—We studied fluid inclusions, trace elements, and oxygen, carbon, and sulfur isotope ratios in minerals from stockwork sulfide–carbonate–quartz ores of the Nikolaevskoe gold deposit confined to volcanic island arc porphyry intrusions. The study shows that fluid inclusions in quartz were homogenized at 260–200 ºС and those in later formed calcite, at 227–205 ºС. The fluids contain aqueous K– Mg–Na chloride solutions with salinity of 4.1–9.6 wt.% NaCl eq. Raman spectroscopy revealed СO2 (29–34 mol.%), CH4 (40–55 mol.%), and N2 (8–30 mol.%) in the fluids. According to LA-ICP-MS data, quartz has low contents of Al (11.7–102 ppm) and Ti (0.05–0.64 ppm), which indicates its deposition from weakly acid low-alumina fluids at <350 ºС. The REE patterns of calcite show accumulation of heavy lanthanides (LaN/YbN = 0.2–0.9), evident of high fluid alkalinity, and negative Ce (0.39–0.82) and positive Eu (1.99–5.25) anomalies. The negative Ce anomalies are due to meteoric water in the fluid and the fluid interaction with limestones. The positive Eu anomalies reflect the high-temperature (>250 ºC) environment that existed before the calcite crystallization. The Y/Ho ratio in calcite (28.7–54.1) suggests that the fluid has magmatic components and components extracted from limestones and contains seawater. The δ18 values (3.6–5.6‰) of the fluid testify to the participation of magmatic and meteoric waters in the ore formation. The δ13 values (–4.1 to 1.4‰) point to carbon of magmatic nature and carbon extracted from limestones. The heavy sulfur isotope composition of pyrite (6.75 to 9.87‰) and arsenopyrite (8.7‰) might be due to sulfur supply from the host rocks or to the participation of [SO4]2- of seawater in the ore-forming process. According to the results obtained, the Nikolaevskoe gold deposit is an island arc (Au ± Cu)–quartz–sulfide deposit transitional between porphyry and epithermal types.