中国西北地区中新生代盆地砂岩型铀矿地质与成矿作用

【作者】 吴柏林；

【导师】 刘池阳；

【作者基本信息】 西北大学 ， 地球化学， 2005， 博士

【摘要】 本文以后生蚀变及流体地质地球化学、砂岩铀矿形成中的有机地质作用特征为研究重点,并结合成矿作用及其演化的动力学作用、成矿作用过程的定年、古气候环境的演变、成矿物质来源、铀的迁移形式和沉淀机理等共七个方面对中国西北地区典型砂岩型铀矿（鄂尔多斯、吐哈、伊犁地区）有机—无机地质成矿作用及其演化进行了研究和对比;在此基础上总结了研究区砂岩型铀矿成矿作用和矿床类型的演化特征;提出了世界主要砂岩型铀矿产铀盆地的六种演化模式;并对水成铀矿成矿理论及找矿方法、思路阐述了作者的一些看法和新的认识。 对于流体地质作用,作者应用蚀变矿物学及包裹体地球化学从定性和定量两方面对基底蚀源区、成岩、氧化、漂白和绿色蚀变、矿化、原生岩石（还原带）带等流体的成分、物理化学性质,C、H、O、S同位素进行了揭示。认为成矿作用流体并非来自深部基底,而是源自于浅部常温的大气降水;但深部来源的气态有机质却是东胜矿床漂白蚀变、绿色蚀变及铀矿化的重要原因,也是成矿流体具低温热液性质的主要起因。成岩期流体一般矿化度较高,温度也偏高,其酸碱性、氧化还原性处于动态变化之中;此阶段硫酸盐还原细菌作用还原SO₄^2-是H₂S及相应黄铁矿形成的重要原因。氧化蚀变流体铀含量低,含有机质成分少,矿化度和温度均较低;经氧化蚀变流体作用后的岩石其Th/U,Ra/U比值达大的峰值。漂白蚀变流体性质为还原强酸性,经漂白蚀变流体作用后的岩石其高岭石含量高,Th/U、∑烃含量达大的峰值;而Orgc、∑Fe明显偏低。在矿化蚀变阶段,流体或其蚀变岩石中U、Orgc、Mo、Re、δD、δ¹⁸O含量最高,Th/U比值最低,流体性质为（中性～弱碱性）—（弱还原～还原）。绿色蚀变为强还原强碱性,此阶段温度、PH均达大的峰值,Eh、δ³⁴S达低的峰值。吐哈和伊犁矿区缺少明显的漂白和绿色蚀变,表明东胜矿床成矿作用流体相比前两个地区曾达到强还原性阶段。这也是东胜矿床和吐哈,伊犁矿床的一个重大差别。 关于有机地质作用特征。东胜矿床气态有机质作用特征明显,固体有机质作用次之;伊犁地区生物细菌作用特征明显,气态有机质作用次之;吐哈地区则是固体有机质、气态有机质、生物细菌共同作用的结果。吐哈、伊犁矿区固体有机质主要是镜煤;东胜矿床则为含沥青质的镜煤有机质,约含2%～10%的沥青质成分。干酪根类型东胜、吐哈为Ⅲ型（即腐殖型）,但由于生物细菌作用,伊犁矿区有机质具一定腐泥型特征;有机质成分源于陆相高等植物。干酪根形成环境吐哈、伊犁矿区为成岩作用阶段,而东胜地区为深埋后生作用阶段（油气生成阶段）。有机质分解后产生的黄腐酸和腐殖酸是铀迁移的重要载体;铀被在还原环境下腐殖质分解产生的小分子有机化合物还原而御载沉淀富集。三个矿区广泛发育的碳酸盐化主要是由固体有机质脱羧基作用造成,但是东胜矿床碳酸盐化也有煤型气作用的特征。 由于矿区所在盆地的后期改造抬升、油气逸散充注、古气候的干热演化以及铀成矿这四种地质作用结果的年代特征具有相似的吻合性,且地球动力学作用造成了油气藏的迁移,充注,形成强的还原环境、有利的干热古气候条件以及提供了铀沉淀反应所需的还原剂。所以,砂岩型铀矿成矿作用并不是孤立发生的,是在统一的动力学背景下有机—无机共同作用的结果。故而,气态有机质圈闭构造（新构造褶皱、断裂、岩性岩相界面等）是成矿有利的因素;气态有机质的逸散方向有利于砂岩型铀矿的形成。 新构造褶皱和断裂（盖层断裂、复活性基底断裂）造成地下水流速、流向等动力机制的改变,以及促使深部气态有机质沿断裂的迁移、扩散,它们是吐哈、伊犁矿区铀矿化主要的赋矿场所。东胜矿区则主要是基底复活性断裂（气态有机质的主要通道）控矿。东部中晚侏罗世、晚白垩-古新世、中新世地层的抬升造成气态有机质向上、向北的扩散运移是东胜矿床“板状”矿体及漂白蚀变形成的主要因素。而西部在晚侏罗-早白垩纪、晚白垩-古新世、中新世期间地层的抬升造成气态有机质向东部顺地层的扩散运移是东胜矿床绿色蚀变及“似卷状”矿体形成的重要因素。 对于成矿物质来源,由于含矿层本身是高铀背景值地层,加上氧化作用、漂白作用等造成岩石更多还原

