Study On Mineralogical And Chronological Characteristics And Genesis Mechanism Of Megacrystalline Uraninite In The Kangdian Region

The natural uraninite is usually small and often exists in magmatic rocks as accessory minerals,but there are few reports about megacrystalline uraninite.Therefore,the study of megacrystalline uraninite is helpful to improve the degree of mineralogy research of uraninite,and to guide the uranium prospecting work of the Kangdian region in the next step.In addition,the available data indicate that there are few or no uranium mineralization events in the Neoproterozoic,which is a global phenomenon,and the Neoproterozoic is the most important uranium mineralization epoch in the Kangdian region.The discovery of megacrystalline uraninite,which is the special mineral,makes the Kangdian region an ideal region for the study of Neoproterozoic uranium mineralization,and also makes the Kangdian region become an important window for the study of Neoproterozoic uranium mineralization in China and even the world.The discovery of megacrystalline uraninite is one of the most outstanding achievements in uranium prospecting and uranium mineralogy in the Kangdian region and even in China in recent years.However,no detailed research has been carried out on the mineralogy,chronology and genetic mechanism of the megacrystalline uraninite,which seriously restricts the research degree of uranium mineralization in the Kangdian region.This paper takes the megacrystalline uraninite as the research object,which is discovered in uranium-rich veins of the Haita 2811/A19 uranium occurrences,the Datian 505 uranium deposit and the Mouding 1101 uranium occurrences in the migmatite strata of the Kangdian region in recent years,to carry out field geological investigation and laboratory analysis and comprehensive research work.This study focuses on uraninite and its associated minerals such as titanite,and on the basis of the previous work,the methods of EPMA,SEM,TIMA,XPS,LA-ICP-MS trace,LA-ICP-MS U-Pb isotope,LA-ICP-MS Sm-Nd isotope and other methods are used for in-depth and systematic analysis.We studied key geological problems such as 1)the mineralogical characteristics of megacrystalline uraninite;2)the formation age of megacrystalline uraninite;3)the genetic types,forming conditions and sources of ore-forming materials of megacrystalline uraninite;4)the coupling relationship between the formation of megacrystalline uraninite and migmatization,and on this basis,the genetic mechanism of megacrystalline uraninite is discussed and summarized.Through the research work of this paper,the following points have been obtained:1)The megacrystalline uraninite,which is discovered in uranium-rich veins of the Haita 2811/A19 uranium occurrences,the Datian 505 uranium deposit and the Mouding1101 uranium occurrences,have the characteristics of good crystal shape,large particle size with the maximum particle sizes up to cm,high cell parameters and low oxygen content coefficient,and uranium element mainly exists in the form of tevalent.The megacrystalline uraninite has a high content of Th O₂ in terms of chemical composition,and its mineral symbiotic assemblages have obvious characteristics of"accessory mineral"assemblages of magmatic rocks.The above characteristics of megacrystalline uraninite indicate that it was formed in a large deep and reduction environment with high temperature and a slow temperature drop.2)On the basis of the previous research data,the age of the titanite minerals co-existing with the uraninite is considered to be the same age of the formation of the megacrystalline uraninite in the migmatite of the Kangdian region,which was formed at about 770-790 Ma,indicating that the mineralization age of the megacrystalline uraninite on the Kangdian region is Neoproterozoic,confirming the existence of Neoproterozoic uranium mineralization events.In addition,based on the EPMA age of the megacrystalline uraninite（240 Ma）,it shows that the megacrystalline uraninite in the Haita area may have been affected by the eruption of the Emeishan mantle plume,leading to the destruction or reset of U-Pb system.3)By comparing with uranium minerals in typical uranium deposits around the world,the in-situ trace elements of the megacrystalline uraninite indicate that their genetic attributes are magmatic type（pegmatite type）,which is related to partial melting under high temperature and low-pressure metamorphism.The high temperature and low pressure（P=0.17～0.35 Gpa,T=670～891℃）of the calculated geochemical parameters of the titanite minerals associated with uraninite represent the crystallization conditions of uraninite.The in-situ Nd isotopic compositions of titanite and uraninite are uniform and have consistent and negativeεNd（t）（-16.9～-5.2）,indicating that the ore-forming materials are from the older crustal materials in the deep.4)By comparing the formation age of the migmatite and megacrystalline uraninite,geochemical characteristics of Uraninite and its symbiotic titanite in uranium-rich veins and migmatite/uranium-rich veins,diagenetic and ore-forming pressures of migmatites and uraninite represented by titanite,and age of the protolith of migmatite and Two-stage Nd isotopic model age represented by uraninite and titanite(T_2DM=1881～2752Ma)in the Kangdian region,shows that there is no direct relationship between migmatization and uranium mineralization.At the same time,based on the uranium enrichment phenomenon in the process of migmatization and the phenomenon that the host rocks of uranium occurrences are migmatite or metamorphic rocks,it is speculated that migmatite may provide part of uranium sources for the formation of the megacrystalline uraninite.It is found that the formation age of migmatite（830～860 Ma）and megacrystalline uraninite（770～790 Ma）indicates that migmatization and uranium mineralization may be responses to the splicing and cracking events of the Rodinia supercontinent,respectively,and breakup event of supercontinent provides heat source and storage space for uranium mineralization.All uranium-rich veins in uranium occurrences of the Kangdian region are constrained by secondary fracture or schistosity,surrounding rock,the genetic type,metallogenic material source and formation conditions,and the uranium-rich veins are consistent with the surrounding granites in terms of diagenesis,ore-forming age and material source.However,the unequal relationship of zircon Hf content between uranium-rich veins and nearby granites indicates that there is no magmatic evolution differentiation relationship between them.Therefore,this study suggests that the Neoproterozoic uranium mineralization represented by the megacrystalline uraninite is closely related to the partial melting of uranium-rich basement in the extensional and decompression background triggered by the cracking event of the Rodinia supercontinent.The genetic mechanism is that the felsic magmas formed by low degree partial melting（deep melting）of uranium-rich metamorphic sedimentary rocks intrude into the schistose or fissures of shallow migmatite and metamorphic rocks after aggregation and migration,and form independent uranium-rich veins in the reduction environment with high temperature and slow temperature decline.It may also be an important mechanism of uranium mineralization in the Kangdian region and the western margin of the Yangtze in the Neoproterozoic.