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The Baituyingzi Mo (Cu) deposit is a newly identified porphyry deposit located in the southern part of the Xilamulun Mo metallogenic belt, Northeast China. Mo (Cu) mineralization mainly occurs as quartz veinlets and stockwork veinlets in the potassic altered Baituyingzi monzogranite porphyry. A detailed investigation of fluid inclusions and O H isotopes from hydrothermal veins at Baituyingzi allow us to establish the evolving history of the fluid system, as well as understand factors controlling Cu Mo precipitation in CO 2 -rich fluids. Six vein types are recognized at Baituyingzi, which belong to five hydrothermal stages: (1) UST quartz vein, formed during magmatic-hydrothermal transitional stage; (2) A 1 vein (sulfide-barren quartz vein) and A 2 vein (quartz–pyrite–chalcopyrite ± molybdenite vein, Cu mineralization), related to potassic alteration; (3) B vein (quartz–molybdenite ± chalcopyrite ± pyrite vein, Mo mineralization), formed during the potassic-sericitic transitional stage; (4) D vein (quartz–pyrite ± molybdenite ± chalcopyrite vein), associated with sericitic alteration; and (5) late stage L vein (quartz–carbonate vein). Four types of fluid inclusions have been observed in these veins, i.e., liquid-rich inclusions (L-type), vapor-rich inclusions (V-type), polyphase high-salinity inclusions (S-type), and CO 2 -rich inclusions (C-type). UST quartz veins contain abundant S- and co-existing V-type inclusions, accompanied by minor melt inclusions, indicating that the parent fluids were likely in the form of bubbly magma (~ 10 wt% NaCl equiv.) that was derived from a deep magma chamber. A 1 veins are dominated by coexisting S- and V-type inclusions that homogenized at temperatures ranging from 400° to 548.5 °C and pressures between 300 and 750 bar, which may represent the result of parent fluid immiscibility (vapor-brine) at depth. Apart from S- and V-type inclusions, Cu-bearing A 2 veins are especially rich in isolated L-type inclusions (homogenized at 347 to 430 °C) that share the same salinity range as V-type inclusions (homogenized at 375 to 470 °C), suggesting that the vapor gradually contracted to a liquid phase at around 350 bar. Because the system was still in a lithostatic condition (revealed by textures of A 2 veins), Cu precipitation at Baituyingzi was thus largely controlled by temperature reduction (to below 400 °C) near the vapor-liquid surface in relatively oxidized fluids (indicated by the precipitation of abundant hematite). C-type inclusions (homogenize at 275 to 350 °C) seem to be more widely developed, and they commonly co-exist with L-type inclusions (homogenized at 340 to 380 °C) of similar salinity in B veins, implying that they unmixed at temperatures below 350 °C and pressures below 200 bar. Considering that the system had already fluctuated to hydrostatic conditions (revealed by clear open-space filling textures), as well as considering the occurrence of minor feldspar-destructive alteration halos in B veins, Mo precipitation would likely be a result of water-CO 2 immiscibility at temperatures below 350 °C with a sharp pressure reduction to below 200 bar and an increase in fluid acidity. D veins contain plentiful L- and minor C– and S-type inclusions that homogenized at lower temperatures (240 to 350 °C) and pressures (30 to 150 bar). The wide feldspar-destructive alteration halos indicate that the fluids became progressively more acidic and reacted intensely with wall-rocks. L veins only contain low temperature (168 to 280 °C) and low salinity (0.4 to 10.9 wt% NaCl equiv.) L-type inclusions, implying that meteoric water was introduced to the barren magmatic fluids, leading to precipitation of a large amount of remnant material. δ 18 O and δD isotope ratios in quartz (δ 18 O fluid = 3.54‰ to 7.51‰, δD = − 107‰ to − 87‰) also confirm a primary magmatic origin for ore-forming fluids in earlier veins, except for L veins. Therefore, fluid evolution at Baituyingzi was characterized by relatively continuous, system-scale cooling along an ascent path gradually straddling the vapor-brine, vapor-liquid, and water–CO 2 surface in the vapor-dominated H 2 O CO 2 NaCl fluid system, whereas the subtle temporal change in P-T-redox conditions and acid balance of the evolutionary paths of the magmatic fluid may be responsible for Cu and Mo separation at the Baituyingzi CO 2 –rich systems. Additionally, water-CO 2 immiscibility seems to be an additional important mechanism that complicates Mo precipitation in CO 2 –rich systems. [ABSTRACT FROM AUTHOR]