K-Ar ages of Au-Ag mineralization and hydrothermal alteration at the Otoge deposit were determined to be 3.93±0.15, 3.99±0.10, 4.10±0.10 and 4.15±0.10 Ma for four sericite separates, proving that the Au-Ag bearing quartz veins at deep level and the sericite zone of the clay deposit at the surface of present level were formed contemporaneously. The ages are also close to those of Au-Ag-Pb-Zn mineralization at the Yatani deposit, 3 km northeast of the Otoge deposit, suggesting that mineralization in both areas occurred within a series of hydrothermal systems. The eruption ages for welded tuff of the upper part of the Otoge Formation were determined to be 4.85±0.13 Ma (biotite, K-Ar age) and 4.60±0.30 Ma (zircon, fission track age), predating the mineralization.

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Experiments on ion exchange of Ca²⁺, Fe²⁺ and Mn²⁺ between grossularalmandine-spessartine garnet solid solution and 2N aqueous chloride solution were conducted at 600°C and 1 kb under relatively reducing conditions in order to determine the stability relation and mixing property of the garnet solid solution. The experimental results revealed that calcium ion is slightly preferentially partitioned into aqueous chloride solution compared with ferrous and manganese ions. Grossular, almandine and spessartine form an almost continuous solid solution except for the semicircular region along the grossular-almandine join, where mineral assemblages of fayalite+anorthite and hedenbergite+anorthite+wustite appear instead of the garnet solid solution. Mixing properties of the garnet solid solution were evaluated from the experimental results by considering the formation of FeCl₃⁻_aq and MnCl₃⁻_aq in the aqueous phase. Almandine-spessartine solid solution shows slightly negative deviation from ideality. On the other hand, grossular-spessartine solid solution shows slightly positive deviation. Mixing property of grossular-almandine solid solution was not well determined due to the large unstable region.

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