seafloor (de Ronde et al., 2003) and others are associated
with subvolcanic intrusions above sealevel (Lydon,
2007). In both cases, mineralizing fluids transport metals
to the site of deposition.
There are two sources of mineralizing fluids in submarine arc volcanoes: (1) metals derived through leaching of
pre-existing rock by ambient surface water; and (2) metals
derived from a degassing magma body. In the first case,
seawater or groundwater is heated and acidified by reaction with degassing sulfur. Fluids associated with convergent margin magmas are very acidic and corrosive
because these magmas release tremendous amounts of sulfur; this combines with H 2 O to produce H 2 SO 4 , sulfuric
acid. These hot, acid waters are produced when heated
by a magma body, resulting in development of a hydrothermal convection cell. This draws in cold water to be
heated and acidified above the magma body, which then
rises to the surface along faults and fractures, which
become upflow channels. Hydrothermal circulation and
mineralization are enhanced in calderas, where major
faults favor fluid circulation above a shallow magma
body. The circulation of hot, acidic solutions through the
surrounding rock leaches and breaks down primary igneous minerals into clays – hydrothermal ore deposits are
invariably associated with a lot of extremely altered
rocks – and such alteration liberates a lot of metals, which
are carried away by the acidic fluids. In the second case,
metals are released by magmatic degassing of metalenriched volatiles. Most ore-bearing fluids reflect the
combined effects of both hydrothermal leaching and magmatic degassing, and the relative contributions are likely
to vary in time and space (Yang and Scott, 1996).
However the metal-bearing acid solutions are generated, the metals these transport will ultimately be deposited where and when the fluid interacts with a different
chemical environment, especially where the rising acid
fluid mixes with higher pH fluids, such as seawater. This
can form massive sulfide deposits associated with backarc
basin hydrothermal vents, which are very much like black
smokers associated with mid-ocean ridges. Another style
of mineralization occurs if the fluid rises into a region
where P is low enough that the fluid boils away, leaving
the metals behind. This is what happens in shallow
subvolcanic intrusions such as porphyry deposits
(Sillitoe, 1997). Phase separation (boiling) is also common in arc and even backarc basin hydrothermal systems.
Some ore deposits that form in other tectonic environments reflect the generation of immiscible sulfide liquid,
which separates from the silicate melt and sinks to the bottom to form mineral deposits, such as the giant Bushveld
deposit of S. Africa. This mode of mineralization is not
common in island arcs because the magma is too rich in
H 2 O and O 2 to form much reduced sulfur (sulfide) in the
magma body.
The composition of the host magma can also exert
important controls on mineralization. For example, sulfide
deposits that are commonly associated with felsic igneous
activity (both intrusive and extrusive) are typically Pb, In,
Bi, and Sb (+/À Au, Ag)-rich, compared to those associated with predominantly mafic igneous rocks, which tend
to be more Cu, Zn and Co (+/À Au, Ag)-rich (Timm et al.,
2012). Similarly, styles of mineralization vary according
to whether igneous activity is submarine (volcanogenic
or Kuroko-type massive sulfides) or at shallow depth
beneath a large volcano (porphyry and epithermal
deposits).
The hydrothermal system associated with Brothers Volcano in the Kermadec arc, NZ (Figure 2) is a good example of submarine arc hydrothermal mineralization.
Hydrothermal vents are found at three sites in the caldera
at 1,300–1,800 m water depth associated with dacite and
rhyodacite domes above a mafic substrate. Hot (max.
305
C), acidic (pH > 1.9) fluids vent through sulfide
chimneys up to 7 m tall (de Ronde et al., 2005). Hydrothermal vents are concentrated along caldera faults. Two
main types of chimneys predominate: Cu-rich (up to
28.5 wt% Cu) and more common Zn-rich (up to 48.5 wt
% Zn; de Ronde and 18 others 2011). Brothers hydrothermal fields only ~3 km apart provide useful examples of the
range of hydrothermal processes and sources seen in submarine arc volcanoes. The Cone hydrothermal site is
today more strongly affected by degassed magmatic components than is the NW Caldera site. These two Brothers
vent fields represent near end-members of a continuum
that spans magma-dominated and seawater-dominated
hydrothermal system.
Porphyry Cu-Au deposits are another important style of
arc mineralization. This mineralization is found in shallow
plutonic rocks affected by rapid uplift, cooling, and
depressurization of upper crustal magma chambers,
releasing metal-bearing magmatic fluids. Porphyry
Cu-Au and epithermal deposits seem to be more common
in atypical arc settings and in association with unusual,
especially highly potassic, igneous rocks (Sillitoe, 1997).
Summary
Igneous activity at convergent plate boundaries is one of
the most important manifestations of plate tectonics.
Convergent-margin basalts demonstrate that crust and
sediments are being recycled back down into earth’s mantle in the underlying subduction zone, and that some of
this deeply-recycled material has escaped up into and
mixed with overlying mantle and caused it to melt. Basaltic magmas rise from the mantle above the subduction
zone towards the surface. Among all Earth magmas, those
produced at convergent margins contain the highest concentrations of magmatic volatiles, especially water. Long,
curved alignments of volcanoes in the ocean define island
arcs, and similar alignments on continental margins mark
where the flux of magma out of the mantle above a subduction zone is especially great. Some mantle-derived
magmas don’t rise to the surface but stall in the crust.
The interaction of mantle-derived magma with
pre-existing crust sometimes leads to fractional crystallization and secondary melting, forming magmas that are
MAGMATISM AT CONVERGENT PLATE BOUNDARIES
405
with subvolcanic intrusions above sealevel (Lydon,
2007). In both cases, mineralizing fluids transport metals
to the site of deposition.
