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

Abstract：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. 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. 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 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 30×10-6), making it the probable uranium source. The high U content of the melt inclusions (U, 14×10?6-25×10?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.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.