Subvolcanic hydrothermal systems: Implications from hydrothermal minerals in hydrovolcanic ash

Hydrothermal minerals, including silica, 7 Å-kaolin minerals, pyrophyllite, di-octahedral smectite, and alunite in ash of phreatic/phreatomagmatic eruptions from eight volcanoes in Northeastern Japan were derived from subvolcanic hydrothermal systems. Comparison with internal structures of some extinct volcanoes indicates that hydrovolcanic ash was derived from cores of stratocones at depths of 250–1500 m or greater. The hydrothermal mineral assemblages are similar to those of acid–sulfate type alteration around hydrothermal ore deposit districts. They represent hydrothermal temperatures up to 340 °C. Relative abundance of Ca–K–Na alunite is not correlated with other acid–sulfate minerals because its precipitation reflects only fluid composition, whereas hydrous aluminosilicate and silica minerals reflect base-ion leaching by acidic fluids. Temporal variations in composition and abundances of mineral assemblages at some volcanoes imply the evolution of acid–sulfate alteration zones.