粤北石角围花岗岩型铀矿床沥青铀矿LA-ICP-MS原位U-Pb定年研究

张伟盟; 严杰; 钟福军; 潘家永; 刘文泉; 赖静; 周堂波

doi:10.15898/j.cnki.11-2131/td.201901160007

粤北石角围花岗岩型铀矿床沥青铀矿LA-ICP-MS原位U-Pb定年研究

In situ LA-ICP-MS U-Pb Dating of Uraninite from the Shijiaowei Granite-type Uranium Deposit, Northern Guangdong Province

摘要

摘要: 石角围花岗岩型铀矿床位于粤北下庄铀矿田东部，沥青铀矿是矿床的主要矿石矿物，也是厘定成矿年龄的理想对象。前人采用同位素稀释法（ID-TIMS）和电子探针U-Th-^totalPb化学定年法获得的成矿年龄为38~138Ma，但前人年龄变化范围大，可靠性有待考究，难以有效约束矿床的成矿时代。本文利用LA-ICP-MS原位微区分析技术，对石角围矿床矿石中沥青铀矿开展了原位U-Pb定年。研究表明：沥青铀矿的²⁰⁶Pb/²³⁸U年龄为52.46~56.89Ma，加权平均年龄为54.68±0.53Ma（MSWD=1.19，n=18）。本次沥青铀矿原位U-Pb定年与前人相比更好地避免了矿物包裹体、后期次生变化、显微裂隙等因素的影响，获得的沥青铀矿原位U-Pb同位素年龄代表矿床的成矿年龄。本研究获得的石角围矿床成矿年龄（~55Ma）与华南花岗岩型铀矿床主成矿期（~50Ma）相一致，指示石角围矿床铀成矿作用与华南岩石圈局部伸展作用下的断裂构造活动密切相关。

Abstract:
BACKGROUNDThe Shijiaowei uranium deposit (No.339 deposit) in the Eastern Guidong granite complex of Nanling region, is one of the large type deposits in the Xiazhuang uranium ore field. Uraninite, the main ore mineral of the Shijiaowei deposit, is the ideal mineral for geochronology. Previous studies show that the uranium mineralize ages of Shijiaowei deposit is 38-138Ma acquired by ID-TIMS and EMPA.
OBJECTIVESTo provide new insight for uranium metallogenic mechanism and background of the Shijiaowei deposit by a new reliable uranium mineralization age determination method.
METHODSIn situ LA-ICP-MS U-Pb dating method was used to constrain the formation age of uraninite from the Shijiaowei deposit.
RESULTSThe ²⁰⁶Pb/²³⁸U age of uraninite was 52.46-56.89Ma with the weighted average age of 54.68±0.53Ma (MSWD=1.19, n=18). The in situ U-Pb dating of the uraninite avoids the influence of mineral inclusions, secondary changes and micro-cracks on dating.
CONCLUSIONSTherefore, the in situ U-Pb isotopic age of the uraninite determined by LA-ICP-MS represents the ore-forming age of the deposit. The ore-forming age (about 55Ma) is consistent with the main metallogenic period (about 50Ma) of the South China granite-type uranium deposit, indicating that the uranium mineralization of the Shijiao ore deposit is closely related to fault tectonic activity induced by the local extension of the lithosphere in South China.

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