The Toyoha Mine is reputed to be one of the typical epithermal lead and zinc vein-type deposits in Hokkaido. The ore is composed mainly of sphalerite, galena and pyrite with an important amount of silver minerals and also include accessarily chalcopyrite, tetrahedrite, marcasite, pyrrhotite, magnetite, hematite etc. Any characteristic feature indicating a xenothermal condition has not been known.
Recently the tin minerals such as cassiterite and stannite were found by the present author from Izumo Vein, one of the important veins of Toyoha Mine. The tin minerals occur almost always in association with finegrained sphalerite and rarely with quartz, pyrite or coarse-grained sphalerite. Both minerals have, in general, about the same grain size as that of the associated sphalerite, but a grain of cassiterite is always an aggregate of minute crystals, 10-20μ in diameter. The formation temperatures estimated by fluid inclusion study of quartz associated with cassiterite, of coarse-grained sphalerite and of banded quartz are in the range of 241°-230°C, 218°-215°and 202°-190°C respectively. Quantitative chemical analysis of stannite by EPMA is not performed yet but Cu, Fe, Sn and S are detected in a good amount with a trace of Zn.
In Hokkaido there have been known two mines where tin minerals occurred, one Suttu Mine for stannite and cassiterite and the other Kutosan Mine for tetrahedrite containing an appreciable amount of Sn. Sn-free tetrahedrite, stannite and cassiterite in a sample from of Kutosan Mine are newly described. It is known that many base metal deposits in West Hokkaido show a zonal distribution from the inner zone of Cu, Pb and Zn to the outer one of Au and Ag. It is an interesting fact that both Toyoha Mine and Suttu Mine are located in the central part of the inner zone and show the characteristics of xenothermal ore deposits.
