| The study area is located in southeastern Ordos basin,tectonically in the transition among Yi-Shan slope and Wei-Bei uplift,geographically between the cities Yan'an and Xianyang,and about4.7×104 km2 in size.There has been Diantou small deposits of uranium discovered in 1950s-1960s in southeast margin of the basin,but the work degree of uranium exploration and scientific research is generally low due to the confinement of topographic and geomorphic conditions.In recent years,Bureau of Geology,CNNC continued increasing the investigation on uranium exploration and scientific research,and this thesis is supported by the Scientific Research in Production projects undertaken in southern Ordos basin by Central Geological Exploration Foundation.Based on digesting the results of former study,the thesis made a full use of the collected scientific research data and drilling data,combined macro study with micro study,approached along major evolutional chain of structure-sedimentation-uranium metallogenesis,focused on regional uranium metallogenic geology,defined key ore-controlling factors and clarified uranium metallogeinc mechanism,constructed ore-forming model,delineated uranium potential prospects,and have supplied direct accordance for the realization of enlargement of deposit and prospection of new district.Through the studies on sedimentation,the uranium content is thought to be generally high(3.30×10-69.01×10-6)in sands of braided stream phase which deposited in early Zhiluo Age.This is obviously an internal-derived uranium source.In this region,the braided stream sands are generally 20-60m thick with abundant coal lines,carbon debris,and pyrites as reductive materials,and supply favorable ore-reserving space for the formation and hosting of sandstone type uranium mineralization.In upper Zhiluo Formation,thick mudstone and silty mudstone have been discovered as the roof of uranium ore-reserving sands,and the coal layers and thick mudstone on top of Yan'an Formation as the floor of the sands,and moreover,the roof and floor are thick and continuously extend.Such a mud-sand-mud structure is the most favorable stratigraphic assemblage for uranium metallogenesis of interlayer oxidation zone type in this region.According to the studies on organic material,ore-forming fluid and epigenic alteration it is concluded that oil-gas is active in southeast of the basin,and that the hydrocarbon source rocks are mainly lacustrine clayish shale of upper Triassic Yanchang Formation.By means of scanned electronic microscope?SEM?,fluid inclusion,thin section observation,electronic probe microscopic analysis?EPMA?as well as dating of uranium mineralization,the sequences among diagenesis,oil-gas charging and uranium mineralization have been determined:sedimentary diagenesis in late Jurassic?the first stage of oil-gas charging?diagenic consolidation with the formation of the first secondary incretion of quartz in early-middle stage of later Jurassic?the secondary uranium mineralization at the end of late Jurassic?the second secondary incretion of quartz in early Cretaceous?the second stage of oil-gas charging?primary uranium mineralization in later Cretaceous-Paleogene?the oil-gas continuously charging at later stage?reductive blading alteration of paleo oxidation zone?ore reservation and superimposition.Through the studies of this thesis on structure-sedimentation-uranium metallogenic evolution,the paleo-geomorphic framework during uranium metallogenesis was restored for the first time in Bin-Chang district on south margin of the basin,and an evolution sequence was established from structure via sedimentation to metallogenesis which revealed the controlling role played by structural-sedimentary evolution in uranium metallogenesis.It was raised that the braided stream developing in the sedimentation of Mid-Jurassic Zhiluo Formation in Bin-Chang district had been controlled by regional down-cutting valey.The late Jurassic geomorphology was higher in north and lower in south,paleo interlayer oxidation zone occurred from north to south,and preliminary uranium mineralization formed at slope and depressed position.The early Cretaceous geomorphology changed to be higher in south and lower in north due to Yanshanian movement,paleo interlayer oxidation zone occurred from south to north at lower part of Zhiluo Formation and on top of Yan'an Formation,and main industrial grade uranium mineralization formed.Pleistocene to present geomorphology succeeds to be high in south and low in north,and interlayer oxidation zone continues developing,but because the uranium source was blocked by Weihe river,uranium cannot be further superposed and enriched,and only the previously formed orebodies were reworked,forming a complex distribution of uranium mineralization.A uranium metallogenic model was constructed based on structural-sedimentary evolution identified above.The thesis innovated to raise that the fold cover on south margin of the basin could be regarded as one of the key ore-controlling factors of uranium metallogenesis.Structures on south margin of the basin are more complex than north margin,generally occurring on structural slope of Wei-Bei monocline which,however,is not an integrated table but with a series of folds and faults developing nearly parallel to the basin margin.Especially,the wide and smooth synclines control both the sands of braided stream in lower member of Zhiluo Formation and the flowing direction of ancient water at later stage,as well as the scale and extension direction of interlayer oxidation zone on slope belt.The comprehensive studies propose that superposed area of syncline,interlayer oxidation zone developed on the slope belts in both wings of the syncline,and sands of braided stream is the best potential area for uranium exploration on south margin of the basin.During comprehensive studies on drilling data together with field geological survey conducted in this thesis,a series of maps,totally 20 sheets,have been compiled for study area,such as uranium geological map,contour map of sands thickness of target layers,map of sedimentary facies,map of ore-forming elements,map of predictive results,and so on.Accorcing to comprehensive studies on uranium mineralization it is basically defined that uranium mineralization in southeastern margin of Ordos basin is mainly controlled by braided stream sands,structure together with epigenic alteration.For instance,uranium mineralization in Binxian-Xunyi district is contolled by the syncline of basin cover,as well as the interlayer oxidation zone developed on both wings of the syncline and braided stream sands,and mineralization in Diantou-Jiaoping-Miaowan is mainly controlled by braided stream sands in mono-cline structure and NE striking fault.Uranium mineralization is manly hosted in the transition between grayish green or grayis white epigenic altered sands and grey primary sands.After comprehensive analysis of uranium mineralization and determination of key ore-controlling elements,6 pieces of uranium prospective areas have been predicted in southeastern margin of the basin,including 2 pieces of class I prospective area,2 pieces of class II and 2 pieces of class III.By use of modified volume method,uranium resources was estimated to be 51000 tons?of which hard sandstone type is 15000 tons?in lower member of Zhiluo Formation,and 24000 tons in lower Cretaceous,totally prognosticated to be 75000 tons uranium in south margin of Ordos basin.On the basis of map compilation and perspective prognosis,uranium potential was comprehensively evaluated in south margin of the basin,and some recommendations were proposed for exploration deployment,such as increasing investigation in southeast margin of the basin,as well as expanding prospective space inside the basin.In general,uranium source is abundant in south margin of the basin,and a continuous structural slope belt is developing,the thickness of ore-hosting sands is suitable and homogeneous,and the reaction of structure-sedimentation-epigenic alteration is significant,therefore,uranium potential is large in study area.It is proposed to increase the uranium geological exploration in Jiaoping-Miaowan-Xunyi district in southeast margin of the basin,focusing on the possible breakthrough of prospecting in both sides of Jinghe river in southern basin,and extending the work further to internal of the basin to discover new prospective area. |
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