Petrogeochemical Characteristics And Genesis Of Manglai Uranium Deposit In Erlian Basin

The Manglai uranium deposit is located in the Tabei Sag in the southwest of Manite Depression in the central and eastern part of Erlian Basin,between the Bayanwula large uranium deposit and the Saihan Gaobi medium uranium deposit,which belongs to the central axis of the entire uranium metallogenic belt of the ancient river valley.Currently,the research degree in this area is relatively low.To study of the geological characteristics,petrological characteristics and the element geochemical characteristics of the uranium deposit,summarizes ore-controlling factors and uranium mineralization,discuss the genetic mechanism of the deposit,is not only helpful to explore the regional metallogenic regularity of the uranium metallogenic belt in the ancient valley of Erlian Basin,can also provide theoretical basis for the further exploration and mining of uranium ore.This paper is based on summarizing the previous research results and field borehole core observation,By means of thin section identification under microscope,?track etching,electron probe analysis and element geochemical analysis,the geological characteristics,petrological characteristics of ore-bearing layers,characteristics and genesis of uranium minerals,trace elements and rare earth geochemical elements of the Menglai uranium deposit in Erlian Basin were studied,the uranium metallogenic geological conditions and metallogenic processes are summarized,and the uranium metallogenic model is established.The following understandings are obtained:(1)The Manglai uranium deposit is controlled by the phreatic interlayer oxidation zone,and the ore body mainly occurs in the gray sand body at the bottom of the upper member of Saihan Formation in plate shape,which is composed of channel sand body under a set of braided river depositional system.The central bank and the river detention with a sand thickness of 55?70m and a sand content of 60?70%are the main areas for uranium mineralization.(2)The composition and structural maturity of the target sandstone of the Manglai uranium deposit are low,and the rock types are mainly lithic sandstone and feldspar lithic sandstone,which are base-type cemented and heterobase supporting structure.The diagenesis has mainly gone through the processes of compaction,cementation,metasomatism,dissolution,recrystallization and thermal fluid reformation,and can be divided into six diagenetic stages:sedimentary stage,early diagenetic stage A,early diagenetic stage B,middle diagenetic stage A,surface diagenetic stage and hydrothermal reformation stage,among them,the uranium mineralization is closely related to the topogenetic stage and the thermal fluid transformation stage.(3)Uranium minerals are mainly distributed in clumps,banded and dispersed adsorbents,in which clumps and banded uranium minerals are evenly distributed in or around the pyrite,while scattered uranium minerals are mainly distributed in the clay minerals of cemented materials,symbiosis with stellate fine pyrite.Uranium minerals are mainly coffinite,pitchblende and phospermuranite.The coffinite is formed around the edge of quartz and hemiidiomorphic granular pyrite,and is related to the dissolution of quartz under alkaline conditions.Pitchblende mainly occurs in the vicinity of pyrite and mechanite and is formed by the superposition of pyrite and organic matter by multi-stage reduction.Phospermuranite is a secondary uranium mineral with reticulated vein and villous cemented pyrite,which is the result of late fluid modification.(4)Spider web of trace elements shows that the pattern characteristics of uranium ore are similar to that of oxidized sandstone and reduced sandstone,indicating that they have the same provenance characteristics.The elements such as Th,Mo,Li,Be,Cs,Cu,Pb,Zn and Sb increase obviously from surrounding rock to uranium ore,among which Th,Sb and Pb have the highest correlation with U,which can be used as prospecting indicator elements.(5)The normalized distribution curve of rare earth chondrites shows that the three types of sandstone have the same provenance,sedimentary and diagenetic evolution environment and tectonic background,and the provenance is from the passive continental margin.The standardized pattern diagram of REE in North American shale shows that the sandstone presents positive Ce anomaly,REE enrichment is obvious in ore,and Eu is negative anomaly,which indicates that the mineralization is a reductive environment and may have been reformed by late reductive hot fluid.(6)The formation of the deposit is mainly controlled by geological conditions such as structure,uranium source,stratigraphic and sedimentary system,development of oxidation zone and so on.Mineralization is restricted by diagenetic evolution,trace elements and rare earth elements.Ore-forming process can be divided into stages of sedimentary pre-enrichment(K1S2),oxidation between phreatic aquifer metallogenic stage(K₂-E₁),thermal fluid transformation stage(E₁-E₂),mineral phase(E₂-Q)four stages.