Research On The Relationship Between Fluid Interaction And Uranium Mineralization In Tongliao Area, Inner Mongolia

The Songliao Basin is one of the major uranium-producing basins in northern China.A few large-scale uranium deposits such as Qianjiadian and Baolongshan have been identified in Tongliao area in the southwest of the basin.In recent years,new deposits such as Dalin and Hailijin have been newly discovered through uranium exploration in the Tongliao area,indicating that there are still great prospects for mineralization.However,the study on the relationship between fluid activity and uranium mineralization is relatively poor and worth further work in order to clarify the formation of such a big-scale uranium deposit.Therefore,this thesis takes Baolongshan and Dalin sandstone-type uranium deposits in Tongliao area in the southwest of Songliao Basin as the study object.With the guidance of petrology,mineralogy,geochemistry and sandstone-type uranium mineralization theory,the geological characteristics of uranium deposits,epigenetic alteration characteristics of sandstone in the Yaojia Formation,the types of ore-forming fluids and their relationship with uranium mineralization are studied and analyzed.The following main results have been achieved:?1?The study suggests that the alteration minerals related to uranium mineralization in sandstone of the Yaojia Formation mainly include kaolinite,pyrite and carbonate.Kaolinite often adsorbs uranium and fills the pores of sandstone in the form of matrix.On the one hand,pyrite exhibits framboidal pyrite as an alternative to organic matter and coexists with uranium minerals;on the other hand,it exists as colloidal pyrite with ankerite and uranium mineral.Uranium-containing carbonate is mainly ankerite in the form of automorphic-semi-automorphic,and uranium mineral exists on its surface.?2?According to the output form and isotopic geochemical characteristics of altered minerals,their genesis has been analyzed.Among them,the kaolinite has a complicated origin,which can be either the product of atmospheric precipitation leaching or diagenetic organic acid fluids,or it can be formed under the injection of the CO₂-rich thermal fluid produced by diabase degassing;pyrite can be divided into framboidal pyrite and colloidal pyrite enclosing framboidal pyrite based on its morphology.The former is a biological origin with strong loss?³⁴S characteristics,and the latter is related to thermal fluid activity because of the enrichment of ?³⁴S;carbonate minerals have three phases,followed by primary sparry calcite formed in the sedimentary diagenesis stage,calcite vein and dawsonite formed during the injection of CO₂-rich fluid,and finally,the ankerite formed under the mix of thermal fluid and groundwater.?3?Combining macroscopic sandstone characteristics and microscopic observations,it is believed that there are groundwater,CO₂-rich thermal fluid produced by diabase degassing,and oil-gas fluid in the study area.The activity of groundwater influences the entire period from the formation of the Yaojia Formation to the uranium deposit;the thermal fluid riched in CO₂ is formed during the period of strong regional tectonic-magmatic activity at the end of the Nenjiang period,and is the product of degassing of basic magmatic activity;oil-gas reducing fluid except for organic acid controlled by primary carbonaceous organic matter,it is composed of deep oil and gas.With continuous structural uplift activities,it can be transported along the fault to the Yaojia Formation and increase the reduction capacity.?4?Using fluid inclusion analysis methods,the study believes that the ore-forming fluids of uranium deposits in the area have the characteristics of medium-low temperature,low salinity,and superposition of multiple fluids.During the structural inversion period,the Yaojia Formation was influenced by the CO₂-rich thermal fluid,which has the characteristics of higher temperature?198-223??and low salinity(0.71-4.34 wt%Na Cl_eq).With the gradual mixing of uranium and oxygen-bearing groundwater,the ore-forming fluid evolved into the characteristics of medium temperature?155-232??and higher salinity(4.18-9.98 wt%Na Cl_eq).In the end,because of the intermittent thermal fluid activity,the ore-forming fluid is dominated by groundwater,characterized by low temperature?83?145??and low salinity(0.88?6.45 wt%Na Cl_eq).?5?the Yaojia formation in the Baolongshan-Dalin uranium ore belt mainly undergoes the sedimentary diagenesis stage,the CO₂-rich fluid injection stage,and the mineralization stage.The evolution process of the geochemical environment is weak alkali?weak acid?alkali-Overalkaline?weakly alkaline?weakly acidic-neutral,this process plays an important role in controlling the formation of altered minerals and the migration and enrichment of uranium.According to the combination of altered minerals and fluid activity,the uranium mineralization process is divided into three stages:the preliminary enrichment stage,CO₂-rich fluid injection,and interlayer oxidation coupling uranium mineralization.