Modern Mining October 2016

DIAMONDS

independent experts we con- sulted – was that all the stones were between 100 and 200 car- ats when mined with two above 200 carats. So clearly there is a potential for Liqhobong to pro- duce large stones.” The pilot plant has now been dismantled and removed from site and the new operation now coming on stream is – to all intents and purposes – a brand new ‘greenfield’ mine. It will see the 8,6 ha main pipe – which contains a 23 Mct indicated and inferred resource at a grade of 28 cpht – being mined to a depth of 383 m over a 15-year mine life at a rate of 3,6 Mt/a – which will result in 1 million carats a year being produced. The process plant is a twin- stream (2 x 250 t/h) facility which utilises a conventional flowsheet comprising scrub- bing, screening, three stages of crushing, concentration via DMS

and final recovery using X-ray machines. The settings on the crushing circuit are such that any large stones in the 100-carat to 400-carat range that are liberated by the scrubbers should make it through to final recovery intact and undamaged. As regards the mining, a split shell design has been adopted for the open pit, with a 14 m bench and 28 m double bench design. Advantages of this approach include a reduced risk of ramp failure (once two splits join then a concentric ramp results), lower capex for the initial mining fleet (due to lower waste strip- ping) and less in-pit traffic congestion with multiple ore and waste faces available. The projected strip ratio is 2:1 over the life of mine. In terms of carat production, Liqhobong will be bigger than Gem Diamonds’ Letšeng, the current flagship of Lesotho’s diamond mining industry. Letšeng – which has an extremely low grade of just under 2 cpht – produces approxi- mately 100 000 carats a year, which is only a tenth of Liqhobong’s projected production, but is highly profitable because of the high quality of its stones and its proven ability to produce large diamonds on a regular basis. In 2015, it received an average of US$2 299 per carat for its production. Liqhobong’s diamonds, by contrast, are only expected to fetch an average price of US$165 per carat escalated over the life of mine. As

a detailed two-year mine plan and schedule was in place. The Liqhobong property hosts two kimber- lite pipes, a satellite pipe and the main pipe, as well as associated blows and dykes, with the cluster forming part of the Lemphane-Robert kimberlite belt of northern Lesotho. The two pipes were discovered in the 1950s but early efforts to mine at Liqhobong were concentrated on the 1,6 ha satellite pipe, which has a higher grade but generally produced poorer quality diamonds. European Diamonds built a plant in 2005 to treat the satellite pipe ore and this was later upgraded and expanded by Kopane and Firestone. The upgraded facility – or pilot plant, as it was known – operated from 2011 to 2013, treating ore from a trial mining campaign conducted on the main pipe. The intention was to generate some cashflow while the feasibil- ity study on mining the main pipe was being completed. “Around 1,18 Mt was mined and this yielded over 325 000 carats,” said Brown. “The plant was, however, not really fit for purpose and the trial mining exercise actually lost money. A major problem was that the gap on the primary crusher was less than 20 mm which resulted in large stones being broken. We subsequently did some reconstitution work on the broken diamonds, analysing ten stones including one of 74 carats. Our conclusion – and that of the

A split shell design has been adopted for the open pit.

24  MODERN MINING  October 2016