【Abstract】 Through study emphasisly from epigenetic alteration and fluid geochemistry、 the characteristic of organic geologic process in the formation of sandstone type uranium deposit, and combining with the mineralization of its dynamic evolution、 dating in mineralization process 、 the evolvement of paleo-climate environment、 the substantial source of ore-forming event、 the ways of uranium migration and its mechanism of sedimentation, this paper studies and contrasts organic and inorganic geologic metallogeny and their evolvement of typical sandstone type uranium deposit （Ordos, Turpan-Hami 、 Yili basin） in the Northwest area in China Based on this, the author summarized the evolvement feature of metallogeny of sandstone type uranium deposit and their types, holded up six patterns of the evolvement of basins which are the main ones that produces sandstone type uranium deposit in the world, stated some of the author’s own opinions and new apprehend on the Metallogenic theory of hydatogenic uranium deposit and the means and ideas of ore finding.For the fluid geologic process, from the qualitative and quantitative analysis, the author revealed the liquid ingredient and its physical and chemical property and the C、 H、 O、 S isotope of basement rock、 the time of diagenesis, oxidation, bleaching and green alteration, mineralizing, autogenetic rock （zone of reduction ） by using alteration mineralogy and inclusion geochemistry; and consider that the metallogenic fluid is not from deep-seated basement rock , but superficial and normal temperature atmospheric precipitation; but the gas organic matter from deep is of importance to bleaching alteration, green alteration and metallization of uranium in Dongsheng, and the main cause of metallogenic fluid having the property of thermal fluid of low temperature. The degree of mineralization and temperature of fluid in the diagenesis time are higher,and its acidity and alkalinity and Eh are in the state of modification;and the reduction process of reducting SO42" for the sulfate reduction bacterial is the important cause of the form of H₂S and the pyrite.The content of uranium and organic matter is lower in the oxidizing liquid,and the temperature and degree of mineralization are also lower.The ratio of Th/U and Ra/U of the oxidized rocks reach their big peak.Bleaching alteration fluid is reductive and highly acidic,and the ratio of Th/U and totalcontent of hydrocarbon and kaolinite in the bleachead rocks are the highest,but the content of its orgc and ￡Fe is clearly lower. In metallization alteration phase, the content of 5Ds 518Ch U, Orgc> Mo, Re is the highest in fluid or in the alteration rocks, and the fluid property is （neutral^alkalescent） — （faintly reductive—reductive） . green alteration fluid is highly reductive and alkal, in this phase the values of Temperature> PH all reach their own big peak .There are no phases clearly of bleaching and green alteration in the deposits of Turpan-Hami and Yili deposits, it demonstrates that compared with these two areas , metallogenic fluid had reached highly reductive phase in Dongsheng. It is also the big difference between Dongsheng and the others （Turpan-Hami , Yili ore area）.In the feature of organic geologic process, the influence that gas organic matter to uranium deposit is the the most important in Dongsheng deposit, solid organic matter take second place;In Yili deposits, the influence of biological bacteria is the most obviously,that of gas organic matter second; In Turpan-Hami, the three influences mentioned above jointly determine its feature.The solid organic matter in Turpan-Hami and Yili ore area is maimly vitrain;but it cantains about percent two to ten of asphaltene in the Dongsheng ore area.The type of organic-carbon is humus.but it tends to the type of sapropel in Yili ore area.The organic matter originates from higher plants of land facies.The formation enviroment of organic-carbon is in the process of diagenesis in Turpan-Hami and Yili ore area.but it is in the process of later phase of diagenesis（that is the phase of forming the gas-oil）.The Fulvic acids and Humic acids that are decomposed into from organic matter are the main vehicle for the migration