Theoretical System of Sandstone-Type Uranium Deposits in Northern China

Many theoretical results on sandstone-type uranium mineralization in northern China obtained by the uranium research team of the Tianjin Center of Geological Survey in recent years are presented.From the source sink system of uranium-producing basins,sedimentary environment of uranium-bearing rock series,ore-forming fluid information,evolution of tectonic events,basin formation and development,we redefine and classify uranium orebodies,redox zoning,and ore-controlling structural styles.We then systematically propose a theoretical system of sandstone-type uranium deposits in northern China.We conclude that sandstone-type uranium deposits in northern China are mainly found in sedimentary environments such as rivers,deltas,and alluvial fans in the Mesozoic and Cenozoic lowstand systems tract and in gray sandstone layers in the vertical redox zoning.The orebodies are controlled by the tectonic slope belt,which is in the shape of a strip on the plane,and spreads in a layer or plate on the section.Vertical(ups and downs)tectonic movement triggers large-scale phreatic flow in the basin,which is the real driving force for controlling the ore-forming fluid.The theoretical system of sandstone-type uranium deposits in northern China should be based on global tectonic movement and environmental changes and take into account factors such as basins as a unit to study mineralization background,ore concentration areas as objects to study mineralization,and the correlation between regional tectonic movement and metallogenic process as a breakthrough point to study tectonic events and metallogenic events.It should also be based on different basin types to establish metallogenic models.The innovative research results and ideas are summarized with the aim of promoting the continuous improvement of sandstone-type uranium mineralization theory in northern China.

Geology,Mineralogy,and Isotopic Characteristics of the Zhonghe Ag-Pb-Zn Deposit,Western Henan Province,Central China

Located in Luoning County,western Henan Province,Central China,the Zhonghe Ag-Pb-Zn ore field,is a newly discovered deposit in the Xiaoshan District.Ore bodies controlled by NNW Faults occur as veins within the Paleoproterozoic Xiong’er Group or the Early Cretaceous porphyritic granite.Given that the Zhonghe deposit has been covered by thick Quaternary sediments,the paragenetic mineral assemblage was determined mainly by microscopic observations,including the quartzsiderite-pyrite alteration (StageⅠ),polymetallic sulfide precipitation (StageⅡ),silver mineralization(Stage Ⅲ),and quartz-carbonate stage (StageⅣ).The host minerals for silver are diverse,such as freibergite,pyrargyrite,polybasite,argyrodite,canfieldite,argentite,and native silver,whereas those for Pb and Zn are galena and sphalerite,respectively.In order to constrain the ore-forming components of the Zhonghe deposit,a combined in-situ analysis was conducted on represented sulfides from StageⅡand Stage Ⅲ.In-situ δ^(34)S values of the analyzed sulfides,including the pyrite,chalcopyrite,sphalerite,and galena,display a relatively narrow range (0.90‰-5.0‰),which is close to magmatic sulfur source.The ^(206)Pb/^(204)Pb and^(207)Pb/^(204)Pb ratios show a narrow range (17.140-17.360 and 15.385-15.490),whereas the ^(208)Pb/^(204)Pb ratios exhibit a broad variation (36.601-37.943),indicating a contamination of the Xiong’er Group.Synthesis of geochronological and geochemical data from the Xiaoshan District,we contend that the ore-forming materials of the Zhonghe Ag-Pb-Zn deposit are originated from the lower crust,which has presumably been influenced by the large-scale lithospheric delamination of the eastern North China Craton during the Early Cretaceous.In consideration of the geological setting,mineralogy,and geochemical compositions,we suggest that the Zhonghe Ag-Pb-Zn deposit is characterized as intermediate sulfidation type epithermal deposit,and may be a potential exploration target for porphyry Mo-Cu deposits.

