The Qiyugou gold orefield is situated in the Eastern Qinling. It is a magmatic hydrothermal system, which includes cryptoexplosive breccia-type, vein-type, and porphyry gold deposits. However, the gold sources and physical–chemical conditions of precipitation of the newly discovered porphyry gold deposit, the Qi189, have not yet been well studied. It also hinders the comprehensive understanding of magmatic–hydrothermal gold systems in the Qiyugou gold orefield. Thus, detailed field and microscopic observations, C–O–He–Ar isotopic analysis, in situ major/trace element analysis, and thermodynamic studies on the Qi189 porphyry gold deposit have been carried out in this study. This deposit has three hydrothermal stages: the (I) early quartz–K-feldspar–molybdenite–magnetite stage, (II) middle quartz–sericite–pyrite–Te–Bi-mineral stage, as well as (III) late calcite–quartz stage. The carbon and He–Ar isotopic compositions suggest deep-seated sources of ore-forming fluids with mantle component proportions of up to 37 %. Oxygen isotopic compositions indicate the gradual involvement of more meteoric water in ore-forming fluids during stages I–III. The meteoric water input and reduction in temperature and pressure in the early stage II increased oxygen fugacity and decreased the pH values of ore-forming fluids. Then, in hematite and sulfides formation during late stage II, the oxygen and sulfur fugacities of ore-forming fluids gradually decreased. Precipitation of native gold and Au-bearing telluride grains with quartz-sericite alteration in stage II is significant for Au enrichment, which occurred under acidic (pH = 3.64–5.71), low-logfO2 (<-34.86), low-logfS2 (−13.07 to −11.13) and low-logfTe2 (−15.55 to −10.32) conditions. Therefore, abundant mantle components and changes in the pH, logfO2, logfS2, and logfTe2 of the ore-forming fluids play important roles in gold enrichment. Combined with previous studies, this work also provides some new evidence for identifying a magmatic-hydrothermal gold system in this district, including similar sulfide trace elements, δ13CV-PDB compositions, and changes in logfO2 and logfS2 of various types of gold deposits in a gold orefield.