准噶尔盆地基底特征与砂岩型铀矿成矿作用

【作者】 宋继叶；

【导师】 秦明宽； 蔡煜琦；

【作者基本信息】 核工业北京地质研究院 ， 矿产普查与勘探， 2014， 博士

【摘要】 准噶尔大型叠合盆地面积约13×104km2，为我国新疆第二大沉积盆地，其所处大地构造位置、盆地类型、主要盖层沉积相及区域铀矿化等条件与伊犁典型产铀盆地相似，总体上具备了砂岩型铀矿成矿的一些基本条件。但是，至今仍未取得找矿突破，为此，铀矿地质勘查决策部门和生产单位均急需了解或回答：“该盆地砂岩型铀矿成矿前景到底如何？”、“有无形成大型、特大型砂岩型铀矿的潜力？”、“有，则到何处去找”以及“如何实现快速突破”等问题。本文在前人研究工作的基础上，选取准噶尔盆地基底为研究对象，针对基底结构属性、构造属性、含铀性及其岩浆演化地壳成熟度和构造成矿有利度等4方面问题进行研究。在板块构造理论和砂岩型铀矿区域成矿理论的指导下，通过与伊犁产铀盆地基底特征的对比研究，探讨盆地基底特征与砂岩型铀矿成矿作用，分析准噶尔盆地砂岩型铀矿成矿潜力并进行成矿预测。论文通过野外地质调查，结合室内岩石化学分析、活性铀浸出实验以及锆石LA-ICPMS分析测试，对上述研究内容开展了研究工作，取得了如下几点认识：（1）准噶尔盆地基底为陆壳性质，具有由前寒武纪结晶基底与海西期褶皱基底组成的“双层”基底结构。证据如下：盆地范围内布格重力异常值均为负值（平均为-65×10-5m/s2），反映其地壳厚度至少在35km以上；平均莫霍面埋深在39km以上，与世界稳定地台的平均莫霍面埋深持平（39.614km）；航空磁力异常在盆地内识别出了3个大的、与前寒武纪结晶基底相对应的正磁性块体；大地电磁测深、人工地震也均获得了盆地盖层下存在前寒武纪结晶基底与褶皱基底的信息。同时，同位素地质测年取得了从晚奥陶世至早前寒武纪的地质年代信息，为上述推断提供了有利证据。这种由前寒武纪结晶基底与海西期褶皱基底组成的“双层”基底结构，与伊犁、吐哈、鄂尔多斯等典型产铀盆地的基底相似，具备了砂岩型铀矿成矿的基底条件。（2）同位素地质年代学数据分析表明，准噶尔盆地周缘从晚泥盆世开始出现构造岩浆活动后，在晚石炭世进入岩浆活动高峰期，二叠纪在北缘阿勒泰和西准达拉布特又有构造岩浆活动出现，形成了盆地周缘广泛分布的火成岩带，构成了盆地周缘褶皱基底剥蚀区的主体。多期的构造岩浆作用形成了盆地周缘广泛发育的基底富铀建造和/或富铀地球化学块体。航空放射性测量在盆地周缘识别出了12片铀含量高值和偏高值（区）带，地面伽玛能谱测量也落实了东准克拉美丽地区、北准阿勒泰地区以及西南喀拉达板地区部分铀含量高、偏高的中酸性侵入岩与火山岩以及部分地层；进一步的全岩铀含量和活性铀浸出实验分析表明：盆缘中酸性侵入岩和火山岩具有接近或稍高于地壳平均花岗岩铀含量的特征，但相比于伊犁产铀盆地蚀源区的铀含量则有所不及；存在铀的迁出现象，但没有吐哈盆地的铀迁出活跃。总体而言，准噶尔盆地具有提供砂岩型铀矿成矿物质的能力，但略逊于伊犁盆地南缘与吐哈盆地南缘褶皱基底。（3）东准克拉美丽、西准达拉布特和北准阿勒泰-富蕴-布尔根等地区典型花岗岩体的岩石地球化学分析表明，岩石具有富硅、富碱、高钾（K2O/Na2O＞1）和高FeO/MgO（＞1）值的特征，属于高钾钙碱性系列，岩体形成时的古地壳厚度大多在35km以上；富集大离子亲石元素（K、Rb、Th、U）和高场强元素（Hf、Y、Nb、Ta）。这意味着准噶尔盆地周缘岩浆演化已达到了成熟的阶段，铀在这个过程中不断在地壳中聚集，形成了具有成熟陆壳性质的富铀花岗岩体，为砂岩型铀矿成矿提供了物质基础。多数岩石主微量元素特征与A2型碱性花岗岩一致，属于后碰撞或板内花岗岩类，说明岩浆活动结束后盆地整体进入了后碰撞的弱伸展构造环境，总体上具备了有利于砂岩型铀矿成矿的稳定构造背景。Sr、Nd、Pb及O同位素研究成果表明：褶皱基底花岗岩的成岩物质有多种来源，既有地幔物质的加入，又有年轻地壳部分熔融产物和地壳物质的混入；其中，地幔物质占65%以上，一定程度上说明，相比于成岩物质来源于成熟古陆壳的花岗岩类，上述演化成熟的岩体内却不能富集如陆壳改造型花岗岩内那么多的铀。相应岩石中锆石的铀含量绝大多数低于1000×10-6，表明岩石形成时古铀背景值较低，与成岩物质中铀含量“先天不足”相一致，这说明在准噶尔盆地找到超大型砂岩型铀矿床困难较大。（4）准噶盆地总体上经历了隆升剥蚀（P-T2）→下降沉积（T3-J21）→隆升剥蚀（J22-J3）→下降沉积（K）→隆升剥蚀（E-Q）等5个过程。其中，从晚三叠世开始至西山窑组沉积结束（约60Ma）的地质时期内，在相对稳定的构造背景下，形成了原生还原沉积建造、含矿建造并形成了多种类型的砂岩型铀矿化；中侏罗世-晚侏罗世至白垩纪，在由挤压转化为伸展的构造环境中活化改造再次成矿；新近纪以来北天山强烈隆升，破坏了盆地（尤其是南缘）原有的成矿条件，甚至可能“铲掉了”本已形成的铀矿化，这可能是盆地至今未取得砂岩型铀矿找矿突破的重要原因之一。（5）综合上述研究成果认为：准噶尔盆地具备了砂岩型铀矿成矿的基底条件、构造条件，演化成熟的富铀的海西期火成岩为铀成矿提供了一定的物质来源，在北三台凸起和陆梁凸起不同方向的斜坡带上具备了形成层间氧化砂岩型铀矿的条件，是有望取得一定找矿突破的远景区。但是，盆缘褶皱基底铀含量有限、铀的迁移能力不强，再加上成矿物质来源不足以及新生代构造运动的负面影响，使盆地的成矿规模受到了一定的限制。然而，通过全方位细致的科研和勘查工作，在盆地内找到中-大型砂岩铀矿床还是有可能的。更多还原

