西南北海道，洞爺湖地域の新生代後期の熱水活動

Hydrothermal ore deposits such as Kuroko, vein, metasomatic and disseminated types are extensively distributed near the Toya Lake area in southwestern Hokkaido, Japan. The hydrothermal alteration zones related to ore formation are characterized by chemical features with neutral-solution reactions including chlorite-illite, epidote-chlorite-illite, mixed layer ± smectite, and smectite ± clinoptilolite zones and those with acidic-solution reactions including quartz, alunite, pyrophyllite ± kaolinite, kaolinite ± smectite ± quartz zones. Based on stratigraphic evidence of host rocks in addition to assignment of alteration zones and K-Ar ages of altered minerals from the surface and the drill holes, the hydrothermal activities are divided into four stages: stage-I (15.4-11.0 Ma), stage-II (8.8-6.0 Ma), stage-III (5.2-3.9 Ma) and stage-IV (ca.3-1 Ma).<br>The metallogic sequences with hydrothermal alteration in the Toya Lake area are characterized by 1) Toya-Takarada and Horobetsu Kuroko deposits, and a chlorite-illite zone hosted by middle Miocene volcanic rocks in stage-I; 2) Hakuryu and Horoto-Higashi Au-Ag vein deposits, and acidic to intermediate alteration zones as well as in the Nakatoya area hosted by Miocene to Pliocene submarine-terrestrial volcanic rocks in stage-II; 3) Kitayuzawa Au-Ag vein deposits and intermediate to acidic alteration zones with ores including Cu-Pb-Zn-Au-Ag hosted by upper Miocene submarine volcanic rocks, mudstone, and intrusive rocks, and Plio-Pleistocene terrestrial volcanic rocks in stage-III; and 4) Abuta and Horobetsu S and FeS deposits with a kaolinite± smectite zone hosted by Plio-Pleistocene terrestrial volcanic rocks in stage-IV.<br>Hydrothermal activity in the eastern area of the Toya Lake in stage-II resulted in the Nakataoya high sulfidation alteration zone with K-Ar ages of 7.0-6.3 Ma and a δ³⁴S of +23.5 ‰ for alunite, and a subsequent low sulfidation alteration zone bearing Au-Ag deposits with K-Ar ages of 6.8-6.0Ma for illite. This hydrothermal activity in the Nakatoya area may be closely related to the intrusion of Miocene granitic rock. In the western area of the Toya Lake, the granitic rocks with K-Ar ages of 7.3-7.2Ma for feldspar had a thermal metamorphic effect on the surrounding rocks, resulting in the formation of secondary biotite with K-Ar ages of 8.8-5.7Ma.<br>Hydrothermal activities often accompanied by the high sulfidation continued in stage-IV in southwestern Hokkaido. This zone was affected by the upper Miocene hydrothermal activity in the N-S trending uplift zone along the volcanic front in the northeast Honshu inner arc.