Palmietfontein kimberlite pipe, South Africa—A case history

Abstract

The Palmietfontein kimberlite pipe is located 150 km northwest of Johannesburg, South Africa. It was emplaced at the contact between mafic rocks of the Bushveld complex and syenites of the Pilanesberg complex, and coincides with the intersection of two major faults. Palmietfontein is one of the larger known kimberlite pipes in South Africa; it has a surface area of 12 ha and is diamondiferous. The present geophysical study was designed to assist in planning an extensive program of trenching and drilling. Unweathered kimberlite has geophysical responses very similar to the country rock at Palmietfontein. Weathering and alteration of the upper 50 m of the pipe, however, have resulted in various physical changes, which has made the target amenable to investigation by various geophysical techniques. The surveys used in this study are gravity, electrical, seismic refraction, and airborne and ground magnetics and electromagnetics (EM). The boundary of the pipe was accurately defined, and the dip of the wallrock contact was determined by using various models and combinations of techniques. A small satellite body of kimberlite was also discovered during the course of this investigation. The most suitable techniques for kimberlite prospecting, particularly when the top portion of the kimberlite is weathered, are airborne EM and magnetics, combined with the Slingram ground-EM system. For more quantitative results, gravity and seismic surveys should be used.