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The Earth's crust, on which we live and depend, is in large part the product of
millions of once-active volcanoes and tremendous volumes of magma
that did not erupt but instead cooled below the surface. Such
persistent and widespread volcanism has resulted in many valuable natural
resources throughout the world. ... Over
many thousands of years, heated groundwater has concentrated valuable minerals,
including copper, tin, gold, and silver, into deposits that are mined throughout
the world.
-- Excerpt from: Brantley, 1994, Volcanoes of the United States:
USGS General Interest Publication
Most of the metallic minerals mined in the world,
such as copper, gold, silver, lead, and zinc, are associated with magmas
found deep within the roots of extinct volcanoes
located above subduction zones. Rising magma does not always reach the
surface to erupt; instead it may slowly cool and
harden beneath the volcano to form a wide variety of crystalline rocks
(generally called plutonic or granitic rocks).
Some of the best examples of such deep-seated granitic rocks, later exposed
by erosion, are magnificently displayed in California's Yosemite National Park.
Ore deposits commonly form around the
magma bodies that feed volcanoes because there is a ready
supply of heat, which convectively moves and circulates
ore-bearing fluids. The metals, originally scattered in
trace amounts in magma or surrounding solid rocks, become
concentrated by circulating hot fluids and can be redeposited,
under favorable temperature and pressure conditions, to
form rich mineral veins.
-- Excerpts from: Kious and Tilling, 1996, This Dynamic Earth: The Story of Plate
Tectonics: USGS General Interest Publication
The active volcanic vents along the spreading mid-ocean ridges
create ideal environments for the circulation of fluids rich
in minerals and for ore deposition. Water as hot as 380 degrees C
gushes out of geothermal springs along the spreading centers.
The water has been heated during circulation by contact with
the hot volcanic rocks forming the ridge. Deep-sea hot
springs containing an abundance of dark-colored ore minerals
(sulfides) of iron, copper, zinc, nickel, and other metals are
called "black smokers." On rare occasions, such deep-sea ore
deposits are later exposed in remnants of ancient oceanic
crust that have been scraped off and left ("beached") on top
of continental crust during past subduction processes. The
Troodos Massif on the Island of Cyprus is perhaps the best
known example of such ancient oceanic crust. Cyprus was an
important source of copper in the ancient world, and Romans
called copper the "Cyprian metal"; the Latin word for copper
is cyprium.
-- Excerpts from: Kious and Tilling, 1996, This Dynamic Earth: The Story of Plate
Tectonics: USGS General Interest Publication
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Mount St. Helens, Washington
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Mining claims for copper, gold, and silver were staked in the
St. Helens mining district north of the volcano
as early as 1892. ... Mining fever broke
out about 1900, and hundreds of claims were staked in the Spirit Lake area as
prospectors sought high-grade vein deposits. About 14 tons of copper ore from
the Sweden Mine were hauled to a Tacoma smelter in 1905 and used to cast
the bronze statue of Sacajawea for the Lewis and Clark Exposition held in
Portland, Oregon, which commemorated the 100th anniversary of their expedition.
... Other mines in the area included the Margaret (Earl) group. Although
thousands of prospect pits and more than 11,000 feet of underground workings
were dug, the veins proved difficult to work and contained only modest amounts
of gold and silver. By 1929, most of the mines had been abandoned, although
exploratory work continued sporadically until the eruption of 1980.
-- Excerpt from: Pringle, 1993, Roadside Geology of Mount St. Helens National
Volcanic Monument and Vicinity: Washington Department of Natural Resources,
Division of Geology and Earth Resources Information Circular 88
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