粤北302铀矿床成矿流体与成矿物质来源研究:H、O、Sr、Nd同位素证据

摘要

粤北地区是中国花岗岩型铀矿最为重要的大型矿集区.位于粤北诸广复式岩体东部的302铀矿床是一个规模大,埋藏较深,矿化垂幅较大的花岗岩型铀矿床.矿体呈似脉状、扁豆状或透镜状产于印支期油洞岩体和燕山早期长江岩体的断裂蚀变带内,矿石矿物以沥青铀矿为主.近年来,研究者们对该矿床的成矿流体与成矿物质来源一直存在争议.文中对该矿床进行的氢、氧同位素研究表明,成矿流体的δ18OH2O值为-7.09‰~3.24‰,δDH2O值为-105‰~-63‰,反映成矿流体主要由深循环的大气降水通过水-岩相互作用演化形成,δDH2O值的偏低可能主要由成矿流体的热液沸腾作用和/或大气降水在深循环过程中与少量有机水(δDH2O=-90‰~-250‰)的混合引起.成矿期萤石的εNd(t)值(-11.9~-11.4)和稀土元素分布模式与赋矿围岩长江岩体十分相似而与油洞岩体区别明显,萤石落在Gd/Yb-Nb/Ta图解中的长江岩体及其邻近范围内,表明302矿床的成矿物质主要来源于赋矿围岩长江岩体.%The 302 uranium deposit, located at the eastern part of the Zhuguangshan granite composite in northern Guangdong province which is the most important large ore-concentrated area of granite-hosted uranium deposit in China, is a large-scale, deeply buried and largely vertical mineralized granite-hosted uranium deposit. Uranium ore bodies occur mainly as vein type, flat beanpod-like and lens-like, and are hosted within hydrothermally altered fault zones in the Indosinian Youdong granite and the early Yanshanian Changjiang granite. And the dominant uranium-bearing mineral is pitchblende. Despite intensive research over the past few decades, the source of ore-forming fluid and ore-forming material of this uranium deposit remains controversial. The results of hydrogen and oxygen isotopes in this article show that δDH2O and δDH2O values of ore-forming fluids range from-7.09‰to 3.24‰and from-105‰to-63‰, respectively, indicating that ore-forming fluids were mainly formed by deeply circulating meteoric water which have experienced water-rock interaction. The relatively low δDH2O value may be caused by hydrothermal boiling action of ore-forming fluids or mixing up with small amount of organic water (δOH2O-90‰to-250‰) in the process of deep circulation. TheεNd(t) values (-11.9 to-11.4) and rare earth element distribution patterns of the fluorite of metallogenic period are similar to those of the Changjiang granite but obviously different from those of the Youdong granite. Fluorites are in or near the scope of Changjiang granite in Gd/Yb-Nb/Ta diagram, indicating that uranium source of the 302 deposit was mainly derived from the Changjiang granite.