Induced‐polarization response of zeolitic conglomerate and carbonaceous siltstone

Three case studies investigating induced‐polarization (IP) responses of a zeolite‐bearing conglomerate and of two carbonaceous siltstones are presented. The IP response of these noneconomic geologic materials can either mask or mimic the response from sulfide mineralization which is sought by electrical field surveys. The nonsulfide rock types which produced unusually high responses on IP field surveys were sampled by core drilling for chemical, mineralogical, and electrical laboratory study. The electrical response of core samples was measured in a four‐electrode sample holder over the 0.03–1000 Hz range. Geologic description of the core, petrographic examination of thin sections, mineral identification by x‐ray diffraction (XRD), and chemical analysis of samples supplemented the electrical measurements. A surface phase response of 20 mrad was obtained from field surveys over the Gila conglomerate at an Arizona location. Core samples of the Gila were examined in thin section, and clast surfaces were found to be coated with a thin layer of zeolites. These zeolites project into pore spaces in the conglomerate, and thus are in intimate contact with formation waters. A series of laboratory experiments suggests that zeolites cause most of the observed IP response. Phase responses as high as 100 mrad were measured with field surveys over siltstone and limestone sequences in western Nevada. Samples recovered from the Luning and Gabbs‐Sunrise formations include siltstones containing small amounts of amorphous carbon. These siltstones are very conductive electrically, and the high‐phase response is attributed to polarization of the carbon‐pore water interface. Low porosity in these carbonaceous siltstones enhances the phase response.