Magnetic and gravity investigation of kimberlites in north-central Botswana

Abstract

The Orapa kimberlite field of Botswana is one of the world’s major diamond producing regions. Within this field, there are several small kimberlite pipes that have not been completely explored in terms of their lateral extent, depth, and diamond potential. Two such pipes, BK54 and BK55, were found during a ground gravity and magnetic survey, and subsequent drilling confirmed the presence of kimberlite material. To determine the physical properties of these pipes, their lateral extent, depth, and thickness were estimated using a gravity and magnetic analysis and 2.5D and 3D modeling. Tilt derivatives of the magnetic data indicated that BK54 has a northwest-trending elliptical shape and BK55 has a roughly circular shape. Residual gravity anomaly maps indicate that BK54 does not have a density anomaly whereas BK55 is associated with a gravity maximum. The 3D gravity and magnetic inversion modeling constrained by magnetic susceptibility measurements indicates that BK54 is smaller in volume than BK55 and that neither pipe is thicker than 125 m. The difference in shape and the lack of a gravity anomaly over BK54 implies a different formation mechanism for each kimberlite pipe. Although several mechanisms are suggested, BK54 may have formed by a more explosive eruption producing more tuffistic material in the crater and diatreme facies. The gravity and magnetic analysis also found that the kimberlite pipes, while small, are larger in extent than was determined by drilling and warrant additional drilling to determine their economic potential.