Research On Characteristics Of Fluid Inclusion And Mineralization Of Youqialang Lead-Silver Deposit In Naqu, Tibet

The Youqialang lead-silver deposit is located in Mesozoic magmatic arc in Gandise- Nyainqentanglha Range, Tibet, which have good mineralization geological conditions and ore potential.According to measure the stratigraphic section and renew the geological maps, The mineralization geological conditions of Youqialang is researched. After analysing the ore-bearing character and mineralization structures, not only the characteristics of ore-body and the composition of ore are summarized, but also the metallogenic stages of magmatic hydrothermalism and supergene metallogenic stages are divided. Based on these work, the order table of generated minerals is drawn. Combined with the data of the fluid inclusion which is first researched in this area and the sulfur and lead isotopes of sulfides, the mineralization and genesis of Youqialang deposit is analysed. A metallogenetic model was established in the end of the thesis.Research indicates that quartz in ore-bearing veins contains two types of primary fluid inclusions. TypeⅠis aqueous inclusions with V/T (vapor/total) of 15%-30%. Locally found typeⅡinclusions characterized by H2O-CO2 in component, some of which have two phases of CO2 at room temperature. According to the wide range of their homogenization temperatures (160～250℃) and the average temperature (210℃), it is inferred that the ore-forming temperature of the deposit is mesotherm. The salinities range from 1.65 to 12.29%, and their peak values range from 3 to 5%. A certain portion of data with higher salinities indicate that the source of high-salinity magmatic hydrothermal fluids. The variation of salinity show that the fluids were mixed with heated groundwater.The sulfur isotopes (δ34S) of galena and sphalerite range from -5.92 to -9.47 (%oCDT), closing to the value of ilmenite-type granite, which indicates that the source of minerals is relevant to the corresponding rocks. The sulfur isotopes balance temperature of galena and sphalerite is 214℃which is close to the homogenization temperature of fluid inclusions. The narrow range of lead isotope values and their position projected onto diagram of tectonic model indicate that the ore lead was mainly derived from crust, especially upper crust.In conclusion, the author suggests that the wallrocks hardly provide mineralizing substance and the ore-forming solution mainly from postmagmatic hydrothermal is the fluid of mid-high temperature and moderate salinity. Mineralizing hydrothermal migrated upwards along the tensional cracks with the composition being changed by mixture of heated underground water and ore being formed in the favorable places. Therefore, the Youqialang lead-silver deposit is a mesotherm hydrothermal infilling and replacement deposit. A metallogenetic model was established in the end of the thesis.