甘肃安家岔金矿床铅同位素地球化学研究

LEAD ISOTOPE GEOCHEMISTRY OF THE ANJIACHA GOLD DEPOSIT,GANSU PROVINCE

通过对安家岔金矿床铅同位素地球化学特征研究及其与邻区地层、岩浆岩和有关矿床铅同位素的对比结果表明:①本矿床矿石铅是一种以高放射成因铅为主的混合型铅,且明显表现出有大气降水参与热液成矿的铅同位素地球化学特征,反映本矿床属于沉积-强烈改造型金矿床;②本矿床成矿物质的直接来源主要为志留系;③本区的金矿成矿作用和主要与志留系重熔有关的岩浆岩(Ⅰ类岩浆岩)的关系最为密切,因此,在志留系展布区内,Ⅰ类岩浆岩密集出现,可作为本区金矿找矿的重要标志。

Located in Xihe County,Gansu Province and in the west of the Xicheng lead-zinc ore belt, the Anjiacha gold dpeosit is a micro-fine-grained disseminated gold in Devonian strata,with its natural type belonging to quartz vein type and altered rock type.Being of multiphase and multi- stage composite origin,the ore deposit might be metallogenically divided into three(pre-ore, principal ore-forming and post-ore)phases consisting of eight mineralization superimposition or transformation of multiphase and multistage mineralizations.Having analysed various geological bodies and mineralized bodies in the deposit for lead isotope compositions and compared these data with lead isotope compositions of strata,magmatic rocks and gold deposits of various ages in neighboring areas,the author has concluded that lead isotope compositions of the Anjiacha gold deposit vary in a very wide range and are of mixed type dominated by radiogenic lead.The iso- tope compositions of ore lead depend not only on initial lead isotope compositions of the source rock but also on U,Th contents and U/Pb,Th/Pb ratios of the ore-bearing rocks.What is more,they have something to do with lead isotope geochemistry of the hydrothermal system. Regional geological setting and the above lead isotope data demonstrate that ore lead of the de- posit was derived mainly from Silurian and partly from Proterozoic Bikou Group,Middle Devoni- an sediments and magmatic activity.A comparison of lead isotope geochemical characteristics also reveals that source magmatic rocks might be classified into three types,derived respectively from ①remelting of Silurian materials,②remelting of materials of Bikou Group,and③remelting of deeper materials.Granite from remelting of Silurian materiald was closely related to the ore- forming process of this deposit,and served as the main heat source besides providing part of ore- forming materials.Therefore,the Anjiacha gold deposit exhibits lead isotope geochemical charac- teristics of sedimentary-strongly transformed ore deposit.