西藏甲玛铜多金属矿床流体包裹体特征及地质意义

Characteristics of Fluid Inclusions in Jiama Copper-Pollymetallic Ore Deposit,Tibet and Its Geological Significance

甲玛矿床是位于西藏冈底斯成矿带中段的超大型铜多金属矿床。成矿相关岩体和主矿体中的流体包裹体岩相学、显微测温、激光拉曼探针及扫描电镜/能谱分析结果表明:与成矿有关的流体包裹体主要有富液相、富气相和含子矿物三相包裹体;流体包裹体均一温度变化范围为225～500℃,流体盐度(w(NaCl))平均值为36.2%,成矿流体主要为一种中高温、高盐度的NaCl-H2O体系;气相成分除H2O外,富含CH4、H2S、CO2、N2等不混溶体系。CH4、C2H4、C3H6等有机质的出现,说明成矿物质形成于还原环境中。据成矿压力估算成矿深度为2.2～8.3km,流体密度平均为0.89～0.98g/cm3。结合包裹体中的氢氧同位素测试结果认为,成矿流体主要来源于岩浆水,后期有大气降水的参与。含子矿物多相包裹体与不同气相充填度的液相包裹体、气相包裹体共存,且均一温度相近,而盐度相差很大,表明成矿流体经历了沸腾作用。

The recently discovered Jiama super large copper-polymetallic deposit is located in the middle section of Gangdese metallogenic belt,Tibet.Systematic studies including petrography,microthermometry,laser Raman microprobe and SEM/EDS were carried out on the primary fluid inclusions in metallogentic granite and major ore bodies.The analytical results show that dominant fluid inclusions from skarn ore related to metallogensis include liquid-rich,gas-rich two-phase and daughter-minerals bearing polyphase types.Homogenization temperatures of the fluid inclusions range from 225 ℃ to 500 ℃.The average of the salinities of ore-forming fluid is 36.2% NaCleq and the densities vary from 0.89 to 0.98 g/cm3.The ore-forming fluids belong to NaCl-H2O type characterized by high-moderate temperature,high salinity,low density and strong reduction.Besides H2O,the gas compositions are immiscible system rich in CH4,H2S,CO2 and N2 etc.The occurrence of organic matter,such as CH4,C2H4 and C3H6,suggests that the ore-forming materials were formed in a reduced environment.Based on estimation of mineralization pressure and the relationship between pressure and depth in fracture zones,the mineralization depth is considered to be at 2.2-8.3 km.The analytical results of hydrogen and oxygen isotopic elements show that ore-forming fluids are mainly derived from magmatic water mixed with meteoric water later.The coexistence of daughter-minerals bearing multi-phase inclusions and those liquid-and gas-rich inclusions with different filling degrees,together with their similar homogenization temperatures and much different salinities,indicates that ore-forming fluids had experienced boiling.