Timing of Au-Ag mineralization and related volcanism at Otoge, Yamagata Prefecture, Northeast Japan

Abstract

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.