Desaturation effects of pyrite–sand mixtures on induced polarization signals

Abstract

Induced polarization (IP) is an acknowledged method in ore exploration and can be applied to evaluate the metal content in dumps containing the residues of ore processing facilities. Existing models explain the relationships between ore content and grain size of the ore particles with IP parameters. However, the models assume full water saturation of the ore containing samples, which is often not the case in field conditions at dump sites. Hence, our study investigates the effect of desaturation on the resulting IP signal. We used six different sand-pyrite mixtures with varying amount and grain sizes of the pyrite particles. Evaporative drying desaturated the samples. Complex conductivity spectra were recorded in the frequency range between 0.02 and 1000 Hz at certain saturation levels. The resulting spectra indicate an decrease of the conductivity amplitude with progressing desaturation. This effect agrees with the second empirical Archie equation. The saturation exponent of the conductivity amplitude shows values slightly larger than one. The measured spectra were processed by a Debye decomposition. We observe a nearly constant total chargeability during desaturation. This finding is in agreement with existing models that relate the total chargeability to the metal content in the sample. However, the mean relaxation time decreases remarkably during the drying process, whereas the normalized relaxation time, which considers the ratio between the mean relaxation time and the resistivity of the embedding material, does not indicate any dependence on water saturation. This behaviour contradicts existing models that predict a decreasing relaxation time with increasing water salinity, which results from evaporative drying. In order to explain the observed effects, we propose a conceptional model that compares a mixture of pyrite particles in an embedding material (sand, water, air) with an electrical RC circuit. The pyrite grains behave as small condensers that are charged and discharged via the conductive background material. According to this simple physical model, the relaxation time is proportional to the resistivity of the embedding material. A resistivity increase while desaturation causes an increase of relaxation time as observed in our experiments. This conceptional model is in good agreement with other experiments that change the resistivity in the background material by varying water salinity or clay content. The capacitive behaviour of the dispersed particles is characterized by the normalized relaxation time that depends on the grain size and the volume content of the pyrite particles.