Ore Mineralogy and Formation Condition of Epithermal Gold – Silver Deposits in the Southwestern Hokkaido, Japan

The southwestern Hokkaido mining district is well known for the occurrence of many epithermal ore deposits ofNeogene Tertiary period.The ore mineral assemblages in the epithermal gold-silver deposits from the southwestern Hokkaido such as Kobetsuzawa, Shin Otoyo, Suttsu, Chitose, Teine, Date, Koryu and Toya mines are examined. The determinations of their minerals assemblages are based on the study of samples collected during field works in the southwestern Hokkaido and from the Hokkaido University Museum collection. Combinations of microscopic, electron-microprobe, and BSE-based image analyses of polished sections were used to evaluate the chemical composition of ore minerals. Most of gold-silver in the magmatic-hydrothermal systems of the southwestern Hokkaido are enriched in tellurides and Te-bearing minerals, suggesting major carriers of precious metal in the ore. Other minerals are sulfides, sulfosalts and oxides group of ores, although each deposit is characterized by the occurrence of some difference ore minerals assemblages. The presences ofenargite or famatinite in some deposits suggest the deposition in the higher sulfidation state of epithermal mineralization style. The measurement ofhomogenization temperature and salinity of fluid inclusions in some gangue minerals such as quartz associated with ores from those deposits suggest that the formation temperature in general are around 170 — 260 °Cwith salinity less than 4 wt.% NaClequiv.Boiling indications in fluid inclusions of some deposits give the possibility this phenomenainfluence to the ore precipitation.