Meteoric water component in magmatic fluids from porphyry copper mineralization, Babine Lake area, British Columbia

Abstract

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 δ 13 C 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.