Hydrothermal alteration and oxygen and hydrogen isotope geochemistry of the D-68 zone Cu-Zn Massive Sulfide Deposit, Noranda District, Quebec, Canada

Abstract

Pervasive hydrothermal alteration zones in quartz-feldspar porphyry domes underly all massive sulfide lenses in the D-68 Zone Cu-Zn deposit, Noranda. Alteration pipes are mineralogically zoned and contain chloritic cores consisting of stringer sulfides, enveloped by sericitic haloes. Silicified rocks are found locally. Alteration took place at nearly constant volume. Na depletion, and K enrichment relative to the least altered rocks, are found in all alteration zones. Fe and Mg have been added to the chloritic zone and subtracted in the sericitic and silicic zones. Ca and Si are enriched mainly in the silicic zone. Al, Ti and Zr were the least mobile of the elements studied. Whole-rock δ18O values vary from +5.6 to +6.2 per mil in chloritized rocks, +5.8 to + 7.3 per mil in sericitized rocks and + 7.2 to + 8.3 per mil in silicified rocks. δD values for two chloritized samples are − 63 and − 70 per mil whereas in two sericitized samples they are close to −62 per mil. Quartz from the chlorite alteration zone is isotopically heavier (δ18O = 8.6 per mil) than that from the sericite alteration zone (δ18O = 6.4 per mil), suggesting equilibration with different hydrothermal fluid or different temperature of alteration. Assuming an alteration temperature of 300° + 50°C the fluid in equilibrium with quartz and chlorite had δ18O and δD values of about 1.5 ± 2.0 per mil and −23 ± 5 per mil, respectively. The fluid in equilibrium with quartz and sericite had δ18O and δD values of about −0.5 ± 2 per mil and −30 ± 5 per mil, respectively. On the basis of isotopic data, seawater was probably the major constituent of the hydrothermal fluids.