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Operations at Letšeng – the world’s highest diamond mine – push ahead around the clock at this internationally famed operation, which continues to yield gemstones of magnificent quality.

In discovering the origins of the world famous Lesotho Promise, a 603 carat gemstone of remarkable quality, visitors to Lesotho’s Letšeng Diamond Mine have to travel steep winding roads surpassing 3 200m before reaching their destination. On arrival, the view is majestic, with sweeping vistas of snow capped peaks in all directions bearing testimony to this remarkable geological find, first discovered in 1957.

Owned jointly by London Stock Exchange listed entity, Gem Diamonds (70%) and the Kingdom of Lesotho (30%), this is a complex operation, with the loading and haulage component carried out by a 100% Caterpillar equipment fleet run by specialist contractor, Matekane Mining Investment Company (MMIC).

As John Houghton, Letšeng’s assistant general manager responsible for production explains, it’s critical that the earthmoving fleet keeps pace with the mine’s 365 day 24/7 programme, despite the constant threat of inclement weather. “Snow, which can fall at any time of the year, can close in suddenly, bringing temperatures down to as low as minus 15 degrees Celsius, but that doesn’t slow down the mining operation, which remains at full tilt,” says Houghton.

Lesothoentrepreneur and managing director of MMIC, Sam Matekane, secured the contract mining account for Letšeng in October 2007, and MMIC currently has approximately 200 personnel on site, supported by a technical maintenance team of 26 from Barloworld Equipment. (Barloworld Equipment is the exclusive Caterpillar dealer in southern African.) The mine’s technical capabilities have been further enhanced following Letšeng Diamond Mine’s R8 million investment in new workshop facilities.

Despite the challenges of a depressed global economic climate, Letšeng remains one of the few mines around the world that hasn’t stepped back on production. Mining is intense, entailing the excavation and haulage of around 450 000t per month of kimberlite and around 660 000t of country rock (basalt), with extraction occurring in two places: at the main pipe and the satellite pipe, both of which have currently reached a depth of around 120m.

Final depths for the satellite and main pipe are estimated at 450m and 550m, respectively, with an estimated life of mine at current production volumes of more than 30 years. Output on a daily basis equates to approximately 37 000t of which around 22 000t is waste and the rest ore, the latter being processed via the mine’s two dense media separation plants.

“The stripping ratio on the satellite pipe is around 4:3:1 (or 4,3t of basalt for every ton of ore),” expands Houghton, illustrating the importance of the effective planning and utilisation of the earthmoving fleet.

“When you consider that Letšeng typically yields below 2 carats per 100 tons it’s clear that we expect maximum efficiency from MMIC and our process plant contractors to keep working costs at a minimum.”

Cat 740’s thrive at altitude

Currently, Letšeng Diamond Mine carries out a standard drill and blast programme, followed by excavation and overland truck haulage, in this case using MMIC’s Caterpillar 740 articulated trucks (ATs).

MMIC’s Caterpillar fleet includes eighteen 740 ATs – 17 of which are constantly in production, with one in reserve – with the oldest Cat 740 AT in operation having already recorded 30 000 hours (or around five years in service). Added to these units are two Cat 385C and two 365 hydraulic excavators; support equipment in the form of two D6 and D8 dozer, respectively, and a Cat 824 wheel dozer; two 140H motor graders; two 980 front end loaders; two wheeled excavators, comprising a 316 and a 318 unit, both fitted with hydraulic hammers; plus a 730 converted to a water cart. Dozers are used on the waste dumps; to control the pit floors; and for tailings processing, with the bulk of their efforts focused on rehabilitation, namely, topsoil stripping and stacking.

“The success of our relationship with MMIC depends on close teamwork to ensure that Letšeng’s production targets are maintained and exceeded,” explains senior salesman, Tom Ferreira, from Barloworld Equipment’s Bloemfontein branch. Ferreira covers the Lesotho region and is responsible for the Letšeng account.

A view which MMIC’s contracts director, Ferreira Coetzee, says is supported by Sam Matekane’s decision to opt for a 100% Caterpillar fleet to maximise service and after-sales support.

“Given the challenging underfoot conditions, the Cat AT’s are far better suited than rigid off-highway trucks in this terrain due to their all-wheel drive capability, especially during the heavy snowfalls traditionally experienced in the autumn and winter months,” explains Coetzee. “As production ramps up going forward, we anticipate that MMIC will need to expand its 740 fleet to around 30 units to keep pace with Letšeng’s demand.”

MMIC works on a three shift system, with the average truck haul distance estimated at around 2,3km. Productivity improvement assessments are ongoing and include maintenance of the haul roads and underfoot conditions at the loading and tipping sites.

With the Cat 740s taking care of the haulage, the core responsibility of loading these class leading workhorses are the Caterpillar 385 and 365 hydraulic excavators.

“The Cat 365 units are used to load the kimberlite (typically a blend of ore from the main and satellite pit) – utilising some 45% of the AT fleet – with the Cat 385s loading the basalt,” says Coetzee. “The blast size ranges at around -750mm for the basalt, and -450mm for the kimberlite ore, with fragmentation at between 5 and 10% for oversize, which is where we use our Cat wheeled excavators for final size reduction. Obviously, the objective is to minimise the amount of rehandling needed to optimise efficiencies.”

As a possible allied extraction approach, Coetzee says that the mine is also considering the use of a “rip and load” methodology following an in-depth feasibility study conducted by Barloworld Equipment in conjunction with Caterpillar’s Work Tools Centre in The Netherlands. Kimberlite has a relatively low UCS, so rip and load (using a specialised bucket incorporating an excavation ripper) could, in certain applications, contribute to a reduction in the need to blast in specified mining areas.

In the meantime, the quest for the next Lesotho Promise, and its famed predecessor, the 601 carat Lesotho Brown (named for its distinctive colour) continues around the clock, joining a steady stream of Letšeng diamonds that are today worn by some of the world’s best-known celebrities.

Adds Houghton: “There are more than 100 kimberlite pipes in Lesotho, but only a handful are said to be diamond bearing. Letšeng has certainly proven to be the most successful.”