Borehole radar application to kimberlite delineation at Finsch Diamond Mine

Abstract

Finsch Mine is a world-class diamond mine in the Northern Cape province of South Africa, producing roughly 2.4 million carats annually. The mine is currently conducting resource evaluation to examine the feasibility of block caving below the current extraction level, which is located 630 m below surface (Block 5). Resource evaluation requires the accurate determination of the kimberlite pipe morphology. As part of the evaluation, borehole radars (BHR) were deployed down two boreholes that fanned outwards at ~30° to one another from a drilling station at 650 m level in the host dolomite. Both holes dipped at ~60°. Both struck the near-vertical flank of the diatreme ~120 m below their collars. Single-hole profiling as well as cross-hole BHR scanning surveys were run to determine whether coherent reflection from the kimberlite?country rock interface could be obtained, and to establish the limits on the operational parameters such as range and resolution. Coherent reflections from the kimberlite pipe contact were clearly observed from distances up to 60 m through the dolomite country rock. The kimberlite?dolomite interface appeared to be sharp, cemented, smooth, and curved. The BHR results showed that the kimberlite pipe surface was not that of a smooth cone. Inversion by interactive forward modelling revealed three horizontal rounded ridges, or corrugations, spaced by ~30 m. These penetrated 4 m to 8 m into the country rock, across the portion of the flank covered by the survey. The range, resolution, and accuracy obtained from the BHR surveys at Finsch Mine are of high quality. The survey results improve knowledge of the detail of, and confidence in, pipe morphology significantly.This allows for more-accurate geological modelling and resource estimations, meaningful project evaluation, better tunnel design, and ultimately, sound investment decisions.