Abstract
Investigation area is located 40 km east of Asahikawa, central Hokkaido, Japan, where the road tunnel approximately 3 km long was constructed. The tunnel passes through the Mesozoic sedimentary rock composed mainly of slate of Hidaka metamorphic belt. The rock distributed from the entrance to 175 m of the tunnel is composed of hydrothermally altered slate. Hydrothermal alteration in this area is accompanied by arsenopyrite. The leached arsenic concentration exceeded 0.01 mg/L in many samples. Arsenic content was high in a vein-type altered zone, such as silicified veins being the main part in the hydrothermally altered zone and contact zones with intrusion of altered andesite. It was also high at local alteration zones away from silicified zones. In particular, the latter zone showed the highest As content. Arsenic in those vein-type altered zones was distributed in sulfides-filled fracture zones and in altered host rock at the contact zones. Arsenic in the contact zones was soluble in water and its concentration after leaching often exceeded 0.01 mg/L, even though the As contents were lower.
