nenjiang era.