Silica-Rich Deposits
Very fragile and porous (50 % porosity) silica-rich deposits with a spongy
appearance are formed by strips of silica filaments, which are associated with
barite and Fe-oxyhydroxides. Silica-rich deposits have the appearance of colloidal
(amorphous, shapeless) material varying in color from milky white to light brown.
Other silica-rich deposits found in association with massive sulfides and sulfide
mounds show a similar texture, and contain more than 90 % SiO 2 . Opal is the
major silica-containing phase in the deposits, which probably have more than one
origin. For example, the silica-rich deposit found on the Southeastern Seamount
(East Pacific Rise, 13°N) contains remnants of pyrite, chalcopyrite, covellite,
digenite, chalcocite and idaite and was thought to have originated from the
replacement of sulfide minerals (Hekinian and Fouquet 1985), while other silicarich deposits composed of opaline products with rod-like filaments were attributed
to the growth of bacteria-like material (Juniper and Fouquet 1988).
Opal is also encountered as a replacement product of wurtzite associated with
the black smokers at 21°N on the EPR. Thus, another origin for the silica-rich
products is thought to be primary, low-temperature precipitation of hydrothermal
fluids directly on top of or within the interstices of volcanics or forming interlayered rims in the sulfide chimneys themselves. The chemistry of this type of
deposit indicates an enrichment of SiO 2 (81 %), of Fe 2 O 3 (5.7 %), traces of Na 2 O
(1–2 %), MgO (0.3 %) and H 2 O (ignition lost 10–11 %). It is also deprived of
most transitional metals, large ion lithophile (LIL) elements such as strontium
(Sr \ 60 ppm), and LREE (Light rare earth elements).
Hydrated silicates have been found to be forming in recent hydrothermal fields
on the ridge axis of the EPR at 21°N and 13°N. Low-temperature, shimmering
water escaping from small cracks and fissures encountered on pillow lava and
sheet flow terrain precipitates milky white and yellowish red coatings on the rock
surfaces. Usually the side of the rock exposed to the hydrothermal fluid will show
the altered crusts, which range in thickness from less than 1 mm up to 50 mm. The
white precipitates have an irregular contact with the surface of the glassy basalt
margin. The contact is marked by concentric lamellae a few microns thick made
up of various hydrated components such as kaolinite (clay), mixed-layer smectite,
serpentine, and boehmite. Other samples show an abundance of mixed-layer
chlorite-smectite associated with small amounts of kaolinite (Haymon and Kastner
1986). The exit temperature of the hydrothermal fluids is responsible for giving
rise to these alteration crusts and was measured to be less than 60 °C. The elements composing the crusts are amorphous Al- or Mg- silicates and chemical
analyses have shown that the MgO is 13–22 %, SiO 2 is 27–43 % and Al 2 O 3 is
18–25 % by weight. The minor element constituents are Ca (1–4 %), Na 2 O
(\ 0.5–2 %) and Fe 2 O 3 (1–5 %).
Fe-bearing nontronite is one of the most common hydrothermal precipitates
found on the sea floor. It is a light-green, yellowish-green soft and/or semi-consolidated aggregate sometimes associated with amorphous Fe-oxyhydroxides but
Types of Hydrothermal Deposits
157
Very fragile and porous (50 % porosity) silica-rich deposits with a spongy
appearance are formed by strips of silica filaments, which are associated with
barite and Fe-oxyhydroxides. Silica-rich deposits have the appearance of colloidal
(amorphous, shapeless) material varying in color from milky white to light brown.
Other silica-rich deposits found in association with massive sulfides and sulfide
mounds show a similar texture, and contain more than 90 % SiO 2 . Opal is the
major silica-containing phase in the deposits, which probably have more than one
origin. For example, the silica-rich deposit found on the Southeastern Seamount
(East Pacific Rise, 13°N) contains remnants of pyrite, chalcopyrite, covellite,
digenite, chalcocite and idaite and was thought to have originated from the
replacement of sulfide minerals (Hekinian and Fouquet 1985), while other silicarich deposits composed of opaline products with rod-like filaments were attributed
to the growth of bacteria-like material (Juniper and Fouquet 1988).
Opal is also encountered as a replacement product of wurtzite associated with
the black smokers at 21°N on the EPR. Thus, another origin for the silica-rich
products is thought to be primary, low-temperature precipitation of hydrothermal
fluids directly on top of or within the interstices of volcanics or forming interlayered rims in the sulfide chimneys themselves. The chemistry of this type of
deposit indicates an enrichment of SiO 2 (81 %), of Fe 2 O 3 (5.7 %), traces of Na 2 O
(1–2 %), MgO (0.3 %) and H 2 O (ignition lost 10–11 %). It is also deprived of
most transitional metals, large ion lithophile (LIL) elements such as strontium
(Sr \ 60 ppm), and LREE (Light rare earth elements).
Hydrated silicates have been found to be forming in recent hydrothermal fields
on the ridge axis of the EPR at 21°N and 13°N. Low-temperature, shimmering
water escaping from small cracks and fissures encountered on pillow lava and
sheet flow terrain precipitates milky white and yellowish red coatings on the rock
surfaces. Usually the side of the rock exposed to the hydrothermal fluid will show
the altered crusts, which range in thickness from less than 1 mm up to 50 mm. The
white precipitates have an irregular contact with the surface of the glassy basalt
margin. The contact is marked by concentric lamellae a few microns thick made
up of various hydrated components such as kaolinite (clay), mixed-layer smectite,
serpentine, and boehmite. Other samples show an abundance of mixed-layer
chlorite-smectite associated with small amounts of kaolinite (Haymon and Kastner
1986). The exit temperature of the hydrothermal fluids is responsible for giving
rise to these alteration crusts and was measured to be less than 60 °C. The elements composing the crusts are amorphous Al- or Mg- silicates and chemical
analyses have shown that the MgO is 13–22 %, SiO 2 is 27–43 % and Al 2 O 3 is
18–25 % by weight. The minor element constituents are Ca (1–4 %), Na 2 O
(\ 0.5–2 %) and Fe 2 O 3 (1–5 %).
Fe-bearing nontronite is one of the most common hydrothermal precipitates
found on the sea floor. It is a light-green, yellowish-green soft and/or semi-consolidated aggregate sometimes associated with amorphous Fe-oxyhydroxides but
Types of Hydrothermal Deposits
157