METEORIC WATER COMPONENT IN MAGMATIC FLUIDS FROM PORPHYRY COPPER MINERALIZATION, BABINE LAKE AREA, BRITISH-COLUMBIA

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 accepte d magmatic isotopic values. The delta D compositions of high-salinity fluids from early mineralized veins range from -100 parts per thousand to -135 parts per thousand (standard mean ocean water), whereas fluid inclusions in late veins and breccias have delta D values between -13 8 parts per thousand to -153 parts per thousand, and unmineralized, re gional veins have delta D values from -94 parts per thousand to -150 p arts per thousand. Multiple 6D analyses of fluids from inclusions are within +/-5 parts per thousand 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 delta(1 3)C values of vein carbonate and delta D values of inclusion fluids in dicates that postentrapment reequilibration did not significantly alte r isotopic compositions of fluid inclusions. Incorporation of external ly derived low-D material into the mineralizing intrusions, either fro m the influx into the melt of deeply convected, evolved meteoric fluid s or by crustal assimilation of D-depleted country rock, is required t o explain the observed D-depleted values of the magmatic fluids.