Electrical properties of hydrothermally altered dacite from the PACMANUS hydrothermal field (ODP Leg 193)

Abstract

Ocean Drilling Program Leg 193 drilled two sites at the PACMANUS hydrothermal field, Snowcap and Roman Ruins. We investigated electrical properties of core samples recovered from these sites to study the effect of hydrothermal alteration on the electrical properties of dacite. Most of the samples are completely altered and have a high porosity (0.16–0.43). When saturated with seawater, they have a high electrical conductivity (0.05–0.3 S/m) and low formation factor (12–103). The only fresh sample has a low porosity (0.01), low electrical conductivity (0.002 S/m), and high formation factor (1920). The samples show a wide range in tortuosity, which can be explained by the large variety of volcanic textures and changes in pore space structure due to hydrothermal alteration. Samples from the Roman Ruins field that exhibit siliceous and sulfate–silica–clay alteration styles have a high surface conductivity, which is related to high clay contents and the presence of clay minerals with high cation exchange capacity such as vermiculite and surface conductivity has a significant effect on formation factor values. In contrast, siliceous samples from the Snowcap field have lower surface conductivity, clay content, and cation exchange capacities. Borehole conductivity measurements (induction log) indicate a non-negligible influence of electronic conduction due to the presence of sulfide minerals. This indication is confirmed by laboratory measurements: several samples show a high frequency dependence in their electrical properties which is probably related to the presence of disseminated pyrite. Intense fracturing, brecciation, high porosity, hydrothermal alteration and the presence of pyrite mineralization are the main factors controlling electrical conductivity of the hydrothermally altered formation.