Quaternary Volcanic‐Related Acid Hydrothermal Alteration in the Ugusu Silica Stone Deposit, Western Izu Peninsula, Central Japan: Geology and Alteration

Abstract

AbstractAcid alteration areas accompanying Quaternary volcanoes are widespread in the western Izu Peninsula, central Japan. The Ugusu alteration area is the largest among them and is mined for “silica stone” at the silica body in the core of the alteration area. Silica zone defined by previous studies is subdivided into highly leached brecciated silica zone and residual silica zone, which extend along a NNW‐SSE–NNE‐SSW direction of fractures/faults. Fe‐rich, alunite, advanced argillic alteration, and intermediate argillic alteration zones occur toward the outside surrounding the two silica zones. The ascent of acid hydrothermal fluid would be responsible for the silica zones and surrounding alteration zones at an earlier stage, while the hydrothermal brecciation and silica veins were caused by a limited supply of silica‐saturated fluids at later stages. Based on the available mineral stability relations and fluid inclusion thermometries, the formation temperatures are estimated to be: >300°C for the residual silica zone; >290°C for the diaspore association in the advanced argillic alteration zone; and <260°C for the kaolinite association in the intermediate argillic alteration zone. The later stage quartz druses have been formed at 200–260°C. The Ugusu–Fukata acid hydrothermal systems were active at 1.5–1.2 Ma, which were temporally related to the Tanaba Andesite volcanism. Hydrothermal system at the Seikoshi gold–silver deposit survived until 0.7 Ma after the volcanism. In the western half of the Izu Peninsula, subduction of the Philippine Sea plate underneath the Suruga Trough caused nearly N–S‐trending maximum horizontal compressive stress (σHmax) and the resultant formation of similarly trending alteration areas and Au‐Ag vein‐type deposits in the Ugusu‐Toi‐Seikoshi area. From a practical viewpoint, at the Ugusu silica stone deposit, the fracture‐controlled vertical morphology of the silica body provides an important guide for exploration. Because the alteration zones occur both in the lower and upper sides of the silica bodies, it is important to make sure to which side the alteration zones correspond.