新疆望峰金矿成矿流体的He、Ar同位素示踪

Helium and Argon Isotopic Tracing of Ore-forming Fluid from the Wangfeng Gold Deposit in Xinjiang

本文采用稀有气体同位素质谱方法,通过分析望峰金矿石中载金黄铁矿流体包裹体He、Ar同位素组成,对成矿流体进行示踪研究。结果显示,黄铁矿流体包裹体3He/4He比值为0.00473～0.01079Ra,40Ar/36Ar比值为301～413,具地壳放射性成因氦同位素组成和大气降水成因氩同位素组成,总体显示由大气降水改造而成的地壳流体特征。望峰金矿成矿流体中He同位素组成异常,是成矿前大气降水与高U、Th含量古老容矿围岩作用遭受放射性成因4He稀释、成矿时发生流体减压沸腾综合作用的结果,Ar同位素组成异常是成矿前大气降水下渗获取容矿围岩放射性成因40Ar的结果,成矿流体是大气降水深循环的产物。

The Wangfeng gold deposit is located in the Bingdaban ductile shear zone along the northern margin of Mid-Tianshan.Helium and Argon isotopic compositions of fluid inclusions in pyrite were analyzed to trace the source of ore-forming fluid by using an inert gas isotopic mass spectrometer after analyzing variables that may affect He-Ar original isotopic compositions of the ore-forming fluid.The analytical results indicate that 3 He/4 He ratio and 40 Ar/36 Ar ratio vary respectively from 0.00473 Ra to 0.01079 Ra and from 301 to 413,helium and argon isotopic compositions synthetically exhibit the crust-derivation of the ore-forming fluid,which was the product of the deep circulation of atmospheric water.Meanwhile,in contrast to typical crust-derived fluid,Wangfeng ore-forming fluid has higher 4 He and 40 Ar,resulting in lower 3 He/4 He and higher 40 Ar/36 Ar than typical He-Ar isotopic compositions of crust-derived fluid.An analysis of main country rocks and ore-forming fluid inclusions reveals that ore-hosting country rocks with high U,Th concentrations produced much more radiogenic 4 He which reduced 3 He/4 He ratio in comparison with normal radiogenic He isotopic composition,that atmospheric water with atmosphere-derived He-Ar isotopic compositions subsided through brittle crannies and captured radiogenic He existing in country rocks,and that some radiogenic 40 Ar was obtained synchronously,which reduced 3 He/4 He ratio preliminarily and raised 40 Ar/36 Ar ratio at the pre-mineralization stage.Then,ore-forming fluid boiled due to hydrothermal decompression at the ore-forming stage,which brought 3 He/4 He ratio down.In summary,the ore-forming fluid of the Wangfeng gold deposit was crust-derived and reformed during the process of fluid evolution.