Sub-volcanic development of kimberlite pipes: Evidence from the Lace and Voorspoed (Group II) kimberlites, South Africa

The Lace and Voorspoed kimberlites occur on the Kaapvaal Craton (South Africa), and form part of the Kroonstad Group II kimberlite (orangeite) cluster. The Lace kimberlite is composed of a main pipe and a satellite blind pipe, the latter of which does not reach the current land surface (~ 30 m below the current land surface), and is not observed connecting with the main pipe at depth. The main pipe increases in size from ~ 100 m to ~ 250 m in diameter at depth. The Voorspoed kimberlite pipe is the largest of the cluster and is dominantly infilled with massive layers (up to 200 m thick) of resedimented volcaniclastic kimberlite (RVK). Coherent kimberlite (CK), identified at all three pipes, is described here in order to constrain their formation. ct volcaniclastic kimberlite (VK) at the Lace main pipe is also described here and is characterised by a highly clastic fragmental, quartz-rich, and mud-rich interclast material. The VK is interpreted to form by resedimentation of extra-crater deposits into a deep (~ 1600 m) open vent. from both Lace occurrences is characterised by coherent to globular segregationary textures, generally high proportions of crustal xenoliths, and the complete absence of Karoo basalt xenoliths; which are ubiquitous in VK infilling South African Cretaceous kimberlites. The increase in pipe size with depth associated with the occurrence of CK at the main pipe and the occurrence of CK within the satellite blind pipe strongly favours an interpretation of formation in a hypabyssal setting, rather than by pyroclastic processes. CK is also described from the Voorspoed kimberlite where it occurs at depth along the south-western margin of the pipe. The CK is xenolith-rich, the xenolith population is entirely from the immediate sidewall lithologies, and has an incipient globular segregationary texture. The contact with the RVK in the centre of the pipe is sharp, indicating that the CK was cross-cut by a period of eruption prior to the formation of spatially associated RVK. The pipe margins at similar depths along the northern and eastern sections of the pipe are characterised by the occurrence of contact breccias, which have a gradational change to lower degrees of brecciation away from the pipe. The CK at Voorspoed is interpreted to form by the intrusion of magmatic kimberlite at depth, which invades a previously formed contact breccia and incorporates large proportions of crustal xenoliths from the immediate sidewall. occurring at the Lace and Voorspoed kimberlite are interpreted to form in a sub-volcanic hypabyssal setting during a stage of pre-conditioning of the country rock prior to eruptive emplacement of the pipe. A model for the development of a sub-volcanic embryonic pipe is presented for the formation of CK at the Kroonstad kimberlite cluster.