Botswana’s newly established Ghaghoo diamond mine, owned and operated by London-listed Gem Diamonds, will be operated on a ‘small mine basis’ until the value of the resource is fully understood, Gem Diamonds tells Mining Weekly.

The Ghaghoo mine was officially opened on September 5 at an event attended by Botswana President Lieutenant General Seretse Khama Ian Khama. Gem Diamonds’ wholly owned subsidiary, Gem Diamonds Botswana, holds a 25-year mining licence for Ghaghoo, which is located in the south-east portion of the Central Kalahari Game Reserve.

Gem Diamonds Technical Services COO Alan Ashworth tells Mining Weekly that production tunnels are currently being developed at the Ghaghoo mine, where personnel are undergoing training ahead of increased production levels planned for early next year.

In 2011, Gem Diamonds issued a technical report that tabled a three-phased approach to the operation of Ghaghoo mine. However, Ashworth says the development plans have changed a little since then.

“The original plans contemplated a formal, phased expansion programme for the underground mine, whereas what we have done is to build a small underground mine without a feasibility study which we will now operate until we have ascertained the full value of the ore in terms of grade and dollar per carat,” he says.

Ashworth explains that the information gathered from this initial phase will be used to inform a detailed feasibility study. “Once we have this feasibility information, we will then decide on a way forward in terms of mine production levels, the expected life-of-mine (LoM) and subsequent expansion projects,” adds Ashworth.

Gem Diamonds’ aim for the Ghaghoo mine in 2015 is to ramp up production to mine and treat 720 000 t/y of ore, producing 210 000 ct of diamonds. Based on the production results, Gem Diamonds will review options post-Phase 1 for the future feasibility of the mine.

Mine Planning
The kimberlite pipe at Ghaghoo has a suboutcrop area of about 11 ha area under the Kalahari sands. At current production rates, the estimated LoM is ten years from an estimated resource of 100-million tons, leaving plenty of opportunities for expansion.

Gem Diamonds plans to slowly build up to a production rate of 720 000 t/y, with steady monthly outputs of 60 000 t. “We expect to reach the full production rate at the end of the first quarter of 2015,” he says.

Ashworth says that the worst- case scenario envisaged is that Gem Diamonds will continue to mine at the Phase 1 production rate. “However, we expect the reserve value to be such that it will make sense to begin to examine mine expansion scenarios to fully exploit the entire kimberlite resource,” adds Ashworth.

Although initially targeting a small, high-value section of the pipe, Gem Diamonds sees opportunities to mine the full area of the pipe.

Processing the Ore
Gem Diamonds opted for auto- genous milling as the primary ore-processing technique with advantages being its simplicity and reduced plant footprint. The alternative would be to employ various stages of crushing, which would require a larger and more complex plant, he explains. “Milling does the bulk of the work in terms of processing the ore, the product from which goes straight to dense-media separation processes for further concentration before final recovery,” says Ashworth, adding that the use of a mill helped reduce the capital cost.

He notes that autogenous milling is not a new technique for diamond mines, as it is used extensively in Russia and is gaining popularity in Southern Africa.

The milling plant at Ghaghoo has started operating and is currently undergoing final commissioning aimed at determining the optimal settings.

Challenges
The greatest challenge that faced Gem Diamonds in rolling out the Ghaghoo project was gaining access to the kimberlite pipe, which entailed sinking a decline through about 70 m to 80 m of Kalahari sand before entering the more competent country rock, Ashworth tells Mining Weekly.

Another significant issue arose once the mine reached its current depth of 154 m. This was as a result of an earlier-than-expected water ingress.

Ashworth tells Mining Weekly that, while Gem Diamonds expected to encounter some groundwater while developing the mine, such quantities were not expected at this shallow depth and were only envisaged to become significant at about 400 m.

“Somehow, water from the main aquifer at 400 m has found its way up to the level we are at now,” he explains, adding that systems to drain the water have been implemented to handle the water ingress at its current rate. “We do not expect the current ingress rate to increase until we get closer to the aquifer, at which point we will have implemented a full dewatering system.

Letšeng Diamond Mine, Lesotho
In addition to the developing Ghaghoo mine, Gem Diamonds has one other producing mine – the Letšeng mine, in Lesotho.

The Letšeng mine is renowned for its regular production of large top colour white diamonds, making it the highest-average dollar per carat kimberlite diamond mine in the world. Since Gem Diamonds acquired the mine in 2006, Letšeng has produced four of the 20 largest white gem-quality diamonds ever recorded.