minerals (Table 1). These minerals precipitate from
diverse processes, including: heating of sea water;
cooling of hydrothermal fluid; mixing between sea
water and hydrothermal fluid; reaction of hydrothermal minerals with fluid, sea water, or fluid–sea
water mixtures; reaction between hydrothermal fluid
and seafloor rocks and sediments; and reactions that
are mediated or catalyzed biologically. This diversity
in the processes and environments of mineral precipitation results in the deposition of many different
minerals and elements (Tables 1 and 2). High concentrations of strategic and precious metals are
found in some deposits (Table 2). The deposits
are potentially valuable, if economic and environmentally safe methods of mining them can be
developed.
Chimneys can be classified broadly by composition into four groups: sulfate-rich, copper-rich,
zinc-rich and silica-rich structures. Copper-rich
chimney compositions are indicative of formation at
temperatures above 3001C. Sulfate-rich compositions are characteristic of active and immature
chimneys. Many chimneys are mineralogically
zoned, with hot interior regions enriched in copper,
and cooler exterior zones enriched in iron, zinc, and
sulfate (Figures 1 and 2). Mounds exhibit a similar
gross mineral zoning, and those which are exposed
by erosion in ophiolites often have silicified (quartzrich) interiors (Figure 2). Seafloor weathering of
deposits after active venting ceases results in dissolution of anhydrite, and oxidation and dissolution
of metal-sulfide minerals. Small deposits that are not
sealed by silicification or buried by lava flows will
not be well preserved in the geologic record
(Figure 3).
Chimneys as Habitats
Chimney and mound surfaces are substrates populated by microbial colonies and sessile organisms
such as vestimentiferan and polychaete worms, limpets, mussels, and clams. It is likely that pore spaces
in exterior regions of chimney walls are also inhabited by microbes. All of these organisms that are
dependent on chemosynthesis benefit from the seepage of hydrothermal fluid through active mineral
structures, and from the thermal and chemical gradients across mineral structures. The structures provide an interface between sea water and
hydrothermal fluid that maintains tolerable temperatures for biota, and allows organisms simultaneous access to the chemical constituents in both
sea water and hydrothermal fluid. However, organisms attached to active mineral structures must cope
with changes in fluid flow across chimney walls
(which sometimes occur rapidly), and with ongoing
engulfment by mineral precipitation.
Some organisms actively participate in the precipitation of minerals; for example, sulfide-oxidizing
microbes mediate the crystallization of native sulfur
crystals, and microbes are also thought to participate
in the precipitation of marcasite and iron oxide
minerals. Additionally, the surfaces of organisms
provide favorable sites for nucleation and growth of
amorphous silica, metal sulfide and metal oxide
crystals, and this facilitates mineral precipitation and
fossilization of vent fauna (Figure 3).
Fossil Record of Hydrothermal Vent
Organisms
Fossil molds and casts of worm tubes, mollusc shells,
and microbial filaments have been identified in both
modern ridge hydrothermal deposits and in Cretaceous, Jurassic, Devonian, and Silurian deposits.
This fossil record establishes the antiquity of vent
communities and the long evolutionary history of
specific faunal groups. The singular Jurassic fossil
assemblage preserved in a small ophiolite-hosted
deposit in central California is particularly interesting because it contains fossils of vestimentiferan
worms, gastropods and brachiopods, but no clam or
mussel fossils. In contrast, modern and Paleozoic
faunal assemblages described thus far include clams,
mussels and gastropods, but no brachiopods. Does
this mean that brachiopods have competed with
molluscs for ecological niches at vents, and have
Figure 3 On left: a time series of seafloor photographs showing the morphological development of a chimney that grew on top of
lava flows erupted in 1991 on the crest of the East Pacific Rise near 9150.3
0 N (Haymon et al., 1993). Within a few days-to-weeks after
the eruption, anhydrite-rich ‘Stage 1 Protochimneys’ a few cm high had formed where hot fluids emerged from volcanic outcrops
covered with white microbial mats (top left). Eleven months later, the chimney consisted of cylindrical ‘Stage 2’ anhydrite-sulfide
mineral spires approximately one meter in height, and as-yet unpopulated by macrofauna (middle left). Three and a half years after the
eruption, the cylindrical conduits had coalesced into a 7 m-high chimneys structure that was covered with inhabited Alvinelline worms
tubes (bottom left). On right: photomicrographs of chimney samples from the eruption area that show how the chimneys evolved from
Stage 1 (anhydrite-dominated; top right) to Stage 2 (metal-sulfide dominated) mineral compositions (see text). As the fluids passing
through the chimneys cooled below B3301C during Stage 2, the CuFe-sulfide minerals in the chimney walls (middle right) were
replaced by Zn- and Fe-sulfide minerals (bottom right).
