西藏曲水县达布斑岩型铜钼矿床金属沉淀机制探讨

Mechanism of metal precipitation in Dabu porphyry Cu-Mo deposit,Quxu Country, Tibet

西藏曲水县达布斑岩型铜钼矿床位于冈底斯成矿带中东段,其矿化体主要产于含矿斑岩体与围岩的内外接触带中。文章以矿石内Cu、Mo矿化石英脉中的流体包裹体为研究对象,探讨了成矿金属沉淀的机制。通过详细的显微镜下鉴定,发现Cu、Mo矿化阶段的流体包裹体类型均以L型为主,但Cu矿化阶段的V型包裹体明显较Mo矿化阶段多,而S型包裹体较少。Cu、Mo矿化石英脉中,常见L型、V型、S型流体包裹体共存的现象,且它们的均一温度范围非常接近,说明成矿流体经历了不混溶作用,使得高盐度流体与低密度气相流体发生分离。单个流体包裹体激光拉曼光谱测试显示,在Cu、Mo矿化阶段的气相包裹体中均检测到CO2的特征峰和H2O峰,而在Mo矿化阶段的气相包裹体仅检测到CH4特征峰,说明Mo矿化阶段的流体的相对还原性更强;同时,检测到硬石膏、赤铁矿、磁铁矿、黄铜矿、黄铁矿等子矿物,但硬石膏、赤铁矿、磁铁矿主要分布于Cu矿化阶段,说明Cu矿化阶段的流体氧化性相对较强。对单个流体包裹体进行同步辐射X射线荧光分析(SR-RXF)显示,Cu、Au等金属主要富集于流体包裹体气相中,表明Cu、Au元素可能是气相运移。对Cu、Mo硫化物沉淀的一系列化学反应研究表明,呈氧化态以及酸度低(HCl浓度低,即偏中性)的流体会促进黄铜矿饱和;活性较高的H2S和略呈还原性的流体有利于辉钼矿的沉淀。综合分析认为,铜矿化与偏中性、结晶分异程度相对较低的花岗闪长斑岩关系密切,而钼矿化与酸性、结晶分异程度相对较高的花岗斑岩关系密切。

The Dabu porphyry Cu-Mo deposit in Quxu County of Tibet lies in the mid-eastern Gangdise metallogenic belt,with its mineralization mainly existing in the inner and outer contact zone between the porphyry orebody and the wall rock. In this paper, fluid inclusions in quartz veins of Cu, Mo mineralization were studied to deliberate the mechanism of metal precipitation. Detailed microscopic observations show that L-style inclusions were prevailing in both Cu and Mo ore-forming stages, the quantity of V-style inclusions at the Cu ore-forming stage was significantly more than that in Mo ore-forming stage, and the distributions of S-type inclusions at the two stages was quite different from each other. Laser Raman spectroscopy was used to analyze typical individual fluid inclusion. At Cu and Mo ore-forming stages, CO2 and broad H2 O peaks were detected in V-style inclusions, whereas CH4 peaks were only detected in the vapor style inclusions of Mo mineralization. These data indicate that fluids at the Mo ore-forming stage was reducible. Simultaneously, anhydrite, hematite, magnetite, chalcopyrite, pyrite and other minerals were detected, whereas oxide minerals（ anhydrite, hematite and magnetite） were mainly existent at the Cu ore-forming stage, suggesting that the fluids at the Cu ore-forming stage was more oxidized. Meanwhile, synchrotron radiation X-ray fluorescence analysis（ SR-RXF） was used to analyze individual fluid inclusions too, and the results show that Cu, Au and some other metals were enriched in V-style inclusions, indicating that Cu, Au might have been carried by vapor. L-type, V-type and S-type inclusions are commonly coexistent in Cu, Mo mineralized quartz veins, with very similar homogenization temperatures, which indicates that ore-forming fluids experienced immiscible process,leading to the separation of high-salinity fluid from the low-density vapor fluid and the selective entry of Cu into the vapor phase, while Mo remained in the high-salinity fluid as an incompatible element. A series of chemical reactions causing the precipitation of Cu, Mo sulfides show that the oxidized and low acidity fluids（ low HCl concentration i. e., close to neutral p H） facilitated saturation of chalcopyrite, and the highly active H2 S and slightly reductive fluids benefited the precipitation of molybdenite. A comprehensive analysis shows that copper mineralization is closely related to neutral p H and a relatively low degree of fractional crystallization of granodiorite porphyry, whereas molybdenum mineralization was closely related to acidity and a relatively high degree of fractional crystallization of granite porphyry.