Sedimentary Characteristics Of The Jurassic Zhiluo Formation In Northern Ordos Basin And Implications For Uranium Mineralization

As an important uranium deposits region of China,several ultra sandstone-hosted uranium deposits have been found in Dongsheng,Daying and Hangjinqi in northern Ordos Basin.The Jurassic Zhiluo Formation has been proved as major reservoir of these sandstone-hosted uranium deposits,its formation and distribution are controlled by sedimentary characteristics of reservoir sandstone bodies.Therefore,it is of great significance to study the sedimentary characteristics of uranium reservoir.whereas the study on the sedimentary facies and its evolution of the Zhiluo Formation is still weak and controversial.Most of the studies are concentrated in the northeastern Ordos Basin while the rest is less involved.In view of this,this thesis takes northern Ordos Basin as the study area.With emphases on the study of distribution,division and correlation,sedimentary characteristics of lower stratum Zhiluo Formation and its relationship with uranium mineralization.The results on sedimentary characteristics and its relationship with uranium mineralization of the Zhiluo Formation could provide important implication for exploration of uranium deposits in northern Ordos Basin.On the basis of outcrops measurement and well data analysis,systematic results are obtained about distribution,division and correlation of the Zhiluo Formation.It's shown that the Zhiluo Formation can be divided into two members in the whole northern Ordos Basin,the upper(J2Z2)and the lower(J2Z1).The lower member can be divided into two sub-members(J2z1-1,J2Z1-2)further.With outcrops,core and well data analysis,the major sedimentary facies types identified in the Zhiluo formation are fluvial system and deltaic system.On the basis of thin section observation,combined with previous studies,some results are obtained about provenance of the Zhiluo Formation.It is shown that the Zhiluo Formation has three major provenance in the north,northwest and west.Furthmore,the provenance of the Zhiluo Formation mainly comes from Precambrian metamorphic rocks of Yin Mountains and Daqin Mountains in the north of study area,and acid magmatic rocks of late Paleozoic in addition.The general direction of provenance is from northwest to southeast.During deposition of the lower sub-member,the main sedimentary facies include gravelly,sandy braided river deposits and braided river delta deposits in the study area.The major sedimentary facies transformed into sandy braided river and meandering river deposits during deposition of the upper sub-member.Then meandering river system widely developed during deposition of the upper member.Generally,four evolutionary stages can be summarized on the whole deposition of the Zhiluo Formation,which are initial,expansion,peak and atrophy stage individually.There was a climate transition from wet and reductive to dry and oxidative environment during deposition period of the Zhiluo Formation.In order to find out the relationship between skeletal sandbodies and uranium mineralization,the empirical formulas are used to quantitatively recover the size of skeletal sandbodies of uranium reservoir.The thickness of braided river sedimentary system's single channel sandbodies ranges from 2m-10m.The width of sandbodies is about 380m-3800m.and width/thickness ratio is about 150-400.With large scale,high aspect ratio,universal connectivity,it is considered that the channel of braided river sedimentary system of lower stratum Zhiluo Formation is the best reservoir of uranium deposits.On the basis of actual exploration practice.Western margin of Ordos Basin have greater potential for uranium exploration.