【作者】 肖新建；

【导师】 陈肇博； 李子颖；

【作者基本信息】 核工业北京地质研究院 ， 矿产普查与勘探， 2004， 博士

【摘要】 鄂尔多斯盆地是我国主要的产油、气、煤盆地之一。盆地东北部东胜地区铀矿床是近年来发现的大型砂岩铀矿床。该砂岩铀矿床具有独特的成矿地质特征。本文主要从砂岩铀矿床成矿地质背景、矿床地质特征、后生蚀变特征以及成矿地质流体等方面入手,并着重以砂岩铀矿成矿过程中低温流体演化和流体成矿作用地球化学过程为主线,对该砂岩铀矿床进行解剖,最后提出了东胜砂岩铀矿床的成矿模式。对鄂尔多斯盆地特征和东胜砂岩铀矿成矿地质背景分析表明,印支-燕山运动和喜马拉雅运动,使刚性块体发生断裂、倾斜、抬升和陷落,打破了整个盆地刚性块体的一致性。燕山期构造运动特别是中侏罗世末和早白垩世末的造山运动在该区造成抬升,发生沉积间断,地表水、地下水渗入,油气的向上和横向上的运移,为研究区铀成矿提供较强的流体动力和物质来源;新构造运动总体使盆地处于上升剥蚀的构造背景下,为铀的活化和向盆地内运移和多次层间氧化作用的叠加创造了有利条件。进一步研究表明,盆地东北部的三组基底构造发育,东胜地区还可能受西缘深部的热物质以及东部山西境内的岩浆岩类物质的热作用。另外,环鄂尔多斯盆地北部东胜地区周围,西北有北东向的狼山岩体,北部为近东西向的阴山岩体,这些花岗岩、火山岩建造可能东胜地区砂岩铀矿床提供铀的来源。对东胜砂岩铀矿的矿床地质特征研究表明,直罗组为含矿层。直罗组可分为上下两段。上段为杂色(紫红色、棕红色、降紫色)泥岩、粉砂岩;下段可分为两个亚段,上亚段为灰绿色的砂岩,下亚段为灰色砂岩。直罗组下段是含矿层,矿体主要产于下亚段的灰色砂岩中,部分产于上亚段的灰绿色砂岩中。赋矿的直罗组砂岩为岩屑长石砂岩。从岩石的化学成分看,直罗组砂岩源岩可能为阴山地区和/或狼山地区的早期的花岗质岩或花岗变质岩类。东胜砂岩矿体形态呈不明显的卷形,更多的呈板状、层状、透镜状。铀主要以吸附形式存在,铀矿物以铀石为主,含有一定量的钛铀矿,可能还有晶质铀矿。从砂岩中炭屑有机质的含量、S的含量、Mn的含量以及Fe元素变化特征可知直罗组下段的灰绿色砂岩经历过氧化作用。从U、Se、V、Mo和Re等元素含量在单个钻孔及钻孔剖面中变化特征可知东胜砂岩铀矿经历过氧化作用成矿。通过计算可知东胜地区直罗组砂岩背景铀含量为(4.05~4.62)×10-6,表明其是富铀砂岩。本论文对东胜砂岩铀矿床的后生蚀变分带作了研究。平面上的砂岩蚀变分带特征表明砂体经历过层间氧化作用。垂向上后生蚀变分带为:直罗组下段下亚段为灰色-灰白色砂岩(含<WP=4>矿段)的绿化(Ⅱ)(主要为长石的绿帘石化)+ 碳酸盐化带;上亚段为灰绿色砂岩的绿化(Ⅰ)(主要为黑云母绿泥石化,也有长石绿泥石化和绿帘石化)+ 粘土化 + 碳酸盐化带;直罗组上段杂色细砂岩的褐铁矿化 + 粘土化 + 碳酸盐化带。而且主要的后生蚀变作用发生在灰绿色的砂岩中。东胜地区砂岩铀矿床存在三类流体的蚀变作用:第一类为酸性氧化性流体的蚀变作用,主要有高岭石化,赤铁矿化,褐铁矿化(残留),有机质氧化消失;第二类为弱酸弱碱性还原性油气流体蚀变作用,主要有长石的绿帘石化(少量),早期的碳酸盐化,黑云母和长石的绿泥石化,绢云化,蒙脱石化(少量),黑云母的绿色黑云母化,黄铁矿化;第三类为碱性强还原性热液流体的蚀变作用,主要有长石的绿帘石化,晚期(第二、第三期)的碳酸盐化,蒙脱石化(大量),黄铁矿化,并生成钛铀矿-锐钛矿-铀石组合。碳酸盐化有三期:第一期形成隐晶质、细晶质的方解石胶结物,在砂岩绿色蚀变之前发生;第二期为细晶质方解石胶结物;第三期为亮晶的方解石脉体。碳酸盐胶结物有大量的油气流体包裹体。第二期、第三期碳酸盐化为砂岩铀矿主矿化之后的油气和热液蚀变作用的结果,并掩盖了早期的各种蚀变现象。对东胜砂岩铀矿床成矿地质流体研究表明:有三类流体对铀矿化起着重要作用。第一类为酸性氧化性流体,这类流体是铀矿的主成矿流体,是形成东胜砂岩铀矿床基础。第二类为油气流体。这类流体为砂岩铀矿床提供了部分还原剂,更重要的是对砂岩铀矿床的后生蚀变起着重大作用,产生了碳酸盐化和绿色蚀变。第三类流体为低温热液流体,这类富碱的流体在油气流体所营造的强还原性环境下,对改造古层间氧化型矿体起了重要作用,是生成铀石-钛铀矿-锐钛矿组合的决定因素。本文还分析探讨了晚侏罗-白垩纪时期的构造热事件及其响应。认为该时期的构造热事件使得东胜地区的基底构造发生了构造活化作用,从而在基底构造之上的盖层沉积地层产生了大中小三种尺度上的裂隙系统,有利于流体的运移。最后,在分析流体演化与铀成矿作用过程后,建立了东胜砂岩铀矿床的“三元流体复成因”成矿模式。这种成矿模式实际上为三元流体在不同时期不同主导作用下,建造、构造与改造的三位一体的成因模式。更多还原

