Third dimension of a presently forming VMS deposit: TAG hydrothermal mound, Mid-Atlantic Ridge, 26 degrees N

ODP drilling of the active TAG hydrothermal mound at 26 degrees N on the Mi d-Atlantic Ridge provided the first insights into the third dimension of a volcanic-hosted massive sulfide (VMS) deposit on a sediment-free mid-ocean ridge. Sulfide precipitation at this site started at least 20.000 years ago and resulted in the formation of a distinctly circular, 200-m diameter, 50 -m-high pyritic mound and a silicified stockwork complex containing approxi mately 3.9 million tonnes of sulfide-bearing material with an average of 2. 1 wt% Cu and 0.6 wt% Zn in 95 samples collected from 1-125 m below the seaf loor. The periodic release of high-temperature hydrothermal fluids at the s ame location for several thousand years with intermittent periods of hydrot hermal quiesence is the dominating process in the formation of the TAG hydr othermal mound. Distinct geochemical, mineralogical and isotopic zonation a s well as a complex assemblage of sulfide-anhydrite-quartz bearing breccias can be related to this process. Geochemical depth profiles indicate extrem ely low base and trace element concentrations for the interior of the mound , which clearly contrasts with published analyses of samples collected from the surface of the TAG mound. This is explained by continued zone refining during which metals were mobilized from the interior of the mound by upwel ling, hot (> 350 degrees C) hydrothermal fluids. Mixing of these fluids wit h infiltrating ambient seawater subsequently caused redeposition of metals close to the mound-seawater interface. The sulfur isotopic composition of b ulk sulfides (+4.4 to +8.2 parts per thousand, delta(34)S; average + 6.5 pa rts per thousand) is unusually heavy when compared to other sediment-free m id-ocean ridge deposits and implies the introduction of heavy seawater sulf ur to the hydrothermal fluid. The slight increase in sulfur isotope ratios with depth and distinct variations between early, disseminated sulfides rel ated to wallrock alteration, and massive as well as late vein sulfides indi cates widespread entrainment of seawater deep into the system. Fluid inclus ion measurements in quartz and anhydrite reveal high formation temperatures throughout the TAG mound (up to 390 degrees C) at one time with an overall increase in trapping temperatures with depth. Lower formation temperatures close to the paleo-seafloor indicate local entrainment of seawater into th e mound. Formation temperatures for a central anhydrite-bearing zone range from 340-360 degrees C and are slightly lower than the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360-369 degrees C). Fluid inclusions in quartz and anhydrite from the stockwork zon e are characterized by formation temperatures higher than 375 degrees C, in dicating conductive cooling of the hydrothermal fluids or mixing with ambie nt seawater prior to venting. Formation temperatures for quartz from an are a of extremely low heat flow at the western side of the mound reach up to 3 90 degrees C, implying that this area was once part of a high-temperature h ydrothermal upflow zone. The low heat flow and the absence of anhydrite wit hin this part of the mound are strong indications that the recent pulse of high-temperature hydrothermal activity is not affecting this area and provi des evidence for significant changes in the fluid flow regime underneath th e deposit between hydrothermal cycles.