Geophysical signatures of some recently discovered large (> 40 ha) kimberlite pipes on the Alto Cuilo concession in northeastern Angola

Abstract

This paper presents a comparison of geophysical responses from several large kimberlite complexes discovered and delineated on the Alto Cuilo concession in the diamond fields of northeastern Angola in the years 2005 to 2008. Several geophysical methods were used in combination with geochemical and mineralogical prioritization techniques to guide exploratory, delineation and bulk sample drilling, in order to rapidly identify and evaluate the kimberlite bodies. The kimberlites were emplaced through Karoo Supergroup sandstones and shales, have eruption ages contemporaneous with the sand-dominated Cretaceous-age Calonda Formation, and are covered by sand-dominated poorly consolidated sediments of the Kalahari Group. Given that sand-dominated non-kimberlite lithologies are magnetically transparent, a low level, high resolution helicopter-borne magnetic gradiometer survey proved to be exceptionally effective in discriminating kimberlite targets, even for low-amplitude anomalies (e.g. 1–2 nT). The helicopter magnetic data outlined approximately 244 probable kimberlite targets and drilling of 103 targets confirmed 80 new kimberlites greater than 5 ha in area. Most kimberlites take the form of well-preserved crater edifices containing a full range of crater-related kimberlite lithologies. Ground gravity and electromagnetic surveys were conducted over all kimberlites prioritized for follow-up investigation. Geophysical responses were ground-truthed against magnetic susceptibility and density measurements, which were routinely collected on all drill cores. The geophysical signatures resolved by the three independent geophysical methods were surprisingly variable and are inferred to be sourced primarily in the crater facies materials, which demonstrate characteristically variable lithologies. Geophysical interpretations guided the drill targeting at all stages of the program at Alto Cuilo, from exploration to evaluation. Combined with geochemical and mineralogical prioritization techniques, the geophysical signatures in magnetic, gravity and electromagnetic data provide a sound basis to guide exploratory, delineation and mini bulk sample drilling of kimberlites buried under 10–70 m of overburden. The significant variability in the geophysical responses from kimberlites that are similar in size, structure and geometry highlights the importance of applying all three independent geophysical methods in order to effectively achieve kimberlite exploration and evaluation goals. The effectiveness of a multidisciplinary approach to kimberlite evaluation is demonstrated in the rapid assessment of a cluster of large kimberlites discovered at Project Alto Cuilo.