Model of a kimberlite pipe

Abstract

The model of a kimberlite pipe depicts a hypothetical pipe having a diameter of 300 m at a level equivalent to the post-erosional (present-day) surface of the major pipes in the Kimberley area. Below this reference level the pipe configuration and composition illustrate features typical of the Kimberley occurrences. Above this surface the pipe contacts have been projected upwards and outwards at an angle of 82°. This average figure is based on a study of the wallrock slopes of eleven kimberlite pipes, emplaced in a variety of host rocks. The length of the upward projection is equivalent to the estimated amount of erosion which has taken place in the Kimberley area since the emplacement of the kimberlites. The amount of erosion is estimated to be approximately 1400 m. This figure is based on an investigation of the depth of formation of kimberlite sills, supplemented by geochemical and diagenetic studies of sedimentary rocks of the Karroo system which occur in the vicinity of the pipes. Near the original land surface the pipe model is based on the geology of the Orapa pipe in Botswana which is typical of the slightly eroded pipes found in the western and northern parts of central and southern Africa. The geographical separation of slightly eroded pipes and deeply eroded pipes suggests a substantial difference in their age or geomorphological history. The model pipe may serve as a means of identifying the relative depth to which a particular kimberlite pipe has been eroded and of showing the changes in its character which may take place with increasing depth.