Argyle diamond mine at the Ekati mine, both 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 extracted. 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
Williamson diamond mine a result of this poor performance, Tanzania invited De Beers to purchase back into the mine, which the company did. In 1994 De Beers bought 75% of the Williamson mine, with Tanzania retaining the remaining shares. Since 1994, De Beers has acted to improve the performance of the Williamson diamond mine by trimming the work force, making capital investment in equipment, and bringing in expertise in technology, technique, and exploration. Although the mine is still viewed as a marginal performer due to its low ore grade, De Beers has identified avenues for development that would keep the mine operating into