Abstract:Porphyry Cu deposits are usually thought to form in magmatic arc settings, although some porphyry deposits also occur in collisional orogen settings, such as Gangdese Porphyry Cu Belt (GPCB) in south Tibet. Genesis of these deposits remain poorly understood. Here we report detailed fluid inclusions and HO isotopes data on Qulong, the most giant deposit in GPCB. The results indicate:①Fluid that induced Ksilicate alteration and CuMo mineralization at Qulong, are not dominated by generallyaccepted highsalinity brine, whereas they are prevailed by magmatic vapor exsolving from magma chamber with near critical density, moderate salinity (~9% NaCl) and high temperature (550~650℃). ②Earlystage alteration and mineralization in the deposit should occur at a relatively high pressure condition (105±15~90±20 MPa) that the vaporlike fluids trapped at that condition were in the one phase field, which correspond to a paleodepths between 4.2±0.6 km and 3.6±0.8 km based on lithostatic pressure. So at least 3~3.5 kmthick rock formation above the deposit were denudated after formation of the deposit (~16 Ma). ③Greater density contrast between moderatesalinity vaporlike fluids and melt would have led to more continuous separation of the fluid from magma chamber during exsolution, resulting in pervasive but poor Ksilicate alteration, largescale but lowgrade CuMo mineralization, and the less direct relationships between individual porphyry intrusions and mineralization at Qulong. ④Cu (Mo) precipitation in the deposit are induced by decrease of pressure and reduce of S in the vaporlike fluids, and are not caused by decrease of temperature, which is the generally accepted mechanism for metal precipitation in porphyry Cu system. In summary, Qulong is a kind of porphyry Cu deposit (PCD) formed by lowdensity vaporlike fluids, and show very different characteristics of alteration and mineralization with the PCDs formed by highsalinity brine.