Characteristics Of Paleo-valley Oxidation Zone And Uranium Mineralization Of Lower Cretaceous Saihan Formation In Basaiqi Area,Erlian Basin

Erlian basin is an important oil-bearing,uranium and coal bearing basin in northern China.In recent years,remarkable achievements have been made in uranium prospecting.In the Basaiqi area in the middle of Erlian Basin,an paleo-valley sand body with a length of about 300 km has been delineated,and Bayanwula,Saihangaobi and Hadatu paleo-valley sandstone uranium deposits have been successively implemented.Taking the ancient valley of the upper Saihan formation in Basaiqi area as the research object,through core observation,sample collection and comprehensive analysis of data,this paper focuses on the analysis of the type,distribution characteristics,development style,identification marks and development stages of the oxidation zone of typical uranium deposits in the paleo-valley,and summarizes the spatial relationship between the distribution of oxidation zone of typical uranium deposits in the paleo-valley and uranium mineralization,The paleo-valley type uranium metallogenic model under different development styles of oxidation zone is established.The study shows that the basaiqi paleo-valley in Erlian Basin crosses Wulanchabu and Manite depressions and is distributed along the long axis of the secondary depression in a NE direction.The basin has experienced three evolution stages: fault depression stage,depression stage and fault depression transformation stage.The depression stage closely related to sandstone type uranium mineralization can be further divided into four stages: early depression development stage of Saihan formation,depression development stage of Erlian period,structural inversion stage and Cenozoic depression basin development stage.The upper member of Saihan formation,the prospecting target layer,was deposited in the early stage of depression development.At this time,the secondary depressions in the middle of the basin have the characteristics of "narrow depression,deep valley,pan connectivity and overlapping sedimentation",which controls the development of large-scale fluvial facies sand bodies and provides space for oxidation and uranium mineralization.Different parts of the paleo-valley formed different types and directions of oxidation zones due to different degrees of uplift and denudation during the structural inversion period.Bayanwula section in the eastern section of the valley is mainly lateral phreatic interlayer oxidation zone,Saihangaobi section in the middle section is mainly vertical phreatic oxidation zone,and Hadatu-qiaoergu section in the western section is mainly clockwise interlayer oxidation zone,corresponding to Bayanwula Typical ancient Valley uranium deposits in Saihangaobi and Hadatu.Through the comparison and summary of regional tectonic evolution,identification of development stages of oxidation in rock cores and metallogenic age,it is considered that the formation of uranium mineralization of typical deposits in basaiqi paleo-valley mainly experienced two stages of oxidation.The first stage was from the end of Erlian to the late Eocene,which was a large-scale oxidation mineralization in the region,forming Bayanwula,Saihangaobi The main uranium mineralization of Hadatu deposit.The second stage started from the early Miocene and ended at the end of Miocene(before the deposition of tonggur formation),which has superimposed and enriched the uranium mineralization formed earlier.The comprehensive study shows that the paleo-valley sandstone type uranium deposit in basaiqi ancient valley of Erlian Basin takes the paleo-valley sand body in the upper part of Saihan formation as the main metallogenic site,and forms oxidation alteration of different types,scales and styles through vertical,lateral or forward oxygenated water supply.Different types of epigenetic oxidation determine the possibility of various types of uranium ore(chemical)bodies in the ancient valley,Saihangaobi type phreatic oxidation zone type uranium mineralization mainly developed by vertical recharge,Bayanwula type phreatic interlayer oxidation zone type uranium mineralization developed by lateral recharge area,and Hadatu type interlayer oxidation zone type uranium mineralization developed by clockwise(along river strike)recharge area.