桂北沙子江铀矿床成矿年代学研究：沥青铀矿U-Pb同位素年龄及其地质意义

苗儿山矿田为中南地区五大铀矿田之一,其内分布有我国最大规模碳硅泥岩型的铲子坪铀矿床及诸多花岗岩型铀矿床,沙子江矿床为矿田内重要的花岗岩型铀矿床之一。沥青铀矿是理想的铀矿床直接定年样品,同时,也是U-Pb同位素研究的理想矿物。本次研究以沥青铀矿为对象进行U-Pb同位素分析,获得了沙子江矿床早、晚两期铀成矿作用的年代分别为104.4Ma和53.0±6.4Ma,结合铲子坪矿床主成矿期年代74.1±9.9Ma,它们可能分别代表了苗儿山矿田3期主要铀成矿作用的时代。沙子江矿床等时线拟合所得高的初始Pb值反映了该期成矿作用之前存在铀的预富集作用。3期成矿作用与华南地区基性脉岩年代数据统计反映的岩石圈伸展期次相对应,暗示了铀成矿受控于华南岩石圈伸展这一大的动力学环境。

Geochronology of the Shazijiang uranium ore deposit, northern Guangxi, China: U-Pb ages of pitchblende and their geological significance

Miaoershan uranium ore field is one of the five large-scale uranium ore fields in center South China, in which includes the Chanziping uranium ore deposit, the largest scale carbonaceous-siliceous-pelitic rock type uranium ore deposit in china, and some granite-hosted uranium ore deposits. Pitchblende is ideal sample to date uranium mineralization directly. Meanwhile, it is also good mineral for U-Pb isotope measurements. U-Pb isotope measurements of pitchblende samples from the Shazijiang uranium ore deposit in northern Guangxi of China, indicated that there existed two mineralization stages, 104.4Ma and 53.0±6.4Ma, respectively. Combined with the major mineralization age of 74.1±9.9Ma for the Chanziping uranium ore deposit, three major mineralization stages were investigated for the Miaoershan uranium ore field with 104.4Ma, 74.1±9.9Ma and 53.0±6.4Ma. High initial Pb isotope ratio obtained by pitchblende U-Pb isochron method for the Shazijiang uranium ore deposit suggested that pre-mineralization had already occurred before this major mineralization stage. Three major mineralization stages were consistent with epochs of lithospheric extension in South China as indicated by mafic dikes geochronology, implying that uranium mineralization was constrained by the geodynamic setting of lithospheric extension.