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Argyle Diamond Mine

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The Argyle Diamond Mine, owned by Rio Tinto, is located in the Kimberley region, in the northeastern part of Western Australia. It is the world’s largest diamond mine in terms of volume,[1] accounting for about one-fifth of the world’s natural diamond production. Since opening in 1983, the mine has recovered about 560 carats of rough diamonds, worth US$6 billion.[2] Diamond production at the mine peaked in 1994 with 42.8 Mcts.[3]

The mine was first constructed as an open-pit mine after the central pipe of diamond-bearing rock named AK1 was discovered by geologists in 1979. When the production of the open-pit mine neared completion in 2001, feasibility studies were carried out to determine if taking the mine underground with the construction of a block cave would be cost-effective. In 2005, Rio Tinto decided it was, and announced the beginning of the construction of an underground mine.[4] Meanwhile, open-pit operations at the mine ended in 2008.[5]

Contents

[edit] History

[edit] Early Exploration

Exploration of diamonds in Kimberley began in 1972 when Ashton Mining Ltd. began to search for diamonds under the Kalumburu Joint Venture.[6] Early studies confirmed that rock found in the Kimberley area was very similar to diamond-bearing rock located in the northern region of Western Australia. As exploration commenced, traces of small diamonds and alluvial deposits were discovered. The idea to build a diamond mine in the region was proposed by a geologist with diamond exploration experience in Africa.

[edit] CRA Enters the Picture

In 1976 CRA, later becoming Rio Tinto Ltd., assumed management of the exploration group known as the Ashton Joint Venture.[7] CRA set specific exploration objectives to justify the cost of building an open-pit mine. The first objective was to discover a diamond bigger that a quarter of a carat. The second objective was to find a diamond-bearing pipe with a high diamond grade. A specific search method of collecting sediment samples from the bottom of water ways was used to meet both of these objectives.

[edit] Discovery of Diamonds

By 1979 a sufficient amount of alluvial diamonds were found from samples taken at Smoke Creek. Crews continued to extract sediment samples as they worked their way along the creek.[8] The main pipe, called AK1, was found when one geologist actually stumbled across a small diamond lodged in an anthill. It took another three years to access the central ore body—the AK1 pipe.

[edit] Establishing an Open-pit Mine

The decision to construct a permanent open-pit mine site at the AK1 deposit came in 1983. The Argyle Diamond Mine Joint Venture and the Ashton Exploration Joint Venture were established in place of the Ashton Joint Venture. Construction of an open-pit mine, processing plant, and supporting infrastructure commenced in 1984, taking 18 months to complete. The total cost of construction was estimated at AU$450 million. The mine, named after the lake north of the site, was officially commissioned in 1985.[9]

[edit] Composition of AK1

The Argyle ore body marked the first occurrence of commercial diamonds not found embedded in kimberlite—the usual host rock in which diamonds are discovered—but lamproite. AK1 actually stands for Argyle Kimberlite because geologists assumed the diamond-bearing rock would be kimberlite.[10] Similar to kimberlite, lamproite is a type of volcanic rock. The diamonds produced from the Argyle mine were unique—harder than diamonds mined elsewhere in the world. Some speculate that the diamonds from the Argyle mine could be as old as 4.25 billion years.[11] Most of the diamonds from the mine are small brown or yellow diamonds. The mine also produces a small amount of near-colorless and colorless rough diamonds and is also reputed for producing over 90 percent of the world’s supply of rare pink diamonds.[12]

[edit] Open-pit Operations

The actual open-pit mine site became fully operational in 1985. An open cut measuring 1.2 miles (2 km) long and 0.6 miles (1 km) wide was dug to expose the AK1 pipe. In total, the entire open-pit mine covered an area of 741 acres (300 ha).[13] Approximately 80 million tons of rock was extracted from the open cut on an annual basis, producing about 10 million tons of ore. After processing, the ore yielded about 30 million carats of rough diamonds per year.[14] The ore was broken up using the drill-and-blast method. The amount of rock liberated with each blast was equal to about 3,000 tons.[15] The resulting overburden was then loaded and removed with a truck and shovel operation. Excavators capable of holding up to 45 tons of ore were used to load the rock into haul trucks.[16] Once loaded, the diamond bearing ore was transported to a primary crusher located 1.6 miles (2.5 km) from the pit.

[edit] Ore Processing

Once the ore was delivered to the processing plant by haul truck, diamonds were extracted from the ore using a six-stage process. The first stage involved placing the mined ore into a primary crusher that reduced them down to a size no bigger than six inches (15 cm).[17] The primary crusher had the capacity to crush approximately 2,500 tons of ore per hour. A secondary crusher was also used—the product from this crusher got conveyed to a primary stockpile. The ore was extracted from the primary stockpile and conveyed to high-pressure rollers where it was crushed to 1.2 inches (3 cm). The ore was then cleaned, screened, and separated according to size. Oversized ore pieces larger than 0.6 inches (1.5 cm) were crushed more and material smaller than 0.1 inches (0.15 cm) was rejected. The ore in between was conveyed to a Heavy Media Separation (HMS) Plant Feed Stockpile.

