The rocks exposed in Lesotho, belong mainly to the upper part of the Karoo Supergroup (Carboniferous to Jurassic) which covers vast areas of south and central Africa. This widespread series of rocks consist of flat-lying conformable sandstones and shales extensively intruded by dolerite dykes and overlain by vast outpourings of basaltic lavas.
In Lesotho most of these rocks have been assigned to the Drakensberg Group, Clarens Formation and Molteno Formation. The Molteno Formation is composed of massive white grits and sandstones with occasional shaly layers (rarely carbonaceous) followed by red beds composed of varicoloured sandstones, alternating with red, green and purple shales, and mudstones. Carbonaceous beds are absent and reptile fossils are abundant. The overlying Clarens Formation (traditional Cave sandstone) is composed of massive, fine grained, aeolian sandstones. The Drakensberg Group (Stormberg lavas) comprises the fullest sequence of basaltic rocks (>1,600m) and dolerite intrusions exposed in southern Africa. These lavas are tholeiitic in composition and are chemically comparable to the dolerites with an indicated age range from 187 Ma to 155 Ma, or younger for the flows on the highest peak (Thaba Ntlenyana) at 3,482m.
The concentration of Kimberlite pipes and dykes in Lesotho is among the highest in the world. Many of these Kimberlites have been studied because of the suite of mantle peridotites that they contain. Others have received periodic economic investigation; interest in Lesotho diamonds includes the Letseng, Kao and Liqhobong Kimberlites, in addition to the current study of Mothae.
The Kimberlite intrusions in Lesotho post-date the Karoo Supergroup and have been generally accepted as of Cretaceous age (Haughton, 1969). However, the oldest age of a post-Karoo Kimberlite in southern Africa (150 Ma at Swartruggens), is very close in age to the youngest Karoo basalt. An isotopic age of 87 Ma was obtained for the Mothae Kimberlite (Davis, 1977) and other South African Kimberlites group in the range 82 Ma to 96 Ma.
The information in this section which is of a scientific or technical nature has been derived in part from the technical report entitled "Mothae Kimberlite Project, Lesotho, Independent Technical Report" dated February 12, 2007 prepared by Dr. Norman Lock (BSc, PhD, CGeol FGS, MGSSA, PrSciNat) of MSA Geoservices (Pty) Ltd., who is a "qualified person" within the meaning of this term in National Instrument 43-101. A copy of the report is available on SEDAR at www.sedar.com.