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 massive sulphide deposit associated with the Uitkomst Complex, in Mpumalanga, South Africa. The mineralisation consists of flatlying massive sulphides, which are magnetic, dense and conductive. Geophysics has been used to assist the exploration program for further resources. Methods that were tried include surface magnetic, gravity and controlled source audio magnetotelluric (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, standard impulse type time-domain DHEM did not give good results until transformed into step-type data. However, strong responses were obtained from frequency-domain measurements. Since all of the holes were vertical, no useful cross component data could be collected and the location of the sources suffered from radial symmetry. In contrast, the DHMMR was able to resolve on which side of the hole the conductors lay. CSAMT successfully defined the disseminated and massive mineralisation at depth. Surface magnetic and gravity surveys were useful for outlining relatively shallow mineralisation outside of the main complex. Qualitative interpretation, and modelling of the gravity and magnetic data, showed a good correlation with the CSAMT results.