essentially of H 2 O, Na, Sr, K, Mg, Ca, Ba, SO 4 , HCO 3 , F and Cl (See Chap. 2),
penetrates the rocks beneath the sea floor through fissures and pores as it descends
into the crust and lithosphere to where it encounters a source of heat which causes
it to react with the surrounding, warmer rocky environment.
To give a simple explanation, fluid circulation in a hydrothermal system can be
divided into four zones (Fig. 6.2): (1) The ‘‘recharge zone’’ is where cold
(2–13 °C) seawater enters into the oceanic crust and a low temperature
(100–450 °C) chemical reaction starts to take place with the surrounding formations. The warm water loses some of its constituents such as Ca
+2 , and SO 4
-2 and
precipitates anhydrite (CaSO 4 ). Similarly, the Na, K, Sr, Br and F of seawater will
enter into the silicate lattice of the rock to replace other cations. (2) The ‘‘acidification zone’’ is where seawater changes its composition and becomes a more
corrosive fluid. At this stage, another chemical reaction takes place with the loss
from seaweater of Mg
+2 that is incorporated into the oceanic crust. This occurs
when the basaltic rocks and their major mineral constituents such as plagioclase
and olivine are transformed into clay. (3) At the ‘‘reaction zone’’ the temperature
of the descending fluid rises after it has reached the vicinity of a magma reservoir.
The depth could be anywhere from 500 m up to 2 km within the crust. Thermodynamic studies (Von Damm and Bishoff 1987) on vent fluids have estimated that
the temperature in the reaction zone is on the order of 390–400 °C. Under these
conditions, higher temperature mineral assemblages such as amphiboles, talc and
other hydrated silicates will be produced from the alteration and leaching of the
basalt-dolerite complexes which form the crust. Minerals such pyroxene and
olivine will give rise to ferromagnesian silicates and plagioclase will give rise to
epidote types of hydrated minerals. (4) In the ‘‘discharge zone’’ the hydrothermal
hot fluid carrying metals that have been leached from the underlying rocks will
return to the sea floor and will then react with ambient sea water and give rise to
sulfide deposits. Metals can also be contributed from the magma reservoir itself
(Yang and Scott 1996).
Other reactions during H
+ exchange with Ca and Ba from the silicate minerals
which combined with the SO 4
2- of sea water will give rise to anhydrite (CaSO 4 ) and
barite [(Ba
2+ (fluid) ? SO 4
2+ (sea water) = BaSO 4 (solid)]. Anhydrite, barite and
hydrated silica are compounds that are commonly found coating sulfide chimneys.
In summary, seawater itself is metal deficient and its sulfur content is found in
the form of sulfates. Later, after descending into the lithosphere where seawater
comes into contact with hot rocks in the reaction zone, the seawater becomes
chemically altered and gives rise to H
+ and free Mg
2+ , Ca and SO 4
2- . During such
a reaction, the hydroxyls (OH
- ) and Mg
2+ ions freed from seawater will go into
the rock to form hydrated silicates (Fig. 6.2). At this stage, seawater will have
changed its composition to become a ‘‘hydrothermal fluid’’ enriched in HS (sulfuric acid) with a pH of 3–5. It is now Mg
2+ free and has become very corrosive
with the ability to further leach metals from the rocks found in the reaction zone.
The fluid reaches its maximum temperature (450 °C) near the magma reservoir
where heavy metals are leached from the rocks. After leaving the area near
the heat source, the temperature of the fluid will drop to near 330–350 °C
Hydrothermal Fluid Circulation and Sub-Crustal Alteration
149
penetrates the rocks beneath the sea floor through fissures and pores as it descends
into the crust and lithosphere to where it encounters a source of heat which causes
it to react with the surrounding, warmer rocky environment.
To give a simple explanation, fluid circulation in a hydrothermal system can be
divided into four zones (Fig. 6.2): (1) The ‘‘recharge zone’’ is where cold
(2–13 °C) seawater enters into the oceanic crust and a low temperature
(100–450 °C) chemical reaction starts to take place with the surrounding formations. The warm water loses some of its constituents such as Ca
+2 , and SO 4
-2 and
precipitates anhydrite (CaSO 4 ). Similarly, the Na, K, Sr, Br and F of seawater will
enter into the silicate lattice of the rock to replace other cations. (2) The ‘‘acidification zone’’ is where seawater changes its composition and becomes a more
corrosive fluid. At this stage, another chemical reaction takes place with the loss
from seaweater of Mg
+2 that is incorporated into the oceanic crust. This occurs
when the basaltic rocks and their major mineral constituents such as plagioclase
and olivine are transformed into clay. (3) At the ‘‘reaction zone’’ the temperature
of the descending fluid rises after it has reached the vicinity of a magma reservoir.
The depth could be anywhere from 500 m up to 2 km within the crust. Thermodynamic studies (Von Damm and Bishoff 1987) on vent fluids have estimated that
the temperature in the reaction zone is on the order of 390–400 °C. Under these
conditions, higher temperature mineral assemblages such as amphiboles, talc and
other hydrated silicates will be produced from the alteration and leaching of the
basalt-dolerite complexes which form the crust. Minerals such pyroxene and
olivine will give rise to ferromagnesian silicates and plagioclase will give rise to
epidote types of hydrated minerals. (4) In the ‘‘discharge zone’’ the hydrothermal
hot fluid carrying metals that have been leached from the underlying rocks will
return to the sea floor and will then react with ambient sea water and give rise to
sulfide deposits. Metals can also be contributed from the magma reservoir itself
(Yang and Scott 1996).
Other reactions during H
+ exchange with Ca and Ba from the silicate minerals
which combined with the SO 4
2- of sea water will give rise to anhydrite (CaSO 4 ) and
barite [(Ba
2+ (fluid) ? SO 4
2+ (sea water) = BaSO 4 (solid)]. Anhydrite, barite and
hydrated silica are compounds that are commonly found coating sulfide chimneys.
In summary, seawater itself is metal deficient and its sulfur content is found in
the form of sulfates. Later, after descending into the lithosphere where seawater
comes into contact with hot rocks in the reaction zone, the seawater becomes
chemically altered and gives rise to H
+ and free Mg
2+ , Ca and SO 4
2- . During such
a reaction, the hydroxyls (OH
- ) and Mg
2+ ions freed from seawater will go into
the rock to form hydrated silicates (Fig. 6.2). At this stage, seawater will have
changed its composition to become a ‘‘hydrothermal fluid’’ enriched in HS (sulfuric acid) with a pH of 3–5. It is now Mg
2+ free and has become very corrosive
with the ability to further leach metals from the rocks found in the reaction zone.
The fluid reaches its maximum temperature (450 °C) near the magma reservoir
where heavy metals are leached from the rocks. After leaving the area near
the heat source, the temperature of the fluid will drop to near 330–350 °C
Hydrothermal Fluid Circulation and Sub-Crustal Alteration
149