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 HO isotopes data on Qulong, the most giant deposit in GPCB. The results indicate:①Fluid that induced Ksilicate alteration and CuMo mineralization at Qulong, are not dominated by generallyaccepted highsalinity 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℃). ②Earlystage alteration and mineralization in the deposit should occur at a relatively high pressure condition (105±15~90±20 MPa) that the vaporlike 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 kmthick rock formation above the deposit were denudated after formation of the deposit (~16 Ma). ③Greater density contrast between moderatesalinity vaporlike fluids and melt would have led to more continuous separation of the fluid from magma chamber during exsolution, resulting in pervasive but poor Ksilicate alteration, largescale but lowgrade CuMo 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 vaporlike 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 lowdensity vaporlike fluids, and show very different characteristics of alteration and mineralization with the PCDs formed by highsalinity brine.