湘东明月峰地区金管冲铀矿床沥青铀矿电子探针U Th Pb化学定年及其地质意义

Electron Microprobe U-Th-Pb Chemical Dating of Pitchblende and Its Geological Significance of Jinguanchong Uranium Deposit in the Mingyuefeng Area,Eastern Hunan

金管冲铀矿床位于湘东明月峰地区,属花岗岩外接触带型铀矿床,主要铀矿化类型为沥青铀矿方解石和沥青铀矿微晶石英萤石。为厘定金管冲地区的铀成矿时代与成矿期次,开展了详细的野外地质调查、岩相学、电子探针化学分析以及U Th Pb化学定年研究。结果表明,两类沥青铀矿的UO_(2)含量高、PbO与SiO_(2)含量低,且w(CaO)与w(UO_(2)+PbO)、w(SiO_(2)+CaO+FeO)与w(PbO)以及沥青铀矿年龄均无明显相关性,表明沥青铀矿形成时较为稳定,受后期改造作用影响弱。金管冲铀矿床中沥青铀矿方解石型铀矿化加权平均年龄为(90.7±3.2)Ma(MSWD=1.9,n=17),沥青铀矿微晶石英萤石型铀矿化加权平均年龄为(70.1±2.4)Ma(MSWD=2.1,n=20),显示该矿床在晚白垩世至少经历了两阶段的铀成矿作用。综合前人已有数据,认为金管冲铀矿床铀成矿作用与华南岩石圈拉张减薄事件较为吻合,可能是古太平洋板块俯冲后的产物。

The Jinguanchong uranium deposit is located in Mingyuefeng area of east Hunan province,which belongs to granite outer contact zone type uranium deposit,and the main types of uranium mineralisation are pitchblende-calcite and pitchblende-microcrystalline quartz-fluorite.Detailed field geological investigations,petrography,electron microprobe chemical analyses,and U-Th-Pb chemical dating studies have been carried out to determine the age and stage of uranium mineralisation in the Jinguanchong area.The results show that both types of pitchblende exhibit high UO_(2) contents and low PbO and SiO_(2) contents.There is no obvious correlation between w(CaO)and w(UO_(2)+PbO),w(SiO_(2)+CaO+FeO)and w(PbO)and the age of the pitchblendes,which indicate that the pitchblendes are relatively stable at the time of their formation and weakly affected by the later modification effects.The weighted average age of pitchblende-calcalite type uranium mineralization in Jinguanchong uranium deposit is calculated as(90.7±3.2)Ma(MSWD=1.9,n=17)using U-Th-Pb chemical dating method.For the pitchblende-quartz-fluorite type uranium mineralization,the weighted average age is determined as(70.1±2.4)Ma(MSWD=2.1,n=20).These results indicate at least two stages of secondary uranium mineralization occurred primarily during late Cretaceous.Combined with the previous dating results,it can be concluded that uranium mineralization in the Jinguanchong uranium deposit aligns with the lithospheric extension and thinning event in South China,which may be the result of the subduction of the Paleo-Pacific plate.