Vivisection and autopsy of active and fossil hydrothermal alterations of basalt beneath and within the TAG hydrothermal mound

Abstract

The TAG (Trans-Atlantic Geotraverse) active hydrothermal mound is 30-40 m thick and consists 4 to 5 X 10 6 tons of pyritequartz-, and anhydrite-rich breccias. A vertically and laterally zoned stockwork, approximately 100 m in diameter, extends at least 125 m below the mound surface. Petrographic and mineralogic studies of core drilled during ODP Leg 158 were used to reconstruct the sequence of alteration processes of crustal rocks beneath and within the mound. The first stage of hydrothermal alteration is the chloritization of the basaltic basement beneath the mound as a result of interaction with Mg-bearing hydrothermal solutions, which are different from the zero Mg fluids presently discharged by the Black Smoker Complex, near the center of the TAG mound. In the deepest part of the stockwork underlying the central part of the mound, the basalt is almost completely chloritized (with minor quartz and pyrite) and Cr-spinel is the only primary mineral left. Beneath the mound margins, the chloritization process is restricted to millimeter- to centimeter-thick halos parallel to the exposed surfaces of basalt fragments.