Argyle was the fourth-largest diamond-producing mine in the world by volume, averaging annual production of . Production peaked in 1994, when were produced. Argyle's open pit mine produced over of rough diamonds. Most of Argyle's gem-quality production was in
brown diamond. 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 diamonds in
pink, purple and red tones, which are very rare and in high demand, therefore commanding premium prices. The pink diamonds were processed and sold as polished diamonds by a specialised team based in Perth to customers worldwide. The mine had ore processing and diamond sorting facilities on site. Once diamonds were removed from the ore and acid washed, they were sorted and shipped to Perth for further sorting and sale. A significant quantity of diamonds were cut in India, where low costs of labour allowed small diamonds to be cut for a profit; this was especially relevant to the Argyle mine, which on average produced smaller rough diamonds than other mines.
Diamond characteristics The diamonds produced at the Argyle diamond mine were of an average low quality. Only 5% of mined diamonds were of gem quality, compared to a worldwide average of 20%. (Author Janine Roberts contends that the "near-gem quality" rating is subjective and misleading because these diamonds can be cut into gems if desired.) The remaining 95% 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 was 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.
Annual Diamond Tender Each year from 1984 to 2021, a small collection of the best pink diamonds was offered in an exclusive, invitation-only, sale known as the Argyle Pink Diamond Tender. For every of rough pink diamonds produced by the mine, only polished was offered for sale at the tender. In March 2009, Argyle announced their first tender of rare blue diamonds. The "Once in a Blue Moon" collection was sourced over several years, and comprised a range of precious blue and violet diamonds, which weighed in total . In 2016, the tender became the highest-selling tender in its 20-year history, according to the Diamond Investment & Intelligence Center. In conjunction with the mine's closure, the 2020 tender set further records, as it was the second last such sale. Following the closure, the final tender in 2021 delivered more record breaking results.
Reserves Initial proven reserves of the Argyle mine were 61 million
tonnes of ore, with an average
ore grade of per ore tonne, about . Further estimated reserves of 14 million tonnes of ore, at a grade of 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-per-tonne (500 to 600 mg) 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 per metric ton. Alluvial deposits of diamonds are believed to have been exhausted. In 1989–1990, drill samples were taken from over 300 m below the floor of the pit. At the end of 1989 around 238 million tons of ore, with an estimated grade of 3.7 carats per ton were quoted as a resource. Much of this resource was below the open pit and was the subject of an underground mining study carried out in the mid-1990s. In the late 1990s, part of the west wall of the open pit mine, containing 25 million tonnes of waste rock, began to collapse. In 1998, the decision was made to remove or "cut back" the unstable part of the west wall which freed up further economic ore in the south part of the open pit. This cut back cost around $100 million. Argyle announced on 2 March 2018 that its estimated Ore Reserves decreased by 13 million tonnes to just 16 million tonnes. They mined 5 million tonnes in 2017. A more conservative view on future production performance and grades could bring forward the economic shut-off criteria. An exploration decline was constructed at a cost of
A$70 million to evaluate the economics of mining diamonds from the diamondiferous pipes below the floor of the open pit; these reserves would be mined underground (by
block caving), rather than the open-pit method used up to then. In late 2005, Rio Tinto concluded that the operation was economically feasible. Pre-production construction of the underground mine commenced in early to mid-2006.
Economics The Argyle diamond mine was economically feasible because its large reserves and high-grade ore offset a low average diamond value. The estimated value of Argyle diamond production was only US$7 per carat ($35/g); this compared to values of $70 per carat ($350/g) for diamonds produced at the
Diavik mine and US$170 per carat ($850/g) at the
Ekati mine, both in Canada. However, Argyle had two to four times the concentration of diamonds (ore grade) of these mines. This made extraction economically feasible, as mine costs are mostly related to the amount of ore processed, not the amount of diamond extracted.
Underground expansion In 2005,
Rio Tinto was given the go-ahead to a future expansion project, moving it from an open pit to an underground mine. This plan was postponed; in September 2010 Rio Tinto announced fresh plans to develop an underground mine beneath the existing pit, increasing annual production to 9 million tonnes of ore. The project was completed by 2013, however with more expensive operations to run deeper mining, the mining costs soon began to outweigh the diamond yield. ==Closure==