新疆望峰金矿成矿流体的He、Ar同位素示踪

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0.00473~0.01079 Ra,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.

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

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 3He/4He ratio and 40Ar/36Ar 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 4He and 40Ar, resulting in lower 3He/4He and higher 40Ar/36Ar 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 4He which reduced 3He/4He 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 40Ar was obtained synchronously, which reduced 3He/4He ratio preliminarily and raised 40Ar/36Ar ratio at the pre-mineralization stage. Then, ore-forming fluid boiled due to hydrothermal decompression at the ore-forming stage, which brought 3He/4He ratio down. In summary, the ore-forming fluid of the Wangfeng gold deposit was crust-derived and reformed during the process of fluid evolution.