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Sunset Valley
Mining Company
Mining Company
Open-pit mining
El Chino, located near Silver City, New Mexico, is an open-pit copper mine
Open-pit mining, also known as open-cast mining, open-cut mining, and strip mining, refers to a method of extracting
rock or minerals from the earth by their removal from an open pit or borrow.
The term is used to differentiate this form of mining from extractive methods that require tunneling into the earth. Open-
pit mines are used when deposits of commercially useful minerals or rock are found near the surface; that is, where the
overburden (surface material covering the valuable deposit) is relatively thin or the material of interest is structurally
unsuitable for tunneling (as would be the case for sand, cinder, and gravel). For minerals that occur deep below the
surface—where the overburden is thick or the mineral occurs as veins in hard rock— underground mining methods
extract the valued material.
Open-pit mines that produce building materials and dimension stone are commonly referred to as quarries. People are
unlikely to make a distinction between an open-pit mine and other types of open-cast mines, such as quarries, borrows,
placers, and strip mines.
Open-pit mines are typically enlarged until either the mineral resource is exhausted, or an increasing ratio of
overburden to ore makes further mining uneconomic. When this occurs, the exhausted mines are sometimes converted
to landfills for disposal of solid wastes. However, some form of water control is usually required to keep the mine pit from
becoming a lake.
Extraction
Open-pit mines are dug on benches, which describe vertical levels of the hole. These benches are usually on four metre
to sixty metre intervals, depending on the size of the machinery that is being used. Many quarries do not use benches,
as they are usually shallow.
Most walls of the pit are generally dug on an angle less than vertical, to prevent and minimise damage and danger from
rock falls. This depends on how weathered the rocks are, and the type of rock, and also how many structural
weaknesses occur within the rocks, such as a fault , shears, joints or foliations.
The walls are stepped. The inclined section of the wall is known as the batter, and the flat part of the step is known as
the bench or perm. The steps in the walls help prevent rock falls continuing down the entire face of the wall. In some
instances additional ground support is required and rock bolts, cable bolts and shotcrete are used. De-watering bores
may be used to relieve water pressure by drilling horizontally into the wall, which is often enough to cause failures in the
wall by itself.
A haul road is situated at the side of the pit, forming a ramp up which trucks can drive, carrying ore and waste rock.
Waste rock is piled up at the surface, near the edge of the open pit. This is known as the waste dump. The waste dump
is also tiered and stepped, to minimise degradation.
Ore which has been processed is known as tailings, and is generally a slurry. This is pumped to a tailings dam or
settling pond, where the water evaporates. Tailings dams can often be toxic due to the presence of unextracted sulfide
minerals, some forms of toxic minerals in the gangue, and often cyanide which is used to treat gold ore via the cyanide
leach process.

Rehabilitation
After mining finishes, the mine area must undergo rehabilitation . Waste dumps are contoured to flatten them out, to
further stabilise them. If the ore contains sulfides it is usually covered with a layer of clay to prevent access of rain and
oxygen from the air, which can oxidise the sulfides to produce sulfuric acid, a phenomenon known as acid mine
drainage. This is then generally covered with soil, and vegetation is planted to help consolidate the material. Eventually
this layer will erode, but it is generally hoped that the rate of leaching or acid will be slowed by the cover such that the
environment can handle the load of acid and associated heavy metals. There are no long term studies on the success
of these covers due to the relatively short time in which large scale open pit mining has existed. It may take hundreds to
thousands of years for some waste dumps to become "acid neutral" and stop leaching to the environment. The dumps
are usually fenced off to prevent livestock denuding them of vegetation. The open pit is then surrounded with a fence,
to prevent access, and it generally eventually fills up with ground water. In arid areas it may not fill due to deep
groundwater levels.
Typical open cut grades
Gold is generally extracted in open-pit mines at 1 to 2 ppm (grams per ton) but in certain cases, 0.75ppm gold is
economic. This was achieved by bulk heap leaching at Alkane Minerals Ltd. Peak Hill mine in western New South Wales,
near Dubbo, Australia.
Nickel, generally as laterite, is extracted via open-pit down to 0.2%. copper is extracted at grades as low as 0.15% to
0.2%, generally in massive open-pit mines in chile, where the size of the resources and favorable metallurgy allows
economies of scale.
Materials typically extracted from open-pit mines include:
Clay
Coal
Coquina
Diamonds
Gravel and Stone (stone refers to bedrock, while gravel is unconsolidated material, as found in glacial or fluvial
deposits)
Granite
Gritstone
Gypsum
Limestone
Marble
Metal Ores, such as Copper, Iron, Gold, Platinum, Palladium and molybdenum
Open-pit mining