Uranium Isotope Variations(234U/238U and 238U/235U)and Behavior of U-Pb Isotope System in the Vershinnoe Sandstone-Type Uranium Deposit,Vitim Uranium Ore District,Russia

The U-Pb isotope system and uranium isotope composition(235U/238U and 234U/238U)were studied in a number of samples from the vertical section of the uranium ore body at the Vershinnoe sandstone-type deposit,Vitim uranium ore district,Russia.These parameters were determined to broadly vary.Deviations of the 234U/238U ratio from the equilibrium value indicate that the uranium ore was not completely conserved during the postore stage,and uranium was determined to continue migrating at the deposit.Comparison of the U-Pb isotope age value and 234U/238U isotope ratio provides an insight into the migrate direction of uranium in the ore body.The broad variations(137.377–137.772)in the 238U/235U ratio over the vertical section of the ore body can be explained by the different settings of the samples relative to the ore deposition front and changes in the redox conditions when this front shifted.The fact that theδ238U and K234/238 values are correlated indicates that the transfer of the 234U isotope into the aqueous phase may have been coupled with isotope fractionation in the 238U-235U system during the postformation uranium migration within the orebody.

Origin of Dispersed Organic Matter within Sandstones and Its Implication for Uranium Mineralization:A Case Study from Dongsheng Uranium Ore Filed in China

Carbonaceous debris(CD),common dispersed organic matter(i.e.,DOM),is widely disseminated in sandstones from uranium-bearing strata from the Dongsheng uranium ore field of the northern Ordos Basin.Compositions of maceral,element and biomarkers of CD were investigated through a series of methods with optical microscope,elemental analyzer and gas chromatography-mass spectrometry analyses(GC-MS)to study origin of CD.The results show that CD,centrally distributed nearby channel erosion surface,decreases with the increased distances to channel erosion surface,which indicates the CD might be related to the coal seam from the upper unit of the J2y Formation or synsedimentary plant from the J2z Formation.Macerals of CD are composed of vitrinite(i.e.,V),inertinite(i.e.,I),and minerals,including that V is primary.Compared with the coals from the J2y Formation classed into vitrinertite-V(V+I>95%,V>I),CD is grouped into vitrite(V>95%).Although,CD and coal are similar in element composition,the former is of lower organic carbon,H,N,and higher S.The(C27+C29)/(C31+C33)ratios of n-alkanes biomarkers indicate that the percentage of woody plants accounting for vegetation composition of CD predominate over that of coal,which is also evidenced by the higher C/N ratios and oleanane contents of CD.The evidence is also supported by plant branch buried in sandstones.The distribution characteristics of CD and differences in vegetation types between CD and coal suggest that CD might be not from the coal seam from J2y.The tissue preservation index,gelification index,ground water level index,and vegetation index reflect that the paleoenvironment of CD is controlled by fluctuating water,which is also supported by the existences of round CD.Compared with peat,sedimentary paleoenvironment where CD deposits is of weaker reducibility,higher salinity by analyzing Pr/Ph ratios and gammacerane index.Distributions of n-alkanes carbon number of CD with the presence of unknown complex mixtures show that microbial activities exist in sand bodies.Differences in hydrodynamic intensity,redox condition,and microbial activity intensity between sedimentary paleoenvironment of CD and peat,show that CD is born in synsedimentary sandstone environment not in peat.Hence,it comprehensively draws conclusions that immature‘non-peatborn’CD is formed from the trunk,stem,branch,root fragments buried in sandstones,depositing in(micro)allochthonous positions by the influences of fluctuating water.The DOM from synsedimentary plant debris might play more roles in adsorption and complexation,and microorganisms may participate in uranium mineralization,which could provide certain guidance for uranium exploration and mining.

