广西苗儿山铀矿田向阳坪矿床成矿时代研究

Research on Metallogenic Epoch of Xiangyangping Deposit in Miaoershan Uranium Ore Field, Guangxi

苗儿山矿田是中国南方重要的花岗岩型铀矿富集区,向阳坪铀矿床为近年来在苗儿山中段新落实的中型矿床,野外铀矿化特征显示该矿床存在多期次成矿作用。本文在详细的矿物学研究基础上,利用LA-ICP-MS U-Pb同位素分析技术,对向阳坪矿床中的沥青铀矿开展了成矿年代学研究,结合电子探针元素面扫描方法,厘定成矿流体性质。41个沥青铀矿LA-ICP-MS有效测点数据显示,其^(206)Pb/^(238)U加权平均年龄可分为2组,分别为(57.4±1.1)Ma和(43.1±2.1)Ma,与苗儿山矿田主要成矿期相吻合。铀矿物电子探针元素面扫描和微量元素特征结果表明,早期热液流体富含U、Fe、S,黄铁矿结晶造成的局部还原环境形成了早期沥青铀矿;晚期流体相对富稀土、Ca、Si,沿裂隙进入黄铁矿和早期沥青铀矿后,导致早期铀矿物中铀元素活化、迁移,形成第二期相对富含稀土、Si、Ca成分的沥青铀矿。综合分析认为,向阳坪铀矿床成矿作用可能受晚燕山期古太平洋板块对亚洲东部的斜向俯冲动力体制的控制,且经历了多期次成矿作用。

In-situ analyses are important techniques in earth science researches.However,these techniques are rarely used in hydrothermal uranium deposits in South China.The lack of high-precision age of uranium minerals restrains the understanding of the metallogenies of hydrothermal uranium deposits.The typical uranium deposit including the Shazijiang,Shuanghuajiang,Menggongjie,Zhangjia uranium deposits,et al.Xiang-yangping uranium deposit is the latest medium-sized deposit in the middle of Miaoershan in recent years.All uranium orebodies are vein-type in shape.The uranium minerals are mainly pitchblende,uramphite,coffinite,and the gangue minerals are mainly quartz,pyrite,calcite and fluorite.The characteristics of uranium mineralization in the field show that the deposit may be the products of the multistage mineralization.Pitchblende is not only ideal sample to date uranium mineralization directly but good mineral for measurement of U-Pb isotope testing as well.Based on the detailed mineralogical research,the pitchblende LA-ICP-MS U-Pb isotopic analysis and electron probe element surface scanning were carried out.The uranium metallogenic chronology and ore fluid properties of the Xiangyangping deposit were revealed.The results of 41 LA-ICP-MS analysis spots of pitchblende show that the^(206)Pb/^(238)U weighted average ages can be divided into(57.4±1.1)Ma and(43.1±2.1)Ma two groups,which are consistent with the main metallogenic epoch of the Miaoershan ore field,which indicating that there is an early uranium metallogenic event of about 100 Ma in Xiangyangping uranium deposit.The results of LA-ICP-MS show that the pitchblende is characterized by low total amount of rare earth elements,but the chondrite-normalized patterns of rare earth elements are rich in light-REE,with obvious negative Eu anomalies.It is indicated that the uranium source rocks may be the Douzhashan wall rock granite.This study suggests that the early hydrothermal fluid is rich in U,Fe,S,and the local reducing environment caused by pyrite crystallization forms early pitchblende.The late fluid is relatively rich in REE,Ca and Si.The pyrite and early pitchblende are altered by the late fluid,which results in the uranium activation and migration,and precipitation of the second phase pitchblende that rich in REE,Ca and Si.The comprehensive analysis suggests that the mineralization of the Xiangyangping uranium deposit may be controlled by the late Yanshanian subduction dynamic system between Ancient Pacific plate and Eastern Asia,and exists polyphase mineralization.