粤北书楼丘铀矿床沥青铀矿原位U-Pb年龄和元素特征及其地质意义

In-Situ U-Pb Age and Elemental Characteristics of Pitchblende in Shulouqiu Uranium Deposit and Its Geological Significance

为深入研究华南长江铀矿田内“交点型”铀矿化类型,对书楼丘矿床中位于基性岩脉与硅化碎裂岩交切部位的沥青铀矿开展原位微区U-Pb定年及元素成分研究.激光剥蚀电感耦合等离子体质谱分析结果显示,书楼丘沥青铀矿的U-Pb加权平均年龄为71.3±1.1 Ma;微量元素以富集W、Bi、Mo等亲硫元素,亏损Nb、Ta、Zr、Hf、Th等高场强元素为特征;稀土元素配分模式表现为轻稀土微富集,轻重稀土分异不明显,总量较低,Eu呈现明显的负异常,与富铀围岩长江岩体的稀土元素配分模式基本一致,暗示其铀源可能主要与长江岩体关系密切,与脉型铀矿床稀土元素配分模式的对比及ΣREE-(U/Th)、ΣREE-(LREE/HREE)N图解指示其为低温、中低盐度热液成因,δCe值指示沥青铀矿的成矿环境为还原环境.矿床内“交点型”铀成矿作用主要受南雄断裂80~60 Ma期间拉张活动、产铀长江花岗岩体和基性岩脉的共同控制,进而在交切的构造部位形成了铀矿化信息.

In order to penetrate into discussion the“intersection type”uranium mineralization in the Changjiang uranium ore field,South China.Here,it collected representative samples from geological structure that formed during the process of mafic dick penetrating silicified surrounding rock and then carried out a study applying in-situ U-Pb dating and elementary analysis via LAICP-MS system.Results show that the weighting average age of pitchblende in this region is 71.3±1.1 Ma.Besides,trace elements are characterized by the enrichment of sulfophilic elements(such as W,Bi,and Mo)and by the depletion of high field strength elements(like Nb,Ta,Zr,Hf,and Th).The signature of rare earth element(REE)curve shows low total amount with negative Eu anomalies,slight enrichment in light REE,and indistinct fractionation between light and heavy REEs.The uranium source of pitchblende probably relates to the Changjiang granite,as indicated by a REE pattern comparison between them.In addition,the pitchblende was formed under low-temperature and low-middle salinity condition.This is validated through a combination ofΣREE-(U/Th)plot,ΣREE-(LREE/HREE)N plot,and comparison against REE distribution pattern in veintype uranium deposits.Moreover,δCe value indicates reducing environment.In this region,the pitchblende was controlled by the interplay among Nanxiong fault(during 80-60 Ma),uranium-rich Changjiang granite,and mafic dyke,therefore forming the“intersection type”uranium mineralization in the active region.