[S-EM14] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM14-05] Physical and microscopic analysis of excessive electrical conduction along sulfide-filled microcracks in hydrothermal vein systems

*大田 優介¹、茂木 透¹、市原 寛¹、久保 大樹²、夏目 樹¹ (1.名古屋大学大学院環境学研究科、2.京都大学大学院工学研究科)

キーワード：熱水系、IP効果、比抵抗

Subsurface resistivity distribution explored by geo-electromagnetic soundings must be translated into subsurface rock types and structures by appropriate interpretation. Practical analysis of rock samples is essential for the proper interpretation. This presentation presents the physical and mineralogical features of several drill core samples obtained at the southern foothills of Mt. Ontake, Nagano Prefecture, Japan to interpret significantly low resistivity areas based on magnetotelluric surveys (Ichihara et al., 2018). We measured complex resistivity using the spectral induced polarization (IP) method and obtained elemental maps using the scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX). We clarified that these core samples contain various conductive sulfide minerals and exhibit low resistivity and strong IP effects. Their resistivity is too low to be explained by pore fluid, and the chargeability of most samples is more than 50%. In addition, SEM-EDX analyses showed that the areas of sulfide mineralization were restricted to vein-like areas. Noble metals such as Ag and Au were also detected in this vein. They suggest that a network of sulfide minerals precipitated in the micro vein system precipitated by past hydrothermal activity may contribute to the low resistivity. However, sulfide network in vein is not known as a cause of low resistivity. Therefore, we have used numerical methods to investigate the effect of the micro vein system on the resistivity of the entire rock mass.