The Kaiama gold deposit, located within the Proterozoic basement complex of northcentral Nigeria, is hosted predominantly in structurally controlled quartz-sulfide veins emplaced within mylonitized quartzite and talc schist units. The mineralization is spatially associated with NE–SW-trending ductile shear zones, interpreted as subsidiary structures of the regionally extensive Anka–Yauri fault system. While gold occurrences in Nigeria have been widely documented, the origin and evolution of the ore-forming fluids have continued to be a subject of debate. This study integrates fluid inclusion petrography, microthermometric analysis, and stable isotope (δ¹⁸O and δD) geochemistry to unravel the physicochemical conditions and fluid sources involved in the mineralization process. Detailed petrographic examination identifies a sulfide assemblage comprising pyrite, galena, chalcopyrite, chalcocite, and sphalerite, with quartz, sericite, and feldspar as the dominant gangue minerals. Fluid inclusion petrography revealed three distinct fluid types: Type I (carbonic aqueous three-phase fluids), Type II (vapour rich biphasic fluids), and Type III (liquid rich biphasic fluids), whose coexistence indicates fluid mixing as a key ore-forming mechanism. Microthermometric measurements yielded homogenization temperatures ranging from 169 °C to 339 °C and salinities between 0.4 and 15.3 wt% NaCl equivalent, consistent with low-to moderate-temperature, moderately saline hydrothermal fluids. Stable isotope compositions of fluid inclusions (δ¹⁸O_water = +1.57 ‰ to +7.07 ‰; δD_water = −114 ‰ to −33 ‰) point to a mixed fluid source, involving both metamorphic and meteoric components. Collectively, the results suggest that structurally focused fluid flow and mixing of contrasting fluid sources played a pivotal role in the precipitation of gold at Kaiama.