Fluid inclusion characteristics and ore genesis of the Banduo Pb-Zn deposit in Songduo area, southern Tibet

The Banduo Pb-Zn deposit in Songduo area of southern Tibet is located along east Gangdise metallogenic belt. Three orebodies have been delineated so far, which are veined and lentoid in form and are controlled by NE-trending faults. The major metallic minerals are galena, sphalerite and chalcopyrite, together with some other metallic minerals such as arsenopyrite, pyrite and pyrrhotite. The major nonmetallic minerals are quartz and sericite, and other nonmetallic minerals are muscovite and calcite.The deposit can be divided into three metallogenic stages:Ⅰ. quartz-arsenopyrite-pyrite-pyrrhotite stage; Ⅱ. quartz-pyrite-sphalerite-chalcopyrite-galena stage; Ⅲ. quartz-calcite stage, with the major metallogenic stage being the second stage. In order to determine the ore-forming physical and chemical conditions and characteristics of ore-forming fluids, the authors selected the quartz grains formed at the major metallogenic stage to study the petrography and micro-thermometry of fluid inclusions. The results show that fluid inclusions in quartz are mainly composed of vapor-liquid two-phase inclusions, with a small amount of pure gas and pure liquid inclusions. The main ore-forming fluid is NaCl-H₂O characterized by medium temperature(163.5~344.5℃, 266.3℃average), low salinity(0.70%~7.15%NaCl eqv, 3.39% NaCl eqv averagely) and low density (0.65~0.95g/cm³, 0.79g/cm³ averagely). The fluid inclusion study indicates that boiling occurred at the early stage of mineralization, then decrease of temperature and pressure of the fluids should be the main factor for Pb and Zn concentration and mineralization. Based on comprehensive ore-formation geological conditions, geological characteristics of the deposit, and fluid inclusions studies, the authors tentatively hold that the deposit is a mesothermal vein type Pb-Zn deposit.