Two-stage Mineralization and Formation Process of the Toyoha Deposits

The formation process of the Toyoha deposits is discussed from considerations on geological environ ment, mineral association and on the result of fluid inclusion studies. It is well known that mineralization at the Toyoha deposits is divided into two stages, the earlier and the later ones. More than 15 veins are arranged in major three directions, E-W, NW-SE and N-S. Most of the veins of E-W system are earlier ones, while those of NW-SE and N-S systems were formed by the later mineralization. Pyrite, sphalerite and galena are common and abundant minerals throughout two stages. In the earlier stage, arsenopyrite is found at eastern lower part (Harima), whereas hematite increases towards western upper side (Chikugo, Tajima). In the later one, pyrrhotite and graphite associated with tin and tungsten minerals occur at eastern and southeastern areas (Sorachi, Izumo), while carbonates and Sb-minerals are found at western and northwestern areas (Soya, Oshima). Temperature and salinity of fluid inclusions have a decreasing tendency from Izumo to Soya, and from Harima to Tajima. From these evidences together with the existence of heat source at southeastern deeper zone, the model for the formation of the Toyoha deposits is proposed. By this model, the consistent explanation for the occurrence of overall minerals in the mine has become possible. The model also leads to some important suggestions for exploration.