Summary:
Letšeng is a primary diamond deposit consisting of two adjacent, vertical, steep-sided, downward tapering pipes. The pipes are named Main Pipe (15.05 ha) and Satellite Pipe (4.36 ha).The current erosion levels of the Satellite and Main Pipes are in the upper diatreme and crater settings. The Main and Satellite Pipes are infilled with a variant of Kimberley-type pyroclastic kimberlite (KPK) as well as coherent (CK), hypabyssal (HK), and resedimented volcaniclastic kimberlite (RVK) (Hetman et al., 2018).
Letšeng has undergone much less erosion than kimberlite pipes in other areas of southern Africa because of its location in the Lesotho Highlands at ~3,100 masl (Stanley et al., 2015). The estimated level of erosion at Letšeng is 400-600 m (Hetman et al., 2018). Basalt is the dominant crustal xenolith type within the Letšeng pipes, ranging from less than a centimetre to large rafts tens of metres in size (megaxenoliths1). The basalts are underlain by sandstones and siltstones of the Clarens Formation; xenoliths of these rocks are present in the Letšeng kimberlites but are small and most easily identified petrographically. Basement xenoliths occur in both pipes with garnet granulite being notably common in some units.
The Main and Satellite kimberlites are vertical, steep-sided and carrot-shaped volcanic pipes. The pipe contacts (kimberlite-basalt) are smooth and striated. Brecciated basalt country rock (some carbonate-cemented) occurs locally adjacent to the pipe contacts but these ‘marginal breccias’ are less common than in other KPK-infilled pipes especially those hosted in granitoid country rock. Both pipes contain megaxenoliths of these brecciated basalts (derived from wall rock during pipe formation), as well as megaxenoliths of basalt breccias with variable amounts of kimberlite within them.
Both pipes are infilled with a variant of Kimberley-type pyroclastic kimberlite (KPK), including KPK with extrusive features (KPKe), as well as coherent (CK), hypabyssal (HK) and resedimented volcaniclastic kimberlite (RVK) (Hetman et al., 2018). The pipe shapes and pipe infills show many similarities to other KPK-infilled pipes, including those from the type area, Kimberley in South Africa as documented in Hetman et al. (2018). The high abundances of locally-derived, unsorted basalt xenoliths with a wide range of sizes from microxenoliths (< 1 cm) to megaxenoliths (> 1m including large rafts) is also typical of KPKs.
The presence of KPKe and RVK indicates an upper diatreme to crater setting of the Letšeng pipes, consistent with the estimated limited erosion in this area of Lesotho. There are only a few other KPKinfilled pipes with preserved crater zone deposits (e.g. Orapa in Botswana and Williamson in Tanzania).
The Letšeng KPK also has several unique characteristics compared to KPK at other localities: (i) the olivine displays complex morphologies and widespread breakage; (ii) olivine macrocrysts are small and many have serrated edges (iii) olivine microcrysts are coarse; (iii) there is an uncommonly high abundance of melilite in the groundmass; (iii) some of the melt-bearing pyroclasts have characteristics similar to those found in Fort à la Corne-type kimberlites (FPK), the other main class of pyroclastic kimberlite. These atypical features can be attributed to the marginal or melnoitic nature of the magma and the upper diatreme to crater setting of the Letšeng pipes (Hetman et al., 2018).
Local Geology
The northern Lesotho region has kimberlite intrusions with both pipe and dyke (sheet) morphologies. The kimberlites in the region are often found in groups with one large “main” intrusion and one or more smaller “satellite” intrusions, as seen at Letšeng. The Main and Satellite pipes and associated dykes at Letšeng form part of the northwest-south-east trending Lemphane - Robert kimberlite belt (e.g., Rapopo, 2017; Ward, Bowen and Fourie, 2017) in which the kimberlite pipes are associated with an extensive kimberlite dyke swarm. Pipes are found at Mothae, Kao and Liqhobong. Dykes can have blows where they become wider. Many dykes are diamondiferous but none has proven to be economical.
Main Pipe Geology
The understanding of the geology is that the pipe consists of 13 internal domains - 11 kimberlite domains and two basalt xenolith/basalt breccia domains. Some of the kimberlite domains are separated by steep, sharp or gradational, cross-cutting internal contacts and have been shown to extend to depth in drill cores. However, the geology of the main K1 portion of the pipe (i.e., not K4, K6 or K2) is complex with shallower, irregular contacts between domains K1A, K1B-1, K1B2, K1C and the respective domains in the south of the pipe (RFW-K1S grouping). This is consistent with the upper diatreme / crater setting of Main Pipe in the portion mined and drilled to date, as is the presence of RVK in the K2 domain.
There is a significant amount of textural and component variability between the various K1 domains. The K1A and K1B-1 domains comprise similar components but have distinctly different textures, with K1A characterised by higher abundance of notably broken and closer packed olivine. The K1C and K1B-2 domains are similar texturally to K1A and K1B-1 (respectively) but contain different relative proportions of melt-bearing pyroclast types (all these domains have mixed melt-bearing pyroclasts populations). In the southern portion of the pipe, broadly equivalent domains are defined as RFW-K1As and RFW-K1B1s based on differences in texture, melt-bearing pyroclast type ratios, and country rock xenolith types.
The K2 domain comprises PK and RVK and is highly heterogeneous internally in terms of the texture, grain size, fabric and spatial distribution of the RVK (which has not been modelled separately). All the kimberlite in the K2 domain is a different phase of kimberlite to the K1 kimberlite, being characterised by a distinct melt-bearing pyroclast population. The K2 domain is not equivalent to ‘K1North’ of Venmyn Rand (2012) which extended further south.
The K6 domain is also internally variable in terms of texture, the proportion of mantle-derived garnet and mantle xenoliths, and basalt xenolith abundance; which defines a crude east-west ‘zonation’ and persists to depth. The K4 domain comprises at least two HK phases.
Satellite Pipe Geology
The Satellite Pipe is dominated by Kimberley-type pyroclastic kimberlite (KPK) and less abundant Kimberley-type pyroclastic kimberlite with extrusive features (KPKe) and resedimented volcaniclastic kimberlite (RVK) with minor amounts of coherent kimberlite (CK) classified as hypabyssal kimberlite (HK).
The understanding of the geology of Satellite is that the pipe consists of six kimberlite domains: southern volcaniclastic kimberlite (SVK), northern volcaniclastic kimberlite (NVK), green volcaniclastic kimberlite (GVK), KIMB7 and the central coherent kimberlite (CCK). The last domain (GVK-M; also referred to as GVK-SVK-Mixed) is a mixed domain dominated by GVK and significant SVK blocks with minor amounts of NVK and KIMB7.