Esoteric and mundane geophysics for diamondiferous pipe exploration

Abstract

Geophysical techniques have been shown to be effective in diamond exploration. Airborne magnetics and EM can in most cases accurately detect and map most kimberlite and lamproite pipes. The variations in observed response are large and controlled by both geological and physical factors. The controls on EM for example are multiple and include geological fades: whether diatreme or crater; degree of brecciation and porosity, depth of weathering and groundwater salinity. There is strong evidence that the source of unweathered kimberlite and lamproite magnetic anomalies is predominantly remanent. Viscous magnetisation may become important in weathered rocks. Observed local anomalies may thus be of normal or reversed polarity compared to a non-magnetic background. There are many sources of confined geophysical anomalies with lateral extent around the nominal 300 m minimum diameter of economic diamondiferous pipes These include amphibolites, collapse breccia pipes, massive sulphides, skarns, inhomogeneous regolith, culture, alkaline diatremes, confined mafic and ultramafic bodies, local sediment patches and magnetite concentrations in granite. Some simple geophysical tests can help discriminate these possible sources in many cases. Diamond grades within economic pipes are known to be strongly zoned, grade also varies greatly between separate intrusions within one pipe. Detailed ground geophysics can usually map out the surface extent of many diatreme and crater facies kimberlites or lamproites, and at times clearly identify separate intrusions within one kimberlite or lamproite. Such detailed geophysics is likely to be far more cost-effective than the alternatives such as grid drilling or pitting.