中蒙边境及邻区铀矿床产出环境、地质特征、形成作用和找矿标志

Geological setting,features,origin and exploration criteria of uranium deposits occurring within the China-Mongolia border region and its neighboring areas

中蒙边境及邻区位于西伯利亚板块、塔里木板块和华北克拉通的结合部位,是全球范围内重要的铀多金属成矿带之一。受多期次构造岩浆活动影响,该区前侏罗纪变质岩块体和中新生代火山-沉积岩分布广泛,深大断裂纵横交错,各类铀矿床(矿化区)星罗棋布。根据围岩类型,结构构造及成矿过程可将该区铀矿床划分为6种类型:(1)火山岩型;(2)砂岩型;(3)岩脉型;(4)褐煤型;(5)交代岩型;(6)磷灰盐型。其中火山岩型和砂岩型铀矿床具有重要经济意义。区域矿产地质研究表明,中蒙边境产出的大部分铀矿床(矿化区)与前侏罗纪变质岩块体具有密切时空分布关系。前侏罗纪变质块体可划分为2部分:(1)前寒武纪高级变质岩;(2)古生代中、低级变质岩。铀的亲石元素特性致使其在壳-幔物质发生分异时富集于地壳的硅铝层。鉴于在地壳长期演化历史中,古老变质岩已具备有较高的铀含量,那么它们在显生宙构造-岩浆活动中就为铀的富集成矿提供有利的物质条件。显生宙构造运动的形式除了断裂活化外,也包括陆相沉积盆地的上隆和下陷。铀在地壳硅铝层中的富集是通过2种方式实现的:(1)陆壳的深熔和岩浆的分异作用;(2)富铀岩体(层)的风化剥蚀和再沉积活动。研究结果表明,铀的富集过程十分缓慢,其中火山岩型铀矿床的形成作用就是长英质岩浆活动的组成部分。火山型铀矿床主要出现在中蒙边境最东端蒙古国境内,它们是中蒙古—额尔古纳地块伸展构造环境(裂陷槽为大量高钾长英质火山岩所充填)中构造-岩浆作用及相关流体活动的产物。长英质火山杂岩体内产出的若干处大型铀矿床(区)和铅-锌-银-铀矿床即是很好的佐证。一般来讲,具有强烈分异特点的富碱性火山岩及相关铀矿床大都在侏罗纪—白垩纪构造-岩浆作用对前侏罗纪岩体(层)发生强烈叠加改造部位产出,其形成作用可能与晚侏罗世—早白垩世构造-岩浆活动有关。同位素年代学(铀矿床铀-铅同位素测年)研究结果表明,铀矿体的形成时间为153~136 Ma,该时间段与其所在的多尔诺德组安山-玄武岩和流纹岩的形成时间基本一致,同时,与俄罗斯远东地区斯特尔特苏维卡(Streltsovsk)超大型铀矿床的形成时代(136~134 Ma)相吻合。矿区范围内富钾流纹岩铀含量较高(300×10^(-6)左右),暗示了这套火山岩可能为铀矿床的矿源层。另外,流纹岩中熔融包裹体铀含量(14~25)×10^(-6)进一步印证了上述推论的可靠性。与富钾长英质岩浆作用有关的热液活动对早期含铀岩体(层)的叠加改造可导致铀的进一步富集,进而形成大规模和高品位铀矿体。大量黄铁矿、方铅矿、闪锌矿和白铁矿等硫化物的存在暗示了成矿作用是在还原条件下形成的。大多数砂岩型铀矿床分布在中新生代断陷盆地内,这些盆地一般为各类沉积岩(物)所充填,其中河流相、三角洲相和浅海沉积相(物)为铀矿床的容矿围岩。在所有上述沉积岩(物)中,辫状河流相沉积岩(物)是最重要的含矿层位。砂岩型铀矿床大都是断陷和凹陷带构造运动最后阶段构造-沉积联合作用的产物。盆地周缘前侏罗纪富铀岩体(层)的风化剥蚀,为砂岩型铀矿床的形成提供了丰富的物质来源。早期构造运动(176~156 Ma)为古潜水面氧化提供了有利条件并且形成了低品位铀矿化区。在晚白垩世(96 Ma)到渐新世(35Ma)时期,古陆块体抬升与沉降活动期间为氧化作用的发生创造了有利条件,并且为主要砂岩型铀矿床的形成奠定了基础。中蒙边境火山型和砂岩型铀矿床独特的地质、地球化学特征受到国内外地质学家的广泛关注。对于这些矿床的地质环境,地质和地球化学特征以及其容矿围岩的系统研究将极大地提高人们对于铀矿床成矿作用的理解。与此同时,对这些铀矿床的成因类型和勘查标志的研究也将在中蒙边境及其邻区开展隐伏铀矿床的综合评价中发挥重要作用。

