Metals come from rocks and minerals in the Earth’s crust. Minerals
are naturally-formed chemical elements or combinations of elements
that have specific chemical compositions and physical properties.
Metallic and nonmetallic minerals occur in ordinary rocks throughout
the Earth’s crust, but only a few minerals contain high enough concentrations of metals to be mined profitably.
Certain metals, such as copper, lead, and zinc have a strong natural
affinity for the element sulfur, and they combine with it to form minerals
called sulfides. Probably the most familiar sulfide mineral is
fool’s gold (pyrite), which is composed of iron and sulfur.
The mining and processing of sulfide minerals has
historically been the source of most environmental concerns with metals extraction.
Identifying deposits where geologic
processes have concentrated sulfide minerals
is a continuing challenge for exploration geologists.
They search for mineral deposits that contain rich enough
concentrations of metal-bearing minerals to economically justify mining.
Metallic mineral deposits can be dispersed through entire mountains
and can cause environmental impacts naturally — whether or not they
are mined. For example, the mineralized deposits on the facing page are
a natural source of acidic and metal-bearing water that enters the
Special geologic processes lead to the development of mineral deposits
having high concentrations of metal-bearing minerals. These types of
mineral deposits are rare, and they occur in very diverse locations. Large
mineral deposits are being mined today from various environmental and
geographic settings, such as high mountainous rain forests located in
Indonesia, arid deserts in Arizona, and the treeless Arctic tundra of Alaska.
The settings where mineral deposits occur can play a significant role in
determining the nature and the extent of environmental concerns at specific
mine locations. The potential environmental impacts of mining the same
type of mineral deposit can be very different in different locations and
settings. For example, mining in arid parts of Arizona has different potential
impacts on surface water and groundwater quality than if the same mining
had occurred in areas of temperate climates, such as the Rocky Mountains
or the midwest. Although many metallic mineral deposits have been
identified through exploration, only a few deposits are large enough and
have a metal content great enough to support commercial operations.
The economically important part of a mineral deposit is known as the
“ore” or “orebody” (Fig. 2).
Once an orebody is identified within a mineral deposit, geologists determine
its form. The form of the orebody is important for two reasons: the shape of
an orebody helps determine the best way to mine it, and the orebody form
influences the potential environmental impacts associated with mining.
Although every mineral deposit has distinctive features, they generally exist
in two common forms. In one form, the orebody can have dimensions
(length, width, and depth) measured in miles (kilometers) and can include a
large volume of rock at or near the surface. These ore deposits are most
efficiently mined from surface excavations called open pits.
The other general orebody form is one characterized by tabular shapes in
which either the vertical or horizontal dimension is much greater than the
other — at the most one or two miles (1 to 3 km) in depth or length. These
types of deposits can extend to considerable depth and are most commonly
mined by underground mining techniques. Large massive orebodies
occurring at depths greater than about 1000 feet (350 meters) also must
be extracted by expensive underground mining techniques.