Petrological control on chargeability with implications for induced polarization surveys

Abstract

Induced Polarization (IP) surveys are commonly used to visualize the physical property of chargeability and are a widely applied tool to investigate subsurface geological structure. In this work we explore the petrological control on chargeability to better understand the underlying geological features that IP surveys may image. We report the results of petrophysics, automated mineralogy, and image analysis from a case study of eleven samples from the Salmon Canyon cobalt‑copper deposit in Idaho, USA with the aim of enhancing comprehension of the relationship between mineralogy and petrophysics. While total sulphide volume is positively related to chargeability, total number of sulphide grains and chargeability have a significantly stronger association. Of the sulphide minerals present in these samples, chalcopyrite is the predominant influence on chargeability response, followed by pyrite, while cobalt-bearing sulphides appear disassociated with response. Specifically, chalcopyrite grains less than 200 μm diameter yield the greatest association to chargeability while grain sizes larger than 200 μm have an association that is notably less significant. Grain shape parameters were examined for potential causation to this grain size drop in chargeability but do not yield any distinct relationship. However, chalcopyrite grain characteristics do exhibit strong correlations with the metamorphic minerals stilpnomelane and tourmaline, implying that chalcopyrite grain abundance and shape are modified during metamorphism. A strong relationship between the presence of stilpnomelane and total number of chalcopyrite grains implies that the process of stilpnomelane growth during metamorphism leads to enhanced conductive ore dissemination and hence chargeability. Furthermore, tourmaline presence correlates with chalcopyrite grain abundance and specifically greater tourmaline is linked with more elongate (less round) large chalcopyrite grains. The results of this case study implies that IP in part images metamorphic processes.