Abstract:

The Lawu copper polymetallic deposit occurred at Xainza-Poindo Cu-Ag-Pb-Zn-Au mineralization sub-zone in Gangdese magmatic-tectonic metallogenic belt. The petrographic observation and microthermometry study were carried out on the fluid inclusions from garnet in anhydrous skarn stage (Ⅰ), quartz in early sulfide stage (Ⅲ) and calcite in late sulfide stage (Ⅳ). The petrographic observations present that various types of fluid inclusions occurred in hydrothermal minerals of different ore-forming stages, which include mainly vapor-liquid two-phase H₂O inclusions, subordinately liquid H₂O inclusions, with occasional vapor-liquid two-phase CH₄ inclusions, as well as NaCl daughter mineral-bearing multiphase inclusions occurred in quartz. And the studying results show that their homogenization temperatures vary from 95 ℃ to 476 ℃, while their salinities are 1.57%-37.33% NaCl eqv and their densities range from 0.68 g/cm3 to 1.23 g/cm³, overall belonging to the ore-forming system of medium-high temperature, medium-low salinity and medium density; and the mineralization pressures, calculated on the basis of homogenization temperatures and salinities mentioned above, vary from 24.63 MPa to 133.61 MPa, while the ore-forming depths range from 2.46 km to 9.64 km, indicating that this mineral deposit formed in mesogene mineralization environment. The data of fluid inclusions in different ore-forming stages indicate that, the mineralization of this deposit is a process of the homogenization temperatures, salinities, ore-forming pressures and depths explicitly decreasing, and of the densities slightly increasing. H-O isotopic studies have shown that, the ore-forming fluids are dominated by the initial mixture of magmatic and meteoric water in the main ore-forming stage, but a great amount of precipitation is added in the following mineralization stages, thus the ore-forming fluids evolved into meteoric water in the later oreforming stage. The boiling events took place in stage Ⅲ, resulting the precipitations of ore-forming elements and forming the ore-bodies. Therefore, it is believed that the boiling is severed as the main mechanism for the metallic sedimentation of this mineral deposit.

Key words: fluid inclusion, H-O isotope, meteoric water, ore-forming fluid, Tibet