S. Petersen et al., Third dimension of a presently forming VMS deposit: TAG hydrothermal mound, Mid-Atlantic Ridge, 26 degrees N, MIN DEPOSIT, 35(2-3), 2000, pp. 233-259
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.