MAGNETIC AND GRAVITY INVESTIGATION FOR KIMBERLITES, NORTHERN BOTSWANA

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 the 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. In order to determine the physical properties of these pipes, their lateral extent, depth and thickness was estimated using a gravity and magnetic analysis and two- and three-dimensional modeling. Tilt derivatives of the magnetic data indicated that BK54 had a northwest-trending elliptical shape and BK55 had a roughly circular shape. Residual gravity anomaly maps indicate that BK54 does not have a density anomaly while BK55 is associated with a gravity maximum. Three-dimensional gravity and magnetic inversion modeling constrained by magnetic susceptibility measurements indicate that BK54 is smaller in volume than BK55 and neither pipe is thicker than 125 meters. The difference is shape and the lack of a gravity anomaly over BK54 implies a different formation mechanism for each pipe. While several mechanisms are suggested, BK54 may have formed by a more explosive eruption producing more tuffistic material in both the crater and diatreme facies. The gravity and magnetic analysis also found that the pipes, while small, are larger in extent than determined by drilling and warrant additional drilling to determine their economic potential.