Characteristics Of Fluid Inclusions In Longmala Pb-Zn Deposit, Tibet

Longmala Pb-Zn deposit is located in Gangdese Nyainqentanglha plate-Nyainqentanglha back arc uplift belt, It is an important skarn Pb-Zn deposit in Menba-JINDA Pb-Zn-Ag-Cu-Mo polymetallic ore in Nyainqentanglha metallogenic belt. The strata of deposit is mainly exposed the Permian Lielong formation(P2l); the Permian Luobadui formation(P1l), the Permian Wu Lu dragon group(P1w), Carboniferous pangdo group formation(Cpn). The main metallogenic stage of skarn deposits divided into skarn stage, quartz sulfide stage and supergene stage.Based on a clear understanding about metallogenic geological background and geological characteristics of Longmala Pb-Zn polymetallic deposits. According to the results of field and indoor work, compared the mineralization process of the deposit with the classical skarn type ore deposit metallogenic evolution, It is believed that the deposit is divided into the following three stages from early to late. A. Hydrothermal stage. The hydrothermal period can be subdivided into skarn stage.①The early skarn stage,the mineral assemblage is garnet and diopside + wollastonite, the stage of skarn minerals in the mining area is the most developed.②The late skarn stage. Mineral assemblage actinolite + epidote + magnetite.③The oxide phase. Minerals are mainly altered phlogopite and magnetite.quartz sulfide stage.①The early stage of sulfide:The development of ore minerals are sphalerite, galena, chalcopyrite andpyrite, pyrrhotite, accompanied by intense chloritization, epidotization and silicification.This stage is the main ore-forming stages of mineralization. ② In the late stage of ore sulfides, late stage of ore-forming stages, the mineral assemblage isquartz+calcite+a small amount of pyrite, molybdenite, galena, quartz and calcite in the vein or patchy occurrence in the fracture of skarn. B. supergene stage, Mainly due to weathering and leaching, Oxide minerals formed,such as Lead oxide and Malachite.Study on fluid inclusions of each stage in Longmala Pb-Zn deposit indicates that,garnet and pyroxene inclusions of early skarn stage mainly the three-phase inclusions and two-phase fluid inclusions,the gas-liquid inclusion salinity ranged from 2.74%~16.89% NaCl eqv, the critical temperature between 466~548 degrees celsius, Three phase inclusions of salinity changed in the range of 39.76%~55.1% NaCl eq, the critical temperature at 464~543 degrees celsius, Main minerals in Inclusions is halite and sylvite;the fluid density changes in the range of 0.692~1.101g/cm3, pressure changes in the 90.5~382.2 MPa range.The early quartz-sulfide stage fluid inclusions in quartz mainly for the two-phase inclusions and a small amount of three-phase inclusions, gas-liquid inclusion salinity value between 1.91%~7.02% NaCl eq, Critical temperature of 296~437 degrees celsius. Three phase inclusions of salinity changed in the range of 39.68%~46.34% NaCl eq, critical temperature at 372~473 degrees celsius, fluid density changes in the range of 0.791~1.066g/cm3, pressure changes within the scope of 99.8~286.8 MPa. Late stage of Quartz sulfide, the main of inclusions in quartz and calcite are gas-liquid two-phase,the inclusions Critical to the liquid phase, The salinity value between 0.88%~6.88% NaCl eq, the homogenization temperature between 274~361 degrees celsius, the density of g/cm3 in 0.674~0.865, pressure between 23.1~114.5 MPa. Study on fluid inclusions indicates that, temperature, salinity, density and pressure of the fluid in the process of mineralization in Longmala deposit are decreased.Study of hydrogen, oxygen isotope in the inclusion which from the Longmala metallogenic period show that, the water of the ore fluid mainly from the initial mixing of magmatic water,and played a leading role in mineralization. The late of mineralization, ore-forming fluid mixed with the meteoric water.In the process of drilling core logging in Longmala deposit,For the first time found Biotite granite to be closely related with mineralization.Through the test and analysis of zircon U-Pb age, showed that the average age is 55.7 + 0.3Ma(n=26, MSWD=3.6), determined the Rock forming age of biotite granite. Three molybdenite samples from Longmala Pb-Zn deposit were analyzed by Re-Os isotope test, results showed that the Re-Os model at the age of 51.8 + 0.7Ma and 54.3 + 0.8Ma, with an average age of 53.3 + 3.7Ma(MSWD=2.7). There is a close relationship in the time and space between Mineralization and biotite granite from Longmala deposit, The two together for Longmala Pb-Zn deposit metallogenic time plays a restraining role.Through in-depth analysis of the Longmala deposits, combination ore fluid evolution, metallogenic epoch metallogenic model and metallogenic mechanism with the same type of deposits in the eastern section of Gangdese, such as yaguila, dongzhongla, dongzhongsongduo, lawu, mengyaa, leqingla, xingaguo. established the metallogenic model of Longmala deposit, as a positive significance of the regional metallogenic regularity of ore deposit.