When the world's richest diamond mine starts reaching the end of its life, what do you do? Obviously, you extend it. That is what Debswana is planning at its Jwaneng mine in southern Botswana, which produced 13,5 million carats of gem quality stones last year, writes Business Day.
The question is, "How?"
Debswana, a joint venture between the Botswana government and mining company De Beers, has a choice to make when the open-pit mine reaches the end of scheduled production in 2022.
By that stage, the pit will be 650m deep. The question is whether to widen a hole that is 2,4 km long and 1,6 km wide to permit digging lower as an open pit, or whether to begin an underground mining operation.
The company has had to publicly change its mind on the issue. A document produced in October last year to commemorate 25 years of operations at the mine states that it would go underground.
But Jwaneng's GM, Balisi Bonyongo, now says they are not ruling anything out.
"New information has come out of the pre-feasibility work that indeed we need to consider a super-pit concept more, rather than blindly going underground," he says. Expanding open-pit operations to the necessary depth of 1000m - the "Cut 9 plan" - would mean moving vast volumes of earth and would be expensive.
But it is preferable to underground mining, as the extraction rate is higher and it is safer, Bonyongo says.
The company will decide this year which option to take. The future of this mine matters for Debswana. While it produces less by volume than the company's Orapa mine in central Botswana, Jwaneng's stones are larger and more valuable.
One found in 1993 weighed 446 carats.
This mine alone accounts for 70% of the company's earnings.
Bonyongo is unwilling to say how much the next step could cost. About R2,1bn is being spent on an expansion program known as Cut 8 to widen the circumference of the hole to deepen the mine from 350m to 650m.
This will more than double the volume of material moved, from 135000 tons a day to 350000. Cut 8 is due to start producing diamond-bearing kimberlite ore by 2016.
While he says the company would prefer to do open cut - that is where its skills lie and going underground would require different equipment and different engineering skills - it will be guided by the results of the research it is conducting. "That is all dependent on what the study trade-off tells us."
While open-pit may be preferable, it will be more expensive and disruptive. The mine yields about 10-million tons of ore a year. The current "stripping ratio", or proportion of waste to ore, is 4:1, meaning for every ton of ore mined, there are four tons of waste.
Under the Cut 8 program, that ratio will rise as high as 12:1. And if they go to Cut 9, rather than underground mining, "you're looking at 18-19," says James Kirby, Jwaneng's operations manager.
Thirty years ago, this site was an unremarkable patch of ground, covered, like much of the area, with a layer of dry sand. It was only thanks to the local ants that its riches were ever identified.
The ants were digging 20m to 30m below the surface to an unknown layer of kimberlite ore. Kimberlite is hydrophilic, or water-containing, and the ants used it as a water source. In consequence, they carried microscopic diamonds to the surface. The presence of these diamonds in the anthills led geologists to look at the area seriously.
The anthills are long gone. And Botswana is a richer place.