Geophysical surveys at the Nkomati Mine, Mpumalanga, South Africa

Abstract

Nkomati Mine, a joint venture between Anglovaal Mining and Anglo American, exploits a magmatic Ni-Cu-Co-PGE massivesulphide deposit associated with the Uitkomst Complex, inMpumalanga, South Africa. The mineralisation consists of flatlyingmassive sulphides, which are magnetic, dense andconductive. Geophysics has been used to assist the explorationprogram for further resources. Methods that were tried includesurface magnetic, gravity and controlled source audiomagnetotelluric (CSAMT) surveys, plus downhole electromagnetic(DHEM) and downhole magnetometric resistivity (DHMMR)surveys.The downhole methods were the most effective for deep targets. Because of the highly conductive nature of the sulphides, standardimpulse type time-domain DHEM did not give good results untiltransformed into step-type data. However, strong responses wereobtained from frequency-domain measurements. Since all of theholes were vertical, no useful cross component data could becollected and the location of the sources suffered from radialsymmetry. In contrast, the DHMMR was able to resolve on whichside of the hole the conductors lay.CSAMT successfully defined the disseminated and massivemineralisation at depth. Surface magnetic and gravity surveys wereuseful for outlining relatively shallow mineralisation outside of themain complex. Qualitative interpretation, and modelling of thegravity and magnetic data, showed a good correlation with theCSAMT results.