INDUCED POLARIZATION SURVEYING AS AN ACID ROCK SCREENING TOOL FOR PROPOSED HIGHWAY ALIGNMENTS

Highway construction agencies are increasingly aware of the adverse environmental impact that can be caused by the blasting and disposal of sulphide-bearing rock. Once fragmented, the surface area available for sulphide oxidation and hence the risk of acid rock drainage is greatly increased, unless the rock contains enough natural buffering capacity to neutralize the pH. If engineers are aware of the presence of sulphide zones during the highway planning stage then confirmatory drilling and geochemical testing can be targeted effectively, and steps can be taken, if necessary, to reduce rock cuts in those areas and dispose of the blasted material in a responsible fashion.

In December, 2001, the New Brunswick Department of Transportation sponsored a field trial of geophysical surveying in order to assess its suitability as a screening tool to locate near-surface sulphide zones. Dipole-dipole induced polarization (IP) was chosen as the primary geophysical method given its ability to detect low-grade disseminated mineralization. Supplementary information was provided by resistivity and VLF-EM surveys sensitive to lateral changes in electrical conductivity, and by magnetic field surveying chosen for its sensitivity to the magnetic susceptibility of pyrrhotite. Geological and geochemical analyses of samples taken from several IP anomalies identified along 4.3 line-km of proposed highway confirmed the effectiveness of the screening technique. The strongest IP effects were observed over a section of graphitic and sulphide-bearing metasedimentary rocks. The question of whether it is possible to distinguish graphite-rich zones from sulphide-rich zones on the basis of their spectral IP responses is currently being investigated through laboratory-scale studies of IP effects and mineralogy in core samples taken from the site.