Classification of slag material in the laboratory and field scale with spectral induced polarisation

Since the demand for raw material is still growing, historical mining dumps are in the focus of geophysical investigations because they can still bear valuable minerals. Moreover, the risk assessment of groundwater contamination due to solutions processes within slag dumps is an important part of dump characterisations. One suitable technique to use is the spectral induced polarisation (SIP) method. Together with accompanying methods, it can lead to an estimation of the potential re-usable mineral resources of slag dumps.
In our study, we investigated several slag samples from five different dumps located in the Harz Mountains. We also conducted laboratory SIP measurements at original Harz ore samples (Rammelsberg ore, Oberharzer vein) and host rock samples (clay shale, greywacke, claystone) for comparison. At a historical
mining dump near Goslar, we also carried out field measurements at two SIP profiles. Based on the spectrum of the imaginary part of the conductivity, σ‘‘, we were able to assign the SIP results to different groups and to classify the samples. Measuring the polarisation magnitude σ‘‘ at a medium frequency (around 1 Hz) enables discrimination between slag material, ore material and host rock, since they could be assigned in a low-polarisable, medium-polarisable or high-polarisable group. Additional information could be obtained by considering the full spectral behaviour of σ‘‘. Since the spectra of the slag samples varysignificantly, any pre-defined models (e.g., Pelton models, constant phase model, etc.) cannot take into account the variability of the observed curves. Instead, we classified five different types of σ‘‘ spectra in the field frequency range (0.1 – 100 Hz): the ascending, descending, constant, maximum and minimum type. Our results show that these types could be recognized in both laboratory and field measurements. In particular, the increasing and maximum type indicated highly polarisable slag material. Our findings enabled
a zonation of different slag types within the dump and a delineation from the host rock. We believe that this classification is more suitable to compare laboratory with field results since the spectral shape depends only to a minor degree on saturation, fluid conductivity, and the representative volume.