粤北302铀矿床围岩蚀变的地球化学特征和成因研究

摘要

With the drill hole ZK23-2 as the object and the microscopic slice identification and the geochemical analysis of fluid inclusions of the ore-forming period as the main means, this paper mainly studied wall rock alteration of the No.302 uranium deposit.The results show that the ore deposit has a variety of alterations, which are composed of such types as alkali feldspathization, silicification, sericitization, hematitization, chloritization, epidotization, carbonation, and kaolinitization.The ore deposit shows obvious vertical and horizontal zoning.From the top to the bottom of the ZK23-2 drill hole, four alteration zones were recognized, i.e., red-sericitization zone→intense silicification red alteration-sericitization zone (uranium enrichment zone)→chloritization-sericitization zone→weak alteration or normal granite zone.Major elements, trace elements and rare elements in the rock show regular variations.The altered rock has preserved the same geochemical characteristics as the ore-hosting Changjiang rock body, as can be seen from the consistency of their distribution patterns of trace elements and rare elements.Homogenization temperatures of fluid inclusions in different alteration zones are all below 220℃ , belonging to moderate-low temperature hydrothermal fluids.As for chemical components of fluid inclusions, inert gas N2 appears at all the alteration zones, and gaseous CO2 exists in the strong alteration zone, suggesting that the dissolution, migration and enrichment of uranium were related to gaseous CO2 and occurred in gaseous N2 environment during the formation of the uranium deposit.In addition, a study of fluid inclusions in the ore-forming period shows that characteristics of different wall rock alteration zones might have resulted from the mixture of ore-forming hydrothermal fluids.%以粤北302铀矿床ZK23-2钻孔为研究对象,通过显微镜下薄片鉴定、岩石地球化学和流体包裹体分析等对该矿床的围岩蚀变进行研究.结果表明,302矿床蚀变种类繁多,蚀变类型主要有:碱性长石化、硅化、绢云母化、红化、绿泥石化、绿(褐)帘石化、碳酸盐化、高岭石化等.矿床表现出明显的垂直分带和水平分带特征.从上至下,将ZK23-2划分为4个蚀变带:红化、绢云母化带→硅化、红化、强绢云母化带(铀富集带)→绿泥石化、绢云母化带→弱蚀变或正常花岗岩带.岩石的主量元素、微量元素和稀土元素均规律变化,微量元素分布模式和稀土元素配分模式均与赋矿围岩长江岩体的趋势一致,说明蚀变作用对它们的影响小,致使蚀变岩石基本保持原岩的地球化学特征.各蚀变带包裹体的均一温度均低于220℃,属于中低温热液,惰性气体N2在各蚀变带包裹体中普遍存在,强蚀变带的包裹体气相成分中均含CO2,表明在铀矿的形成过程中,铀的溶解、迁移以及富集成矿作用与CO2气体密切相关并发生在N2环境中.此外,成矿期脉石矿物包裹体的均一温度和压力为正相关的平滑曲线,表明围岩蚀变的分带特征主要是由成矿热液流体的混合作用引起的.