Geology and Mineralisation
The gold-silver mineralization style at Buho and other nearby deposits are classified as rift-related, low to intermediate sulfidation epithermal. Mineralization is typically associated with precious-metal and base-metal-bearing epithermal veins, characterized by pyrite, chalcopyrite, sphalerite, galena, electrum and tetrahedrite in a gangue of quartz, adularia, calcite, sericite and illite-smectite. Late-stage rhodocrosite and barite are common. Some of the deposits (e.g., Chitose) contain significant base-metals in the deeper parts of vein systems. The silver-gold ratios of the vein deposits are 5-50 and were formed from about 12-1 Ma4.
The Buho gold-silver workings are hosted by Pliocene rhyolitic lava, tuff and tuff-breccia that are underlain by Early Pliocene propylitic-altered, basaltic-andesitic volcanic rocks . Both rock units are overlain unconformably by andesitic volcanic rocks that comprise the southern margin of the Pleistocene Akaigawa caldera complex. The caldera hosts a number of mineral occurrences including the Todoroki gold mine, which produced 188.3 Koz5 of gold, the Meiji mercury mine and several base-metal vein deposits. Buho is perhaps the youngest of these deposits with silica sinter still intact, forming part of the relict hot spring system developed at the intersection of major structures with the caldera ring-fracture zone. At Buho, the major structures trend N070° E and are related to caldera development while subsidiary tensional structures trend north-south. Silica sinter is preserved near the top of the main east-west trending, silicified ridgeline. Immediately below the sinter, two historical exploration adits are developed on north-trending gold-silver-bearing, colloform-banded, quartz-chalcedony-adularia veins.
The historic Nakanosawa and Todoroki gold-silver mines occur outside of the project area, located 7- and 12-km to the east-southeast of the Buho workings, respectively. The larger Todoroki mine (188.3 Koz Au and 7 Moz Ag historic production5) consists of 12 gold-silver quartz veins hosted by Pliocene rhyolitic tuff-breccia units ranging from 0.8 to 4.5 m wide and extended up to 1200 m along strike and 220 m vertical, characterized by average mining grades of 2.3-6.2 g/t Au and 92-257 g/t Ag1.