Geophysics Of Kimberlites In A Magnetically Noisy Environment: Syferfontein And Goedgevondon Case Study

The Goedgevonden and Syferfontein kimberlites are located ~1 km apart and are ~150 km from Johannesburg, and ~25 km NNE of Klerksdorp in South Africa. Both pipes are small (~0.2 ha) and Goedgevonden is diamondiferous, although uneconomic due to low grade and quality. While the location of the Goedgevonden kimberlite has been known since the 1930s, the Syferfontein kimberlite was discovered by a speculative airborne EM survey in 1994. Exploration in the area is complicated due to the presence of thick layers of ferricrete and calcrete which partially resorb and trap heavy minerals making traditional soil sampling methods ineffective. The area around the Goedgevonden kimberlite hosts the highly magnetic shales of the West Rand Group, magnetically variable Ventersdorp lavas and several meters of ferricrete, rendering traditional airborne magnetic data virtually useless due to large numbers of anomalies not associated with kimberlite. However the frequency domain EM method was instrumental in discovering the Syferfontein pipe and also shows a pronounced response over the Goedgevonden pipe. Data for various ground geophysical methods (gravity, magnetic, TDEM, HLEM) were also collected over both pipes and show prominent anomalies. Further follow up with fixed wing and helicopter TDEM shows a significant response over both kimberlites. The EM responses are most likely due to the weathered kimberlite, which is well exposed in the Goedegevonden pipe. In spite of the presence of alluvial diamonds and diamondiferous kimberlites, this region remains under explored; we suggest that EM surveys could prove effective for exploring in this region.