Relations of Uranium Enrichment and Metal Sulfides within the Shuanglong Uranium Deposit,Southern Ordos Basin

Large quantities of metal sulfides are widely distributed in uranium ores from the Middle Jurassic Zhiluo Formation of the Shuanglong uranium deposit,southern Ordos Basin,providing a convenient condition to study the relationship between metal sulfides and uranium minerals.The morphology and composition of uranium minerals and metal sulfides are illustrated to study uranium mineralization and mechanisms of metallogenesis.Uranium minerals can be broadly categorized as pitchblende,coffinite and brannerite.Metal sulfides associated with uranium minerals are pyrite,sphalerite,chalcopyrite and galena.Some assemblages of various metal sulfides and uranium minerals indicate that they are coeval,but the order of formation is different.Based on mineralogical observations,paragenetic sequences for mineral assemblages are discussed.Alteration of Fe-Ti oxides forms Ti oxides,brannerite and pyrite.The formation of chalcopyrite was later than that of pyrite.Clausthalite(Pb Se)replaces sphalerite or shows isomorphism with galena.There are three genetic types of galena,of which typeⅠis related to tectonic thermal events and can interact with uranyl ions to form uranium oxides and Pb;.When sulfur activity is relatively high,Pb;can form new anhedral galena,that is,typeⅡ.TypeⅢof galena is related to the decay of uranium minerals.The genetic order of the main minerals was determined as follows:Fe-Ti oxides>Ti oxides/sphalerite/pyrite>clausthalite/galenaⅠ/chalcopyrite>galenaⅡ/uranium minerals>galenaⅢduring the diagenetic stage.Hydrogen sulfide(H;S)is a decisive factor in the interaction between metal sulfides and uranium.Metal ions can react with H;S,accompanied by precipitation and enrichment of uranium minerals.

Classification of Sandstone-Related Uranium Deposits

Sandstone type deposits are the most common type of uranium deposits in the world.A large variety of sub-types have been defined,based either on the morphology of the deposits(e.g.,tabular,roll front,etc),or on the sedimentological setting(e.g.,paleovalley,paleochannel,unconformity),or on tectonic or lithologic controls(e.g.,tectonolithologic,mafic dykes/sills),or still on a variety of others characteristics(phreatic oxidation type,interlayer permeable type,multi-element stratabound infiltrational,solution front limb deposit,humate type,etc.),reflecting the diversity of the characteristics of these deposits,but making it difficult to have a clear overview of these deposits.Moreover,uranium deposits occurring in the same sedimentological setting(e.g.,paleochannel),presenting similar morphologies(e.g.,tabular),may result from different genetic mechanisms and thus can be misleading for exploration strategies.The aim of the present paper is to propose a new view on sandstone-related uranium deposits combining both genetic and descriptive criteria.The dual view is indeed of primordial importance because all the critical characteristics of each deposit type,not limited to the morphology/geometry of the ore bodies and their relationships with depositional environments of the sandstone,have to be taken into account to propose a comprehensive classification of uranium deposits.In this respect,several key ore-forming processes,like the physical-chemical characteristics of the mineralizing fluid,have to be used to integrate genetic aspects in the classification.Although a succession of concentration steps,potentially temporally-disconnected,are involved in the genesis of some uranium mineralization,the classification here proposed will focus on the main mechanisms responsible for the formation and/or the location of ore deposits.The objective of this paper is also to propose a robust and widely usable terminology to define and categorize sandstone uranium deposits,considering the diversity of their origin and morphologies,and will be primarily based on the temperature of the mineralizing fluid considered as having played the critical role in the transportation of the uranium,starting from synsedimentary uranium deposits to those related to higher temperature fluids.

Uranium Metallogeny in Fault-Depression Transition Region:A Case Study of the Tamusu Uranium Deposit in the Bayingobi Basin

Compared to the sandstone-type uranium deposits in the Ordos Basin and the Songliao Basin,the Tamusu uranium deposit in the Bayingobi Basin formed in fault-depression transition region displays distinctive features.First,the uranium-bearing sandstones and their interlayer oxidation zone extend longitudinally no more than ten kilometers.Second,gravity flow sediments are more common in the uranium-bearing strata.Comprehensive facies analysis indicates that the Upper Member(orebearing horizon)of the Bayingobi Formation was largely deposited in fan deltas that prograded into lakes during period of relatively dry paleoclimate.Spatial distribution patterns of five facies associations along with two depositional environments(fan delta,lake)were reconstructed in this study.The results demonstrated that the depositional systems and their inner genetic facies played different roles in uranium reservoir sandstone,confining beds(isolated barrier beds)and reduction geologic bodies during uranium mineralization process.

