HYDROTHERMAL VENT DEPOSITS
R. M. Haymon, University of California, CA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
In April 1979, submersible divers exploring the midocean ridge crest at latitude 211N on the East Pacific
Rise discovered superheated (3807301C) fluids,
blackened by tiny metal-sulfide mineral crystals,
spewing from the seafloor through tall mineral conduits (see Hydrothermal Vent Fluids, Chemistry of).
The crystalline conduits at these ‘black smoker’
hydrothermal vents were made of minerals rich in
copper, iron, zinc, and other metals. Since 1979,
hundreds of similar hydrothermal deposits have been
located along the midocean ridge. It is now clear that
deposition of hydrothermal mineral deposits is a
common process, and is integrally linked to cracking,
magmatism, and cooling of new seafloor as it accretes and spreads away from the ridge (see MidOcean Ridge Geochemistry and Petrology).
For thousands of years before mid-ocean ridge hot
springs were discovered in the oceans, people mined
copper from mineral deposits that were originally
formed on oceanic spreading ridges. These fossil
deposits are embedded in old fragments of seafloor
called ‘ophiolites’ that have been uplifted and
emplaced onto land by fault movements. The copperrich mineral deposits in the Troodos ophiolite of
Cyprus are well-known examples of fossil oceanridge deposits that have been mined for at least 2500
years; in fact, the word ‘copper’ is derived from the
Latin word ‘cyprium’ which means ‘from Cyprus.’
The mineral deposits accumulating today at hot
springs along the mid-ocean ridge are habitats for a
variety of remarkable organisms ranging in size from
tiny microbes to large worms. The properties of the
mineral deposits are inextricably linked to the organisms that inhabit them. The mineral deposits
contain important clues about the physical–chemical
environments in which some of these organisms live,
and also preserve fossils of some organisms, creating
a geologic record of their existence.
Hydrothermal vent deposits are thus a renewable
source of metals and a record of the physical,
chemical, biological, and geological processes at
modern and ancient submarine vents.
Where Deposits Form: Geologic
Controls
Less than 2% of the total area of the mid-ocean ridge
crest has been studied at a resolution sufficient to
reveal the spatial distribution of hydrothermal vents,
mineral deposits, and other significant small-scale
geologic features. Nevertheless, because study areas
have been carefully selected and strategically surveyed, much has been learned about where vents and
deposits form, and about the geologic controls on
their distribution. The basic requirements for
hydrothermal systems include heat to drive fluid
circulation, and high-permeability pathways to facilitate fluid flow through crustal rocks. On midocean ridges, vents and deposits are forming at sites
where ascending magma intrusions introduce heat
into the permeable shallow crust, and at sites where
deep cracks provide permeability and fluid access to
heat sources at depth.
Fast-spreading Ridges
Near- and on-bottom studies along the fast-spreading East Pacific Rise suggest that most hydrothermal
mineral deposits form along the summit of the ridge
crest within a narrow ‘axial zone’ less than 500 m
wide. Only a few active sites of mineral deposition
have been located outside this zone; however, more
exploration of the vast area outside the axial zone is
needed to establish unequivocally whether or not
mineral deposition is uncommon in this region. The
overall spatial distribution of hydrothermal vents
and mineral deposits along fast-spreading ridges
traces the segmented configuration of cracks and
magma sources along the ridge crest (see Mid-Ocean
Ridge Geochemistry and Petrology).
Within the axial zone, mineral deposition is concentrated along the floors and walls of axial troughs
created by volcanic collapse and/or faulting along the
summit of the ridge crest. The majority of the deposits are located along fissures that have opened
above magmatic dike intrusions, and along collapsed
lava ponds formed above these fissures by pooling
and drainage of erupted lava. Where fault-bounded
troughs have formed along the summit of the ridge
crest, mineral deposition is focused along the
bounding faults and also along fissures and collapsed
lava ponds in the trough floor. Hydrothermal vents
appear to be most abundant along magmatically inflated segments of fast-spreading ridges; however, the
374
R. M. Haymon, University of California, CA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
In April 1979, submersible divers exploring the midocean ridge crest at latitude 211N on the East Pacific
Rise discovered superheated (3807301C) fluids,
blackened by tiny metal-sulfide mineral crystals,
spewing from the seafloor through tall mineral conduits (see Hydrothermal Vent Fluids, Chemistry of).
