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Argyle diamond mine

2007 Schools Wikipedia Selection. Related subjects: Geography of Oceania
(Australasia)

   The Argyle diamond mine ( 16°42′44″S, 128°23′51″E Coordinates:
   16°42′44″S, 128°23′51″E) is a diamond mine located in the north-east of
   Western Australia. The Argyle mine is the largest diamond producer in
   the world by volume, although due to the low proportion of gem-quality
   diamonds, is not the leader by value. It is the only known significant
   source of pink diamonds, producing 90 to 95% of the world's supply. The
   Argyle diamond mine is also notable for being the first commercial
   diamond mine exploiting a volcanic pipe of lamproite, rather than the
   more usual kimberlite pipe. The mine is owned by the Rio Tinto Group, a
   diversified mining company which also owns the Diavik Diamond Mine in
   Canada and the Murowa diamond mine in Zimbabwe.

Description

   A false-color NASA image of the Argyle mine. Blue colours show the
   location of the mine and its depressed elevation as a result of the
   open pit mining technique.
   Enlarge
   A false-colour NASA image of the Argyle mine. Blue colours show the
   location of the mine and its depressed elevation as a result of the
   open pit mining technique.

   The mine covers about 450,000 square metres (110 acres), stretching in
   a mostly linear shape about 1600 metres (5,200 ft) long and 150 to 600
   metres (500 to 2,000 ft) wide. The mine is of open pit construction,
   and reaches about 600 metres (1,900 ft) deep at its deepest point.

Location

   The Argyle diamond mine is located in the Kimberley region in the far
   northeast of the Australian state of Western Australia. It is located
   to the southwest of Lake Argyle in the Matsu Ranges, about 550
   kilometres (340 miles) southwest of Darwin. Because the mine is located
   about 120 kilometres (75 miles) from the nearest settlement (
   Kununurra), a complete residential camp has been constructed on site.
   Most of the 520 workers commute from Perth (over 2,000 kilometres, or
   1,200 miles, away) for alternating two week shifts at the mine. The
   mine has encouraged local employment and has a large number of
   indigenous local people working within the mine.

Geology

   The mine is the first commercial diamond mine (except alluvial mining
   operations) not located on a kimberlite pipe. The pipe is named "AK-1",
   although it is commonly simply called the "Argyle pipe".

   The volcanic pipe is a diatreme, composed of olivine lamproite, present
   as tuff and lava. Peripheral volcanic facies suggest the lamproite
   eruption formed a maar. At the margins of the volcanic pipe the
   lamproite is mixed with a volcanic breccia containing shattered wall
   rock fragments mixed and milled by the eruption. Minerals in the
   marginal facies include zeolite minerals, micas, kaolinite and clays,
   typical of post-eruption hydrothermal circulation.

   Diamonds are found within the intact core of the volcanic pipe, as well
   as within some of the marginal breccia facies and maar facies. However,
   some diamonds are considered to have been resorbed during the
   post-eruption cooling of the pipe and converted to graphite.

   The diatreme pipe formed by explosive eruption of the lamproite magma
   through a zone of weakness in the continental crust.

   The diamonds found at the Argyle pipe have been dated to about 1.58
   billion years of age, while the volcano which created the pipe is aged
   between 1.1 and 1.2 billion years old. This represents a relatively
   short period during which diamond formation could have taken place
   (around 400 million years), which may explain the small average size
   and unusual physical characteristics of Argyle diamonds. Diamonds found
   in the Argyle pipe are predominantly eclogitic, meaning that the carbon
   is of organic origin (see Natural history of diamonds).

   In addition to the pipe itself, there are a number of semi-permanent
   streams that have eroded away portions of the pipe and created
   significant alluvial deposits of diamonds. These deposits are also
   actively mined.

Production

   The Argyle diamond mine leads the world in volume production of
   diamond, averaging annual production of 35 million carats (7,000 kg),
   or about one third of global production of natural diamonds. Production
   peaked in 1996, when 42 million carats (8,400 kg) were produced. Of
   this quantity only 5% is considered gem-quality, with the rest being
   either near-gem quality or industrial grade; this is somewhat below
   world averages of about 20% of mined diamonds qualifying as gem-grade.
   Since the mine's opening in 1985, it has produced over 600 million
   carats (120,000 kg) of diamonds.

