A combined electromagnetic and resistivity survey for exploration for vein graphite: a case study over a Potential Graphite Field in the Sabargamuwa Province, Sri Lanka

Graphite mining in Sri Lanka carries a history of nearly two centuries. Even though the mining activities are extensively carried out, methods of exploration are not up-to-date when compared to rest of the world. Occurrence of vein graphite in the country shows a systematic pattern with respect to the surrounding geology. Understanding of local geology and structures is vital for explorations in extensively deformed terrains like Sri Lanka. Use of immature tools and technology in earlier days restricted the local mining activities to shallower levels, mainly due to the collapsing of pit walls as the excavation depth increases. Deeper subsurface mineralization in such regions can be further prospected with advanced modern geophysical methods. Medagoda in Avissawella region is an area where open pit shallow graphite mining was carried out several decades ago. A Combined electromagnetic (EM) and electrical resistivity survey was conducted to identify the remaining potential of graphite occurrence within the area along survey lines of a pre-arranged grid. Electrical Resistivity Profiling was done along selected survey lines based on the distribution of abandoned pits and mainly to cross check the EM results. Anomalies of resistivity survey matched with EM anomalies with a good precision. Both survey results correlated with abandoned pits to infer the most probable zones of mineralization. Within the depicted mineralized zone, calculated depths to graphite occurrences are ranging 15-30m below surface. Reserve estimation was not possible under the present survey because of largely spaced survey lines and absence of drill hole data for validation. Extended geophysical surveys of Resistivity and Induced-polarization methods are suggested for further clarification of EM results with a higher degree of resolution and to omit the mining dumps of previous excavations.