LECTURE: Mine Tailing Blues: Electrical Imaging of Acid Mine Drainage

Electrical geophysical methods can map enhanced electrical conductivity caused by acid mine drainage in mine tailings deposits. We have investigated quantitative relationships between geophysical responses and the electrical conductivity, acidity and salinity of tailing samples at the Central Manitoba Mine Tailings in Manitoba, Canada. Larger-scale electromagnetic surveys at the site identify zones of enhanced conductivity that were interpreted to be caused by acid mine drainage. Subsequently, high-resolution EM31 and DC-resistivity measurements were made on a profile through a zone of enhanced conductivity and laboratory measurements of pore-fluid salinity and pH were made on an array of tailing samples from the upper 2 m along the profile. Spatial correlation of pH and pore-fluid salinity in the tailings samples confirms that the enhanced conductivity in the tailings is due to acid mine drainage. Contoured cross-sections indicate that the acid mine drainage is concentrated near the base of the oxidized zone in the thicker parts of the tailings pile. The zone of increased acidity extends to the surface on sloping margins causing an increase in apparent conductivity in shallow penetrating geophysical responses. The quantitative relationship between pH and pore fluid salinity shows that the conductivity increase associated with the acid mine drainage is only due in part to conduction by ions from the dissociation of sulphuric acid. Ca2+ and Mg2+ ions also make significant contributions to the conductivity at all values of pH, and Cu2+, Al3+ and Fe3+ ions make additional contributions at low pH. The results of the study demonstrate the feasibility of using electromagnetic surveys to map acid mine drainage, and associated ionic concentrations, in gold mine tailings.