others). The sulfide-filled veins can be seen forming an intricate interconnected
network terminating on top of fault scarps underneath sulfide edifices.
A volcanic or hydrothermal outcrop could later become brecciated due to the
extensive hydrothermal fluid circulation that has altered the rocks through fissures
and cracks, and created veins filled by hydrated Fe–Si clay and metallic sulfides.
Such alteration can loosen the coherent volcanic rocks, which are more easily
broken into fragments that could then be re-cemented during further circulation of
hot fluids and mineral precipitation. Examples of stockwork from the EPR axial
graben were observed by submersible near 12°50
0 N as well as near an ultra-fast
spreading segment of the EPR at 18°30
0 S (Fig. 6.4a–c).
Formation of Hydrothermal Deposits
As explained earlier in this chapter, the formation of hydrothermal deposits occurs
through four steps when seawater enters into the lithosphere through pores and
fissures in the volcanic rocks, then comes into contact with heat, so its composition
is changed and it becomes more corrosive so it can dissolve metals and other
elements from the volcanic rocks before being discharged. The hydrothermal fluid
forms precipitates of metal bearing sulfides on the sea floor and in the country rock
on its way to the surface. When the discharge flow rate of the hydrothermal
geysers is high, chimneys could be formed, and they will then serve as conduits for
subsequent hydrothermal discharge.
The formation of a hydrothermal chimney will occur when a large volume of
fluid with a rapid discharge rate (1–5 m/s) is spewed out onto the sea floor. The
contact between the exiting hot chemically altered fluid and the cold ambient
seawater produces another chemical reaction between the edge of the fluid flow
and the seawater, constructing a thin skin of sulfates and sulfides. More details on
the observed growth of a hydrothermal chimney in an area called the ‘‘Chain site’’
are presented in Chap. 7.
The early stage of mineral precipitation varies with the composition and the
temperature of the fluid. A rapid discharge of hydrothermal fluid will initiate the
formation of a fragile skin of whitish gray colored anhydrate (calcium sulfate),
which will form a protective carapace at the interface between the hot fluid and the
cold seawater. Below 150 °C, anhydrate starts to dissolve in seawater but, before
this happens, there may be a subsequent deposition of amorphous silica, which
lowers the structure’s permeability and prevents the exchange between the fluid and
seawater (Tivey and Delaney 1986). When the protective coating of the chimney
walls is built, thereby preventing any extensive mixing with the ambient seawater,
several other sulfide phases start to form. The most inner part of the chimney is also
the most insulated from seawater and will be formed by the precipitation of
Cu-sulfide at high temperatures ([300 °C) while Zn and Fe will precipitate at lower
temperatures (100–270 °C). The texture of the mineral grains will also vary inward
from dendritic, spikey shaped minerals to more coarse-grained deposits. In the
Types of Hydrothermal Deposits
159
network terminating on top of fault scarps underneath sulfide edifices.
A volcanic or hydrothermal outcrop could later become brecciated due to the
extensive hydrothermal fluid circulation that has altered the rocks through fissures
and cracks, and created veins filled by hydrated Fe–Si clay and metallic sulfides.
Such alteration can loosen the coherent volcanic rocks, which are more easily
broken into fragments that could then be re-cemented during further circulation of
hot fluids and mineral precipitation. Examples of stockwork from the EPR axial
graben were observed by submersible near 12°50
0 N as well as near an ultra-fast
spreading segment of the EPR at 18°30
0 S (Fig. 6.4a–c).
Formation of Hydrothermal Deposits
As explained earlier in this chapter, the formation of hydrothermal deposits occurs
through four steps when seawater enters into the lithosphere through pores and
fissures in the volcanic rocks, then comes into contact with heat, so its composition
is changed and it becomes more corrosive so it can dissolve metals and other
elements from the volcanic rocks before being discharged. The hydrothermal fluid
forms precipitates of metal bearing sulfides on the sea floor and in the country rock
on its way to the surface. When the discharge flow rate of the hydrothermal
geysers is high, chimneys could be formed, and they will then serve as conduits for
subsequent hydrothermal discharge.
The formation of a hydrothermal chimney will occur when a large volume of
fluid with a rapid discharge rate (1–5 m/s) is spewed out onto the sea floor. The
contact between the exiting hot chemically altered fluid and the cold ambient
seawater produces another chemical reaction between the edge of the fluid flow
and the seawater, constructing a thin skin of sulfates and sulfides. More details on
the observed growth of a hydrothermal chimney in an area called the ‘‘Chain site’’
are presented in Chap. 7.
The early stage of mineral precipitation varies with the composition and the
temperature of the fluid. A rapid discharge of hydrothermal fluid will initiate the
formation of a fragile skin of whitish gray colored anhydrate (calcium sulfate),
which will form a protective carapace at the interface between the hot fluid and the
cold seawater. Below 150 °C, anhydrate starts to dissolve in seawater but, before
this happens, there may be a subsequent deposition of amorphous silica, which
lowers the structure’s permeability and prevents the exchange between the fluid and
seawater (Tivey and Delaney 1986). When the protective coating of the chimney
walls is built, thereby preventing any extensive mixing with the ambient seawater,
several other sulfide phases start to form. The most inner part of the chimney is also
the most insulated from seawater and will be formed by the precipitation of
Cu-sulfide at high temperatures ([300 °C) while Zn and Fe will precipitate at lower
temperatures (100–270 °C). The texture of the mineral grains will also vary inward
from dendritic, spikey shaped minerals to more coarse-grained deposits. In the
Types of Hydrothermal Deposits
159