西澳大利亚纳沃日金矿流体混合与金的成矿作用

摘要

流体包裹体研究表明，纳沃日金矿成矿脉体中流体包裹体类型有三种，富CH4、CH4-CO2-H2O和富H2O包裹体。初始成矿流体是富CH4热液，在主矿化阶段，另一来源的CO2-H2O流体与CH4流体发生了不均匀混合，并且在石英-磁黄铁矿阶段最为强烈，造成不同矿化部位的xCH4变化极大，早期近于纯CH4流体被充分“稀释”后，石英-黄铁矿阶段的xCH4显著减小，到成矿晚期只剩CO2-H2O流体。正是由于不同性质流体的混合，造成热液的pH及fO2的升高，流体中的金-硫络合物分解，金沉淀成矿。%The Nevoria deposit,a banded iron formation-hosted gold deposit,occurs within the roof of amphibolite-facies greenstones in the Southern Cross belt,Yilgarn Block,Western Australia.The mineralizing veins include early quartz-pyrrhotite±chalcopyrite veins and late quartz- pyrite veins.Three types of fluid inclusions,namely CH4-rich,CH4-CO2-H2O and H2O-rich ones,were found at Nevoria.Microthermometric data reveal that the initial auriferous fluids were CH4-dominant.Fluid mixing may have caused remarkable variations of CH4/CO2 ratios or xCH4 of CH4-CO2-H2O inclusions,particularly in mineralized quartz-pyrrhotite veins.The CH4-CO2-H2O inclusions in quartz-pyrite vein set have an apparent trend of decreasing xCH4 and increasing molar volume.It can be inferred that,from the quartz-pyrrhotite stage to the quartz-pyrite stage,homogenization temperatures of CH4-CO2-H2O inclusions decreased,and CH4 contents (xCH4) also decreased.The carbonic phase in the inclusions varies from CH4-dominant to CO2-dominant.Phase mixing,resulting in an increase in pH and fO2,together with loss of reduced sulfur by mineral/fluid reactions and precipitation of sulfides,led to the breakdown of the gold-transporting complexes.