Regional geological study was carried out in the famed gold-silver quartz vein mining area of the central part of western Izu Peninsula (ca. 195 km2), which belongs to the southern Fossa Magna geotectonic province of the late Cenozoic Green Tuff orogeny. The constituents, stratigraphy and tectonic development of the area are summarized as follows:
The results of this study, stratigraphic correlation in particular, were discussed in detail in comparison with many previous works. It is concluded that the constituents in this area consist of the lower unit of the Miocene Yugashima formation and the upper unit of the Pliocene Tanaba formation and Nekko dacite group.
The geologic history of the upper unit was initiated by basic volcanism associated with block subsidence (1, 000 m±). The second stage is acid magmatism that occurred in the subsided area and formed many dome structures in the pre-existing rocks. The final stage is seen as Quaternary volcanism and hot spring activity at present.
Most of gold-silver quartz veins tend to cluster around the dome structure. The ore veins were formed in tensional fractures originated in normal faulting related to the dome formation, as best exemplified at the Seigoshi mine. Acid intrusive bodies are inferred to exist under these domes. It is concluded that the gold-silver deposits were formed by the ore solution derived from the hidden subvolcanic intrusives during the second stage of dacitic activity.
