藏南扎西康铅锌锑多金属矿床流体包裹体研究及地质意义

Fluid Inclusions in the Zhaxikang Pb-Zn-Sb Polymetallic Deposit, South Tibet,and Its Geological Significance

扎西康铅锌锑多金属矿床位于藏南拆离系东部,是中国西藏为数不多的以富含硫盐矿物为重要特征的大型铅锌锑银共生矿床之一。矿床赋存于下侏罗统日当组,容矿岩石为含炭钙质板岩、钙质板岩、绢云母板岩、页岩和石英砂岩。矿体严格受近南北向和北东—南西向两组断裂控制,呈脉状、透镜状产出。矿床的形成经历了中低温热液期和表生期。中低温热液期菱铁矿、石英、方解石中的包裹体类型主要为气液两相水包裹体,含少量纯气相水包裹体、纯气相CO2包裹体、气液两相CO2包裹体和CO2-H2O三相包裹体。成矿流体均一温度范围为184～329℃,峰值为255℃;成矿流体盐度w(NaCl)为2.07%～12.05%;密度为0.65～0.86 g/cm3。成矿流体主要为中低温度、低盐度、低密度的H2O-NaCl体系,含少量或微量的CO2和CH4。石英、方解石和硫化物包裹体中δDV-SMOW值变化范围为-165‰～-131‰,δ18OH2O变化范围为-13.7‰～10.21‰,成矿流体来自大气降水下渗循环构成的地热水。成矿过程中可能发生了以气液相分离为主要标志的不混溶作用,推测这种不混溶作用可能是导致硫化物大量沉淀的重要原因。矿床成因类型为沉积-构造-热活动驱动地热系统流体循环形成的中低温热液矿床。

Abstract： The Zhaxikang Pb-Zn-SB polymetallic deposit, which is located in the south Tibetan detachment sys- tem, is one of the few large Pb-Zn-Sb-Ag symbiotic deposits in Tibet that characterized by great variety of sulfosah minerals. The deposit occurs in Lower Jurassic Ridang Formation, with the rocks being carbon-beating calcareous slate, calcareous slate, sericite slate, shale and quartz sandstone. The ore bodies in Zhaxikang deposit are mainly controlled by SN and NE-SW striking tensional faults. The deposit was subjected to two stages： medium-low tem- perature hydrothermal period and hypergenesis period. The inclusions in siderite, calcite and quartz of the medi-um-low temperature hydrothermal period are dominantly vapor-liquid two-phase inclusions, including a small amountof pure vapor inclusions, pure vapor CO2-type inclusions and CO2-H2O three-phase CO2 inclusions. In the medium-low temperalure hydrothermal period, homogenization temperatures of inclusions vary from 184 to 329 ℃ with peak temperature of 255 ℃ , the salinities from 2. 07% to 12.05% , the densities from 0. 65 to 0. 86 g/cm^3. The ore-forming fluid is a medium-low lemperature, low salinity, low density H2 O-NaCl system, with minor or trace amounts of CO2 and CH4. The δD_V-SMOW and δ^18O_H2O values of the fluids in quartz and calcite nclusions varied from - 165‰ to - 131‰ and from - 13.7‰ to 10. 21‰, respectively, therefnre, the water in the oreforating fluids derived from precipitation. The immiscibility which was marked by the escape of gases from the fiu- id occurred during the mineralizalion, whereas this immiscibility resuhed in sulfide mineralization. The genesis of this deposit belongs to medium-low temperature hydrolhermaJ type formed during groundwater convection cycle driven by sedimentary-structural-geothermal activity.