Open-pit mining
:
This is an article about a specific type of surface mining. For a more general treatment, see that article.Open-pit mining, or
opencast 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).
Where minerals occur deep below the surface—where the overburden is thick or the mineral occurs as veins in hard rock— underground mining methods are used to extract the valued material. Open-pit mines are typically enlarged until the
mineral resource is exhausted.
Open-pit mines that produce
building materials are commonly referred to as
quarries. People in some English-speaking countries are not likely to make a distinction between an open-pit mine and other types of open-cast mines, such as quarries,
borrows,
placers, and
strip mines.
When they are no longer productive for extraction of material, open-pit 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.
Open Cut mines are dug on benches, which describe horizontal levels of the mine. These benches are usually on 3 metre or 6 metre levels, depending on the size of the machinery being used. Quarries rarely use benches, as the majority of quarries are dug using (relatively) small-scale machinery.
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 vertical section of the wall is known as the batter, and the flat part of the step is known as the bench. The steps in the walls help prevent rock falls continuing down the entire face of the wall. To support the wall, often
rock bolts are used. Sometimes, de-watering bores are drilled horizontally into the wall to relieve water pressure, 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 surface near the edge of the open cut. This is known as the waste dump. The waste dump is also tiered and stepped, to minimise
erosion.
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.
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. 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 the deep groundwater levels.
Gold is generally extracted in open cut 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.
Nickel, generally as laterite, is extracted via open cut down to 0.2%.
Copper is extracted at grades as low as 0.15% to 0.2%, generally in massive open cut 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*
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, and
molybdenum*
Sand*
Sandstone*
SlateThis list includes only those large open-pit mines for which an article exists in Wikipedia.
*
Adams Mine – controversial abandoned mine in
Kirkland Lake, Ontario.
*
Berkeley Pit - former copper mine in
Butte,
Montana,
United States; now a toxic lake.
*
Chuquicamata –
Chilean copper mine
*
Colomac Mine – gold mine in
Northwest Territories,
Canada.
*
Cerrejón – coal mine in
Guajira Department,
Colombia.
*
Diavik Diamond Mine – diamond mine in
Northwest Territories,
Canada.
*
Ekati Diamond Mine – diamond mine in
Northwest Territories,
Canada.
*
El Chino Mine – copper mine in
Grant County,
New Mexico,
United States*
Hull Rust Mahoning Mine–iron ore mine in
Hibbing, Minnesota, St.Louis County.
*
Kennecott Copper Mine – copper mine in
Salt Lake County,
Utah,
United States; largest open-pit mine in the world
*
Lavender Pit – copper mine in
Cochise County,
Arizona,
United States.
*
Mirny Mine – diamond mine in
Mirny,
Eastern Siberia*
Pascua Lama – gold and silver mine in
Atacama,
Chile (in project)
*
Penrhyn Quarry –
slate mine in
Wales.
*
Pine Point Mine – lead and zinc mine in
Northwest Territories,
Canada.
*
Super Pit – gold mine near
Kalgoorlie, Western Australia.
* The
Big Hole, a diamond mine in
Kimberley,
South Africa that reaches a depth of more than 1,000 m (3,300 ft).
*
Udachnaya pipe – diamond mine in
Yakutia,
Russia.
*
List of minerals*
List of stone*
Ore genesis*
Mineral resource classification
