High-grade lead-zinc-silver are occurs in the Toyoha vein-type deposit, which is located 40 km southwest of Sapporo, Hokkaido, Japan. Several K-Ar ages from 3 Ma to 0.5 Ma were reported for the mineralization. It was originally regarded as one of typical epithermal are veins, but can now be considered a polymetallic vein-type deposit, because of the discovery of tin, indium, tungsten and molybdenum minerals from the veins. Main gangue minerals are quartz, calcite, rhodochrosite, chlorite, sericite and kaolin mineral. Illite occurs at the upper levels, and gives place to chlorite at the lower. As a whole, the boundary seems to be a shallower than-150 meter level in the northwestern veins of the mining area, and declines to about-400 meter level in the southeastern veins. Chlorite is one of the abundant hydrothermal minerals throughout the alteration zones of Toyoha deposit. Iron content in the chlorite tends to increase generally toward the ore veins and the chlorite in gangue mineral is almost pure iron-chlorite. Iron-rich chlorite can be formed by an are solution which is in equilibrium with rocks at high temperature and reaches to an ore deposition site.