Coexisting vapor-rich and high-salinity “magmatic” fluid inclusions from mineralized quartz ± carbonate veins in porphyry Cu systems of the Babine Lake area are depleted in deuterium (D) relative to accepted magmatic isotopic values. The δD compositions of high-salinity fluids from early mineralized veins range from −100‰ to −135‰ (standard mean ocean water), whereas fluid inclusions in late veins and breccias have δD values between −138‰ to −153‰, and unmineralized, regional veins have δD values from −94‰ to −150‰. Multiple δD analyses of fluids from inclusions are within ± 5‰ for 14 of 17 duplicate analyses, which suggests that mixing of fluid-inclusion populations did not contribute significantly to the D-depleted values. A correlation between δ13C values of vein carbonate and δD values of inclusion fluids indicates that postentrapment reequilibration did not significantly alter isotopic compositions of fluid inclusions. Incorporation of externally derived low-D material into the mineralizing intrusions, either from the influx into the melt of deeply convected, evolved meteoric fluids or by crustal assimilation of D-depleted country rock, is required to explain the observed D-depleted values of the magmatic fluids.