Review: Geophysical methods for kimberlite, prospecting

Abstract

Kimberlites occur as intrusive igneous rocks, generally as dykes, and pipes and some small diatremes or volcanic vents. The pipes are generally irregular structures and have not been found to exceed 2 km in diameter. The physical properties of kimberlites are variable. Physical properties from a large number of sources are tabulated. Western Australian pipes have shown some similarity in features to the Orapa (Botswana) pipes and the choice of geophysical prospecting methods for these pipes should be according to these similarities. Geological interpretation of aerial photography and satellite imagery are useful in the screening and classification of magnetic anomalies in kimberlite environments. Aerial photography may serve as a basic tool for studying the structural environment of kimberlites and the identification of intrusions, particularly dykes. Fracture analysis may aid in locating kimberlites. There may be considerable variation in the amplitude of magnetic response over kimberlite pipes. Within pipes that exhibit large magnetic response, there are large local inhomogeneities, probably caused by a combination of the explosive emplacement mechanism, different volcanic pulses, and different weathering. The gravity method is useful for detecting extensions of existing kimberlite pipes, and defining the position of the intrusion at depth. Over large pipes where kimberlite is weathered, a negative anomaly of a few tenths of a milligal may be expected. The unweathered kimberlite at depth gives a positive anomaly, and must be more dense than the average density of basalt. Since the anomalies over quite large pipes in basalt are small (1 or 2 milligal), accurate terrain corrections and allowances for any substantial variations in the thickness of the overburden and for any regional gravity gradient are important. Kimberlite material is electrically conductive compared with host country rocks, and resistivity methods are useful in delineating large bodies. Electromagnetic techniques have proven effective in some areas in defining the extent of kimberlite pipes and shape of dykes. Despite success in some Scandinavian countries and Canada the electromagnetic method for kimberlite prospecting in tropical and subtropical countries has not shown convincing results because of the conductive overburden in these areas. Induced polarisation has sometimes served to define sharply the vertical edge of weathered pipe rock. Owing to the enrichment of radioelements including potassium, thorium and uranium in kimberlite the radiometric technique might be expected to be of use in kimberlite prospecting. However, weak total-count anomalies have been obtained over pipes, with some potassium radio-activity measured either over the pipe or along its contact.