藏南扎西康铅锌多金属矿绢云母Ar-Ar年龄及其成矿意义

The Zhaxikang Pb-Zn Polymetallic Deposit:Ar-Ar Age of Sericite and Its Metallogenic Significance

扎西康脉状铅锌多金属矿位于特提斯喜马拉雅东段中部,是该带内规模最大的多金属矿床,扎西康矿脉受控于数条南北向正断层,这些断层在青藏高原后碰撞伸展阶段形成。矿区发生了两次成矿事件,分别为铅锌成矿期和富锑成矿期,并且后期富锑成矿对早期铅锌成矿具有叠加改造作用。在后期富锑成矿期中方解石-黄铁矿阶段发现了与闪锌矿、黄铁矿等矿物共生的绢云母。本文采用Ar-Ar同位素测年方法对该绢云母进行了测试分析,获得绢云母的坪年龄为12.28±0.45 Ma,对应的等时线年龄为11.7±3.1 Ma。两个年龄在误差范围内相等,表明该绢云母的Ar-Ar年龄具有地质意义,代表了绢云母的形成年龄。该Ar-Ar年龄指示了扎西康富锑成矿期发生于12Ma左右。特提斯喜马拉雅成矿带在主碰撞成矿阶段发育一期金(银)矿化事件,而扎西康绢云母Ar-Ar年龄表明后碰撞阶段矿化事件可以持续到12Ma。由于矿体发育于南北向断裂之中,因此,暗示了扎西康地区NS向正断层系统发育时间早于12Ma。早期铅锌硫化物成矿期矿石普遍破碎,说明受到构造挤压作用影响,而富锑成矿期矿石表现为脉状充填,并无构造挤压破碎,可以推测12Ma后藏南NS向正断层系统可能处于持续伸展环境。

Located in the middle part of the Tethys-Himalaya,the Zhaxikang Pb-Zn polymetallic deposit is the biggest ore deposit in this belt.The Zhaxikang ore veins are controlled by several normal faults nearly striking NS,developed at the post-collision stage of the Tibet Plateau.Two stages of mineralization has been identified,and the first phase,Pb-Zn mineralization,has been overprinted by Sb-bearing that happened later. Among the mineral assemblage of the calcite- pyrite stage during Sb-bearing mineralization,sericite has been found coexist with quartz,calcite,pyrite and minor sphalerite.Dating of the sericite using Ar-Ar isotopes shows that plateau age of this mineral is 12.28±0.45 Ma,and its isochron age is 11.7±3.1 Ma.Two dated ages have the same deviation range,indicating the geological validity of the dating,and therefore the age of Sb-bearing mineralization.Tethys Himalaya has developed an Au（Ag）mineralization event in main collisional orogenic setting,while the Ar-Ar age of Zhaxikang indicates that mineralization in late collisional stage would continue to 12 Ma.The mineralization developed in the NS fault belt suggests that the fault system should predate this age,earlier than 12 Ma.The first stage of Pb-Zn mineralization has experienced compression supported by broken rocks distributed among the belt,while the late phase of Sb-bearing mineralization develops along the veins,manifesting that the NS-striking fault system has undergone extension since 12Ma.