of uranium,and uranium is precipitated and forms rich ores by reductive little molecule chemical compounds.The wide carbonation is formed by decarbonation of the organic matter,but it is also affected by coal gas in Dongsheng ore area.The feature that the consistency of times indicated by the four geologic events that later stage reformation and uplift, dissipation injection of the gas organic matter , metallogeny of uranium deposit and dry-hot aero-climate in basin, show that migration and injection of oil and gas caused by geodynamical action formed highly reductive circumstance and offered needed reducer for precipitation reaction of uranium. So, the metallogeny of sandstone type uranium deposit doesn’t occur isolatedly, it is the result of organic and inorganic interaction in the united dynamical setting. Therefore, the trap structure（neotectonic folds, faults and lithologic-lithofacies interfacies etc.） is favorable condition of metallogeny. The direction of dissipation of gas organic matter is advantageous for the formation of sandstone type uranium deposit.Alterting the dynamical mechanism of moving rate and direction of underground water,and forming dissipation injection of gas organic matter along the faults,neotectonic folds and faults（cover faults and reviving basement faults） are the main ore-holding tectonic in Turpan-Hami and Yili ore area.But in Dongsheng ore area,the reving basement faults are the main ore-holding tectonec.The migration of gas organec matter upward and in the direction of north made by later reformation and uplift in the eastern area in Ordos basin on time of J2-3, K2-E1 and Ni is the important factor forming bleaching alteration and the "orebody of plate shape". The migration of gas organic matter along the data in the direction of east made by later reformation and uplift in the western area on time of J3-Ki,K2-Eiand Ni is the main factor forming thegreen alteration and the "similar roll orebody" in Dongsheng uranium deposit.About the source of metallogenic substance. One side, the ore-holding strata itself have high value of uranium, in addition to the high Th/U ratio caused by oxidizing and bleaching action, all these show that the two alterations offer substance for metallogeny of uranium deposit; the other, volcanic rock and granite also have severe depletion of uranium in denuded area, show that they are one of source of metallogenic substance. In conclusion, for metallogeny of sandstone type uranium deposit, the ore-holding strata itself and volcanic rock and granite in denuded area are all the important source of metallogenic substance.In Dongsheng , Turpan-Hami and Yili area, the uranium migrates and occurs in U02 （CO3） /\U02 （C03） 2’ ?. uranium organic complex and U02 （OH） 2H2O .When the Eh value fall, it react with reducer such as H2s H2S> C0> CHun gas organic matter ,and then uraninite and bilibinite come into being. In addition, the adsorption by organic matter .. clay and clastic debris to uranium have great significance to precipitation of uranium.According to the feature of organic and inorganic action of metallogeny of uranium deposit and dynamical evolvement of the basin, the basin models that uranium mine occur are devided . The main sandstone type uranium deposits in the world occur in the tectonic area of "faintly neotectonic district" and "second orogenic belf’.one of the types is the oxidation excedinly.that is," cy-c a p h c y ft — cup n a p b h h "type, its characteristic is of having a long time of interlayer oxidizing action, abundant provision of the groundwater% the great scale of interlayer oxidizing zone. Dongsheng is of deacidizing extreme model, there are rich resource in basin ,such as oil, gas and coal, and it has high "deacidizing atmosphere". This two types wholly are situated at" faint new tectonic movement district". From the scale of ore deposit,it can be see that both ought to belong the super large-scale deposit（when the oxidizing time is not long enough, the quantity of reducer that induce reaction which will give birth touranium mineral would have critical significance）. So, in the light of this theory, there are still tremendous prospecting potential in Dongsheng field.更多还原