Conductor identification in tropical terrains—Case histories from the Itapicuru greenstone belt, Bahia, Brazil

Abstract

An airborne electromagnetic (EM) and magnetic survey was conducted to prospect for massive sulfide ore bodies in the Itapicuru greenstone belt in the state of Bahia, Brazil. Because tropical weathering is intense in this region, most of the recorded airborne EM anomalies were caused by a conductive overburden. Standard procedures to select targets for ground investigation (follow‐up) were found only partially satisfactory because such procedures resulted in anomalies in conductive environment being largely ignored. Because of this difficulty, a new scheme of target selection was developed which is based on a statistical analysis of conductance within a given lithologic unit. Four case histories of ground geophysical surveys, which were carried out during the follow‐up of airborne anomalies are described. The case histories focus on problems likely to be encountered in environments of intense tropical weathering. In areas covered by a conductive overburden, the EM response is sensitive to a proper choice of field equipment, frequency, and coil separation. As a result of the geophysical survey, bodies of volcanogenic massive sulfides were found in the Itapicuru greenstone belt, but so far none is of economic importance.