   Most of Argyle's gem quality production is in brown diamonds. These
   diamonds are usually difficult to sell, although Rio Tinto has seen
   some success in a decade-long marketing campaign to promote brown
   diamonds as champagne and cognac toned. In contrast, the company has no
   problems selling pink and red diamonds, which are very rare and in high
   demand, therefore commanding premium prices. The pink diamonds are
   processed and sold as polished diamonds by a specialised team based in
   Perth to customers world wide. The highlight of the coloured diamond
   industry calendar is the annual Pink Diamond Tender. Access to its
   collector's edition catalogue and website access in itself is highly
   sought after.

   The mine has ore processing and diamond sorting facilities on site.
   Once diamonds are removed from the ore and acid washed, they are sorted
   and shipped to Perth for further sorting and sale. A significant
   quantity of diamonds are cut in India, where low costs of labor allow
   small diamonds to be cut for a profit; this is especially relevant to
   the Argyle mine, which on average produces smaller rough diamonds than
   other mines do.

Diamond characteristics

   The diamonds produced at the Argyle diamond mine are of on average low
   quality. Only 5% of mined diamonds are of gem quality, compared to a
   worldwide average of 20%; of the remaining 95%, they are about evenly
   split between classifications of "near gem quality" and industrial
   grade. 80% of Argyle diamonds are brown, followed by 16% yellow, 2%
   white, 2% grey, and less than 1% pink and green. Despite the low
   production volume of pink and red diamonds, the Argyle mine is the only
   reliable source in the world, producing 90 to 95% of all pink and red
   diamonds. Most Argyle diamonds are classified as type 1a (see material
   properties of diamond), and have low levels of nitrogen impurities,
   their colour resulting instead from structural defects of the crystal
   lattice. Argyle diamonds tend to fluoresce blue or dull green under
   ultraviolet light, and blue-white under X-ray radiation. The most
   common inclusion is unconverted graphite, followed by crystalline
   inclusions of orange garnet, pyroxene, and olivine.

Reserves

   Initial proven reserves of the Argyle mine were 61 million metric tons
   (tonnes) of ore, with an average ore grade or diamond concentration of
   6.8 carats (1.36 g) per tonne, about 400 million carats (80,000 kg).
   Further estimated reserves of 14 million tonnes of ore, at a grade of
   6.1 carats (1.22 g) per tonne, 85 million carats (17,000 kg), also
   existed. As of 2001, reserves and resources in the open-pit mined area
   contain 220 million tonnes of 2.5 to 3.0 carat (500 to 600 m) per tonne
   graded ore, sufficient to sustain current production rates until 2007.
   The ore grades at the Argyle mine are unusually high, with most
   commercial diamond mines averaging grades of 0.3 to 1.0 carats (60 to
   200 mg) per metric ton. Alluvial deposits of diamonds are believed to
   have been exhausted.

   In 1995, drilling samples taken from about 300 metres below the floor
   of the pit indicated the possible presence of about 100 million tons of
   ore, with an estimated grade of 3.7 carats (740 mg) per ton. In 1998,
   it was decided to instead move mining operations toward the west ridge
   of the mine, where 64 million tons of ore graded at 2.6 carats (520 mg)
   per ton are located.

   An exploration decline was constructed at a cost of 70 million
   Australian dollars to evaluate the economics of mining diamonds from
   the diamoniferous pipes below the floor of the open pit; these reserves
   would be mined underground (via sub-level caving), rather than the open
   pit method currently used. In late-2005 Rio Tinto Ltd concluded that
   the operation was economically feasible and pre-production construction
   of the underground mine is due to commence in early to mid 2006.

Economics

   The Argyle diamond mine is economically feasible because its large
   reserves and high grade ore offsets a low average diamond weight value.
   The estimated value of Argyle diamond production is only 7 USD per
   carat ($35/g); this compares to values of 70 USD per carat ($350/g) for
   diamonds produced at the Diavik mine in Canada, and 170 USD per carat
   ($850/g) at the Ekati mine (also in Canada). However, Argyle has two to
   four times the concentration of diamonds (ore grade) of these mines.
   This makes extraction economically feasible, as mine costs are mostly
   related to the amount of ore processed, not the amount of diamond which
   is extracted.

History

   Small quantities of alluvially deposited diamonds have been known in
   Australia since the late 19th century, first found by prospectors
   searching for gold. However, no source volcanic pipe deposit was
   apparent. A systematic search of Western Australia for the source of
   these diamonds began in 1969, and on October 2, 1979, the Argyle pipe
   was discovered. Over the following three years, the deposit was
   assessed for economic viability, and in 1983 the decision was made to
   commence mining operations. Alluvial mining operations commenced
   immediately, while the open pit mine was constructed over a period of
   18 months at a cost of 450 million Australian dollars. The mine was
   commissioned in December 1985.
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