Electrical tomography and magnetic imaging of Zeida’s abandoned mine tailings (Morocco)

Abstract:

The Zeida (Pb-Zn) mine was closed since 1985 and contains huge dams of mine waste deposited without adjustment or remediation. These tailings contain large amount of Pb (3000 ppm) and Zn (140 ppm) expressed mainly as galena, wulfenite and barite. The subsurface of the studied area is constituted mainly by weathered granite coved in some places by Triassic red formations. A geophysical survey was conducted by using Electrical Resistivity Tomography (ERT) combined with Magnetic Imagery to explore the subsurface area and understand how the mine waste is interacting with the groundwater. Two-Dimensional ERT revealed the occurrence of rounded structures with high resistivity values corresponding to spherical boulders of weathered granite. Resistivity values within the boulders decrease gradually from the center toward the outer layers. This granite was found to be covered by highly conductive materials from the tailings of the mine reaching a depth extending up to 60 m. Magnetic maps revealed local magnetic anomalies reaching 400 nT due to old buried pipes at the mine site of Zeida. These anomalies correspond to the presence of faults used as pathways to recharge the groundwater reservoir