is a good conductor of electricity and heat, and it is a vital element
of virtually all modern electronic devices. Arizona is the United
States’ largest source of copper, primarily mined from a type of ore
body known as porphyry copper deposits.
The most common approach to extracting metal-bearing ore from a
porphyry copper deposit is by open-pit mining, although such mines also
typically include some underground activities.
This astronaut photograph illustrates three open-pit mines located
west-northwest of the town of Green Valley, Arizona. While the mines
appear to be close to each other, each exploits a separate porphyry
copper deposit. A porphyry copper deposit forms when crystal-rich magma
moves upwards through pre-existing rock layers. As the magma cools and
crystallizes, it forms an igneous rock with large crystals embedded in
a fine-grained matrix, known as porphyry.
Hot fluids circulate through the magma and surrounding rocks via
fractures, depositing copper-bearing and other minerals in
characteristic spatial patterns that signal the nature of the ore body
to a geologist.
The mine pits are recognizable by the concentric lines of benches
cut into the pit sides. The benches allow equipment and personnel
access to the fresh ore (gray) exposed at the bottom of the excavation.
Water may also pool at the bottom of inactive pits, such as in the Twin
Buttes Mine at image upper right (black areas). The open pit areas are
surrounded by an array of sculpted tailings ponds and mine dump areas;
these receive mine waste rock for storage and later leaching for
further recovery of metals. The green color of the water in the
tailings pond at image right is likely due to the presence of leached
The Asarco-Mission complex (image left) is an active producer of
copper and molybdenum (important in making steel and other metal
alloys), processing approximately 53,700 tons of ore per day (as of
2008). The Twin Buttes Mine (image top right), also a producer of
copper and molybdenum, was closed in 1994 but was recently purchased by
a new owner (Freeport-McMoRan Copper & Gold) and may reopen. The
Sierrita Mine at image lower right (also owned by Freeport-McMoRan
Copper & Gold) produces copper, molybdenum, and the rare metal
rhenium. Rhenium is used to make high-temperature alloys for jet engine
parts and in high-octane, lead-free gasoline. This mine can process
over 115,000 tons of ore per day (as of 2007).
Astronaut photograph ISS022-E-26137
was acquired on January 14, 2010, with a Nikon D3 digital camera using
an equivalent 440 mm lens, and is provided by the ISS Crew Earth
Observations experiment and Image Science & Analysis Laboratory,
Johnson Space Center. The image was taken by the Expedition 22 crew. The image in this article has been cropped and enhanced to improve contrast. Lens artifacts have been removed. The International Space Station Program
supports the laboratory to help astronauts take pictures of Earth that
will be of the greatest value to scientists and the public, and to make
those images freely available on the Internet. Additional images taken
by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by William L. Stefanov, NASA-JSC.