The crystalline conduits at these ‘black smoker’
hydrothermal vents were made of minerals rich in
copper, iron, zinc, and other metals. Since 1979,
hundreds of similar hydrothermal deposits have been
located along the midocean ridge. It is now clear that
deposition of hydrothermal mineral deposits is a
common process, and is integrally linked to cracking,
magmatism, and cooling of new seafloor as it accretes and spreads away from the ridge (see MidOcean Ridge Geochemistry and Petrology).
For thousands of years before mid-ocean ridge hot
springs were discovered in the oceans, people mined
copper from mineral deposits that were originally
formed on oceanic spreading ridges. These fossil
deposits are embedded in old fragments of seafloor
called ‘ophiolites’ that have been uplifted and
emplaced onto land by fault movements. The copperrich mineral deposits in the Troodos ophiolite of
Cyprus are well-known examples of fossil oceanridge deposits that have been mined for at least 2500
years; in fact, the word ‘copper’ is derived from the
Latin word ‘cyprium’ which means ‘from Cyprus.’
The mineral deposits accumulating today at hot
springs along the mid-ocean ridge are habitats for a
variety of remarkable organisms ranging in size from
tiny microbes to large worms. The properties of the
mineral deposits are inextricably linked to the organisms that inhabit them. The mineral deposits
contain important clues about the physical–chemical
environments in which some of these organisms live,
and also preserve fossils of some organisms, creating
a geologic record of their existence.
Hydrothermal vent deposits are thus a renewable
source of metals and a record of the physical,
chemical, biological, and geological processes at
modern and ancient submarine vents.
Where Deposits Form: Geologic
Controls
Less than 2% of the total area of the mid-ocean ridge
crest has been studied at a resolution sufficient to
reveal the spatial distribution of hydrothermal vents,
mineral deposits, and other significant small-scale
geologic features. Nevertheless, because study areas
have been carefully selected and strategically surveyed, much has been learned about where vents and
deposits form, and about the geologic controls on
their distribution. The basic requirements for
hydrothermal systems include heat to drive fluid
circulation, and high-permeability pathways to facilitate fluid flow through crustal rocks. On midocean ridges, vents and deposits are forming at sites
where ascending magma intrusions introduce heat
into the permeable shallow crust, and at sites where
deep cracks provide permeability and fluid access to
heat sources at depth.
Fast-spreading Ridges
Near- and on-bottom studies along the fast-spreading East Pacific Rise suggest that most hydrothermal
mineral deposits form along the summit of the ridge
crest within a narrow ‘axial zone’ less than 500 m
wide. Only a few active sites of mineral deposition
have been located outside this zone; however, more
exploration of the vast area outside the axial zone is
needed to establish unequivocally whether or not
mineral deposition is uncommon in this region. The
overall spatial distribution of hydrothermal vents
and mineral deposits along fast-spreading ridges
traces the segmented configuration of cracks and
magma sources along the ridge crest (see Mid-Ocean
Ridge Geochemistry and Petrology).
Within the axial zone, mineral deposition is concentrated along the floors and walls of axial troughs
created by volcanic collapse and/or faulting along the
summit of the ridge crest. The majority of the deposits are located along fissures that have opened
above magmatic dike intrusions, and along collapsed
lava ponds formed above these fissures by pooling
and drainage of erupted lava. Where fault-bounded
troughs have formed along the summit of the ridge
crest, mineral deposition is focused along the
bounding faults and also along fissures and collapsed
lava ponds in the trough floor. Hydrothermal vents
appear to be most abundant along magmatically inflated segments of fast-spreading ridges; however, the
374