Letseng la Terai is a diamond mine located in Lesotho some 70 km of Mokhotlong.
Lesotho is a small mountainous enclave within South Africa with its entire area more than 1,000 m above sea level. About 40% of its population lives below the poverty line of a US$1.25 per day.
Kimberlite pipes were discovered up in the Maloti Mountains by United Nations geologists back in the 1950s. South African adventurers were the first to prove that the pipes were diamondiferous; they were quickly followed by artisanal diggers (1959-1968) and then large mining outfits like Rio Tinto (1967-1972) and De Beers (1977-1982).
In 2004 operations restarted under a new management but in 2006 Gem Diamonds stepped in and purchased 70% of the diamond mine for 118.5 million dollars; the balance of interest is being held by the Lesotho government.
Letseng la Terai is a mine better described by superlatives: At 3,100 m altitude it is the world's highest diamond mine; at just over 2 carats per 100 tones (cpht) it is the world's lowest grade kimberlite mine; at over $1,900 a carat it has the world's highest average cost per carat almost 20 times the industry average; its large diamonds account for two thirds of its revenue; it has the highest percentage of large diamonds (over 10 carats) of any known kimberlite mine; and, 90% of diamonds recovered are of gem quality.
The mine is an open pit truck and shovel operation with ore beneficiated on site. Processing capacity is 5.5 Mt per year, which would produce approximately 100,000 carats per year. Roughs are being shipped to Antwerp, Belgium for sale and marketing.
Up to date the mine produced numerous stones over 100 carats and several stones over 400 carats (up to 603 carats) of which four were included in the world's top twenty rough diamonds and three of them are considered to be the largest diamonds recovered in the 20th century.
The Kingdom of Lesotho is a small landlocked country entirely surrounded by South Africa. It is about the size of Belgium or Maryland and has a population of little over 2 million people of which some 99% are ethnically Basotho (Bantu). Some 90% of the population is Christian and speaks at least one of the official languages - Sesotho or English. Maseru is the capital city and hosts about 175,000 people. Local people are engaged in animal husbandry and agriculture.
The country was founded by king Moshoeshoe, who took refuge in the mountains where he bravely fought Zulu warriors, Afrikaner settlers, and South Africa's British armies. In 1868, Basutoland became a protectorate of the British Empire. It won independence from Britain in 1966. Nowadays, the country is a parliamentary constitutional monarchy.
Lesotho ("the kingdom in the sky") is mountainous and its whole territory lies above 1,000 m in elevation. Only 11% of the land is arable. The Drakensberg mountains are the source of the mighty Orange River; together with the Maloti mountains they form the Lesotho highlands. Much of the country's income comes from water export to South Africa.
Mokhotlong ("the place of the bald ibis") is the main centre of a vast mountainous area in the east of the country. Until the 1950s, it had no connection to the outside world; nowadays an asphalt road drives to it but on winter time the 7,000 people settlement can still be cut off for weeks at a time by snow.
Lesotho has a continental climate with hot summers and cold winters and experiences all four seasons. Climate at the mine is typical of high altitude settlements with snow possible all year round.
In 1957 a UNDP diamond exploration program headed by geologist Peter Nixon discovered two kimberlite pipes at 3,100 m altitude, on top of the Maluti Mountains in northeastern Lesotho, some 70 km from Mokhotlong in a place named the "swamp in the corner" or Letseng-la-Terae in local Sesotho language.
Hearing that kimberlites lay hidden somewhere up in the mountains, the very next year a South African adventurer, colonel Jack Scott, accompanied by a young man named Keith Whitelock, set out prospecting for diamonds. They came up on horses and at times spent up to a week stranded in blizzards that swept the high altitude country but in the end they managed to find diamonds at Letseng. The colonel's booty: 1,800 carats of diamonds.
The Mokhotlong fauna have suffered from substantial human impact through direct competition with livestock, hunting and habitat change resulting from changes to the vegetation and soil erosion. Habitat change has primarily been caused by uncontrolled burning and overgrazing practices that have also caused discernable soil erosion. Vegetation consists of grasses and scrubs.
The Gondwana supercontinent started to break up in pieces about 180 million years ago in mid-Jurassic - Antarctica, Australia, India and Madagascar started moving east thus creating an incipient Indian Ocean.
As a result of the rifting large outpourings of basaltic lavas accumulated on the southeastern part of the African continent in piles that nowadays measure 1.5 km in thickness. They formed the Lesotho highlands and Lebombo mountains.
Following the breakup of the continent explosive volcanic activities brought to surface kimberlite rocks and in some cases diamonds. Hundreds of Cretaceous kimberlites (dykes and pipes) riddle the basaltic Maluti Mountains and the Triassic sedimentary rocks of the lowlands but diamonds have only been found in the parts of the region that are underlain by the Archaean Kaapval craton - an old and stable part of the continental lithosphere.
