Mineralogy, chronology and formation process of the epithermal gold–silver vein deposits in the historical Togi mine, Noto Peninsula, Japan

Abstract

To determine local deposit features in the vein‐type gold–silver ore deposit of the historical Togi mine, Noto Peninsula, central Japan, we investigated the occurrence and chemical compositions of ore minerals, especially silver‐ and/or gold‐bearing minerals, and wall‐rock alteration in the Togo No. 3 tunnel, a major tunnel of the Urukami mining area of the Togi mine. In the tunnel, the stockwork veins which are generally 0.1–1.5 m wide runs in Neogene pyroxene andesite. In the ore‐bearing quartz vein, in addition to the sulfides such as sphalerite, pyrite and chalcopyrite, the following Au and/or Ag ore minerals were identified: electrum with <49.3 mol% Ag, uytenbogaardtite, fischesserite, silver‐rich minerals of the Ag2S–Ag2Se solid solution series, and Se‐rich pearceite. Gangue minerals in the quartz veins are quartz and adularia. The wall rock is silicified, chloritized, and sericitized pyroxene andesite. Multiple generations of adularia grown on quartz suggest that adularia has grown by repeatedly passing the hydrothermal solution through the vein. Based on the characteristic occurrence of the Ag and/or silver‐rich ore minerals such as electrum, fischesserite, and Se‐rich pearceite and a gangue mineral, adularia, and on the wall‐rock alteration characterized by chlorite, sericite, and calcite, the Togi deposit is classified as a Se‐type and low‐sulfidation epithermal gold vein deposit. The K‐Ar ages of illite/smectite mixed‐layer minerals from the altered outcrop and plagioclase from a fresh andesite sample are 17.68 ± 0.41 and 17.52 ± 0.43 Ma, respectively, in agreement with published data of the Besshodake andesite. These results indicate that the hydrothermal fluids generated by the Miocene andesitic magma activity infiltrated the fissures relating to the formation of the Togi‐gawa Nangan Fault at the northern boundary of the Togi mining area, and Au and/or Ag‐rich ore minerals were formed from the hydrothermal fluid with decreasing temperature at the shallow level.