International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064 Index Copernicus Value (2015): 78.96 | Impact Factor (2015): 6.391 Volume 6 Issue 12, December 2017 www.ijsr.net Licensed Under Creative Commons Attribution CC BY 3D Geoelectrical Resistivity Mapping of Tourmaline-Rich Pegmatite in AngwanDoka, Nassarawa State, Northcentral Nigeria O. A. Oyonga 1 , E. Kudamnya 2 , S. I. Ugar 3 1, 2, 3 Department of Geology, University of Calabar, Calabar, Nigeria Abstract: 3D geoelectrical resistivity survey was carried out using the Multiple Vertical Electrical Sounding (MVES) along profile lines to determine the extent of mineralized tourmaline-rich pegmatite within a mining site in Angwan Doka community in Kokona Local Government Area, Nassarawa State, Nigeria. A total of 61 out of 90 soundings within a grid area which covered 200m 2 were selected for convenience of modeling. Resistivity data were modeled using a combination of manual curve matching, data inversion and iteration with IP2WIN software, and 3D data modeling using rockworks software. This integrated approach with a resulting 3D model revealed a major trend of the pegmatite in the NE-SW direction which is in line with the overall regional structural strike in the region. The method can therefore be applied extensively for other related subsurface resistivity investigations. Keywords: 3D model, Tourmaline, Pegmatite, IP2win, Resistivity, Schlumberger 1. Introduction Electrical methods utilize direct currents or low frequency alternating currents to investigate the electrical properties of the subsurface. Electrical resistivity is a fundamental and diagnostic physical property that can be determined by a wide variety of techniques, including electromagnetic induction [1]. The resistivity method is used in the study of horizontal and vertical discontinuities in the electrical properties of the ground, and also in the detection of three- dimensional bodies of anomalous electrical conductivity. It is routinely used in engineering and hydrogeological investigations to investigate the shallow subsurface geology. In the electrical resistivity surveying method, artificially- generated electric current (I) is passed into the ground through two earth connections (electrodes) and the voltage (potential difference (V) is measured across a second pair of electrodes at the surface. The ratio of voltage to current, is the resistance that when multiplied by a factor which takes into account the spacing between the electrodes, gives a parameter known as the apparent resistivity. When the measurement is made over a homogeneous surface, the apparent resistivity is equal to the true resistivity of the ground. However, when the resistance is made over a complicated subsurface structure, the apparent resistivity is a weighted average of the resistivities of the various rocks below the surface [2-4]. Deviations from the pattern of potential differences expected from homogeneous ground provide information on the form and electrical properties of subsurface inhomogeneities [5]. Advancements in software packages have provided opportunities for subsurface modeling of physical properties in 3 dimensions [6], and resistivity is not an exception. For example quasi-3D modeling of resistivity data has been achieved by merging the results from a number of sections acquired and inverted using 2D resistivity imaging [7]. An attempt has been made in this study to model subsurface resistivity in 3D to reveal the trend and extent of tourmaline-rich pegmatite in AngwanDoka, Nassarawa State of Nigeria. The area investigated is part of an existing mining site in AngwanDoka community in Kokona Local Government Area of Nassarawa State in Northcentral Nigeria. It is characterized by significant exposure of tourmaline-rich pegmatite with ongoing wildcat pit mining by local artisans (plate. 1). Mineralization patterns can sometimes be very deceptive even when one may likely assume a general trend based on structural investigation of the area of interest. Reports from geostatistical studies have shown that minerals are sometimes disseminated, i.e. without following a regular trend within a deposit. However, there will always be an overall trend in the sample distribution. The sample value is expected to vary from area to area in the deposit. Some areas are expected to be rich, and some to be poor [8]. The unguided exploitation of this gem is a serious cause for concern, as rich zones of mineralization may be bypassed, while zones which ought to have been avoided may be encountered during the mining process. It is therefore imperative to carry out geophysical investigation in the search for minerals to identify prospective zones within the host rock, and to offer valuable advice on the best areas and best way to mine. The electrical resistivity method which has gained popularity in a wide range of applications including measurements of depth to bedrock, environmental investigation of contamination, groundwater investigations, mineral exploration and archaeology, is a valuable and sensitive tool that can be used to delineate the contrasts in the physical properties of the rocks, in this case the resistivity of rocks. Tourmaline which is the mineral of interest here like some other minerals may not have significant or outstanding physical properties due to either the disseminated nature or low concentration, and thus may not allow for significant contrast in physical properties with the host rock to be revealed. In these and similar cases, geophysicists must rely on detecting rocks that are associated with the minerals, or other minerals with significant physical properties that occur in association with the mineral of interest. The application here is the use of a 3D resistivity modeling approach to produce subsurface images of the area under investigation to show the contrast between the generic migmatite (host rock) and the pegmatite which is the rock with high potentials of tourmaline mineralization in the area of investigation. Paper ID: ART20178541 DOI: 10.21275/ART20178541 1646