西藏拉屋铜多金属矿床的成矿流体特征与成矿机制研究

西藏拉屋铜多金属矿床产于冈底斯构造岩浆成矿带的申扎—旁多铜-银-铅-锌-金成矿亚带内。分别对干矽卡岩阶段(Ⅰ)的石榴石、早期硫化物阶段(Ⅲ)的石英和晚期硫化物阶段(Ⅳ)的方解石中的流体包裹体进行岩相学观察和显微测温研究,研究表明成矿各阶段热液矿物中的流体包裹体主要为气液水两相包裹体,其次为纯液相水包裹体,偶见气液两相甲烷包裹体,石英中也有大量的含NaCl子矿物多相包裹体,其均一温度变化于95～476℃之间,盐度介于1.57%～37.33%,密度变化于0.68～1.23 g/cm3,总体属中-高温、中-高盐度、中等密度的体系;据此计算的成矿压力范围为24.63～133.61 MPa,成矿深度介于2.46～9.64 km,表明该矿床形成于中深成矿环境。不同成矿阶段流体包裹体研究数据表明,该矿床的成矿作用是一个温度、盐度和压力总体显著降低(减小)、密度略渐增大的过程。氢、氧同位素研究表明,成矿流体在主成矿阶段主要为初始混合岩浆水,随着成矿作用进行,大气降水大量加入,到晚期阶段成矿流体逐渐演化成大气降水。成矿流体在Ⅲ阶段(主成矿阶段)发生了沸腾作用,导致成矿元素沉淀形成矿体。因此认为沸腾作用可能是该矿床金属沉淀的主要机制。

Studies on Ore-forming Fluid Characteristics and Mineralization Mechanisms of the Lawu Copper Polymetallic Deposit,Tibet

The Lawu copper polymetallic deposit occurred at Xainza-Poindo Cu-Ag-Pb-Zn-Au mineralization sub-zone in Gangdese magmatic-tectonic metallogenic belt.The petrographic observation and microthermometry study were carried out on the fluid inclusions from garnet in anhydrous skarn stage(Ⅰ),quartz in early sulfide stage(Ⅲ) and calcite in late sulfide stage(Ⅳ).The petrographic observations present that various types of fluid inclusions occurred in hydrothermal minerals of different ore-forming stages,which include mainly vapor-liquid two-phase H2O inclusions,subordinately liquid H2O inclusions,with occasional vapor-liquid two-phase CH4 inclusions,as well as NaCl daughter mineral-bearing multiphase inclusions occurred in quartz.And the studying results show that their homogenization temperatures vary from 95 ℃ to 476 ℃,while their salinities are 1.57%-37.33% NaCl eqv and their densities range from 0.68 g/cm3 to 1.23 g/cm3,overall belonging to the ore-forming system of medium-high temperature,medium-low salinity and medium density;and the mineralization pressures,calculated on the basis of homogenization temperatures and salinities mentioned above,vary from 24.63 MPa to 133.61 MPa,while the ore-forming depths range from 2.46 km to 9.64 km,indicating that this mineral deposit formed in mesogene mineralization environment.The data of fluid inclusions in different ore-forming stages indicate that,the mineralization of this deposit is a process of the homogenization temperatures,salinities,ore-forming pressures and depths explicitly decreasing,and of the densities slightly increasing.H-O isotopic studies have shown that,the ore-forming fluids are dominated by the initial mixture of magmatic and meteoric water in the main ore-forming stage,but a great amount of precipitation is added in the following mineralization stages,thus the ore-forming fluids evolved into meteoric water in the later ore-forming stage.The boiling events took place in stage Ⅲ,resulting the precipitations of ore-forming elements and forming the ore-bodies.Therefore,it is believed that the boiling is severed as the main mechanism for the metallic sedimentation of this mineral deposit.