【Abstract】 Ordos basin is one of the mainly oil-productive, gas-productive and coal-productive basin in China. Dongsheng sandstone type uranium deposit, the latest large deposit, is located in the northeast of the basin. The Dongsheng sandstone type uranium deposit has unique metallogenic features. Based upon detailed studies on metallogenitic geology setting, geological characteristics of ore deposit, epigenetic wall-rock alteration and ore-forming fluid of the deposit, the dissertation discusses the evolution of the low temperature fluid during the mineralization processes and geochemistry characteristics of fluid metallogeny emphasizedly and points out the metallogenic model of the sandstone type uranium deposit.Research on the characteristics of Ordos basin and metallogenic setting of Dongsheng sandstone type uranium deposit indicated that rigid block of the basin occurred faulting, tilting, uplifting and depressing and whole rigid block of basin were disturbed during the Indosinian-Yanshanian movement and Himalayan movement. During the mid-Jurassic and early-Cretaceous orogenic movement especially, uplift of basin, sedimentation interruption, infiltration of the surface-water and groundwater and lateral migration of oil can supply the material source and fluid dynamic for Dongsheng sandstone type uranium deposit. The neotectonic movement the uplifting and denuding of the basin were beneficial for the U element remobilization and multiple interlayer oxidation. Furthermore, around the Dongsheng district there have NE strike Langshan rock mass on the northwest, EW strike Yinshan rock mass on the north, all the granitic rocks and volcanic rocks could supply the uranium source for the deposit.The studies show that the ore-bearing stratum of Zhiluo group has two segments, the top layer is middle-fine motley sandstone and claystone, and the under layer could also be divided into two sub-layers, which top sub-layer is mostly gray-green sandstone and claystone, and under sub-layer is gray sandstone with bearing uranium. According to the mineralogy analysis,the sandstone of ore-bearing stratum is confirmed on lithic arkose. And the chemical composition analysis indicated that there are the same chemical composition between Zhiluo group sandstone and granitic rocks of Yinshan & Langshan area. Shapes of ore bodies show the blurry roll type, more plank type, layer type and lentoid type. The most part of uranium element are absorbed, and the coffinite is the primary uranium mineral, there are also some brannerite and a little uraninite.The contents variety of organic matter, S, Mn and Fe element in the different part of the Zhiluo <WP=6>group under layer show that the gray-green sandstone rock has occurred the interlayer oxidation. And the varieties of U, Se, V, Mo and Re elements on drill and section plane show the interlayer oxidation mineralization of the Dongsheng sandstone type uranium deposit. According to the analysis results, it is obviously the Zhiluo group is rich uranium sandstone rocks with the U contents of 4. 05~4.62×10-6.This paper studied the epigenetic alteration zones of the deposit. The plane alteration zones indicate the interlayer oxidation in the sandstone strata. There is three-layer alteration mode of the deposit in lengthways: the top alterations are limonitize, clay and carbonation which are in the upper layer of Jurassic system Zhiluo group, and middle alterations are chloritization of biotite and plagioclase, epidotization of plagioclase, clay alterations and carbonation in top sub-layer, under-layer alterations are epidotization of plagioclase and carbonation in under sub-layer of Zhiluo group which is the host rock of uranium deposit. In this deposit, there are three type alterations created by three mineralizing fluids. the first type is the acid oxygenic alteration with kaolinization, hematitization, limonitzation and the oxidation of organic matters; the second type is the weak acid & alkalescent reductive oil-gas alteration with epidotization, carbonation, chloritization, sericitization,更多还原