桂东北苗儿山地区花岗岩型铀矿田地质特征及成矿模式分析

Geological Characteristics and Metallogenic Model of Granite-type Uranium Orefield in the Miaoershan Area,Northeastern Guangxi Province

桂东北苗儿山铀矿田是我国重要的花岗岩型铀成矿区,在近几十年内勘查与研究过程中已取得较好的成果。然而,对于该地区的花岗岩型铀矿地质特征及成矿模式认识仍然较为薄弱。在系统搜集、综合分析矿区基础地质资料和前人研究成果基础上,从控矿构造、岩石学、矿物学、热液蚀变、含铀矿物组合类型等方面研究苗儿山地区花岗岩型铀矿地质特征及其成矿模式。结果表明,苗儿山地区铀矿田主要受北北东方向的断裂及次级断裂控制,断裂越发育的地方,铀矿化越明显。豆乍山和张家是主要的产铀地,岩体呈现出高硅、高钾偏碱、强过铝质以及演化程度高等特征。其中黑云母、锆石不仅是苗儿山地区花岗岩有效的产铀特征矿物,同时还指示了其低氧逸度(-16~-17)的有利产铀条件。苗儿山铀矿田围岩蚀变和铀矿物组合类型多样:与铀成矿关系密切的围岩蚀变主要有赤铁矿化、黄铁矿化、碳酸盐化、萤石化;铀矿物组合以铀-赤铁矿型、铀-黄铁矿型、铀-玉髓型、铀-萤石型为主。在燕山晚期华南陆壳拉张背景下,地幔流体和大气降水组成的矿化剂促使产铀岩体中的铀得以活化和萃取。上升的含矿热液在合适的温度、压力、pH、围岩蚀变、容矿空间等条件下沉淀形成了沙子江、向阳坪、张家、红桥等一系列铀矿床。

The Miaoershan uranium ore field located in the northeastern Guangxi province is regarded as the important granite-hosted uranium metallogenic region in China.Although good prospecting results have been conducted in the Miaoershan uranium ore field in recent decades,its geological characteristics and metallogenic model are so far from well-understood.On the basis of the summarization of geological survey data and prior studies,an integrated study in terms of the ore-controlling structure,petrology,mineralogy,hydrothermal alteration,and uranium mineral assemblages,has been conducted on the Miaoershan uranium ore field to discern the geological characteristics and metallogenic model.The results show that the Miaoershan uranium ore field are mainly controlled by the NNE fault and secondary fault.The more developed the fault network,the more obvious the mineralization of uranium.The Douzhashan and Zhangjia plutons,as the major reservoirs of uranium,are characterized by high Si and K and alkali-rich,peraluminous and high degree of magma evolution.Biotite and zircon not only effectively recognize the uranium productive granite but also reflect the beneficial uranium productive condition of low oxygen fugacity(logf(O_(2))from-16 to-17).There are various types of wall rock alteration and uranium mineral assemblage in the Miaoershan uranium ore field.The hydrothermal alteration associated with uranium mineralization includes hematitization,pyritization,carbonatation and fluorization.The uranium mineral assemblages mainly consists of U-hematite,U-chalcedony,U-pyrite and U-fluorite.Hereby,a metallogenic model is proposed for the Miaoershan uranium ore field.Under a crustal extensional setting of the late Yanshanian in South China,the mineralizing agents consisting of mantle fluids and meteoric water facilitate the activation and extraction of uranium from the uranium productive granite.Subsequently the upward uranium-bearing hydrothermal fluid precipitates under the appropriate temperature,pressure,pH,wall rocks alteration and ore storage space,and ultimately formed a series of uranium deposits,e.g.,the Shazijiang,Xiangyangping,Zhangjia and Hongqiao granite-hosted uranium deposits.