Faradaic and Non-Faradaic Spectral Induced Polarization Signatures of Copper and Graphite Mixtures: Disseminated Grain Versus Veinlet Mineral Structures

Abstract

Summary Characterizing electronically conductive minerals with spectral induced polarization (SIP) has been a longstanding goal of mineral exploration. A more comprehensive understanding of how mineral texture (e.g. disseminated grain versus veinlet forms) and mineral type influence SIP signals is required, as the polarization mechanisms at the electron conductor-fluid interface and their effect on SIP signatures remain incompletely understood. We performed laboratory measurements on synthetic copper (Cu) and graphite (Gr) samples prepared in disseminated grain and veinlet forms by mixing fixed concentrations of the electron conductor with a non-polarizable sand. Current density dependent SIP (complex impedance) responses were assessed by measuring at multiple current densities, and non-equilibrium was investigated by making measurements as a function of time. SIP responses that remained independent of current density were interpreted as evidence of non-Faradaic processes, with possible minor Faradaic contributions, being dominant. Conversely, current density dependent complex impedance responses were interpreted as an increased contribution from Faradaic polarization processes involving redox reactions at the electrolyte-electron conductor interface. Veinlet Cu samples exhibited strong current density dependent SIP responses and prolonged equilibration, in contrast to the negligible current density dependence of veinlet Gr and disseminated grain Cu and Gr samples. All samples that exhibited negligible current density dependence could be fit to a Pelton-Cole model, whereas samples exhibiting strong current density dependence could only be fit with a more flexible Debye decomposition model. Gr samples displayed a fluid-resistivity dependent peak in the phase response at a specific frequency consistent with mechanistic models, while Cu samples deviated from predicted behaviour. Measurements on rock cores were consistent with observations on synthetic samples, with the SIP response of a Cu bearing rock core exhibiting a clear dependence on current density, whereas the Gr bearing rock core showed negligible current dependence. The observed dependence of the phase and complex resistivity on current density, mineral type, and texture is consistent with differences in the relative importance of Faradaic and non-Faradaic polarization processes. These results suggest that distinguishing current-density-dependent Faradaic responses from current-independent non-Faradaic responses may improve the discrimination of mineralogical and textural characteristics using SIP measurements, although field-scale applications will be limited due to the lower current densities relative to those used in this laboratory study.