The Yaochong porphyry Mo deposit in Xinxian County,Henan Province,China,is located in the Dabie orogenic belt.Mo mineralization formed in continental collision settings.The hydrothermal ore-forming process can be divided into the early,middle and late stages.The three stages are defined by the quartz-K-feldspar veins with minor mineralization,the quartz-molybdenum-other sulfides veins or stockworks and the barren quartz veins or quartz-calcite veinlets or stockworks,respectively.There are four fluid compositional types determined in hydrothermal quartz from early and middle stages: Pure CO2,CO2-H2O,aqueous and solid-bearing fluid inclusions (FIs).The early-stage quartz contains three of the four types FIs other than the aqueous type.The middle-stage quartz has CO2-H2O,aqueous and solid-bearing FIs,without the pure CO2 FIs.In the late stage,only the aqueous FIs can be observed.The primary fluid inclusions in the early-stage quartz are homogenized at temperatures of 277-380 ℃,clustering at 300-350℃,with salinities mainly ranging from 4.3 to 10.3 wt% NaCl equiv.By contrast,primary fluid inclusions in the middle stage quartz mainly yield homogenization temperatures of 172 to 350 ℃,with most between 260 and 320℃,and salinities of 1.6 to 9.3 wt% NaCl equiv.The estimated pressures decrease from a span of 36-142 MPa in the early stage,to a range of 21-124 MPa in the middle stage,corresponding to a change in depths from 3.6-5.1 km to 2.1-4.4 km.We conclude that the initial ore-forming fluid system of Yaochong is CO2-rich but NaCl-poor,probably representing a new type mineralization and predict that the deeper exploration is potential for ore prospecting.