Structural Control Of Sedimentary-type Uranium Mineralization And Analogue Modelling

The formation of sedimentary-type uranium deposits depends on the coupling of uranium source,uranium migration,precipitation,enrichment and preservation conditions and their reasonable collocation relations,and is controlled by regional tectonic characteristics and tectonic evolution.Taking Erlian basin,an important depositional type uranium deposit in northern China,as an example,the representative uranium-rich sag,Naomugen sag and Hurenbuqi sag,were selected as the key research areas.Based on the analysis of basin deformation characteristics,combination rules and tectonic evolution,the tectonic-physical modeling experiment revealed the formation mechanism and main controlling geological factors of the basin.Combined with borehole data and sample analysis,geological analysis of seismic section in uranium-rich depression and analysis of tectonic subsidence history and tectonic balanced section recovery.The structural deformation characteristics,tectonic-sedimentary evolution history,uranium mineralization geological characteristics and genesis mechanism of Naomugeng and Hurenbuqi sags were systematically studied,and the tectonic-sedimentary model of typical depressions was established.(1)The dynamics of the Early Cretaceous rift in the Erlian Basin were derived from the post-arc expansion induced by the subduction of the Paleo-Pacific plate and the mantle heat at the bottom of the Eurasian plate.There are 4 types of composite structures,series,parallel,oblique and interlaced.The average uranium content in Erlian basin basement is high,which provides abundant uranium deposits for the formation of uranium deposits in the basin.The Cretaceous strata are the main sedimentary bodies in the basin sedimentary cap rocks,and the Saihan Formation of the Lower Cretaceous is the main development horizon of uranium deposits.(2)The main controlling factors of Mesozoic tectonic deformation in Erlian basin were analyzed and studied by physical modeling method.It was proposed that extensional direction is the main controlling factor of Mesozoic tectonic deformation in Erlian basin.The formation and evolution of Erlian basin is mainly due to the tensile stress action in SE135 NW315 direction.Synsedimentation and extension rate are the secondary controlling factors.The direction of extension(equivalent to the change of inclination angle)controls the strike of faults in the rift.The inclination angle determines the plane geometry of the rift zone.The small inclination angle leads to narrow forward rift and the large inclination angle leads to wide and gentle inclined rift.Extension rate controls the formation of secondary faults within the rift and affects the scale of the rift;synsedimentation promotes the development of secondary faults within the rift.(3)The analysis of structural characteristics of uranium-rich sag revealed that the development and distribution of uranium-bearing strata are obviously controlled by synsedimentary faults,and the graben-type and semi-graben-type structural assemblages are developed.The main faults have a large dip angle and gradually slow down to the deep.The analysis of tectonic subsidence history and tectonic equilibrium profile shows that the tectonic evolution of uranium-rich rift has undergone three stages: rifting stage,fault depression transition stage and post-rift thermal subsidence stage.Uranium mineralization is mainly controlled by fault depression transition stage.(4)Based on the theory of sequence stratigraphy,the sequence stratigraphic framework of uranium-bearing strata in Naomugeng and Hurenbuqi sags was established.The uranium-bearing strata are divided into three major sedimentary facies types: river,delta and lake,including five sedimentary subfacies and eight sedimentary microfacies,and two third-order sequence units can be contrasted horizontally.The relationship of depositional system configuration under different sequence framework has been established,and the migration rule of sedimentary system has been revealed.It is found that the uranium deposits are mainly developed in the low-stand system tract and lacustrine transgressive system tract.It is considered that the sedimentary environment near the initial Lake flooding surface is most favorable for uranium mineralization.Saihan Formation of the Lower Cretaceous includes 3 types of uranium mineralization facies: conglomerate,sandstone type,siltstone,mudstone type and coal rock type.The type of uranium mineralization in Naomugeng sag is phreatic oxidation coincided sedimentary-diagenetic type,while that the the Hurenbuqi sag is phreatic oxidation-interlayer oxidation coincided sedimentary-diagenetic type.(5)The distribution regularity of uranium-bearing rock series and uranium deposits in different sags was revealed.The uranium deposits in graben sag are mainly distributed in the center of the sag and the uranium deposits in semi-graben sag are mainly distributed in gentle slope zone.It was considered that the fault-depression transformation period,structural deformation controlling factors and sedimentary environment difference are the main controlling factors of uranium mineralization characteristics and uranium-bearing differences in rift basins.Furthermore,the tectonic-sedimentary control uranium model of typical depression was established.