Geophysical investigation of sulphides mineralisation in Maitengwe Greenstone belt, North-East Botswana.

Abstract

During early 1990s, the then Botswana Geological Surveys (now BGI) used geochemical, geophysical (Horizontal Loop Electromagnetic (HLEM), magnetic and gamma ray) and borehole drilling methods in the Maitengwe Greenstone Belt in order to determine the gold and base metal mineralization potential. However, these geophysical methods had limited spatial resolution, and could not adequately determine the spatial distribution mineralized zones and the geometry of sulphide-rich veins. For this reason, this study utilized aeromagnetic dataset as desktop studies and a follow up using the IP method in conjunction with electrical resistivity imaging (ERI) and Time Domain Electromagnetic (TEM) method to determine the spatial distribution and the geometry of potential mineralized zones. The findings of the study reveal that structural lineaments on the aeromagnetic data of Maitengwe area are mostly oriented NE-SW. Induced polarization and electrical resistivity methods successfully identified zones of potential mineralization in Maitengwe Greenstone Belt. High sulphide mineralization potential zones were identified in all the eight survey lines. Such zones occur beneath 100 m depths and are characterized by chargeability values of 7.4mV/V -12.8 mV/V and resistivity values of 400 Ωm-9205.0 Ωm. Such values of moderate to high resistivity and high chargeability corresponds with silicified mineralized zones. Zones of low resistivity coinciding with high chargeability values are possible alteration zones with significant metallic minerals. The inferred mineralized zones appear sub-vertical to vertical and trends in a NE-SW direction. This is consistent with the direction of the lineaments identified using the high-resolution aeromagnetic data, suggesting that these mineralized zones may be associated and controlled by these structures. The TEM results of 1D soundings correlate with the 2D resistivity models. The results reveal a three-layered earth in which the resistivity increases with depth. Keywords: Sulphide mineralization, geological structures, chargeability, greenstone belt, resistivity, induced polarization, Time domain.