Formation Of Sandstone-hosted Uranium Deposit In The Ordos Basin, Northwest China

The study of uranium deposit has been one of the forefronts and hotspot topics in the field of mineral exploration. Uranium is one of the most common elements in the earth crust. Being a radioactive element, used as fuel in nuclear reactors and weapons so it also got more importance in the field of nuclear power of a country. Uranium has different kinds of primary and secondary minerals and extracted from uranium bearing minerals such as uraninite (UO2) or pitchblende (U3O8). Uranium deposits are classified into different categories based on host rock, structural setting and deposit mineralogy. Sandstone-hosted uranium deposits fall in category of continental facies sedimentary uranium deposits and constitute about 30% of world’s uranium requirement.China has large number of famous uranium deposits in different metallogenic provinces. In northern part of China, the prospecting of sandstone-hosted uranium deposits started since 1990s and a number of roll front type sandstone-hosted uranium deposits exposed from the Yilli Basin, the Tuha Basin and the Ordos Basin. Following discussion mainly focuses on the detailed study of sandstone-hosted uranium deposits in the Ordos Basin, Northwest China. As the second largest sedimentary basin, Ordos Basin has a great potential of petroleum, natural gas, coal and uranium deposits so also called as "Basin of Energy Resources". The most common areas in Ordos’s basin having sandstone-hosted uranium deposits are, Dongsheng area, Longxian area and Huangling area with Shengshangou deposit, Guojiawan deposit and Diantou deposit respectively. The newly founding and explored Dongsheng and Diantou uranium deposits are hosted by Middle Jurassic Zhilou Formation. Dongsheng uranium deposit is the largest and high grade sandstone type uranium deposit of Ordos Basin. Petrological and geochemical studies of Dongsheng uranium deposit provide evidence for source and enrichment of uranium in Zhilou Formation.Petrographic observations show that Lower-Middle Jurassic Zhilou Formation is coarse to medium-grained feldspathic sandstone. Mineralogical analysis reveals that on the basis of concentration of major minerals such as quartz, feldspar and rock fragments, ore bearing host sandstone can be classified as lithic subarkose sandstone with loosely cemented and moderately sorted grains. Color of host rock is dark gray to black due to a mixture of ore minerals and carbonaceous debris. Microscopic thin section observations also show two types of mineral alteration, chloritization and sericitization. Chloritization occurs when chlorite replace biotite and due to this high chlorite content, gray color of uranium host sandstone changed to greenish gray color. So chloritization basically causes green alteration in Zhilou Formation. High content of Chlorite also indicate that host rock undergoes a strong oxidation before going to be green in color, as well as it plays a crucial role in uranium mineralization. On the other hand sericitization occur when sericite replace K-feldspar. Replacement of K-feldspar by sericite is the indication of involvement of hydrothermal processes.Scanning Electron Microscope (SEM) observations show some unusual microbe like mineral structures. EDS analysis was also performed to confirm the composition of these microbe like structures, which is similar to composition of uranium mineral coffinite [(USiO4)1-x (0H)4x]. Some of the coffinite was intimately juxtaposed with secondary pyrite and calcite. High resolution mapping of uranium mineral and trace elements was also performed by SEM.Uranium bearing samples show enrichment of LREEs and depletion of HREEs, as well as negative Eu anomalies and positive Ce anomalies. Negative Eu anomalies indicate reducing environment for uranium enrichment in tetravalent form (U4+). While Ce anomalies indicate oxidizing environment, which is suitable for uranium migration in its hexavalent form (U6+). Study of trace elements and their correlation with uranium mineral show that Pb, Mo and Ba have positive correlation with uranium. So these are considered to be the indicator elements and proved to be helpful during uranium detection and its exploration.Study of stable isotopes along with petrography of diagenetic alteration minerals help out to find the geochemical environment of formation of authigenic minerals and their diagenetic mechanism. For detailed study to find the origin of cementing material as calcite and pyrite of Dongsheng uranium deposit, values of carbon and sulfure isotopes were find out.δ13C values ranges from-2.2‰ to-26.7‰. So the most of δ13C values are greater than-10‰, suggesting a great contribution of organic matter. Similarly the δ34S values of twenty one field area samples for framboidal pyrite ranges from-3.2‰ to-25.2‰ and for euhedral-subhedral pyrite of Zhilou Formation are 4.5%o to 10.2%o that are more negative than the organically derived δ34S. So it indicate that high positive and negative δ34S values are result of bacterial sulfate reduction (BSR).Other than the large uranium deposits, Ordos Basin has also a high gamma ray tuff anomaly in Yanchang Formation of Triassic age. To study the genesis and formation of this tuff interval, fourteen samples of this tuff layer were analyzed to find the content of major elements, trace elements and REEs by ICP-MS analysis. The basic reason of high gamma ray values is the presence of uranium and thorium content in this tuff layer. Data of major elements show that the ratio of SiO2/Al2O3 ranges from 3.28-6.11 with average of 3.74. Similarly the ratio of TiO2/Al2O3 varies from 0.037 to 0.20 with average of 0.06, which indicate tuff layer was originated from a felsic magma. Further Harker diagram also conform it, that Silica content show a positive correlation to Al and K. Values REEs show the enrichment of LREE and depletion of HREE, as well as negative Eu anomaly indicate reducing environment, which is suitable for uranium deposition. To find the formation mechanism of this tuff interval, SiO2 content is plotted against TiO2 which show that almost all samples fall in sedimentary rock area except of one in igneous rocks. It mean tuff layer was formed as a result of deposition of vocanic ashes and other sedimentary material that covered a long distance from source area to host rock. Then the descrimination diagrams for different REEs plot show the Qingling Orogenic Belts of Southern Ordos Basin are the source of this tuff interval in Yanchang Formation of Triassic age. Uranium enrichment took place in tuff layer when uranium bearing fluids of Jurassic strata move downward and educing gases move upward along fissures and fractures upto tuff interval.Consequently, an "Oxidation-Reduction Model" for sandstone-type uranium deposit is proposed which cover role of different geological factors that affect uranium mineralization. This model elucidate the source and geochemistry of uranium deposits in the Ordos Basin, based on the role of organic materials and sulfate reducing bacteria, tectonic activities hydrologic gradient and interlayer pressure. Oxidation process is primary and important for enrichment and migration of uranium in the form of U6+in host fluids. Reduction process is secondary and also very important for uranium deposition in the form of U4+ after enrichment. So both oxidation and reduction processes play important role for uranium mineralization. Sandstone with loosely packed grains, conglomeritic grit, low degree of diagenesis and good permeability is suitable for uranium mineralization. So finally this model summarized the mechanism of uranium deposition in a sandstone bed of Zhilou Formation to form such a large uranium deposit (Dongsheng deposit) in this unique sedimentary basin.