There are two sources of mineralizing fluids in submarine arc volcanoes: (1) metals derived through leaching of
pre-existing rock by ambient surface water; and (2) metals
derived from a degassing magma body. In the first case,
seawater or groundwater is heated and acidified by reaction with degassing sulfur. Fluids associated with convergent margin magmas are very acidic and corrosive
because these magmas release tremendous amounts of sulfur; this combines with H 2 O to produce H 2 SO 4 , sulfuric
acid. These hot, acid waters are produced when heated
by a magma body, resulting in development of a hydrothermal convection cell. This draws in cold water to be
heated and acidified above the magma body, which then
rises to the surface along faults and fractures, which
become upflow channels. Hydrothermal circulation and
mineralization are enhanced in calderas, where major
faults favor fluid circulation above a shallow magma
body. The circulation of hot, acidic solutions through the
surrounding rock leaches and breaks down primary igneous minerals into clays – hydrothermal ore deposits are
invariably associated with a lot of extremely altered
rocks – and such alteration liberates a lot of metals, which
are carried away by the acidic fluids. In the second case,
metals are released by magmatic degassing of metalenriched volatiles. Most ore-bearing fluids reflect the
combined effects of both hydrothermal leaching and magmatic degassing, and the relative contributions are likely
to vary in time and space (Yang and Scott, 1996).
However the metal-bearing acid solutions are generated, the metals these transport will ultimately be deposited where and when the fluid interacts with a different
chemical environment, especially where the rising acid
fluid mixes with higher pH fluids, such as seawater. This
can form massive sulfide deposits associated with backarc
basin hydrothermal vents, which are very much like black
smokers associated with mid-ocean ridges. Another style
of mineralization occurs if the fluid rises into a region
where P is low enough that the fluid boils away, leaving
the metals behind. This is what happens in shallow
subvolcanic intrusions such as porphyry deposits
(Sillitoe, 1997). Phase separation (boiling) is also common in arc and even backarc basin hydrothermal systems.
Some ore deposits that form in other tectonic environments reflect the generation of immiscible sulfide liquid,
which separates from the silicate melt and sinks to the bottom to form mineral deposits, such as the giant Bushveld
deposit of S. Africa. This mode of mineralization is not
common in island arcs because the magma is too rich in
H 2 O and O 2 to form much reduced sulfur (sulfide) in the
magma body.
The composition of the host magma can also exert
important controls on mineralization. For example, sulfide
deposits that are commonly associated with felsic igneous
activity (both intrusive and extrusive) are typically Pb, In,
Bi, and Sb (+/À Au, Ag)-rich, compared to those associated with predominantly mafic igneous rocks, which tend
to be more Cu, Zn and Co (+/À Au, Ag)-rich (Timm et al.,
2012). Similarly, styles of mineralization vary according
to whether igneous activity is submarine (volcanogenic
or Kuroko-type massive sulfides) or at shallow depth
beneath a large volcano (porphyry and epithermal
deposits).
The hydrothermal system associated with Brothers Volcano in the Kermadec arc, NZ (Figure 2) is a good example of submarine arc hydrothermal mineralization.
Hydrothermal vents are found at three sites in the caldera
at 1,300–1,800 m water depth associated with dacite and
rhyodacite domes above a mafic substrate. Hot (max.
305
C), acidic (pH > 1.9) fluids vent through sulfide
chimneys up to 7 m tall (de Ronde et al., 2005). Hydrothermal vents are concentrated along caldera faults. Two
main types of chimneys predominate: Cu-rich (up to
28.5 wt% Cu) and more common Zn-rich (up to 48.5 wt
% Zn; de Ronde and 18 others 2011). Brothers hydrothermal fields only ~3 km apart provide useful examples of the
range of hydrothermal processes and sources seen in submarine arc volcanoes. The Cone hydrothermal site is
today more strongly affected by degassed magmatic components than is the NW Caldera site. These two Brothers
vent fields represent near end-members of a continuum
that spans magma-dominated and seawater-dominated
hydrothermal system.
Porphyry Cu-Au deposits are another important style of
arc mineralization. This mineralization is found in shallow
plutonic rocks affected by rapid uplift, cooling, and
depressurization of upper crustal magma chambers,
releasing metal-bearing magmatic fluids. Porphyry
Cu-Au and epithermal deposits seem to be more common
in atypical arc settings and in association with unusual,
especially highly potassic, igneous rocks (Sillitoe, 1997).
Summary
Igneous activity at convergent plate boundaries is one of
the most important manifestations of plate tectonics.
Convergent-margin basalts demonstrate that crust and
sediments are being recycled back down into earth’s mantle in the underlying subduction zone, and that some of
this deeply-recycled material has escaped up into and
mixed with overlying mantle and caused it to melt. Basaltic magmas rise from the mantle above the subduction
zone towards the surface. Among all Earth magmas, those
produced at convergent margins contain the highest concentrations of magmatic volatiles, especially water. Long,
curved alignments of volcanoes in the ocean define island
arcs, and similar alignments on continental margins mark
where the flux of magma out of the mantle above a subduction zone is especially great. Some mantle-derived
magmas don’t rise to the surface but stall in the crust.
The interaction of mantle-derived magma with
pre-existing crust sometimes leads to fractional crystallization and secondary melting, forming magmas that are
MAGMATISM AT CONVERGENT PLATE BOUNDARIES
405