Gold Mineralization in Izu Peninsula, Central Japan, during Crustal Extension in Response to Double Subduction

Abstract

Izu Peninsula in central Japan, the northern tip of the Izu-Bonin arc, hosts numerous epithermal Au-Ag vein deposits of low-sulfidation style. All have similar vein textures, mineralogy, and alteration. Geochemical data from fluid inclusions in vein quartz, the mineralogy and mineral chemistry of alteration, and stable isotope data indicate that auriferous hydrothermal activity occurred under subaerial conditions. The K-Ar ages of auriferous vein minerals are <1.5 Ma, indicating that the mineralization took place after extensive submarine volcanism for the host rocks. These observations suggest that Au-Ag mineralization was synchronous with the development of an extensional regime of the Izu block after its collision with the Honshu arc after 1.5 Ma. This collision resulted in the shifting of the Izu block far from the trench to the rear position, and the subduction of the Izu block along the Suruga trough to the west and along the Sagami trough to the east. The reararc position of the Izu block and double subduction resulted in crustal extension, upwelling of asthenospheric mantle, and tholeiitic magmatism reflected by mafic dyke swarms and subsequent monogenetic volcanic activity in the Izu peninsula. The timing of the Au mineralization in the Izu Peninsula during the beginning of lithospheric extension is similar to that of the Sado Au-Ag deposit on Sado island in the Japan Sea. Two mineralization events coincide with extensive tholeiitic mafic volcanism and injections of dyke swarms related to the back-arc opening of the Japan Sea. The geological setting of the Au-Ag mineralization in Izu and Sado is also similar to that of the epithermal Au-Ag deposits in northern Nevada, where mineralization was contemporaneous with crustal extension and tholeiitic mafic magmatism derived from the asthenospheric mantle. This study suggests that epithermal Au mineralization at shallow crustal depths is a product of large-scale lithospheric evolution.