小兴安岭西北部永新金矿床成矿流体来源与矿床成因:流体包裹体和H-O-S-Pb同位素证据

Ore-fluid sources and genesis of Yongxin gold deposit in northwestern Xiao Hinggan Mountains: Constraint from fluid inclusions and H-O-S-Pb isotopes

永新金矿是近年在小兴安岭西北部嫩江-黑河构造混杂岩带新发现的大型金矿床,由于研究程度较低,该矿床的成因仍然存在较大争议。为了准确限定永新矿床的成因类型,文章开展了野外地质调查和室内镜下观察。结果显示,永新金矿床矿石矿物主要包括自然金、黄铁矿、闪锌矿、方铅矿和少量黄铜矿等,其中,自然金主要以裂隙金和包裹金形式赋存在黄铁矿中。围岩蚀变类型主要包括钾长石化、硅化,绢云母化,高岭石化、碳酸盐化、绿泥石化和局部冰长石化,其中硅化与金矿化关系最为密切。对永新金矿床开展了流体包裹体分析和稳定同位素(S、Pb、H和O)测试,结果表明,永新金矿床流体包裹体以气液两相为主,从成矿早期到晚期各阶段平均成矿温度由305℃→237℃→202℃→162℃,逐渐降低;盐度w(NaCl_(eq))由7.5%→3.4%→2.9%→1.7%,逐渐减低;流体密度由0.78→0.84→0.89→0.92 g/cm3,微弱增高,但整体均较低。成矿流体为典型中低温、低盐度和低密度流体,成矿深度小于1.1 km,形成于浅成环境。包裹体激光拉曼光谱分析结果显示,流体气相成分以H_(2)O为主,见少量CO_(2),基本不含有CH_(4)和H_(2)。H-O同位素显示,成矿流体主要来自大气降水,并与围岩发生了明显的水-岩反应;S-Pb同位素显示成矿物质与赋矿火山-次火山岩关系密切,具有成因联系。在上述研究的基础上,结合区域典型矿床对比分析,确定了永新金矿床为典型的低硫化型浅成低温热液矿床,与同一地区的众多早白垩世浅成低温热液金矿床可能共同形成于区域性伸展构造背景下。

The Yongxin gold deposit is a newly discovered large gold deposit in the Nenjiang-Heihe tectonic mélange belt in the northwest of northern Xiao Hinggan Mountains. Limited previous studies of this deposit have been conducted but its genetic type is still controversial. In this paper, in order to constrain the genetic type of the Yongxin gold deposit, the authors conducted detailed field geological and petrographic investigation as well as microscopic observation. The results show that the ore minerals of the Yongxin gold deposit include mainly native gold, pyrite, sphalerite and galena with minor chalcopyrite, among which the native gold mainly occurs in quartz and pyrite as microfracture and inclusion gold. The hydrothermal alteration mainly includes potassic alteration, silicification, sericitization, kaolinization, chloritization, carbonation and local adularization, of which the silicification alteration is closely associated with gold mineralization. Detailed fluid inclusion and stable isotope(SPb-H-O) analyses show that the fluid inclusions are dominated by liquid-vapor two-phase type. From the early stage to the late stage of the fluid evolution process, the fluid temperatures decrease gradually in order of 305℃→237℃ →202℃ →162℃. Correspondingly, the salinities w(NaCl_(eq)) decrease in order of 7.5%→3.4%→2.9%→1.7%, and the fluid densities slightly increase in order of 0.78→0.84→0.89→0.92 g/cm3, although the overall variations are limited. The ore-forming fluid has typical medium-low temperature, low salinity and low density,and the mineralization depth is less than 1.1 km, indicating a shallow environment. The results of laser Raman analysis show that gas phase compositions of the fluid is mainly H_(2) O with minor CO_(2), but without detectable CH_(4) and H_(2). The H-O isotopes indicate that the ore-forming fluid could be dominantly of meteoric water with obvious water-rock reaction. The S-Pb isotopic compositions show that the ore-forming materials are genetically related to the volcanic and subvolcanic rocks. On the basis of the above studies and the comparative analysis of regional deposits, the Yongxin gold deposit could be classified as a typical low-sulfidation epithermal Au deposit, similar to many other epithermal gold deposits in the Xiao Hinggan Mountains region, which might have been formed in an extensional background.