粤北下庄矿田仙石铀矿床沥青铀矿电子探针化学年龄及其地质意义

EPMA Chemical Age of Pitchblende and Its Geological Significance in Xianshi Uranium Deposit of Xiazhuang Ore Field, the Northern Guangdong, China

华南晚中生代以来多期铀成矿与岩石圈伸展事件基本对应,高精度的铀成矿年代学测定有利于进一步明确两者的关系。下庄矿田以发育众多与辉绿岩脉有关的"交点型"热液铀矿床为特色,是华南代表性铀矿田之一。选择下庄矿田仙石铀矿床这一典型的"交点型"铀矿床为研究对象,利用光学显微镜、扫描电镜对铀矿化特征进行了详细的岩相学观察,并采用电子探针分析计算了沥青铀矿U-Th-totalPb化学年龄。结果表明:仙石铀矿床铀矿化与碳酸盐脉共生,可分为角砾状、脉状两期矿化;沥青铀矿主要化学组成为UO_(2)、PbO、CaO、SiO_(2)、Na_(2)O、Al_(2)O_(3),且SiO_(2)与PbO含量、Na_(2)O与PbO含量、Na_(2)O与Al_(2)O_(3)含量、Na_(2)O与CaO含量、PbO与Al_(2)O_(3)含量、PbO与CaO含量、Al_(2)O_(3)与CaO含量之间相关系数都小于0.5,反映了沥青铀矿形成之后受改造作用弱,铀矿物U-Th-totalPb体系相对封闭;采用电子探针U-Th-totalPb化学定年法计算,矿化年龄可分为两组,加权平均年龄分别为(123.3±4.5)和(84.1±3.2)Ma,分别对应于角砾状、脉状两期铀矿化。综合前人定年结果,认为仙石铀矿床主要铀成矿作用与华南岩石圈伸展事件具有较好对应关系。下庄矿田乃至华南地区的主要铀成矿作用可能与古太平洋板块俯冲的地球动力学背景有关。

The multi-stage uranium mineralization since Late Mesozoic is broadly corresponding to lithosphere extensional events in South China. The high precision dating on uranium mineralization is helpful for understanding the relationship between uranium mineralization and lithosphere extension. Xiazhuang ore field, one of the representative uranium ore fields in South China, is characterized by hydrothermal uranium deposits located at the intersections of fault and mafic dikes. Xianshi uranium deposit, a typical intersection-located uranium deposit in Xiazhuang ore field, is selected as the research object. Based on detailed petrographic observation by optical microscope and scanning electron microscopy(SEM), the U-Th-totalPb chemical ages are calculated based on oxide compositions of pitchblende determined by electron probe micro-analysis(EPMA). The results shows that uranium mineralization in Xianshi uranium deposit is associated with carbonate veins, and can be divided into earlier brecciaous and later veinlet pitchblende;the pitchblende is mainly composed of UO_(2), PbO, CaO, SiO_(2), Na_(2)O and Al_(2)O_(3). Moreover, the correlation coefficients of contents between SiO_(2) and PbO, Na_(2)O and PbO, Na_(2)O and Al_(2)O_(3), Na_(2)O and CaO, PbO and Al_(2)O_(3), PbO and CaO, and Al_(2)O_(3) and CaO are all less than 0.5, indicating no significant alteration of pitchblende after its formation, which is a prerequisite for calculating U-Th-totalPb chemical age of pitchblende. The calculated U-Th-totalPb chemical ages of pitchblende are divided into two groups with weighted mean values of(123.3±4.5) and(84.1±3.2)Ma, corresponding to earlier brecciaous and later veinlet pitchblende, respectively. Combined with previous published ages of Xianshi uranium deposit, the multi-stage uranium mineralization in Xianshi uranium deposit is identical to regional lithospheric extension events in South China. The uranium mineralization events in Xiazhuang ore field and even across South China may be related to the subduction of paleo-Pacific plate to the eastern China block.