SIMS And Fs-LA-ICP-MS In-situ U-Pb Dating Of Sandstone-type Uranium Deposits

In recent years,the exploration and evaluation of in-situ leaching sandstone-type uranium deposits have gradually become the focus of uranium work in China.At present,sandstone-type uranium deposits have become the most important type in China.In terms of prospecting and theoretical understanding of sandstone-type uranium deposits,the determination of the metallogenic age of uranium deposits is one of the most important contents,which directly affects the understanding of metallogenic mechanism,ore-controlling factors and metallogenic regularity of the deposits.Previous dating of sandstone uranium deposits is based on U-Pb method of whole rock samples?or mixed minerals?.Because the mineralization of sandstone uranium is open,it is theoretically controversial.When interpreting the data obtained,there are some contradictions with the characteristics of deposits and tectonic background.Therefore,there is an urgent need to develop and establish a suitable,high-precision and suitable dating method for popularization and application.Under the current situation that the focus of uranium prospecting in China is shifting to sandstone-type uranium deposits,Exploring and solving the problem of precise determination of sandstone uranium deposits has important theoretical and practical significance for promoting the development of uranium chronology,better guiding the prospecting and exploration of current sandstone uranium deposits,and enriching the practice of determining the direction of deposits.This paper is based on the comprehensive analysis and research of dating and present situation of sandstone uranium deposits at home and abroad,taking pitchblende?GBW04420?as the standard sample,SIMS and fs-LA-ICP-MS technology were combined to test the sample.The SIMS method can be used to measure particles of 5-20?m,while the latter can be used to measure particles larger than 20?m,the two can complement each other.Taking the main sandstone uranium deposits in northern China?Ordos and Yili?as the object of study,the in-situ U-Pb isotope dating of sandstone uranium deposits has been successfully studied and explored,and a series of metallogenic chronologies have been obtained by directly measuring the U-Pb isotopes of the main uranium minerals in situ.A set of technical methods for precise dating of sandstone uranium deposits,including sample preparation and complementary technical means,has been preliminarily established and explored.Sampling method is different from zircon dating and EPMA analysis,it uses the method of thickening 3-5 times of conventional thin section.In terms of instrumental testing,the combination of fs-LA-ICP-MS and SIMS is adopted because the uranium mineral pitchblende in the northern Ordos Basin has a high silicon content,a large particle size and a large amount of required testing.Uranium minerals in the southern margin of the Yili Basin are typical pitchblende with smaller particles and less amount of testing,SIMS method is used to determine the uranium minerals.The data were determined by the above methods,and the chronological characteristics of mineralization were analyzed in combination with the deposit geology and the regional tectonics.The results are as follows:For the Hangjinqi-Daying uranium ore-forming area in the northern Ordos Basin,it can be divided into four stages:The interlayer oxidation zone type main metallogenic period in late Cretaceous-Paleozoic?89±17Ma?;oxidized zone type secondary metallogenic period in late Paleogene?Oligocene,29.1±3.3Ma,37.6±2.9Ma?;the uranium superimposition and integration period in Miocene-Pliocene?12.8±6.6 Ma,16.0±2.1Ma,20.6±1.9Ma?;uranium reconstruction,enrichment and preservation period in Pliocene-Pleistocene?4.67±0.53Ma,4.58±0.46Ma?.In addition,the ages of the uranium in XXX mineralization area in the southwestern Ordos Basin were measured.And the results were late Miocene?8.65±0.86Ma?and Pleistocene?1.21±0.39Ma?.This is the first time to use the SIMS method to measure the in situ ages of the uranium deposit in the Yili basin in Mengqigul.The results can be divided into two metallogenic stages:The main mineralization period in Pleistocene?1.38±0.22Ma?,and the uranium reconstruction,enrichment and preservation period in Miocene-Pliocene?12.8±2.4 Ma?.The comprehensive analysis of the characteristics and rationality of the obtained age data.Compared with the traditional whole-rock U-Pb isochronal ages,fs-LA-ICP-MS or SIMS uranium mineral U-Pb ages have the characteristic that they are significantly younger than whole-rock U-Pb ages which have more older ages before late Cretaceous.The rationality of the ages by different methods can be analyzed and compared by the characteristics of the formation of sandstone-type uranium deposits coupled with the late tectonic events or uplifting effects of the basin.The eras of important late reformation,tectonic uplift and regional denudation events in the Ordos Basin since late Cretaceous include late Cretaceous,early Oligocene,Miocene-Pliocene,and early Pleistocene.And the in-situ ages in the northern Ordos Basin are basically consistent with eras of the above-mentioned tectonic events.This study is in line with the geological realities and has great rationality.Similarly,the same understanding can be drawn from the ages of the Yili Basin.In theory,the sealing temperature of pitchblende is low?about 200??and it contains common lead,but the SIMS?SHRIMP?method can be used to determine ²⁰⁶Pb,²⁰⁷Pb and²⁰⁴Pb,which can be used to correct common lead and ensure the accuracy of the method.LA method can not detect ²⁰⁴Pb,its ²⁰⁶Pb/²³⁸U age is meaningless,but ²⁰⁷Pb/²⁰⁶Pb age can be obtained by Tera-Wasserburg diagram,which also ensures the reliability of the method.SIMS method is expensive,but fs-LA-ICP-MS is economical and applicable,and can reduce the matrix effect more effectively,so the combination of the two methods is the best choice.In summary,a series of supporting technical methods for in-situ dating of sandstone-type uranium deposits was initially established,including the method of preparing the thickened electron probe rock sections,and the technical means of combination and supplement of fs-LA-ICP-MS and SIMS dating these two methods in the test process.Therefore,the combination of fs-LA-ICP-MS and SIMS uranium mineral U-Pb in-situ dating makes the entire test process have the characteristics of flexible sample preparation,high spatial resolution,large test mineral selectivity,high precision,and so on.And it will be the main development direction of sandstone-type?or hydrothermal type?uranium precision dating in the future.