【Abstract】 Junggar large superimposition basin is the second major sedimentary basin in Xinjiang Autonomous Region with the area of about13xl04km2. Its tectonic location, basin types, main sedimentary facies of covering strata and regional uranium mineralization conditions are similar to those of Yili typical uranium-producing basin. In terms of this reason, the basin generally possesses a number of basic favorite conditions for the occurrence of sandstone type uranium mineralization. However, successful breakthrough in U exploration has not been achieved up-to-date. Therefore, there are some crucial problems that the uranium exploration decision-making departments and production units are facing to and urgent to understand or answer:a) how is the sandstone type uranium mineralization prospect? b) is it potential to form large or superlarge sandstone type uranium deposit? c) if it has, where should we go to explore? and d) how to achieve a fast breakthrough?Based on the previous research work, the basement of the Junggar basin has been selected as one of the research objectives. In this paper,4aspects will be studied including the basement structure properties, tectonic properties, folded-basement uranium aboundance and its crustal maturity and tectonic metallogenic favourability degree. Under the guide of the plate tectonics and regional sandstone type uranium mineralization theory, the relationship between the basement characteristics and sandstone-type uranium mineralization in Junggar basin are to be discussed compared to those of Yili uranium-producing basin, and the sandstone type uranium metallogenic potential is to be analysed. What’s more, the work of metallogenic prediction will also be carried out.The field geological survey work was carried out combined with indoor chemical analysis and test, moblile uranium leaching experiment and zircon LA-ICP MS analysis and test. The results are shown as follows:(1) The basement of Junggar basin is of continental crust property composed of Precambrian crystalline basement and the Hercynian fold basement. The evidences are as follows:a) Bouguer gravity anomaly shows negative value (average value=65x10-5m/s2) in basin range reflecting the crustal thickness of35km at least. The average Moho depth of the basin is more than39km which is close to or more than that of the world stable platform (39.614km); b)3large anomalies were indentified during airborne magnetic survey in the basin corresponding to the Precambrian crystalline basement. Moreover, the source information indicating the presence of Precambrian crystal basement and fold basement under the covering strata was obtained by the method of magnetotelluric sounding and artificial earthquake, c) At the same time, isotopic geochronology gave the ages from Late Ordovician to Early Precambrian providing evidence for the above inference. Such "double" layer basement structure is comparable to that in Yili, Turpan-Hami and Erdos typical uranium-producing basin, and indicates that the Junggar basin posseses basic conditions for the metallogenesis of sandstone type uranium deposits to some extent.(2) Isotopic chronological data analysis show that tectonic magmatic activities around the Junggar basin margin began in Late Devonian, and peaked Late Carboniferous. Also, the activities occurred in Permian at the northern margin of Altai and Dalabute belt in western Junggar, which led to wide development of igneous rock belts around the basin margin forming the main body of the folded-basement.At periphery of the basin, the U-rich formations and/or geochemical blocks enriched in uranium are developed widely because of multistage tectonic-magmatic activities.12zones (ares) with high or moderately elevated uranium content have been identified by Airborne Radioactive Survey in the basin. Those are intermediate-acidic intrusions, volcanics and strata enriched in uranium have also been discovered by the ground gamma spectrometric survey in Kelameili area, eastern Junggar, Aletai area in the northern part of the basin and south-western Caladaban region. The further analytic results of geochemical samples and mobile uranium leaching experiments show that, the uranium abundance of those intrusive and volcanics at the periphyry of the basin margin is close to or slightly higher than the average content of granite of crust but a little bit less than those in Yili uranium-producing basin. The remove of uranium did happen in above rocks or strata, but it does not reach the level as much as in Turpan-Hami basin. In general, the folded-basement of Junggar basin could provide some components for sandstone type uranium ore-formation, but its uranium metallogenic potential might be a little worse as compared to that for the southern margin of Yili and Turpan-Hami basin.