CHAPTER 3 Igneous Rocks and Intrusive Activity
86
FIGURE 3.32 High grade gold ore from a mine in Ghana, West
Africa. (Photo by Greenshoots Communications/Alamy)
solidified to produce one of the largest iron
deposits in the world.
Among the best known and most
important ore deposits are those generated
from hot, ion-rich fluids called
hydrothermal (hot-water) solutions.*
Included in this group are the gold deposits
of the Homestake mine in South Dakota;
the lead, zinc, and silver ores near Coeur
d’Alene, Idaho; the silver deposits of the
Comstock Lode in Nevada; and the copper
ores of Michigan’ s Keweenaw Peninsula.
The majority of hydrothermal deposits
originate from hot, metal-rich fluids that
are remnants of late-stage magmatic
processes. During solidification, liquids
plus various metallic ions accumulate near
the top of the magma chamber. Because of
their mobility, these ion-rich solutions can
migrate great distances through the surrounding rock before they are eventually
deposited, usually as sulfides of various
metals (Figure 3.31). Some of these fluids
move along openings such as fractures or
bedding planes, where they cool and
precipitate metallic ions to produce vein
deposits. Many of the most productive
deposits of gold, silver, and mercury
occur as hydrothermal vein deposits
(FIGURE 3.32).
Another important type of accumulation generated by hydrothermal activity is
called a disseminated deposit. Rather than
being concentrated in narrow veins and
dikes, these ores are distributed as minute
masses throughout the entire rock mass.
Much of the world’ s copper is extracted
from disseminated deposits, including
those at Chuquicamata, Chile, and the
huge Bingham Canyon copper mine in
Utah. Because these accumulations contain
only 0.4 to 0.8 percent copper, between
125 and 250 kilograms of ore must be
mined for every kilogram of metal recovered. The environmental impact of these
large excavations, including the problems
of waste disposal, is significant.
*Because these hot, ion-rich fluids tend to chemically alter the host rock, this process is called
hydrothermal metamorphism and is discussed
in Chapter 7.
Another economically important
mineral with an igneous origin is diamond.
Although best known as gems, diamonds
are used extensively as abrasives. Diamonds
are thought to originate at depths of nearly
200 kilometers, where the confining
pressure is great enough to generate this
high-pressure form of carbon. Once crystallized, the diamonds are carried upward
through pipe-shaped conduits that increase
in diameter toward the surface. In
diamond-bearing pipes, nearly the entire
pipe contains diamond crystals that are
disseminated throughout an ultramafic
rock called kimberlite. The most productive
kimberlite pipes are those in South Africa.
The only equivalent source of diamonds
in the United States is located near
Murfreesboro, Arkansas, but this deposit
is exhausted and serves today merely as a
tourist attraction.
C O N C E P T C H E C K 3 . 9
List two types of hydrothermal deposits.
1
86
FIGURE 3.32 High grade gold ore from a mine in Ghana, West
Africa. (Photo by Greenshoots Communications/Alamy)
solidified to produce one of the largest iron
deposits in the world.
Among the best known and most
important ore deposits are those generated
from hot, ion-rich fluids called
hydrothermal (hot-water) solutions.*
Included in this group are the gold deposits
of the Homestake mine in South Dakota;
the lead, zinc, and silver ores near Coeur
d’Alene, Idaho; the silver deposits of the
Comstock Lode in Nevada; and the copper
ores of Michigan’ s Keweenaw Peninsula.
The majority of hydrothermal deposits
originate from hot, metal-rich fluids that
are remnants of late-stage magmatic
processes. During solidification, liquids
plus various metallic ions accumulate near
the top of the magma chamber. Because of
their mobility, these ion-rich solutions can
migrate great distances through the surrounding rock before they are eventually
deposited, usually as sulfides of various
metals (Figure 3.31). Some of these fluids
move along openings such as fractures or
bedding planes, where they cool and
precipitate metallic ions to produce vein
deposits. Many of the most productive
deposits of gold, silver, and mercury
occur as hydrothermal vein deposits
(FIGURE 3.32).
Another important type of accumulation generated by hydrothermal activity is
called a disseminated deposit. Rather than
being concentrated in narrow veins and
dikes, these ores are distributed as minute
masses throughout the entire rock mass.
Much of the world’ s copper is extracted
from disseminated deposits, including
those at Chuquicamata, Chile, and the
huge Bingham Canyon copper mine in
Utah. Because these accumulations contain
only 0.4 to 0.8 percent copper, between
125 and 250 kilograms of ore must be
mined for every kilogram of metal recovered. The environmental impact of these
large excavations, including the problems
of waste disposal, is significant.
*Because these hot, ion-rich fluids tend to chemically alter the host rock, this process is called
hydrothermal metamorphism and is discussed
in Chapter 7.
Another economically important
mineral with an igneous origin is diamond.
Although best known as gems, diamonds
are used extensively as abrasives. Diamonds
are thought to originate at depths of nearly
200 kilometers, where the confining
pressure is great enough to generate this
high-pressure form of carbon. Once crystallized, the diamonds are carried upward
through pipe-shaped conduits that increase
in diameter toward the surface. In
diamond-bearing pipes, nearly the entire
pipe contains diamond crystals that are
disseminated throughout an ultramafic
rock called kimberlite. The most productive
kimberlite pipes are those in South Africa.
The only equivalent source of diamonds
in the United States is located near
Murfreesboro, Arkansas, but this deposit
is exhausted and serves today merely as a
tourist attraction.
C O N C E P T C H E C K 3 . 9
List two types of hydrothermal deposits.
1