[edit] Diamond Extraction

Most of the ore material containing diamonds was processed in a cyclonic separation plant. A heavy media consisting of ferrosilicon powder was mixed with water. The solution separated the diamonds and other heavy minerals from the ore producing a diamond-rich concentrate. This diamond concentrate was fed through a recovery plant that used special x-ray sorters to detect flashes of light emitted from the diamonds because diamonds tend to fluoresce when exposed to x-ray light. The flashes of light signaled a microprocessor to fire an air blaster valve, blowing the diamonds into a collection box. The diamonds were then acid-cleaned, washed, and weighed.

[edit] Equipment Used

[edit] Refurbishment/Renovations/ Recent Projects

[edit] Conversion to an Underground Mine

The process of converting the Argyle Diamond mine from an open-pit operation to an underground operation began in 2003. Work on a block cave underneath the existing pit started with the development of an exploratory decline tunnel measuring 2.5km.[18] The first blast was completed by Roche JR. The information gathered during the construction phase was used in a feasibility study. By December 2005 work on the decline was finished and Rio Tinto determined that developing an underground mine would be cost-effective. The total cost of the project is expected to be around US$760 million, according to a Rio Tinto press release.[19]

The block cave is being developed around a 60 million-ton ore reserve with the productive capacity of 9.5 million tons of ore per year being delivered by a conveyor lift to the surface. It is estimated this amount of ore will yield about 20 to 25 million carats of diamonds a year.[20] Another US$150 million is being spent on the development of a low-grade open-pit operation designed to complement and ramp up underground operations.[21] Combined, the operations will extend the mine’s productivity to 2018.[22]

Full construction on the underground block cave moved ahead in 2006 with a targeted completion date set for early December 2010.[23] Work on the block cave is being carried out by Macmahon Holdings.[24] The underground block cave operation will feature two access declines, one of which will accommodate the conveyor system. About 60 percent of the work has been completed, including about 12 of 21 miles (20 of 34 km) of tunnels planned.[25] Other components of the operation will include the excavation of two underground crusher chambers, vertical development, and cave draw production. Upon completion, the underground Argyle mine will represent one of the largest block caving operations in the world.

[edit] Financial Setbacks

In January 2009 Rio Tinto pulled the plug on the construction of the underground mine. A slowing economy had forced the debt-laden company to halt production for at least three months. Macmahon laid off about 140 of the 330 to 360 employees hired to work on the project.[26]

[edit] References

  1. Diamonds in Australia-Argyle Diamond Mine. 06-03-2009.
  2. The Argyle Diamond Mine. 06-03-2009.
  3. Statistics. Argyle Diamond Mine Official website. 06-03-2009.
  4. Underground Mining. Argyle Diamond Mine Official website. 06-03-2009.
  5. Argyle Diamond Mine. Mining Technology. 06-03-2009.
  6. Argyle History. Argyle Diamond Mine Official website. 06-03-2009.
  7. Argyle History. Argyle Diamond Mine Official website. 06-03-2009.
  8. Argyle History. Argyle Diamond Mine Official website. 06-03-2009.
  9. Argyle History. Argyle Diamond Mine Official website. 06-03-2009.
  10. Open Pit Mining. Argyle Diamond Mine Official website. 06-03-2009.
  11. Diamonds in Australia - Argyle Diamond Mine. World of Diamonds. 06-03-2009.
  12. Rio Tinto to Slow Production at Argyle Mine. JCK Online. 06-03-2009.
  13. Argyle Diamonds. Kununurra Tourism website. 06-03-2009.
  14. Argyle Diamonds. Kununurra Tourism website. 06-03-2009.
  15. Open Pit Mining. Argyle Diamond Mine Official website. 03-06-2009.
  16. Open Pit Mining. Argyle Diamond Mine Official website. 03-06-2009.
  17. Open Pit Mining. Argyle Diamond Mine Official website. 03-06-2009.
  18. Underground Mining. Argyle Diamond Mine Official website. 06-03-2009.
  19. News. Argyle Diamond Mine Official website. 06-03-2009.
  20. Argyle diamond mine underground development, Australia. Mining Weekly. 06-03-2009.
  21. News. Argyle Diamond Mine Official website. 06-03-2009.
  22. Argyle Diamond. Mining Technology. 06-03-2009.
  23. Argyle Diamonds: World’s Biggest Diamond Pit Goes Underground.” International Mining, January 2009, pg 8 – 14.
  24. Argyle Diamonds: World’s Biggest Diamond Pit Goes Underground.” International Mining, January 2009, pg 8 – 14.
  25. Argyle Diamonds: World’s Biggest Diamond Pit Goes Underground.” International Mining, January 2009, pg 8 – 14.
  26. Rio Sacking staff at Argyle diamond mine. The Australian. 06-03-2009.

[edit] External Links