Borehole logging and automated interpretation of drillhole lithology from the Murowa kimberlites, Zimbabwe

Abstract

The first of the diamondiferous Murowa kimberlites was discovered in December 1997 through the follow-up of anomalous indicator stream sample results. From March 1997 until the year 2000 a program of drilling and shaft sinking was carried out to evaluate the resource. Borehole logging formed an integral part of the evaluation process used to define lithologies, measure densities and calculate sample volumes. The five hypabyssal bodies at Murowa are a complex mix of various kimberlite lithologies which make accurate geological logging, especially of reverse circulation holes, difficult. Dual neutron, gamma gamma density and magnetic susceptibility logs were used to estimate kimberlite and host lithology proportions in the reverse circulation holes as a routine part of the geological logging process. Analysis of the physical properties of the various lithologies indicated that any brecciation and mixing of the hypabyssal macrocrystic kimberlite with country rock inclusions is reflected by a reduction in magnetic susceptibility. The lower apparent resistivities of the kimberlite lithologies within the resistive granite host explain why the pipes respond as conductors to surface geophysics. LogTrans, a program developed by the Centre for Mining Technology and Equipment, Brisbane, was used to perform automated interpretation of the geophysical borehole logs. The software calculates the median values and ranges for each physical property for each lithology in a suite of training holes, and then applies these statistics to interpret data from other holes. Data from the diamond drillholes were used for training, and interpretation was carried out on the reverse circulation holes. LogTrans was also used to estimate kimberlite content based on a linear relationship at Murowa between percentage of kimberlite and apparent neutron porosity.