Geological Features of Hydrothermal Altered Rocks Leaching Arsenic and Heavy Metals at the Horobetsu Sulfur Mine Area, Southwestern Hokkaido

Abstract

 Clarifying the leaching behavior of arsenic and heavy metals from hydrothermal altered rocks at mine areas is important to deduce the water quality of mine drainage. Using drill core samples, we discussed geological features of hydrothermal altered rocks which considerably leach toxic elements, especially arsenic, at the Horobetsu sulfur mine area, southwestern Hokkaido. Two Pleistocene andesite lava layers and a volcanic breccia layer have undergone sulfuric acid hydrothermal alteration. The lower part of the volcanic breccia layer particularly shows strongly altered features, consisting of the sulfide zone, acid leached silicified zone and argillic zone. The arsenic concentration in the leachate is different according to alteration mineral assemblages, thus the mineral assemblages would be an important clue to estimate the degree of arsenic leaching. Compared with other alteration zones, the sulfide zone is rich in arsenic content (∼965ppm) and leaches it remarkably (∼113mg/l). Moreover, large amounts of pyrite are contained in the sulfide zone, and a high concentration of arsenic was recognized in the pyrite. These indicate that arsenic was supplied by hydrothermal activities and was added with pyrite precipitation. Silicified veins are sometimes observed in the sulfide zone and contain Fe-sulphate mineral (rhomboclase) which was formed by the hydrolysis of pyrite. Since rhomboclase generates ferric ion by dissolution, it would promote pyrite oxidation accompanied with the release of arsenic.