| The Bilihe gold deposit in Inner Mongolia is located in the Bainaimiao arc within the Wenduermiao-Bainaimiao subduction-accretion belt along the northern margin of the North China Craton. In order to deeply explore the ore formation process of this deposit, the evolutionary characteristics of the ore-forming magma, the oxygen fugacity conditions, and the mechanisms of gold enrichment, this paper conducted petrographic, mineralogical, electron probe, whole-rock geochemical, and melt inclusion studies on 19 samples from Drill Hole No.7 of the ore belt II of the Bilihe Gold deposit. The results indicate that the granite porphyry of the deposit has SiO<sub>2</sub> contents ranging from 60.35% to 91.74%, belonging to a high-K peraluminous calc-alkaline series. Trace element characteristics show enrichment of light rare earth elements and relative depletion of heavy rare earth elements, indicating that the ore-forming magma was generated in a tectonic setting transitioning from subduction influence to post-collision extensional regimes. The variation in plagioclase An values (21.65–54.4) and the presence of zoning structures suggest that the magma underwent fractional crystallization and experienced multi-stage magma replenishment and mixing. Apatite exhibits typical features of magmatic apatite, with generally high F contents (average ~3.15%). Based on △FMQ calculations, combined with laser Raman spectroscopy evidence of magnetite crystals in melt inclusions, it is shown that the ore-forming magma maintained relatively high oxygen fugacity conditions during the crystallization evolution stage. High oxygen fugacity conditions suppressed early sulfide saturation and precipitation, thereby reducing the likelihood of gold being captured by sulfide phases during early magma crystallization. With magma differentiation and volatile enrichment, Cl-rich fluids began to exsolve and gradually dominate the system evolution, facilitating gold migration in the form of chloride complexes, and resulting in precipitation and mineralization during the magma-hydrothermal to early hydrothermal transition stage. The study suggests that the formation of the Bilihe gold deposit is controlled by a high-oxygen-fugacity acidic magma system influenced by crust-mantle interactions, and the magma-hydrothermal transition is the critical stage for gold enrichment. |