Terrestrial volcanic products and lacustrine deposits of Middle Miocene to Pliocene age are widely distributed in the Monbetsu-Kamishihoro Graben which belongs to the Kuril Inner Arc. The Seta deposit located in the southern extremity of the Graben is classified into a hot spring gold deposit, based upon the modes of occurence.
In the Seta mine area, Middle Pliocene lacustrine deposits of the Ashoro Formation which were accumulated in the Tokachi Basin unconformably covers Late Miocene to Early Pliocene terrestrial volcanic products of the Horokapiribetsugawa Formation. The lacustrine deposits are composed of sandstone, mudstone and siliceous deposits associated with a hot spring gold mineralization. A wide variety of sedimentary structure of the lacustrine deposits is observable in this area ; for detailed investigation, a logging of 80 meters of new diamond drill core (GSH-91-1) was necessary, which revealed the sedimentary structure of the basal part of the deposits and the relationship of the Horokapiribetsugawa and the Ashoro Formations. Siliceous deposits have been divided into 5 types, S-I to S-V. Silica is deposited as silica sinter (S-I) on land and/or temporally on shore near the water line, and also conducted into the cold lake water by the thermal water as a thin bedded siliceous deposit (S-II), because the silica gelation quickly proceeds under the influence of current or wave. The hydrothermal eruption broke up the underlying rocks for breccias like a clastic dike and, as a result, scattered breccias, so-called hydrothermat explosion breccias (S-IV) and/or the accidental silica blocks (S-III) in tuff and sandstone. When the hydrothermal eruption took place intermittently in shallow water, silica was deposited as silica complex deposit with sandstone and conglomerate (S-V)
The hydrothermal activity has been divided into seven stages (I to VII). In the Stage I, just prior to form the lake, the acid hydrothermal activity, began, resulting in the silicified and argillized zones in the Horokapiribetsugawa Formation. Stage II, is characterized by the prevalence of hydrothermal explosion breccias on the land of the northern edge of the lake. As mixing of the ascending acid thermal water with the cold groundwater or interstitial water in the basal part of the lacustrine deposits and the argillized rocks of the Horokapiribetsugawa Formation just under the surface of unconformity, adularia formed in sandstone and argillized tuff breccia with kaolin minerals as an alteration product. In the Stages III and V, the hydrothermal activity increased and silica was deposited on the land and/or on the bottom of the lake resulted in silica sinter, thin, bedded siliceous deposit and siliceous complex deposit. Stages IV and VI-1 are similar to Stage II. It may be inferred that the hydrothermal activity was carried on the shore of the lake or the land. In the Stage IV-2, the silica sinter which deposited to a thickness of about 2 meters on land. The hydrothermal activity has silicified to the surrounding rocks and has formed vertical and horizontal quartz veins in these rocks. In the Stage Vll, the hydrothermal activity was attenuated.
These spots of the hydrothermal activity moved from north to south associated with the gold mineralization.
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