diffuse fluids are spewed out from the central orifices (\3–4 cm in diameter) and
through the surrounding basal material made up of thin slabs of Fe-oxyhydroxides.
In addition to the chimney fields, other flat lying and layered, partially indurate
hydrothermal material have completely covered the lava flow and the cinders on a
rift zone at 2800 m depths on the Teahitia volcano. These deposits are composed
of a few centimeter-thick, layered or loosely packed volcaniclastic debris and/or
volcanic ash, which alternate with thin layers of purple-red Fe-oxyhydroxides. The
hyaloclastites are encountered in the vicinity of the loosely packed, flat lying
volcaniclastic-bearing deposits at about 1600 m depths near the summit of the
Teahitia volcano.
More scattered Fe-oxyhydroxide powdery material coating the top of pillow
lava at a depth of about 200 m occurs on the Moua-Pihaa volcano. The diffusion of
water through rocks and between pillow interstices is associated with living colonies of small coral. Ochreous Fe-rich semi-indurate crusts were also recovered at
two dredge stations (stations DR28 and DR29) (one on the summit of Moua-Pihaa
at less than 200 m depth, and the other one on the northern flank at 2000 m depth).
These Fe-rich crusts have the same composition as the flat-lying, layered deposits
described on the summit of the Teahitia volcano.
The lack of sulfide (metallic) deposits on all the intraplate hotspot volcanic
edifices that were explored is puzzling. The fluids from the chimneys that were
collected and analyzed indicate that that they are deprived in transitional metals,
except for iron and manganese, and they are less acid with a lower pH = 5–6
when compared to spreading ridge sulfide deposits. Nevertheless, the hydrothermal
fluids from hotspot edifices are enriched in iron, manganese, silica, lithium, barium
and Rare Earth elements when compared to seawater. The discharge of such a type
of fluid gives rise to iron, silica and manganese hydroxide. These products are
formed in a more oxygenated environment with respect to the sulfide deposits
found on spreading ridges. Assuming that the process of rock alteration is the same
on hotspot volcanoes and in the spreading ridge system, then the hydrothermal
fluids formed in intraplate regions have undergone chemical changes prior to their
exit on the seafloor. These fluids have been mixed with seawater, within the
volcanic edifice, during their ascent. This will lower their temperature and oxygenate their systems. The metallic sulfides will either precipitate within the interior
of the edifice, or form oxides.
The fact that high temperature ([250 °C) polymetallic sulfides precipitate
within the conduits of the edifices is evidenced by the presence of sulfide veins and
veinlets forming pyrite, chalcopyrite, and sphalerite, all of which are associated
with hydrated low temperature (\60 °C) products. These veins and veinlets are
exposed from underneath the hydrothermal deposits during tectonic fracturing, or
are found within the fragmented precipitates of magmato-phreatic explosive
eruptions. Thus, polymetallic sulfides are indeed present in intraplate hotspot
volcanoes, but they are just not visible on the surface. Instead, they seem to occur
in the interior of the volcanic edifices, in the form of veins and veinlets, also called
‘‘stockwork formations’’ in the mining industry.
Hydrothermal Activity of Intraplate Hotspot Volcanoes
341
through the surrounding basal material made up of thin slabs of Fe-oxyhydroxides.
In addition to the chimney fields, other flat lying and layered, partially indurate
hydrothermal material have completely covered the lava flow and the cinders on a
rift zone at 2800 m depths on the Teahitia volcano. These deposits are composed
of a few centimeter-thick, layered or loosely packed volcaniclastic debris and/or
volcanic ash, which alternate with thin layers of purple-red Fe-oxyhydroxides. The
hyaloclastites are encountered in the vicinity of the loosely packed, flat lying
volcaniclastic-bearing deposits at about 1600 m depths near the summit of the
Teahitia volcano.
More scattered Fe-oxyhydroxide powdery material coating the top of pillow
lava at a depth of about 200 m occurs on the Moua-Pihaa volcano. The diffusion of
water through rocks and between pillow interstices is associated with living colonies of small coral. Ochreous Fe-rich semi-indurate crusts were also recovered at
two dredge stations (stations DR28 and DR29) (one on the summit of Moua-Pihaa
at less than 200 m depth, and the other one on the northern flank at 2000 m depth).
These Fe-rich crusts have the same composition as the flat-lying, layered deposits
described on the summit of the Teahitia volcano.
The lack of sulfide (metallic) deposits on all the intraplate hotspot volcanic
edifices that were explored is puzzling. The fluids from the chimneys that were
collected and analyzed indicate that that they are deprived in transitional metals,
except for iron and manganese, and they are less acid with a lower pH = 5–6
when compared to spreading ridge sulfide deposits. Nevertheless, the hydrothermal
fluids from hotspot edifices are enriched in iron, manganese, silica, lithium, barium
and Rare Earth elements when compared to seawater. The discharge of such a type
of fluid gives rise to iron, silica and manganese hydroxide. These products are
formed in a more oxygenated environment with respect to the sulfide deposits
found on spreading ridges. Assuming that the process of rock alteration is the same
on hotspot volcanoes and in the spreading ridge system, then the hydrothermal
fluids formed in intraplate regions have undergone chemical changes prior to their
exit on the seafloor. These fluids have been mixed with seawater, within the
volcanic edifice, during their ascent. This will lower their temperature and oxygenate their systems. The metallic sulfides will either precipitate within the interior
of the edifice, or form oxides.
The fact that high temperature ([250 °C) polymetallic sulfides precipitate
within the conduits of the edifices is evidenced by the presence of sulfide veins and
veinlets forming pyrite, chalcopyrite, and sphalerite, all of which are associated
with hydrated low temperature (\60 °C) products. These veins and veinlets are
exposed from underneath the hydrothermal deposits during tectonic fracturing, or
are found within the fragmented precipitates of magmato-phreatic explosive
eruptions. Thus, polymetallic sulfides are indeed present in intraplate hotspot
volcanoes, but they are just not visible on the surface. Instead, they seem to occur
in the interior of the volcanic edifices, in the form of veins and veinlets, also called
‘‘stockwork formations’’ in the mining industry.
Hydrothermal Activity of Intraplate Hotspot Volcanoes
341