Diamonds are mined underground when:
Open pit mining becomes uneconomic - As the open-pit excavation around a kimberlite pipe goes deeper, we balance the cost of waste removal with the rate of diamond recovery. At an appropriate point in the life of the mine, if the pipe continues to produce a high quality diamonds, and is of a suitable structure, we may decide to mine it underground. Debswana’s Jwaneng Mine in Botswana and DBCM’s Venetia Mine in South Africa are likely to “go underground” at some time in the future.
The kimberlite deposit is not vertical - Kimberlite is usually found in cone shaped, vertical pipes, widening toward the surface, reflecting their volcanic origins. Occasionally, horizontal and sloping deposits are discovered, where weaknesses in the structure of the host rock has allowed the kimberlite to form branches and dykes. If these occur at sufficient depth, we mine them underground. De Beers Canada’s Snap Lake Mine, in Canada's Northwest Territories, has this kind of deposit.
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