U-Blacks Mineralization in Sandstone Uranium Deposits

Ores of infiltration sandstone-hosted uranium deposits in the sedimentary cover are ubiquitous composed of dispersed soot powder mineralization of black,brownish-black colour.Longterm studies of such loose U-ores by analytical transmission electron microscopy(ATEM)proved their polymineral nature.Uranium blacks are composed by at least three different U-mineral forms:oxide(uraninite),silicate(coffinite)and phosphate(ningyoite)which are present in various proportions of ore compositions.Such high dispersed friable uranium formations are difficult to diagnose by traditional mineralogical methods(optical,XRD,IR and X-ray spectroscopy,etc.)which analyze total sample composition(phases mixture);their results characterize the dominant sample phase,omitting both sharply subordinate and X-ray amorphous phases.All research results are based on ATEM methods(SAED+EDS),which are optimal for crystallochemical diagnostics in the mineralogical study of such uranium ores.The article presents the diagnostic characteristics under electron microscope(EM)of uranous minerals from different sandstone deposits with their origin being discussed.

Central Asia—A Global Model for the Formation of Epigenetic Deposits in a Platform Sedimentary Cover

Metallogenic specialization of sedimentary cover in Central Asia is determined by its tectonic setting that governs the hydrodynamic regime(exfiltrational or infiltrational)and as a consequence,the hydrogeochemical zonality(type of water and its gaseous and microcomponent composition).Hydrodynamic conditions(distribution of recharge and discharge areas)determine the direction of stratal water flow and location of mineralization resulted from the change in geochemical,thermodynamic,lithological,structural and other conditions.The exfiltrational regime suggests a dependence of the epigenetic mineralization upon the distribution and degree of preservation of hydrocarbon occurrences.Often,hydrocarbon matter serves as a reducing barrier and ore-concentrating factor during the formation of polymineral concentrations related to stratal oxidation zone.The supergene epigenetic ore-forming processes are induced by the interaction between the Earth’s sedimentary cover and hydrosphere.Sedimentary rocks themselves commonly serve as a source of ore materials.The ore deposition zones on geochemical barriers and ore material source are often located significantly apart from each other.The trend of these processes is determined by the position of ore-bearing depressions in large tectonic blocks.

Mass Transfer during Hematitization and Implications for Uranium Mineralization in the Zoujiashan Deposit,Xiangshan Volcanic Basin

The Zoujiashan uranium deposit in the Xiangshan ore field is the largest volcanic-related uranium deposit in China.Hematite-and fluorite-type ores are the predominant mineralization styles.Hematitization in the Xiangshan ore field is closely associated with uranium mineralization,mainly occurring as hematitized rocks enclosing fluorite-type vein ores developed in pre-ore illitized porphyritic lava.Detailed petrographic and mass balance calculation studies were conducted to evaluate the mechanisms for uranium precipitation and mass transfer during hematitization.Petrographic observations suggest that in the hematitized rocks,orthoclase is more altered than plagioclase,and quartz dissolution is common,whereas in the illitized rocks,pyrite commonly occurs within the altered biotite grains,and chlorite grains are locally found.Mass balance calculations indicate that Na2O and U were gained,K2O,Ca O and Si O2were lost,whereas Fe2O3-t remained more or less constant during hematitization.These observations suggest that the hydrothermal fluids were Na-and U-rich and Ca-K-poor,and the Fe2+used for hematitization was locally derived,most likely from biotite,pyrite and chlorite in the host rocks.The Fe2+is inferred to have played the role of reductant to precipitate uranium,and calculation indicates that oxidation of Fe2+provided by host rocks is sufficient to form ores of economic significance.Consequently,the hematite-type ore is interpreted to be generated by the reaction between oxidized ore fluids and reduced components in host rocks.The development of calcite and pyrite in the fluorite ores suggests that perhaps mixing between the U-rich fluid and another fluid carrying reduced sulfur and carbon may have also contributed to uranium mineralization,in addition to temperature and pressure drop associated with the veining.

