Mineralization Of Dongsheng Uranium Mining Area Of ​​the Ordos Basin Jurassic Diagenesis Constraints

Ordos basin is a suitable place for research on coexistence of multiple energy resource and coexisting mechanism that it contain petroleum, natural gas, coal and uranium etc multiple energy resources in the same basin. Choosing Jurassic sandstone type uranium deposit in Dongsheng area as a keystone to analysis, profound study on mineralization environment and mineralizing mechanism is accomplished from diagenetic aspect. It has not only important theoretic significance for discussion on forming-enrichment circumstance and mineralizing mechanism of multiple energy resource in the Ordos basin, but also practical economic meaning.Based on fieldwork and observation of drill cores, using theories and methods of reservoir geology, geology of ore deposits, geology of petroleum and gas, coal geology and geochemistry, research of correlation among multiple energy resources and diagenesis-mineralizing response of Zhiluo formation in different diagenetic stages are conducted by polarizing microscope, scanning electron microscope, electron microprobe, X-ray diffraction, cathodoluminescence, reflectivity of vitrinite, fluid inclusion, stable isotope, and trace element analysis etc multiple methods. Conclusions are drawn below.1. Jurassic uranium deposits coexist with coal and organic matters (disperse organic matters) in Dongsheng area. It is also the target area where natural gas vaporizes from underlying strata. Formation of uranium deposits correlate to participant of gas, coal and organic matters. Gas, coal and organic matters may react to uranium by means of adsorption, complexation and revivification. They can also change environment of diagenesis and mineralization.2. In syndiagenetic stage, Zhiluo formation in Dongsheng area is open system. Diagenetic environment is oxidation and subacidity that CO₂ is given out in the transition of organic matters and coal. Pore water in sediments contains relatively high uranium that absorption uranium and resolution uranium is keeping in dynamic balance in this diagenetic environment.3. In period A of early diagenetic stage, Zhiluo formation in Dongsheng area is close system. Coal and organic matters in the strata transforms to humus. The transform of diagenetic environment from subacidity and oxidation to alkalescence & revivification, deoxidization and deposition of a part of uranium pre-enriched in syndiagenetic stage take place. But minable uranium deposits can hardly formed in this period because of absence of abundant uranium-rich groundwater and strong absorption of humus and magnalite.4. In period B of early diagenetic stage, humus transforms to kerogen, lignite transforms to long flame coal, adsorbability of flaming coal to uranium is poor . Accompanied by the increase of buried depth, diagenetic environment transform from alkalescence to acidity that short-chain organic acid and CO₂ degraded in maturation of kerogen infuse to pore water with increasing burial depth. Dissolution of CO₂ and corrosion of minerals may produce a great deal of CO₃^2-. It causes uranium existing in pattern of resolution that CO₃^2- band with uranyl ion in pore water and depositional uranium together.5. In epidiagenetic stage, geothermal evolution of organic matters and coal measures tend to inactive. Natural gas escape from Paleozoic strata causing by release of pressure and development of tectonic fractures. At the same time, preconcentrating uranyl ions in the strata are activated by alkalescence surface water from the erosion source area, seeping downwards through pores and cranny in drought- semiarid palaeoclimate. Primary balance of fluid-rock is broken by the mixing of uranium-rich oxidizing fluid from shallower strata and reductive organic fluid from the deeper strata. It is propitious to deposition and enrichment of uranium in the transformation of diagenetic environment from acidity to alkaline and from oxidation to reducing. So this stage is the main stage of uranium mineralization.6. By comparison of mineralization conditions between Zhenyuan area and Dongsheng area, it is discovered that there are some similarities between the two areas on property of sandstone and supply- flowoff-drainage conditions of groundwater. But Zhenyuan area is quite different from Dongsheng area because the former lack for valid interlayer, source of uranium and effective reducing agent, and diagenetic environment is mainly oxidizing conditions. Difference of the two areas causes the gloomy prospect of uranium mineralization in shallower sandstone of Jingchuan formation and Luohandong formation in Zhenyuan area. As a result, exploration target of uranium deposits should transfer to the deeper Huanhe-Huachi formation and Luohe formation in the lower cretaceous system.