oa Interpretation of Airborne Magnetic and ASTER Images over Kimberley and Boshof Areas, Northern Cape Province, South Africa: Implication for the Occurrence of Diamond-bearing Kimberlite Pipes

Interpretation of airborne magnetic and ASTER images acquired from Kimberley and Boshof areas was undertaken. The study area lies between Kimberley and Boshof, which is located in the Free State and Northern Cape Provinces. Geologically the area underlies three major groups of rocks, viz. Archaean granites of the oldest age, which are overlain by sedimentary rocks and andesite lavas of the Ventersdorp Supergroup. The youngest rocks are the Karoo sediments and volcanic rocks which are widespread throughout the study area. The aim of the integrated interpretation of the magnetic and ASTER images is to identify diamond-bearing kimberlite pipes and related targets. In order to achieve the objective, the interpretation of airborne magnetic and ASTER images was supplemented by enhanced Landsat TM (ETM+) images. The magnetic data was processed for mapping structural features and lithological units. In order to improve the spectral and radiometric signatures of the ASTER and ETM+ images, image enhancement techniques such as edge detection filter, band ratioing, histogram equalization and principal component analysis were applied. In addition, colour composite images derived from both ASTER and ETM+ bands were prepared based on the calculation of Optimum Index Factor. The analytic signal and the first vertical derivative of the total magnetic intensity revealed several circular features which resemble the morphology of kimberlite pipes. The ASTER composite image derived from bands 6, 2 and 1 in red, green and blue channels, respectively assisted identification of linear features such as kimberlite dykes and fissures, while colour composite image derived from bands 8, 3 and 1 displayed circular-shaped features, which can be interpreted as potential kimberlite pipes. The results from both sets of images were merged with similar features obtained from the magnetic maps. The resulting merged layers revealed areas which are potentially prospective for the occurrence of diamond-bearing kimberlite pipes. The most interesting zone lies in the central and northwestern parts of the study area, where large number of cross-cutting lineaments and circular-shaped bodies were identified. In general, the targets are controlled by cross-cutting structures and largely confined to areas that are intruded by Karoo dyke swarms.