(3). Geochemistry analysis results of typical granite in Kelameili area, Eastern Junggar, Dalabute belts, Western Junggar and Aletai-Fuyun-Buergen region, Northern Junggar show that rocks studied are generally rich in silicon, alkali, large-ion lithophile elements (LIL) and high field strength elements (HFS) with high ratio value of K20/Na20(>1) and high FeO/MgO ratio (>1). They belong to high K calc alkaline series with the ancient crust thickness mostly being more than35km, when rocks are formed. That is to say, the magma evolution at the periphery of the basin reached a matured stage and the uranium is continuously accumulated in crust during the evolution process and finally U-rich granites originate from matured crust appeared thus providing material base for the sandstone type uranium mineralization.The characteristics of major and trace elements in most granitic rocks are in accordance with those in A2type alkaline granite, and they belong to the post collisional granitoids or with in plate ones which indicate that the basin at that time entered into the post collision tectonic environment at the end of magmatic activity. In other words, a stable weakly extensional tectonic setting being favorable for the formation of sandstone type uranium deposits.Sr, Nd, Pb and O isotopic studies indicate that the material source of studied granite rocks are originated from varies including mantle materials, young crust partial melting products as well as crustal materials. Then, the materials from mantle are accounted for more than65%. That is to say, in a certain extent, as compared with the continental crust-originated (especially matured ancient continental crust) transformed-type granite, the matured crust stated above will not enrich much abundant uranium for "rock material being congenitally deficient". Zircon from studied rocks contains mostly uranium less than1000x10"6which indicates the paleo uranium background value is low. This fact is in accordance with low U-content in granites. This might be the reason why the exploration for superlarge sandstone type uranium deposit in Junggar basin still is quite difficult.(4). During the basin evolution process, Junggar basin generally experienced5stages, it was uplifted and eroded from Permian to Middle Triassic, subsides and received deposition during the period from Late Triassic to early stage of Middle Jurassic, then again uplifted and eroded from late stage of Middle Jurassic to Late Jurassic and subsided and received deposition again in Cretaceous and finally uplifted and eroded in Cenozoic era. During the geological period from Late Triassic to the end of Xishanyao period, the primary reduction sedimentary formations, ore-hosting formations and mult-types of uranium mineralization were developed; The tectonic environment was changed from compressional to extensional during the period from Middle Jurassic to Cretaceous which led to the uranium ore formation again. The strong uplifting of Northern Tianshan from Neogene period destructed basin preexisting metallogenic conditions (especially the southern margin of the basin) even "shoveled" inherent uranium mineralization, which might be one of the important reasons for unsuccessful prospectings for large-sized sandstone type uranium deposit in Junggar basin.(5). Based on the above research results, the suggestions can be obtained as the following. Junggar basin has possessed certain metallogenic conditions for sandstone type uranium mineralization, such as:"double-layerd" structure of the basement, relatively stable tectonic environment from Late Devonian to Eogene, and matured Hercynian igneous rocks which can provide certain source material for uranium mineralization. In different direction slopes of Beisantai covex and Luliang convex, there are four slope zones where the formation of interlayer oxidation sandstone type uranium deposit could be developed. These are perspective areas for uranium exploration. However, the uranium abundance in folded basement is limited, the uranium migration ability is weak, in addition the deficiency of ore mineral sources and negative effect of Cenozoic tectonic movement, the metallogenic scale of sandstone uranium deposit is limited in Juggar basin as compared with U-highbearing Yili and Tuke basins. Nevertheless, on the basis of systematic detailed research and exploration work, it mighted be possible to find median-large scaled sandstone-hosted U-deposits.更多还原