Launched in 1958, the Mirny diamond mine is the deepest open pit diamond mine in the world. The mine is 525m deep, 1.2m wide and has a diameter of 1,200m. Operated by Alroso, the mine yielded approximately two million carats of diamonds annually, when in operation. It was abandoned in 2000 after being exhausted. In 2009, Mirny was converted into an underground mine at a cost of RUR22bn. Commissioned officially in August 2009, the underground mine is expected to breakeven within three
years.
Underground mining
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The underground mine has been developed using a 1,050m deep cage shaft and a 1,036m deep skip (cargo) shaft. The sinking of the cage shaft and the skip cargo shaft started on March 2002 and August 2003, respectively. The shafts were sunk at the rate of 50m a month.
To access the minerals, a 10km mine tunnel was constructed horizontally at a rate of 320m a month. Nearly 350,000m3 of rock mass was excavated from the mine, while the ore body was accessed at a depth of 310m. A head frame was also constructed up to a height of 60m.
The underground mine has an estimated life of 50 years. In its lifetime, the mine will produce one million ton of ore per annum. In 2009, the mine is expected to supply 150,000t of diamond-bearing ore for processing.
Geology
The open pit Mirny mine is located in the Sakha Republic of Eastern Siberia in north-eastern Russia below the Arctic circle.The mine was built over the Malaya Botuobiya kimberlite field that contains four kimberlite-filled pipes of its total eight pipes. Large tracts of permafrost which extended up to a depth of 1,600ft, lied around the mine. The Mirny mine had a highly continental climate with temperatures dropping down to -50F to -70F.
Exploration
The mine was operated using the open pit exploration method since 1958. In the initial phase, the mine was explored to a depth of 600m and gradually deepened to 1,235m. The diamond reserves were assessed at the same depth. Minerals were brought up to the surface of the mine via 20ft tall rock-hauling trucks. The trucks travelled along a spiral road that started from the top of the hole and ended at its basin. The trucks used to take two hours to complete one round-trip.
Due to the harsh climatic conditions in Siberia, mining at Mirny was initially hindered. A harsh winter saw mining equipment including oil, rubber tires and steel rigging freeze or crack. Jet engines were used to melt layers of ice while dynamite was used to blast holes into the frozen ground.
As the mine was surrounded with frozen ground, the processing plant was set up 20 miles away from the mine site. Gradually, with the expansion of the mine a city was built for the mining workers. Named Aikhal, the city was constructed on steel poles to provide support.
Production
In its initial years, the open pit mine annually produced ten million carats of diamonds. As the upper layers of the mine contained high diamond content, a relative high portion of the production was of gem quality.
The bottom of the pit contained 1.5-2 carat of diamonds per ton. As the mining operations reached the bottom of the pit, the production rate slowed to two million carats a year. In its operation between 1957 and 2000, the mine produced $17bn-worth of diamonds.