Two kimberlite pipes (the Main Pipe - 17.2 ha and 495 m in vertical depth; and the Satellite Pipe - 5.2 ha, 655 m) have been emplaced at Letseng along with numerous kimberlite dykes. The event took place some 91 millions years ago in the late Cretaceous period.
Subsequently millions years of weathering resulted in destruction of the weaker minerals and the liberation of diamonds from their rock case - they were found either resting on top of the pipes as eluvial deposits or they were transported over short distances and deposited in alluvial deposits along the banks of two small creeks that drain the pipes.
The kimberlite pipes have low grades (1-3.5 carats per hundred tones) but they frequently produce large stones that significantly raise the value per tonne of kimberlite. The low grade of the kimberlites could be explained by a process of resorption witnessed by rounded and low relief diamond surfaces.
Some 75% of the hardrock diamonds recovered are of gem quality and the average stone size is of 1 carat which is comparable with large-stone diamond placer deposits.
The large stones (over 10.8 carats) recovered at Letseng are part of a special diamond class. An important feature is that these large diamonds are Type IIa stones, nitrogen free, and "D" color and that definitely improves the value per carat - most recent in May 2010 a 27.91 carat diamond was sold for US$58,724 per carat. The most unusual feature is the fact that the Main Pipe diamond population has 19% and the Satellite Pipe 25% Type IIa diamonds.
The abundance of these large, high quality Type IIa diamonds, rather than the grade per se, makes mining the pipes a viable enterprise.
Alluvial operations also manage to produce large stones some well over 100 carats.
Some of the most important diamonds mined at Letseng include: the 601 carat Lesotho Brown (1967); the 123 carat Star of Lesotho (2004); the 603 carat Lesotho Promise (2006); the 493 carat Letseng Legacy (2007); and, the 478 carat Light of Letseng (2008).
Both the Satellite and Main Pipes are mined using traditional open-pit technique of drill, blast, load and haul. In the mining process the phased introduction of larger and more efficient equipment continues to reduce costs, whilst improvements in blasting practices and technology solutions have improved the overall efficiency.
The ramp-up of waste stripping commenced with the introduction of larger, more cost effective, rigid frame dump trucks.
By reacting to economic conditions, diamond prices, waste to ore ratio, and the need to balance mineral resource risk mining was shifted from one pipe to another. During the latter part of 2009, Letseng commenced with a dewatering program of the deepest part of the Main Pipe in order to prepare for future mining activities In January 2010, a body of work was initiated to model the maximum extractable rate of ore from the two Letšeng pipes. The outcome of this work indicates that an extraction rate of up to double that currently being achieved is potentially possible (approximately 10 million tonnes per annum).
As of 2010, management was investigating the possibility of moving underground at the Letšeng Satellite pipe in order to maximize profit through the reduction of waste stripping.
Alluvial diamond deposits are being mined by an independent operator but the ore is also being processed at the mine site.
Since Gem Diamonds took control, Letseng's annual production has risen from 55,000 carats in 2006 to 90,878 carats in 2009. In the first half of 2010 average mine grade was 1.45cpht.
Mine life is estimated at over 25 years.
As of 2010, the two Letšeng plants have the capacity to process 5.5 million tonnes per year from the Satellite and Main pipes.
Primary treatment of the ore include primary crushing, scrubbing of clay and secondary crushing.
Equipment used include 250 mm jaw crushers (800tph), 70-95 jaw crushers, 25 mm cone crushers (145tph), 35mm cone crushers (237 tph), and scrubbers. Next, the crushed ore goes through a Dense Medium Separation (DMS) plant where the rock is separated by ore material containing diamonds.
The Letseng recovery operations are hosted by a separate building made up of five floors that incorporates: Floor 4 hosts sizing screens (45 mm enable the recovery of up to 1000 carat stones) and +2mm to + 16 mm; Floor 3, a buffer storage room for concentrate; Floor 2 where five wet X-ray units sort diamonds based on their luminescence; Floor 1, which hosts two IR dryers that treat the finer size luminescent fraction (+2mm - -8mm) before sending it to the Ground Floor to the glove boxes for manual sorting by a recovery team made of three persons and a manager. At the ground floor the diamonds are picked and recovered, counted, weighed and prepared for export.
Next, diamonds are valued and sent to Antwerp, Belgium where they are boiled in high temperature hydrochloric acid to remove any external kimberlite latching. After cleaning they are revalued and then interested companies are invited to tender for diamonds.
Assessing diamond breakage is very important at Letseng where large diamonds are being recovered on a regularly basis. Various technologies are being investigated to address this.
The company investigates potential cost and efficiency improvements by employing high volume X-Ray machines in place of the DMS units. A production trial was scheduled for the second half of 2010.
The majority of Letseng's electricity is supplied by South African electricity parastatal, Eskom but because of power outages standby electricity generating capacity was installed on site. This capacity is sufficient to operate one of the two processing plants and the mining contractor's plant.