Studies On Geological Characteristics And Fluid Inclusions In The Zhaxikang Lead-zinc-antimony Polymetallic Deposit, Tibet, China

The Zhaxikang lead–zinc–antimony polymetallic deposit located in the south Tibetan detachment system. The deposit occurs in Lower Jurassic Ridang Formation, with the rocks being carbon–bearing 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 main structure of the ore are massive structure, banded structure, disseminated structure and vein–like structure; Its main texture are shape texture, half shape texture, containing texture and account texture. The main metal minerals of the ore are pyrite, sphalerite, galena, stibnite, zinkenite, boulangerite and jamesonite; Non–metallic minerals are mainly quartz, calcite and siderite. Wall–rock alteration includes silicification, calcitization and limonitization.The deposit subjected to two stages: Medium–low temperature hydrothermal period and hypergenesis period. The period of the medium–low temperature hydrothermal period can be divided into five stages: Siderite–sphalerite stage, Quartz–arsenopyrite stage, Quartz–galena–sphalerite–sulfosalt stage, Quartz–stibnite and Quartz–calcite stage. The inclusions in siderite, calcite and quartz of the medium–low temperature hydrothermal period are dominantly vapor–liquid two–phase inclusions, including a small amount of pure vapor inclusions, pure vapor CO₂–type inclusions and CO₂–H₂O three–phase CO₂ inclusions. In the medium–low temperature hydrothermal period, homogenization temperatures of inclusions vary from 184℃to 329℃with peak temperature of 255℃, the salinities of the different hydrothermal stages are 7.17～12.05﹪, 2.24～6.16﹪, 2.07～8.41﹪, 2.41～7.02﹪and 3.87～11.70﹪; the densities range from 0.65 g/cm³ to 0.86g/cm³. The gases in fluid inclusions are chiefly composed of H₂O, followed by a small amount of reducing gases such as CO₂, CH₄, C2H4,C3H6. The homogenization is a medium–low temperature, low salinity, low density H₂O–NaCl system, With minor or trace amounts of CO₂, CH₄ and C3H6. .the pressures of the ore–forming fluids range from 48Mpa to 86MPa and the depth of mineralization range from 1.77 km to 3.19 km.In the quartz and calcite of stage Quartz–galena–sphalerite–sulfosalt, various vapor–liquid ratios commonly coexist with simultaneously trapped pure vapor inclusions, pure vapor CO₂–type inclusions and CO₂– H₂O three–phase CO₂ inclusions. And different kinds of vapor–liquid two–phase inclusions have similar homogenization tempratures. This indicates that immiscibility which was marked by the escape of gases from the fluid occurred during the mineralization, whereas this immiscibility resulted sulfide mineralization.TheδDV–SMOW andδ¹⁸OH₂O values of the fluids in quartz and calcite inclusions were -165‰～-131‰and -13.7‰～10.21‰, the water in the ore–forming fluids derived from the precipitation.