Potassium-argon ages of the epithermal gold-silver mineralization in the Haenam-Jindo area, southwestern Korea

The Haenam-Jindo area, located on the southwestern margin of the Korean Peninsula, was the site of vigorous volcanic activity during the Late Cretaceous and Early Tertiary periods. Large parts of the area record strong hydrothermal alteration, and there exist many clay-alunite and gold-silver deposits. We undertook potassium-argon (K-Ar) age dating of five mineral samples (including adularia, sericite and alunite) from the Eunsan, Moisan and Gasado epithermal gold-silver deposits in this area. The purities of the samples were confirmed by X-ray diffraction analysis. The K-Ar ages of adularia from the Eunsan deposit and adularia and sericite from the Moisan deposit (related to gold-silver mineralization) are 75.0 ± 1.6, 74.7 ± 1.6 and 75.1 ± 1.6 Ma, respectively. The similarity of these ages, combined with the close proximity and similar geochemical characteristics of the deposits, indicates that the mineralization occurred as part of a single hydrothermal system. The K-Ar ages of alunite at the surface and adularia at depth within the Gasado deposit are 82.2 ± 1.9 and 70.7 ± 1.9 Ma, respectively, revealing that the clay-alunite and gold-silver mineralization formed at different ages. K-Ar age data indicate that the gold-silver mineralization in this area occurred mainly at 75-70 Ma, resulting from hydrothermal activity in the Haenam-Jindo area (82-70 Ma). This is the first time that the mineralization of precious metals in Korea has been identified during this period.