The Sino-Mongolia border region and its neighboring areas are located at the convergence zone of the Siberian platform, Tarim plate and North China craton, and is one of the most important uranium metallogenic provinces in the world. Deep-seated faults, pre-Jurassic metamorphic terrane and various types of uranium deposits （mineralized areas） are well developed in the region due to the multiphase tectonic-magmatic events. These uranium deposits can be classified into six types in term of their host rocks, geometry and ore-forming processes： （1） volcanic type; （2） sandstone type; （3） vein type; （4） lignite type; （5） metasomatitic type; （6） phosphorite type, among which the first two types of uranium deposits bear the most important economic significance. 〈br〉 Regional metallogenic studies show that most of the uranium deposits （or mineralized areas） occurring within the Sino-Mongolian border region are closely spatially associated with pre-Jurassic metamorphic terrane consisting of two parts： （1） Precambrian high-grade metamorphic rocks; （2） Paleozoic lightly metamorphic rocks. Since uranium is a lithophile element, it is more easily enriched in the acidic sialic section of the crust during the differentiation of mantle matter. Because these old formations had already been enriched in uranium through the long geological evolution, they might have provided the precondition for economic enrichment of uranium in the Phanerozoic tectonic movements when downfaulted or downwarped continental basins occurred with terrestrial dominated sediments. Where the uranium-enriched geological bodies existing in one region are eroded, all of them can serve as the source for the sandstone-type deposits. The early tectonic event occurring around 176 to 125Ma provided suitable conditions for the oxidization of the groundwater table and the formation of low-grade uranium mineralization area. In the Phanerozoic tectonic-activated regions, the economic enrichment of uranium usually occurred in intensive rejuvenated places of the pre-Jurassic metamorphic terrane. The gradual enrichment of uranium in the sialic crust is mainly achieved through two differential processes：granitization and sedimentary differentiation. However, this combined process is very slow and takes a long time. The ore-forming processes of volcanic type uranium deposits may be an integrated part of the uranium-bearing granitization. 〈br〉 For the volcanic type uranium deposits occurring in the easternmost segment of the Sino-Mongolian border, they were formed during the time of tectonic extension when a number of troughs that were filled with high K-felsic vocanics were formed within the Central Mongol-Argun terrain. Several large-sized Pb-Zn-Ag-U deposits have been identified in the felsic volcanic complexes. Both uranium and fractionated peralkaline magma＆amp;nbsp;were produced by the intensive rejuvenation of the pre-Jurassic metamorphic terrane. The formation processes of the sandstone type uranium deposits might have been genetically related to Late Jurassic to Early Cretaceous igneous activities. Geochronological studies （U-Pb isotopes on uranium ores） demonstrate that the uranium ores formed around 153 to 136 Ma. That time of the uranium ore formation coincides with the formation age of andesitic basalt and rhyolite of the Dornod Formation. Late Jurassic to Early Cretaceous ages are practically equivalent to the formation time of uranium deposits in the Streltsosk caldera in Russia （136-134 Ma）. The high K rhyolite is clearly enriched in uranium （about 30 ＆#215; 10-6）, making it the probable uranium source. The high U content of the melt inclusions （U, 14 ＆#215; 10-6-25 ＆#215; 10-6） from the rhyolite provides the further evidence for the hypothesis mentioned above. The Early formed uranium mineralized zones were intensively overprinted by the hydrothermal events associated with the emplacement of the high K-felsic magma. Widespread pyrite, galena, sphalerite and marcasite suggest formation from metastable sulfur species, which are powerful reductants. Most of the sandstone type uranium deposits occur in the Meso-Cenozoic rift basins filled with various sediments. The uranium-bearing layers formed by amalgamation of braided channels deposited in a fluvial, terrestrial delta and offshore environment. All these sandstone uranium deposits were formed at the last stage of phaneroic tectonic movement when downfaulted or downwarped continental basins occurred with terrestrial dominated sediments. The Early tectonic event （176-156 Ma） provided suitable condition for the paleo-phreatic oxidation and led to the formation of low-grade uranium mineralized zones. During the period of Late Cretaceous （96 Ma） to Oligocene （35 Ma）, the uplifting erosion and sedimentation resulted in suitable condition for the inter-layers oxidation and led to the formation of major sandstone type uranium deposits. 〈br〉 Geological and geochemical features of both volcanic type uranium deposit and sandstone type uranium deposit have attracted much attention among geologists both in China and abroad. The integrated analyseis of the geological setting, geological and geochemical features of these deposits and their related wall rocks will greatly upgrade the understanding of the ore-forming processes of the uranium deposits. Meanwhile, the genetic model and mineral exploration criteria of these uranium deposits can also be used during the comprehensive evaluation of the concealed uranium deposits in the China-Mongolia border region and its neighboring areas.