LEG 158

TAG (Trans-Atlantic Geotraverse) Hydrothermal Mound

The TAG mound, located at the base of the eastern wall of the MAR, exhibits two distinct flat platforms, representing two phases of active growth. During Leg 158, 17 holes were drilled in five areas of the mound including a high-temperature (363 deg C) black-smoker complex, characterized by chalcopyrite and anhydrite deposits, and a lower-temperature (260-300 deg C), sphalerite-dominated white smoker vent field. Fluids from the white smoker are possibly derived from the black smoker fluids by a combination of conductive cooling and mixing with seawater and precipitation of sulfides within the mound. Drilling revealed that upflow beneath the mound occurs as a complex network of channelways through a permeable sequence of pre-existing breccias. Repeated episodes of cementation and replacement are responsible for the complex stratigraphy, present surface morphology, and distribution of vents. Sulfide accumulation appears to be more a process of hydrothermal replacement in the upflow zone rather than direct precipitation on the seafloor.

The upper 10-20 m of the mound consists of massive pyrite and pyrite breccias underlain by pyrite- anhydrite breccias. With increasing depth, quartz-pyrite mineralization and quartz veining in the pyrite breccias becomes dominant (representing the top of the quartz-sulfide stockwork zone), grading into silicified wallrock breccias below 40 mbsf. Chloritized basalt breccias were sampled at depths greater than 100 mbsf. The complex assemblage of breccias in the mound may be the products of multiple episodes of mass wasting, cementation, hydrothermal reworking, and replacement during growth of the mound. Sulfide breccias now at the base of the mound likely formed at the seafloor and were since buried and overprinted by later hydrothermal events; some possibly accumulated through the collapse of large sulfide chimneys and by dissection of pre- existing massive sulfides along active fault scarps.

High-temperature discharge at the top of the mound led to precipitation of massive anhydrite within the black smoker complex. Anhydrite beneath the black smoker complex occurs as thick veins cementing the sulfide breccias of the mound. The presence of these veins at 40 mbsf indicates a large influx of seawater to the top of the stockwork zone. Within the central part of the stockwork, quartz veining is confined to a narrow zone of high-temperature upflow where no significant seawater mixing has occurred. Relatively fresh basalt near the edge of the mound constrains the extent of the stockwork mineralization and intense alteration to an 80-m-diameter, pipe-like feature.