On the unusual characteristics of the diamonds from Letšeng-la-Terae kimberlites, Lesotho

Abstract

The Letšeng-la-Terae kimberlites are situated 3100 m above sea level in the Maloti Mountains of Lesotho, southern Africa. The principal economic bodies are two Late Cretaceous, low grade, 1–3.5 carats/hundred ton (cpht), kimberlite pipes that host high-value diamonds realising US$ 2000–2500/carat ($/ct) in 2008 terms. Locally, the larger kimberlite body is referred to as the Main Pipe (17.2 ha) and the smaller one is called the Satellite Pipe (5.2 ha). These pipes, and their associated eluvial and proximal alluvial deposits, are renowned for yielding large, “D” colour, gem quality diamonds, including + 100 carat (ct) stones. Earlier artisanal effort (1959–1977) and formal mining (1977–1982) produced 335,000 carats (cts), including the 601 ct Lesotho Brown in 1968. In 2003, Letšeng Diamonds Limited re-commenced mining operations and had produced 265,000 cts by the end of July 2008, including 24 + 100 ct diamonds, the largest of which was the 603 ct Lesotho Promise. We report here on the unusual characteristics of the Letšeng diamond population that include: i. 75% gem quality that is more commonly associated with alluvial diamond deposits, ii. large average stone size of ca. 1 carat/stone (ct/stn) that is also more typical of certain alluvial diamond placers, iii. high-yielding, rounded to flattened irregular, resorbed dodecahedral shapes (Main Pipe 67% and Satellite Pipe 87%) with subordinate dodecahedral macle (Main Pipe 32% and Satellite Pipe 12%) and broken (ca. 1%) forms. In both pipes the octahedral component is virtually absent (< 0.1%), iv. economically favourable colour mix (ca. 33% white colour diamonds in both pipes), v. abundance of nitrogen-free, “D” colour, Type IIa diamonds that dominate the internationally recognised “special” stone size fraction which covers all diamonds larger than + 10.8 cts (Main Pipe 32% and Satellite Pipe 51%). During 2008, these larger, “special” diamonds commanded prices in excess of US$ 15,000/ct, contributing ca. 75% of the revenue generated by the Letšeng mine. Furthermore, of the 24 + 100 ct diamonds recovered between November 2003 and July 2008, 18 (75%) were Type IIa “D” colour diamonds that also fetched prices mostly in excess of US$ 25,000/ct. Therefore the Type IIa diamonds boost significantly the revenue per unit measure (in this case, the US$/ton) of the Letšeng-la-Terae pipes, making these low grade kimberlites economic to mine.