This study presents a new occurrence of hydrothermal argentopentlandite, associated with pyrrhotite, chalcopyrite, sphalerite, and galena in albite (± calcite, muscovite, pyrochlore-group minerals, high-Th phases) veins cutting metagabbro of the Sedova Zaimka intrusion, Western Siberia, Russia. The data obtained from petrography, mineral chemistry and fluid inclusion study of the vein-forming minerals do confirm the proposed hydrothermal origin of sulfides and associated gangue minerals from the internally-derived metamorphic fluids generated during potential post-peak contact metamorphic cooling. These ore-forming fluids were Na-dominant and enriched in Si, Al, Fe, Cu, with minor Ca, K, Ta, Nb, U, Th, Zn, Pb, Ni, Ag, Co, Bi, Te, Se, and Cd. They were characterized by moderate temperatures (270–325 °C) and moderate salinities (4.70–7.25 wt% NaCl equiv.) and belong to the NaCl-H2O fluid system with dominant CH4 gas-phase species. It is concluded that the components (metals and other elements) for the vein infill have been transferred from metagabbro by the metamorphic fluid, possibly in a dissolution–transport–precipitation process.