The Geological-geochemical Characteristics And Genesis Of The Main Epigenetic Alteration In The Dongsheng Uranium Area Of The Ordos Basin

The main types of the rock alteration in study area include green alteration, carbonatization,oxidation alteration,bleaching phenomenon.Together with uranium enrichment in Dongsheng deposit,forms a unique series of fluid phenomenon. The paper deals with geological-geochemical characteristics and genesis to provide a rich understanding in generation and accumulation of uranium.Under strong reducing environment condition, the early interlayer oxidation zone was reformed and formed ore-controlling green alteration zone after twice deoxidizations. These two systems both have low organic carbon, low∑S, high Th /U. Uranium mineralizated rocks have the highest organic carbon and the lowest Th/U. The bleaching phenomenon on the top of Yan'an Formation was formed after hypergene oxydic rocks'reduction and reformation in reducing environment. The bleaching rocks have high Th /U and low TFe.In short, green alteration, bleaching phenomenon and uranium mineralizating are formed in a relatively strong environment which relates to large scale hydrocarbon dissipation in this area.The inclusions in the calcite in "calcified wood" to observe and test the composition by the Laser Raman Microprobe, the temperature is tested with the inclusion homogenization method. The results showed that the fluid mainly contained gas component and derived from hydrothermal fluids in the sedimentary basin. Based on the fluid inclusions study in the secondary concrescence of quartz edge and calcite cements in other types of rock alterations, it is found that inclusions have lots of saltwater inclusions and fluid composed with normal temperature atmospheric water. It is concluded that the alterated fluid in this area consists of atmospheric water, epithermal and dissipation hydrocarbon.Based on the carbon and oxygen isotope analyzing in lenticular carbonate and "calcified wood", it is found that carbon source relates fluid action of organic matter; Sulfur isotope analysis in pyrite showed that sulfur source has the characteristic between the biological bacteria and coal; The hydrogen and oxygen isotope in kaolinite showed that it suffered hydrothermal activity rather than weathering; The hydrogen and oxygen isotope in fluid inclusion showed groundwater of mineralizing fluid rooted in hypergene superficial atmospheric precipitation. It is concluded that, the formation of Dongsheng deposit is the result of mixed fluid action by " atmospheric precipitation-dissipation hydrocarbon-coal organic acids-epithermal".Combined with experiment on bleaching and green sandstone by fluid, proved that the clay minerals of oxidation sandstone have changed and precipitated of uranium at the same time under the gas action, it is a good model to explain enrichment of uranium in Dongsheng deposits and the alteration mechanisms.