Geochemical Characteristics And Genesis Of Nalinggou Uranium Deposit In Ordos Basin

The Nalinggou uranium deposit is located in the northeast of Ordos Basin,which belongs to a part of North China platform.The ore bodies mainly occur in the paleointerlayer oxidation-reduction(green sandstone gray sandstone)transition zone of the lower member of Zhiluo Formation(J₂z¹),and they are close to the gray sandstone and mostly occurs in a plate shape.The main ore controlling factors include the paleointerlayer oxidation area,geochemical acidity and alkalinity of rocks,heterogeneity of sand body,composition and morphology of reducing medium and fault structure.Based on the collection of previous research results and field borehole logging,this thesis systematically analyzed the geological characteristics,the types and genesis of uranium minerals,the geochemical characteristics of major elements,minor elements and rare earth elements of Nalinggou uranium deposit using the?track etching,scanning electron microscopy,electron probe,ICP-MS and other analytical and testing methods.The main understandings of metallogenic model are shown below:(1)Uranium minerals are mostly distributed on the edge of quartz and feldspar or in clay minerals,often coexisting with pyrite or other reducing substances such as organic matter.There are mainly two kinds of uranium minerals,coffinite and pitchblende.The coffinite is the main uranium minerals.The pitchblende was formed in the early stage,and the oxygen-containing uranium water precipitated in the acid reducing environment;the uranite was formed in the late stage,in the alkaline reducing environment.The alkaline groundwater help increase the activity of SiO₂,causing the melting of quartz,and converted the pitchblende formed in the early stage into uranite.(2)SiO₂ is the highest component and Al₂O₃ is the second highest component in the sandstone of Nalinggou uranium deposit.The grey sandstone in the uranium ore and main host rock has higher Ca O content,and the degree of calcareous cementation is large.The green sandstone of secondary host rock is second,which further indicates that the enrichment and mineralization of uranium is accompanied by the deposition of CaCO₃.Therefore,calcareous cementation can be an important indicator of uranium prospecting.The correlation among TiO₂,FeO,TFe₂O₃ and Mg O retains positive just like the correlation between the original Ti-Fe and Fe-Mg of magmatic rock.The correlation shows that the source of sandstone in Nalinggou area may come from magmatic rock,and the transport distance is relatively short.The chemical alteration index(CIA)of uranium ore is the most similar and relatively low to that of grey sandstone,indicating that uranium ore in Nalinggou area is formed in a relatively stable environment.The variation index(ICV)of uranium ore and various sandstone components in Nalinggou area is greater than1 except for some samples,indicating that the variation index of uranium ore and sandstone components in Nalinggou area is greater than 1.The content of clay minerals in sandstone and uranium ore in each alteration stage is low,which is not affected by recycling and strong weathering.The tectonic background of diagenesis and mineralization is relatively stable,which is a favorable factor for the formation of uranium ore.(3)Compared with the trace element cobweb map of the samples in Nalinggou area,it is found that the sandstones within the two-stage reduction zone(gray sandstone and green sandstone)are similar,indicating that uranium mineralization and two-stage reduction have the same source features;the correlation coefficients of V,Co,Sr,y,Mo,Sb,Tl,Pb and u are relatively high,among which V,Co,Y,Mo,Sb,Tl,Pb and U are in the positive phase.The W is significantly enriched in uranium ore and sandstone in various alteration stages,which further indicates that sandstone in Nalinggou area may be formed by weathering,denudation and transportation of magmatic rocks around the basin.(4)In all kinds of sandstones and uranium minerals,the variation range of?Eu and?Ce is small.The degree of differentiation between light and heavy rare earth in uranium ores and the degree of internal fractionation of light and heavy rare earth are obviously higher than that of altered sandstones in all stages of redox process.The HREE distribution curve of uranium ore shows that it is located on both sides of the HREE distribution curve of the upper crust,which indicates that a large number of LREE migrated into the uranium ores during the uranium mineralization process,and the HREE migrated internally due to the later fluid transformation,resulting in a large difference in HREE content in uranium ore.(5)The study shows that the metallogenic model of Nalinggou uranium deposit is"paleo interlayer oxidation zone metallogenic model",which can be divided into three main stages:sedimentary preconcentration stage interlayer oxidation metallogenic stage,and later interlayer oxidation secondary reduction reconstruction stage.