HYDROTHERMAL VENT DEPOSITS 379
diverse processes, including: heating of sea water;
cooling of hydrothermal fluid; mixing between sea
water and hydrothermal fluid; reaction of hydrothermal minerals with fluid, sea water, or fluid–sea
water mixtures; reaction between hydrothermal fluid
and seafloor rocks and sediments; and reactions that
are mediated or catalyzed biologically. This diversity
in the processes and environments of mineral precipitation results in the deposition of many different
minerals and elements (Tables 1 and 2). High concentrations of strategic and precious metals are
found in some deposits (Table 2). The deposits
are potentially valuable, if economic and environmentally safe methods of mining them can be
developed.
Chimneys can be classified broadly by composition into four groups: sulfate-rich, copper-rich,
zinc-rich and silica-rich structures. Copper-rich
chimney compositions are indicative of formation at
temperatures above 3001C. Sulfate-rich compositions are characteristic of active and immature
chimneys. Many chimneys are mineralogically
zoned, with hot interior regions enriched in copper,
and cooler exterior zones enriched in iron, zinc, and
sulfate (Figures 1 and 2). Mounds exhibit a similar
gross mineral zoning, and those which are exposed
by erosion in ophiolites often have silicified (quartzrich) interiors (Figure 2). Seafloor weathering of
deposits after active venting ceases results in dissolution of anhydrite, and oxidation and dissolution
of metal-sulfide minerals. Small deposits that are not
sealed by silicification or buried by lava flows will
not be well preserved in the geologic record
(Figure 3).
Chimneys as Habitats
Chimney and mound surfaces are substrates populated by microbial colonies and sessile organisms
such as vestimentiferan and polychaete worms, limpets, mussels, and clams. It is likely that pore spaces
in exterior regions of chimney walls are also inhabited by microbes. All of these organisms that are
dependent on chemosynthesis benefit from the seepage of hydrothermal fluid through active mineral
structures, and from the thermal and chemical gradients across mineral structures. The structures provide an interface between sea water and
hydrothermal fluid that maintains tolerable temperatures for biota, and allows organisms simultaneous access to the chemical constituents in both
sea water and hydrothermal fluid. However, organisms attached to active mineral structures must cope
with changes in fluid flow across chimney walls
(which sometimes occur rapidly), and with ongoing
engulfment by mineral precipitation.
Some organisms actively participate in the precipitation of minerals; for example, sulfide-oxidizing
microbes mediate the crystallization of native sulfur
crystals, and microbes are also thought to participate
in the precipitation of marcasite and iron oxide
minerals. Additionally, the surfaces of organisms
provide favorable sites for nucleation and growth of
amorphous silica, metal sulfide and metal oxide
crystals, and this facilitates mineral precipitation and
fossilization of vent fauna (Figure 3).
Fossil Record of Hydrothermal Vent
Organisms
Fossil molds and casts of worm tubes, mollusc shells,
and microbial filaments have been identified in both
modern ridge hydrothermal deposits and in Cretaceous, Jurassic, Devonian, and Silurian deposits.
This fossil record establishes the antiquity of vent
communities and the long evolutionary history of
specific faunal groups. The singular Jurassic fossil
assemblage preserved in a small ophiolite-hosted
deposit in central California is particularly interesting because it contains fossils of vestimentiferan
worms, gastropods and brachiopods, but no clam or
mussel fossils. In contrast, modern and Paleozoic
faunal assemblages described thus far include clams,
mussels and gastropods, but no brachiopods. Does
this mean that brachiopods have competed with
molluscs for ecological niches at vents, and have
Figure 3 On left: a time series of seafloor photographs showing the morphological development of a chimney that grew on top of
lava flows erupted in 1991 on the crest of the East Pacific Rise near 9150.3
0 N (Haymon et al., 1993). Within a few days-to-weeks after
the eruption, anhydrite-rich ‘Stage 1 Protochimneys’ a few cm high had formed where hot fluids emerged from volcanic outcrops
covered with white microbial mats (top left). Eleven months later, the chimney consisted of cylindrical ‘Stage 2’ anhydrite-sulfide
mineral spires approximately one meter in height, and as-yet unpopulated by macrofauna (middle left). Three and a half years after the
eruption, the cylindrical conduits had coalesced into a 7 m-high chimneys structure that was covered with inhabited Alvinelline worms
tubes (bottom left). On right: photomicrographs of chimney samples from the eruption area that show how the chimneys evolved from
Stage 1 (anhydrite-dominated; top right) to Stage 2 (metal-sulfide dominated) mineral compositions (see text). As the fluids passing
through the chimneys cooled below B3301C during Stage 2, the CuFe-sulfide minerals in the chimney walls (middle right) were
replaced by Zn- and Fe-sulfide minerals (bottom right).
HYDROTHERMAL VENT DEPOSITS 379