青藏高原东缘缅萨洼金矿成矿流体地质地球化学特征

The Miansawa gold deposit at the eastern margin of the Tibetan Plateau: fluid inclusions, stable isotopic and noble gas isotopic data and their metallogenic implications

缅萨洼金矿位于中国中轴构造带的中南段,青藏高原的东缘,赋存于金河-箐河断裂带次级断裂羊坪子韧性剪切带中本文根据对该矿床硫化物流体包裹体的氦氩同位素、硫化物的硫同位素以及与硫化物共生的石英的流体包裹体特征、成分以及氢氧同位素组成的测定,讨论了缅萨洼金矿的成矿流体来源及其矿床成因。结果显示,该矿床硫化物流体包裹体中的3He/4He变化较小,为0.69-0.82,显示了地幔流体参与成矿作用的可能性。而4He的含量变化范围较大,一般在2.19-10.62×10-6cm3STP/g(方铅矿除外)与3He/4He相比,40Ar/36Ar的比值则变化较小,一般为251-509。而硫化物的δ34S同位素变化范围在-1.8-2.2‰,平均值为0.5‰,说明硫的地幔来源。与硫化物共生的石英的流体包裹体的类型主要有富液相的盐水溶液包裹体、富气相的盐水溶液包裹体、三相CO2包裹体、纯液相CO2包裹体以及有机流体包裹体。成矿流体的氢氧同位素则显示成矿流体来源于岩浆水(或地幔流体)与大气降水的混合流体,本文认为,缅萨洼金矿的成矿流体为地幔流体与大气降水的混合流体,是渐新世印度大陆与亚洲大陆碰撞之后,该地区大规模走滑与剪切作用过程中,局部伸展作用的产物。

Miansawa gold deposit is located in the west margin of the Yangtze craton, at the eastern margin of the Tibetan. The gold deposit occurs in the mylonitic granite and (or) plagiaplite, controlled by NNE-trending ductile-brittle ductile shear zone. The gold mineralization occurs mainly as auriferous quartz veins with minor sulfide minerals. Fluid inclusions in pyrite have He-3/(4) He ratios of 0.69 to 0.82 Ra, whereas their Ar-40/Ar-36 ratios range from 251 to 509. The He and Ar isotope data indicate mixing of fluids of mantle and crustal origin. The delta(34)S values of pyrite are from - 1.8 to 2.2 parts per thousand, with an average of 0.5 parts per thousand, suggesting a mantle source. Hydrothermal quartz has delta(18)O values between - 2.5 and 0.7 parts per thousand and deltaD values between - 64 and - 93 parts per thousand, indicating mixing of magmatic and meteoric fluids. The paper concludes that the noble gas isotopic data, along with the stable isotopic data suggest that the ore-forming fluids is a mixture between mantle-derived fluids and meteoric water, the gold mineralization are the results of the collision between India plate and Asia plate, associated with sklip-slip faults in the Oligocene.