HomeDiamonds & GemstonesBulk sampling at Mothae – Lesotho

Bulk sampling at Mothae – Lesotho

Motapa Diamonds and partner Lucara Diamond Corporation have completed diamond recovery work on the 100,000 tonne bulk sample program at the Mothae sample project in Lesotho.

Processing of 15,390 dry tonnes yielded 1,519 stones for 715.79 carats, giving a sample grade of 4.65 carats per hundred dry tonnes (cpht). The sample produced 48 stones greater than two carats, 13 stones greater than five carats and one stone greater than 20 carats. The average size of diamonds recovered from the sample was 0.47 carats per stone.

The samples were processed through a dense media separation (DMS) plant designed to recover diamonds in various size fractions ranging from 2.0 mm to 18 mm. Heavy mineral concentrate produced by the DMS plant was processed over a continuous grease belt for primary diamond recovery, with the exception of the very coarse fraction (+16 mm) which was hand sorted in a secure glove box. All grease belt tailings material was processed through an EVE X-ray diamond recovery unit.

Previously, in early February this year, a 2,452 meter core drilling program was completed and the results used as a basis for geological modelling of the Mothae kimberlite. These results, combined with kimberlite bulk density estimates derived from drill core measurements, provide a global tonnage estimate to a depth of 200 meters below the base of overburden of 38.62 million tonnes. Of this tonnage, the southern lobe of the Mothae pipe, which shows the greatest potential in terms of diamond grade, contains an estimated 23.56 million tonnes, though this is not intended to imply a resource tonnage.

This project is the latest in a country with a long history of diamond production, mainly from alluvial deposits. De Beers mined the Letseng la Terae kimberlite between 1976 and 1982 and produced about 280,000 carats. The Letseng kimberlite was recently put back into production and has produced some of the highest dollar value per carat diamonds in the world. Diamond grades at Letseng are very low, between two and 2.5 cpht of kimberlite, but diamond values are over US$1,000 per carat.

This high average diamond value is a result of consistent but infrequent recovery of very large, exceptional quality type IIA diamonds. Other Lesotho kimberlites of economic interest and in varying stages of development and evaluation include the Liqhobong, Kao and Kolo occurrences.

Mothae is located 6.5 kilometres north-west of the Letseng diamond mine, some 130 kilometres north-east of the capital city Maseru. It is situated within the Lesotho highlands at 2,900 metres above sea level, which is about 200 m lower than the Letseng mine. Access to the Mothae pipe is by tarred road from Maseru and a five kilometre long road leading directly to the Mothae kimberlite has been upgraded to provide year-round access to the site for equipment and material for plant construction and kimberlite excavation.

Kimberlite is not exposed at surface at Mothae and is covered by extensive gravels and peat deposits to an average depth of 4.5 m. Historical exploration work, undertaken prior to 1973 included ground geophysical surveys and excavation of several deep pits. This work indicated that the pipe has an elongated, bi-lobate outline, with a surface area of about 8.8 hectares. It extends roughly 600 m north-south, with widths of 150 m in the north and 250 m in the south.

A pit sampling program undertaken in 1966 by Lonrho yielded a grade of about 2.8 carats per hundred tonnes, similar to that of Letseng. However, given the low grade of the kimberlite and the expected rarity of high-value stones, it is likely that the diamond parcel obtained from the bulk sample, estimated at about 16,000 tonnes, was not of sufficient size to reliably establish diamond value.

Motapa secured exclusive rights to Mothae on the premise that the kimberlite may contain diamonds of similar value to those recovered from the adjacent Letseng mine. Estimation of the diamond value at Mothae will require a large bulk sample of the kimberlite in the order of 100,000 to 150,000 tonnes. To this end, Motapa commenced a major bulk sampling program at Mothae.