桂西大新-钦甲地区辉绿岩脉地球化学与锆石U-Pb同位素年代学及对碳硅泥岩型铀矿床成因的启示

碳硅泥岩型铀矿床是我国铀矿地质工作者建立的铀矿床类型,是我国四大铀矿工业类型之一。桂西地区是我国华南地区一个重要的碳硅泥岩型铀矿床产出区域,包括大新(373)铀矿床等多个典型铀矿床(点)。研究辉绿岩与矿床成因的联系,对重新认识矿床成因有重要意义。本文在对桂西大新-钦甲地区碳硅泥岩型铀矿床和辉绿岩脉野外地质研究的基础上,系统研究了辉绿岩的地质特征、岩石学、地球化学特征以及其中锆石的U-Pb同位素年代学特征,在此基础上,探讨了辉绿岩的成岩与碳硅泥岩型铀矿床成矿间的关系。元素地球化学特征显示Rb、U、Th、Ba等大离子亲石元素的含量都高于MORB值,高场强元素Ta、Nb、Zr、Hf等均相对于MORB有所富集,而HREE元素中等亏损,表明本区辉绿岩属于富集地幔特征的板内碱性玄武岩系列(WPB),属于华南陆内伸展构造背景下软流圈上涌导致富集岩石圈地幔部分熔融形成的铁镁质岩浆发生侵位的产物。辉绿岩脉3个样品的锆石LA-ICP-MS U-Pb年龄为86. 7±0. 9Ma、89. 05±0. 96Ma、91. 6±8. 3Ma,表明形成于晚白垩纪早期,与华南地区与中基性岩脉有关的铀矿床成矿时代具有较好的对应关系,也与华南地区广泛分布的中生代基性岩脉时代一致,对应于华南白垩纪构造应力场中四次重要的拉张活动时代中的一期。辉绿岩脉锆石特征指示了辉绿岩成岩过程对矿源层中铀的活化所产生的重要作用以及成岩与成矿之间的热动力联系;结合元素地球化学、矿床成矿年龄和辉绿岩锆石UPb同位素年龄研究结果表明,区内铀矿床具有多期成矿特点,即"沉积期铀预富集、辉绿岩作用下二次预富集、后期热液再次富集成矿",可能是该类型矿床成矿作用的重要形式。

Geochemistry and LA-ICP-MS zircon U-Pb geochronological dating of diabase dykes and their relationship with mineralization of the carbonate-siliceous-pelitic rock type uranium deposits in Daxin-Qinjia, western Guangxi

Numerous uranium polymetallic deposits, uranium polymetallic occurrences and mineralized spots are widely distributed in western Guangxi. Daxin Sanqisan uranium ore deposit is one of the most important and famous sources of the carbonate-siliceous-pelitic rock-type uranium deposits in southwestern China, which has drawn great attention of many geological researchers and ore deposit experts for its scale, high grade, intensive distribution and abundant associated ores. Sanqisan and Bajiang U deposits and several U occurrences were selected from western Guangxi for the present study. The tectonic environment of diabase dykes is distinguished as the plate basalt (WPB) by geochemical characteristics, which draw the conclusions that the content of SiO₂ in rocks has a large variation, mainly caused by silicides. SiO₂ is obvious negatively correlated with Fe₂O₃, MgO, CaO and FeO, while alkali degree (Na₂O+K₂O) is poor. Trace element enriches high field strength elements Ta, Nb, Zr and Hf, and big ion lithophile elements Rb, U, Th and Ba, and losses Sr. The chondrite-normalized rare earth element (REE) patterns are light REE-enriched and exhibit no Eu anomalies. According to laser ablation LA-ICP-MS U-Pb zircon dating, the ages for the diabase dykes are 86.7±0.9Ma, 89.05±0.96Ma and 91.6±8.3Ma, which is in accordance with the ages of the widely distributed Mesozoic mafic dikes in South China. Characteristics of zircons of the rocks in the study area indicate the diagenetic process of diabase dykes play an important role in the activation and transportation of the uranium, and the dynamical relationship between diagenesis and mineralization. Through studies of the geological, geochemical characteristics, composition and chronology of zircon and metallogenic epoch, this study indicate diagenesis of diabase dykes plays an important role in uranium mineralization. The mineralization is characterized by multi-stages, and U-preconcentration for the first time during the deposition period, the secondary preconcentration by function of diabase, and then uranium concentration by the hydrothermal superimposition, which most likely reflects the mineralizing process and mechanism for the carbonate-siliceous-pelitic rock type uranium deposits.