The J₂z¹ Sedimentary System And Its Relationship With Uranium Mineralization In Bayinwusu Area, Ordos Basin

In this paper,Bayinwusu,in the west of the north of Ordos Basin,is selected as the study area.This paper analyzes the sedimentary background during the sedimentary period of Zhiluo formation of middle Jurassic,type of sedimentary system and spatial distribution characteristics guided by sedimentology,lithofacies palaeogeography,sedimentary petrology,mineralogy,rock geochemistry and hydrologic uranium deposit theory,as well as combined with characteristics of typical sedimentary structures in the field and logging curve,microscopic features,granularity characteristics,types of heavy minerals and their combination characteristics,U-Pb chronology characteristics of detrital zircons and elemental geochemical characteristics.In the meanwhile,combined with the geological features of the uranium deposits found in the study area,this paper establishes the relationship between the sedimentary system and uranium mineralization from the aspects of paleoclimate,stratum,sandstone body,and lithology,further deepening the understanding of their relationship.In the study area,the sedimentary period of the lower Zhiluo formation is dominated by warm and humid climatic environment,and the climate has gradually changed from being warm and humid to being hot and dry during the sedimentary period from the lower to the upper submember.Under this climate transition background,the salinity of paleocurrent in the lower submember of the Zhiluo formation is lower than that of the upper submember.The average value of Fe3+/Fe2+ of mudstone in the lower submember is 0.37 and that in the upper submember is 1.24,indicating that reducing current is dominant in the lower submember,while the upper submember is dominated by weakly oxidized current.It shows that the oxidation ability of paleocurrent in the upper submember is stronger than that in the lower submember.The results of provenance analysis show that the parent rock of clastic material in the lower sumember derives from the Archean Paleoproterozoic metamorphic rocks and the Middle-Neoproterozoic Paleozoic magmatic rocks,which have been formed in the oceanic island arc and active continental margin tectonic environment.The fluvial delta sedimentary system is developed in the lower member of Zhiluo formation and it is mainly composed of delta fac ies.The small scale fluvial facies is developed in the lower submember,while only delta facies is developed in the upper submember.The coal seam and typical biogenetic structure indicate that the study area was located in the delta plain subfacies during the sedimentary stage of the lower member of Zhiluo formation,and the branch channel,crevasse fan,mire and swamp microfacies were identified according to lithology combination and logging curve characteristics.There are four main branch channels dur ing the sedimentary period of the lower submember of the lower Zhiluo formation,which is respectively from the northwest,the northeast and the west of the study area.The small scale fluvial facies is developed in the upstream of the branch channel in the northeast of study area,which is trending from the northeast to the southwest.The channel branches frequently to the southwest and evolves into the branch channel of delta plain subfacies.That is to say,the northeast of the study area is the transition site between fluvial facies and delta facies during the sedimentary period of the lower submember of the lower Zhiluo formation.There is no fluvial facies in the study area during the sedimentary period of the upper submember of the lower Zhiluo format ion and only three main branch channels are developed,which is respectively from the northwest,the northeast and the west of the study area.Crevasse fan microfacies are developed at the edge of the branch channel.Mire as well as swamp microfacies,composed of fine-grained argillaceous sediments,is developed between the branch channels,providing a stable top and bottom aquiclude for the sand body of the branch channel.The results show that the grain size of sandstone in the lower submember is coarser than that in the upper submember,while its maturity is lower than that in the upper submember.It also displays that the sand body scale and thickness are larger than that in the upper submember,and the sand content is higher than that in the upper submember.It indicates that there is stronger hydrodynamic conditions,shorter transport distance of clastic materials,and much sufficient material source supply in the upper submember,further proving it is a process of river retreating,the lake advancing and source area retreating.The branch channel sandstones in the delta plain are mainly medium-coarse feldspar lithic sandstones and lithic feldspar sandstones.The porosity and permeability of sandstones are excellent.The sandstones contain a large number of carbonaceous clasts and pyrites,which are favorable to reducing agent for uranium precipitation and enrichment.There exists a good connectivity in the branch channel sandstones,moderate thickness in the monolayer sandstones,and "mud-sand-mud" structure developing in vertical direction,which provides an excellent fluid unit for the formation of interlayer oxidation zone.The sedimentary microfacies change frequently on the plane and the heterogeneity of the stratum is obvious.The physical barrier effect of the mudstone with poor permeability increases the reaction time between the fluid and the rock,which is conducive to the reduction and precipitation of uranium.The uranium ore bodies which have already been discovered are mainly distributed in the transition positions of sandstone body thickness and sand content as well as different sedimentary microfacies.Combined with the distribution of sand body,sedimentary microfacies and oxidation-reduction zoning,two favorable ore-forming areas,located in the northwest and east of the study area,are delineated in the lower submember of the lower Zhiluo formation,and three,located in the southwest,southeast and east of the study area,are circled in the upper submember.The above areas can be taken as the key places for further uranium exploration.