Study On Metallogenic Mechanism Of REE-U Elements In North China

The northern part of China remains huge rare earths,rare and strategic resources.The Bayan obo Fe-REE-Nb deposit is the world's largest rare earth deposit.As an important strategic resource,uranium is mainly distributed in the sandstone-type uranium deposits of the Mesozoic basins in northern China.The study of the Bayan Obo rare earth deposit and the sandstone-type uranium deposit in the northern basin has important scientific,economic and strategic significance.In the study of the Bayan Obo deposit,we carried out detailed mineralogical work,supplemented the Raman spectrum and X-ray energy spectrum of various rare earth minerals,and confirmed the common niobium-bearing minerals of Bayan Obo by scanning electron microscope(SEM).Scanning electron microscope,EPMA and LA-ICP-MS were used to analyze the chemical compositions of dolomite from coarse-grained and fine-grained dolomite and to distinguish three types of apatites in various ores.Marginal metasomatic texture and metasomatic perforation texture are universal phenomena.The rare earth contents of apatite in coarse dolomite are lower than those in the fine-grained mineralized dolomite.A wide range in SrO contents in dolomite mineral grains from coarse-grained dolomite marble has been observed.The SrO contents in the margin of the dolomite mineral reach up to 1.04 wt%,while the SrO contents in the core are as low as 0.16 wt%,while the Sr content of the fine-grained dolomite is relatively uniform.The differences in the compositions of the coarse-grained and fine-grained dolomite imply different forming processes.Apatite is a superb mineral as a geochemical tracer to the evolution process of REE mineralized hydrothermal fluid and the process of metasomatism.CL properties combined with chemical compositions of apatite provide fast screening tools to assess multistage hydrothermal alteration.There are significant differences between the three types in their compositions and internal structures.The dark areas in apatite under BSE prove to be subject to fluid replacement.The in-situ Th-Pb age of monazite from the banded ore around 450Ma defines the Caledonian main REE mineralization.REE-rich hydrothermal fluid is closed related to the Caledonian hydrothermal fluid from subduction zone which then extract REE and Nb from the Mesoproterozoic carbonatite.The mineralized hydrothermal fluid evolved after the REE-rich apatite and monazite precipitating in the REE main mineralization stage.The mesothermal fluid rich in Na,rich in SO42-,and CO32-reacted with REE-rich apatite,producing the REE depletion in the apatite.The superimposed oxidized,Sr-rich,Na,REE-poor metamorphic hydrothermal fluid leached the REE-rich apatite.The redistribution of REE occurred during the dissolution and re-precipitation process,producing a large number of tiny REE inclusions inside the apatite,and Sr-rich,Na-poor,REE-poor Type III apatite.Multi-stage REE mineralization and reactivation processes did occur in the Bayan Obo deposit.Caledonian REE-rich hydrothermal fluid reacted with sedimentary dolomites,producing the large rare earth deposits.In the study of northern sandstone-type uranium deposits,the sandstones were collected from Daqing-Changyuan area in the northern part of the Songliao Basin,Daying-Nalinggou uranium deposits in the northern part of the Ordos Basin,and the Jingchuan-Pengyang uranium prospect area in the southwestern Ordos Basin.The carbon and oxygen isotope of calcite cement and in situ sulfur isotope of pyrite are used to comprehensively analyze the element coupling variation law and metallogenic control factors.There are different types of reduction media leading to different ore-forming processes in three regions.The uranium mineralization in the Daqing-Changyuan area is located in the sand body of the Upper Cretaceous Sifangtai Formation.Scanning electron microscopy results show that the uranium minerals are mainly coffinite,closely related to plant debris organic matter and pyrite,and uranium mineralization is later than the formation of pyrite.Uranium minerals have short rod,cocci microbial morphology.The data of main element of rocks show the strong correlation between U and S elements.The Sulfur isotope of pyrites is obviously negative.Type IV kerogen shows the characteristics of biodegradation.Uranium mineralization is related to plant debris organic matter and microbial processes.The sandstone from Daying-Nalingou uranium deposit contains high content of organic matter.Uranium minerals distributed around pyrite and altered ilmenite.Uranium mineralization is later than pyrite formation.The content of pyrite is obviously increased in the coal-rich samples of sandstone.Under scanning electron microscopy,coffinite shows the wheat grain-fish ovate shape of microbial morphology and is obviously related to the coal dust organic matter and pyrite.Reduction environment generated by reaction of coal degraded by microorganisms favors the uranium precipitation and mineralization.The hydrocarbons produced by a large number of coal seams in the lower part of the Zhiluo Formation can be transported upwards for a distance,providing a transportable reducing gas for the upper coarse-grained sandstone.The carbon isotope of calcite cement showing negative bias in sandstone indicates the secondary reduction of oil and gas,which may cause superimposed mineralization.The uranium mineralization in the uranium prospecting area of the Jingchuan-Pengyang area in the southwestern Ordos Basin is located in the Luohe Formation.Under the scanning electron microscope,the pitchblende microspheres are mainly distributed in the pores of the fragmental minerals.Aggregates of pitchblende are closely related to colloidal TiO2.The Luohe Formation is composed of aeolian sandstone,mainly red fine-grained feldspar-quartz sandstone,with large-angled cross-bedding.The ore-bearing sandstones in drill holes are gray-gray-green sands,although some anomalies in the red sandstone.The sandstone is well sorted,and has no detrital organic matter,which represents the deposition environment is the arid desert facies.The ore-bearing sand bodies and non-mineralized sand bodies have macroscopic differences in color and microscopic differences in the contents of pyrite.The content of U show strong correlation to the contents of sulfur.The Changqing Oilfield in the northeastern of study area,provides the hydrocarbon-containing fluid migrating along the fault to the uranium deposit.We reveal the metallogenic mechanisms of sandstone-type uranium in the Northern basins in China.The studies of the sandstone-type uranium deposits in different areas in the Songliao Basin,Ordos Basin provide the comprehensive ore-forming models of sandstone uranium deposit,revealing that the reactions between microorganisms and organic matter(plant debris,coal and oil)play a critical role in the reduction of uranium and control the formation of sandstone-type uranium deposit.