HR: 09:30h
AN: S21C-07    [PDF]
TI: High-resolution Seismic Reflection Imaging of Thin, Diamondiferous Kimberlite Dykes and Sills
AU: * Hammer, P T
EM: hammer@eos.ubc.ca
AF: University of British Columbia, Department of Earth and Ocean Sciences, 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AU: Clowes, R M
EM: clowes@lithoprobe.ubc.ca
AF: University of British Columbia, Department of Earth and Ocean Sciences, 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AU: Ramachandran, K
EM: kumaran@eos.ubc.ca
AF: University of Victoria, School of Earth and Ocean Sciences, P.O. Box 3055 STN CSC, Victoria, BC V8W 3P6 Canada
AB: A unique seismic reflection experiment has successfully imaged a thin, diamondiferous kimberlite dyke intruded into granitic host rock. Although the typical dyke thickness is only 1-3 m, it is mapped from the near-surface to 1500 m depth. Such an application of seismic techniques to the diamond exploration and mining industry is unusual because the primary exploration targets are near-vertical kimberlite pipes that often are detected using magnetic and electromagnetic techniques. Subhorizontal dykes and sills do exist but they are poor potential field targets and have not been discovered by these techniques. However, one sub-horizontal structure, the Snap Lake dyke, was discovered in 1997 in the Archean Slave geological province of the Northwest Territories, Canada by tracking indicator minerals in the glacial till overburden. The Snap Lake dyke is a thin, dipping sheet that extends over at least 25 square km and plunges at approximately 15 degrees. The intrusion is richly diamondiferous and currently in the permitting stage for development of an underground mine. Its discovery heightened industry interest in dykes and sills, both in terms of their potential economic value and the information they yield regarding kimberlite emplacement. Since seismic reflection methods are especially well suited for mapping subhorizontal structures, dykes and sills have the potential to be excellent seismic targets. As a result, the Snap Lake seismic program was carried out to evaluate the seismic reflection method as a tool for exploration and deposit characterization of subhorizontal kimberlite intrusions. Snap Lake provides a superb test site for such a study because the dyke's gross geometry and composition have been determined through a substantial drilling program. Prior to the seismic field experiment, drill-core samples from the kimberlite and host rocks were used to measure P velocities and densities. These data were used to generate finite-difference and reflectivity synthetic seismograms in order to explore thin-bed resolution limitations, tuning effects and acquisition parameters. The seismic survey included two 2-d lines designed to obtain comparative datasets between different sources (explosives and vibroseis) and ground types (land and lake-ice). The explosive-source, land data yield a superb image of the thin dyke with high-amplitude reflections mapping the dyke topography to 1300 m depth. Weaker reflections indicate the dyke can be imaged to depths in excess of 1500 m. The vibroseis data detect the dyke only when sources and geophones are on land; they provide an image with nearly equivalent resolution. The dyke is not imaged beneath the ice by either source, due to reverberation and attenuation effects. The thickness of the thin intrusive layer is not directly resolved and 3-d structure makes the interpretation of fine-scale variations in reflectivity and continuity difficult. However, apparent correlations between variations in reflection characteristics and dyke properties (thickness, feathering, structure, and physical properties) suggest that seismic reflection data may be valuable for guiding drilling programs. The results demonstrate that, in the appropriate situation, seismic reflection methods have great potential for use in kimberlite exploration, subsurface mapping, and detailed imaging for mine development purposes.
DE: 0905 Continental structures (8109, 8110)
DE: 0935 Seismic methods (3025)
DE: 0999 General or miscellaneous
DE: 7205 Continental crust (1242)
DE: 8434 Magma migration
SC: Seismology [S]
MN: 2003 Fall Meeting