The Geophysics Of The Jacomynspan Proje

Abstract

African Nickel Ltd applied a number of geophysical methods from May 2011 to March 2013 with state of the art equipment at the Jacomynspan nickel-copper project in the Northern Cape in order to guide drilling and to detect the better mineralised parts of the deposit. The Ni-sulphide mineralisation is hosted in a 100m thick mafic to ultramafic sill that dips 650-750 south. A high resolution aeromagnetic survey was conducted covering the main target zone and the strike extent of historical drilling. The survey successfully delineated the sub-crop of the east-west trending ultramafic rocks. This information was used to plan ground electromagnetic (EM) surveys. Physical property measurements in the form of magnetic susceptibility and electrical conductivity were collected on historical drill core. The relative conductivity readings show the mineralised harzburgite to be more conductive than the gneissic host rock and the tremolite schist mineralisation. It was anticipated when planning the EM techniques, that the current would gather in massive sulphide stringer zones and the net-textured higher grade mineralised harzburgite. Therefore the EM plate model targets could potentially indicate these units rather than the lower grade tremolite schist mineralisation. A moving loop Time Domain Electromagnetic (TDEM) survey was carried out using a Low Temperature SQUID sensor over the main target zone to assess whether multiple stacked conductors exist, to map the best mineralised zones, and to explore for massive sulphide mineralisation. Downhole Electromagnetic (DHEM) surveys were also carried out in order to confirm whether off-hole mineralisation was present. To image both deep and near surface geological structure and mineralisation, vector Audio-frequency Magnetotellurics (AMT) was carried out on the main target zone. The results confirm that the intrusion is steeply dipping to the south and clearly mapped the higher grade harzburgite mineralisation down to 500m depth. The final 3D model was integrated into Micromine software with modelled geology and EM plates. This work confirms that AMT is applicable at Jacomynspan to rapidly and cost-effectively prospect for higher grade mineralisation.