Location Conditions And Mineralization Mechanism Of The Fuda Granite-type Uranium Deposit In Southern Zhuguan

Zhuguang uranium Orefield is one of the areas with the most concentrated granite-type uranium deposits in China,several uranium deposits of different sizes and grades are distributed inside and outside of Zhuguang granite,the Mianhuakeng,Dongkeng and Lanhe deposits are typical representatives,characterized by large scale or high grade.In recent years,the research work mainly focuses on the Mianhuakeng and Shulouqiu deposits in the Changjiang uranium ore field.The research degree of Dongkeng and Lanhe deposits is low,which restricts the understanding of the metallogenic mechanism of uranium deposits which have a high grade a large scale.Previous studies have carried out some research on the scientific problem of"what are the factors affecting the grade and scale of granite type uranium deposit".Most of them have summarized and speculated on their favorable metallogenic conditions from the macro level,but there is a lack of micro level research.This paper takes the Mianhuakeng,Dongkeng,Lanhe and Shulouqiu deposits as the main research deposits,divided the metallogenic stages,carried out major and trace elements analysis,EPMA,LA-（MC）-ICP-MS trace elements and sulfur isotopes of sulfide,LA-ICP-MS isotopic chronology and trace element analysis of pitchblende.The spatial location conditions of high-grade or large-scale uranium deposits in this area are discussed.The source of ore-forming material,the characteristics of ore-forming fluids,the metallogenic age and mechanism of high-grade and large-scale uranium deposits in this area are analyzed.The main results achieved are as follows:The element migration ratios and the result of trace elements in pitchblende show that the contents of Li,Be,W,Mo and Bi in ore-forming fluid are relatively high,the LREE/HREE in pitchblende increases gradually from poor ore to rich ore.The LREE/HREE in pitchblende increases gradually from Mianhuakeng→Dongkeng→Lanhe deposit,which is consistent with the LREE/HREE change trend of granite in each deposit,indicating that the ore-forming material is mainly inherited from granite.The trace elements,∑REE and HREE/LREE of pitchblende in large deposits（Dongkeng and Mianhuakeng deposit）are higher,which are related to the more stonger mineral extraction process.The LA-MC-ICP-MS sulfur isotope test results of pyrite show that theδ³⁴S of pyrite in Dongkeng deposit is-20.09‰～-13.34‰,which is obviously different from theδ³⁴S（-13‰～-5‰）of Mianhuakeng,Shulouqiu and Lanhe deposit,but close to theδ³⁴S of pyrite in early Cambrian carbonaceous slate（-19.9‰）and Nansheba deposit（-14.14‰～-16.56‰）.Therefore,it is considered that the sulfur in Dongkeng deposit mainly comes from carbonaceous slate strata,while Mianhuakeng,Shulouqiu and Lanhe are the result of the mixing of sulfur in granite and Cambrian strata.The results of element mobility calculation and LA-ICP-MS trace element of pyrite show that the ore-forming fluid is mainly medium-low temperature oxidizing fluid,the Dongkeng deposit has a relatively high fluid temperature,and the Mianhuakeng deposit has a higher oxygen fugacity.The trace element thermometer results of sphalerite in Dongkeng deposit show that the fluid temperature is between199.6 and 243.8℃,which belongs to medium temperature fluid.However,the contents of Co and Ni in metallogenic pyrite are significantly higher than those of Mianhuakeng,Shulouqiu and Lanhe deposit,while the content of arsenic is lower.The content of titanium,thorium and niobium in pitchblende of Dongkeng deposit is higher than those of Mianhuakeng,Shulouqiu and Lanhe deposit.Those evidences show that the ore-forming fluid of the above deposits is medium-low temperature fluids.The results of element mobility and geochemical analysis show that low-grade deposits（Mianhuakeng）have high Fe₂O₃ and Mn O migration rates andδCe anomalies,while high-grade deposits（Lanhe and Donkeng）have higher mobility of redox sensitive elements（Mo,V,Tl）.The analysis shows that the relatively high oxygen fugacity of the fluid during the precipitation of uranium minerals is not conducive to the formation of uranium-rich deposits.The LA-ICP-MS and TIMS U-Pb dating results of pitchblende show that there are at least four stages of uranium metallogenic events in Zhuguang area,which are～100 Ma,～90 Ma,～70 Ma and～60 Ma respectively.The main metallogenic age of Dongkeng deposit is 90.0±6.6ma,and there is an earlier metallogenic age of101.5±4.0 Ma;Mianhuakeng deposit includes two mineralization stages of70.4±2.4Ma and 57.9±2.1 Ma,and there may exist a younger metallogenic age;The metallogenic age of Shulouqiu deposit is 68.6±1.3Ma.The results of Tera-Wasserburg diagram and LA-MC-ICP-MS lead isotope of galena show that the ore-forming fluids of Dongkeng,Mianhuakeng and Shulouqiu deposits contain radiogenic Pb accumulated in early uranium-rich geological bodies,and these radiogenic Pb are partially entered into pitchblende and galena during mineralization,and the proportion of radiogenic Pb in Dongkeng and Mianhuakeng deposits is higher.It shows that the formation of large ore deposits is related to the activation and reprecipitation of uranium.Based on the study of typical ore deposits such as Mianhuakeng,Shulouqiu,Dongkeng and Lanhe deposits,it is considered that the main ore-controlling factors in this area are structural,lithologic interface and hydrothermal alteration,among which fault structure is the most important ore-controlling factor.In this paper,the factors restricting the scale and grade of the deposit are expounded,and it is considered that the multi-stage uranium mineralization,higher mineral extraction process and activation-reprecipitation process of uranium are favorable for the formation of large deposits.Sufficient reducing materials,favorable structural conditions and good preservation conditions are favorable for the formation of rich uranium deposits.The excessive silicification,fluoritization,calcitization and post-ore alteration of the deposit are not conducive to the formation of rich uranium deposits.