Association of Sandstone-Type Uranium Mineralization in the Northern China with Tectonic Movements and Hydrocarbons

In the continental basins of Northern China(NC),a series of energy resources commonly co-exist in the same basin.As the three typical superimposed basins of different genesis in the NC,the Junggar,Ordos,and Songliao basins were chosen as the research objects.The favorable uraniumbearing structures are generally shown as a basin-margin slope or transition belt of uplifts with the development of faults,which are conducive to a fluid circulation system.The Hercynian,Indosinian,and Yanshanian movements resulted in the development of uranium-rich intrusions which acted as the significant uranium sources.The main hydrocarbon source rocks are developed in the Carboniferous,Permian,Jurassic and Cretaceous.The mature stage of source rocks is concentrated in the Jurassic–Cretaceous,followed by the multi-stage expulsion events.Influenced by the India-Eurasian collision and the subduction of the Pacific Plate,the tectonic transformation in the Late Yanshanian and Himalayan periods significantly influenced the sandstone-type uranium mineralization.The hydrocarbon reservoirs are spatially consistent with sandstone-type uranium deposits,while the hydrocarbon expulsion events occur in sequence with sandstone-type uranium mineralization.In the periphery of the faults or the uplifts,both fluids met and formed uranium concentration.The regional tectonic movements motivate the migration of hydrocarbon fluids and uranium mineralization,especially the Himalayan movement.

Characteristics of Altered Ilmenite in Uranium-Bearing Sandstone and Its Relationship with Uranium Minerals in the Northeastern Ordos Basin

In recent years,the close relationship between uranium and Ti-Fe oxides in the sandstonetype uranium deposits has been extensively recognized.However,the altered characteristics of ilmenite and its relationship with uranium enrichment still remain unclear.With this paper based on heavymineral sorting of uranium ore selected from the Tarangaole-Nalinggou deposit in the northeastern Ordos Basin,electron probe,backscattering image,energy spectrum and scanning electron microscopy were systematically performed.The ilmenite in the sandstone can be divided into four groups,including unaltered,weakly altered,moderately altered,and strongly altered ilmenite.The alteration of ilmenite in uranium ores is notably more intense than that of the surrounding rocks.In addition,weakly,moderately,and strongly altered ilmenite associated with uranium minerals in uranium ores demonstrate that the more intensity ilmenite altered,the closer its relationship with uranium minerals is.The ilmenite has likely been somewhat altered before mineralization,and the alteration intensifies by later exposure related to an oxygencontaining fluid.The alteration mechanism comprises a process of competitive diffusion between Fe^(2+)and O_(2)-ions.In the early stage,Fe ions was mainly diffused on the particle surface.Subsequently,diffusion of O ions into the particles began to be dominate.Most of the leached iron is stripped or carried away by fluid.In an alkaline and reductive environment,the remaining iron is reduced to form the surrounding pyrite,and TiO_(2)in a form of titanium sol recrystallizes(i.e.,anatase).Backscattering images show that uranium and altered ilmenite are close in space.Coffinite is often distributed along the edges of altered ilmenite as burrs in shape.Colloidal or knitted coffinite associated with anatase is formed in the voids of altered ilmenite.The chemical composition of altered ilmenite varies considerably from the core to edge,and the mineral assemblage sequence is from girdle with ilmenite,to leucosphenite,to anatase,and to coffinite.There is no brannerite that is symbiotic with altered ilmenite.It is considered to be a uranium-containing titanium mineral aggregate caused by the reduction and adsorption of uranium.As the altered product of ilmenite,TiO_(2)is an aggregation agent,increasing the concentration of uranium by adsorption.Together with Fe^(2+)and S_(2)-in secondary pyrite,this aggregate creates a uranium-rich environment in the microzone for the formation of coffinite.Therefore,the alteration of ilmenite plays a geochemical role in the processes of sedimentary,diagenesis and mineralization,in which Fe is removed,Ti is enriched,and U is adsorbed and reduced.