Tomuraushi volcanic group (TVG) is situated at the middle part of Daisetsu-Tokachi volcanic chain, Central Hokkaido, Japan. The basement rocks of the TVG are composed mainly of Miocene altered volcanics and early Pleistocene Tokachi welded tuff. The volcanic activity of the TVG started in middle Pleistocene and can be divided into Older and Younger stages.
     The Older stage (1.0∼0.7 Ma) consists of two polygenetic volcanoes, Koganegahara (total volume> 5 km³) and Goshikigahara (>15 km³). Koganegahara volcano, located in the southwestern area of the TVG, is composed of thin lava flows which effused from E-W trending vents. Goshikigahara volcano, located in the northern area of the TVG, consists of three cones, Kaundake, Goshikidake and Ponkaundake. Thin lava flows and voluminous pyroclastics erupted from central conduit of Kaundake cone, form the lower part of Goshikigahara volcano. At the late stage of activity, voluminous lava flows effused from the eastern and western flank of Goshikigahara volcano, built Goshikidake and Ponkaundake cones.
     After a long dormancy, the volcanic activity of the Younger stage started about 0.3∼0.2 Ma. Both Futamata (>0.08 km³) and Kaunnai (>0.1 km³) volcanoes are monogenetic and effused lava flows from the base of the northern flank of Koganegahara volcano. Tomuraushi volcano (about 5 km³) is composed mainly of thick block lava flows and lava domes with less voluminous pyroclastic fall deposit, which effused from more than 20 eruption centers between Koganegahara and Goshikigahara volcanoes. The primary surface structures of those flows and domes have been preserved.
     In summary, the volcanism of the TVG changed from polygenetic to monogenetic, and the corresponding differences in erupted volume and mineral assemblage of lavas were found.

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The Nasu volcano group is composed of six volcanic edifices, Kasshiasahidake, Sanbon-yaridake, Asahidake, Minamigassan, Futamatayama, and Chausudake, located in the volcanic front of the Northeast Japan arc.
     Kasshiasahidake (ca. 0.54-0.42 Ma), Sanbon-yaridake (ca. 0.36-0.27 Ma), Minamigassan (ca. 0.21-0.03 Ma) have similar geological sequences. Alternation of thin basaltic andesite lava flows and associated pyroclastics developed in the lower part, whereas thick andesitic or dacitic lava flows and minor pyroclastic flows developed in the upper part. Between these two stages, caldera collapse sometimes occurred. On the other hand, Asahidake (ca. 0.21-0.06 Ma), Futamatayama (ca. 0.14 Ma), and Chausudake(ca. 0.04-0 Ma) are composed of andesitic lavas, lava domes, and pyroclastic flows, lacking thin basaltic andesite sequences. From evolutionary historical point of view, Kasshiasahidake, Sanbon-yaridake, and Minamigassan (including Asahidake and Chausudake edifices) edifices construct individual stratovolcanoes, which have similar evolutionary histories. Futamatayama is distinct from these.
     Volume and eruption duration of the three stratovolcanoes are as follows; ca. 200 k.y., 16.2 km³ for Kasshiasahidake, ca. 150 k.y., 7.2 km³ for Sanbon-yaridake, ca. 200 k.y., 17.8 km³ for Minamigassan (including Asahidake and Chausudake). These volumes and eruption rates are less than those of large stratovolcanoes (Akagi, Haruna, Hakone volcanoes etc.) in near the triple junction of plate boundary, but are comparable to those of small stratovolcanoes (Quaternary volcanoes in Shin-etsu Highland).

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The Koryu gold-silver mine is located approximately at 141°19' of east longitude and 42°51' of north latitude in southwestern Hokkaido. The ore deposits are of epithermal gold-silver-quartz vein type, and they are composed of the No. 1-No. 7 veins hosted by the Miocene black mudstone.
     In this study, we have determined homogenization and ice melting temperatures of fluid inclusions in quartz, and chemical compositions of some ore minerals by microprobe analyses for the interpretation of mechanism of gold-silver deposition in the two veins (No. 1 and No. 3).
     Three stages of mineralization are discriminated on the basis of brecciation and cross-cutting relations in the vein, and gold-silver deposition occurred predominantly during the middle stage. The ores ordinarily exhibit banded structure composed of quartz, ±adularia, ±clay minerals, ±carbonates, and gold-silver and other metallic minerals (ginguro).
     The ore minerals are electrum (44-74 atom.%Ag), aguilarite (Se/(S+Se) atomic ratio : 0.25-0.51), pyrargyrite-proustite, pearceite-polybasite, miargyrite, Ag-bearing tetrahedrite (22-24 wt.%Ag) , galena, sphalerite (0.6-1.2 mole% FeS), chalcopyrite and pyrite etc. Ag content of electrum tends to decrease slightly toward the upper levels in the No. 3 vein, whereas FeS content of sphalerite is invariable between the upper and lower levels.
     Fluid inclusions observed in quartz of the middle stage are two phase and liquidrich type. Homogenization temperatures for the No. 1 vein are mainly 220°-270°C with high frequency in the range 250°-260°C. The temperatures for the No. 3 vein are about the same as those for the No. 1 vein, but show higher values (250°-280°C) at 30 mL. Ice melting temperatures of inclusion fluids for the No. 1 and No. 3 veins are from −0.4°C to −1.3°C, and from −0.5°C to −1.4°C, respectively.
     Spatial variations of these homogenization and ice melting temperatures, together with the data previously reported, would indicate that the relatively CO₂-rich ore fluids had boiled on the way to form ore shoots.

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