滇西富碱斑岩型多金属矿稀有气体同位素组成特征及地质意义*

通过对滇西富碱斑岩型多金属矿区稀有气体同位素组成的研究表明,黄铁矿和石英脉等流体包裹体中3He/4He值主要为0.160 8~3.470 0 Ra,远高于地壳特征值,而整体略低于地幔特征值;20Ne/22Ne和21Ne/22Ne平均值分别为11.271和0.032 2,接近地幔同位素组成;40Ar/36Ar和38Ar/36Ar平均值分别为395.51和0.197 6,均高于大气比值,而低于MORB比值;128~136Xe/130Xe值与大气相比均表现出过剩的特征。综合研究表明,滇西多金属矿区包裹体中稀有气体同位素组成在显示含矿流体的幔源特征的同时,又表现出强烈的地壳特征;成矿流体主要源于深部地幔,在参与交代蚀变过程中,其性质由熔浆向热液过渡,同时引发壳幔物质叠加混染,正是这种流体作用构成了滇西新生代富碱斑岩多金属成矿的内在统一制约因素。

Noble Gas Isotopic Compositions and Geological Significance From Alkali-Rich Porphyry Polymetallic Deposits in Western Yunnan

The study of noble gas isotopic data for alkali-rich porphyry polymetallic mining areas in western Yunnan shows that ₃He/₄He ratios of fluid inclusions from pyrite and quartz veins are mainly in the range of 0.160 8 ~3.470 0Ra which are much higher than the crust eigenvalue but are slightly lower than the mantle eigenvalue; the average ₂₀Ne/₂₂Ne ratios and ₂₁Ne/₂₂Ne ratios are average 11.271 and 0.032 2 which are close to mantle isotopic compositions; ₄₀Ar/₃₆Ar ratios and ₃₈Ar/₃₆Ar ratios are in the average of 395.51 and 0.197 6 which are higher than air ratio but are lower than MORB ratio; _128~136Xe/₁₃₀Xe ratios represent surplus characteristics compared with air ratio. From what has been discussed above, noble gas isotopic compositions reveal that ore-bearing fluids have the mantle-derived characteristics and the drastic crustal characteristics in western Yunnan; ore-forming materials mainly derive from deep mantle, and their properties are transformed from magma fluids to hydrothermal fluids in the course of metasomatism and alteration, which leads to initiate crust-mantle mixing simultaneously. It is this fluid effect that plays an important role in the formation of Cenozoic polymetallic deposits in western Yunnan.