Small porphyry-type molybdenum (Mo) mineralization, the Geumeum deposit in the Gyeongsang Basin, South Korea, is associated with the crystallization of a Cretaceous granodiorite, exsolution of magmatic hydrothermal fluids, and related hydrofracturing. Quartz and molybdenite occur with minor amounts of uneconomic chalcopyrite, pyrite, sphalerite, and galena that precipitated from exsolved magmatic fluids and formed hydrothermal fissure-filling vein ores. Three distinct fluid inclusion assemblages responsible for the precipitation of molybdenite are present in the vein quartz. The earliest fluid is represented by low-salinity liquid-rich Type I fluid inclusions, which displayed homogenization temperatures ranging from 298 to 352 °C, and salinities from ∼0 to 9 wt.% NaCl equiv. The Type I inclusions were trapped in a liquid-stable, vapor-absent, one-phase field. The intermediate fluid is represented by CO2-bearing vapor-rich Type IV inclusions, generally showing no visible CO2 liquid phase. These CO2-bearing inclusions totally homogenized by vapor disappearance at 327∼340 °C or vapor bubble expansion at 327∼369 °C, exhibiting near-critical behaviors. The textural relationships and phase equilibria constraints indicate that the Type IV inclusions were derived from a single population, which formed as pressure and temperature fluctuated at near-critical conditions. The latest fluid can be represented by vapor-rich Type II fluid inclusions. No microthermometric data were obtained for these latest assemblages as no visible amounts of liquid phases are evident in small inclusions. Type III inclusions, which commonly refer to halite-bearing inclusions, are absent in the quartz veins studied. The oxygen and hydrogen isotopic fluid compositions of the vein quartz (δ18OSMOW = 4.3 to 6.9‰ and δDSMOW = −65 to −84‰ at 400 °C) is consistent with a magmatic origin with a possible slight influence from meteoric water. Molybdenum mineralization at Geumeum is a product of hypogene hydrothermal processes that were strongly fracture-controlled, highlighting the importance of low-salinity liquid-rich to vapor-like supercritical fluids for the mineralization. It seems likely that the magmas responsible for the formation of the deposit at Geumeum were emplaced at greater depths than those reported for economic porphyry copper deposits in the world. The deposit could thus have survived long periods of erosion, and represent the weakly mineralized “base” of porphyry systems in the Gyeongsang Basin, South Korea.