ABSTRACT

Chemical compositions of epidote from active and ancient geothermal systems were summarized. Iron and aluminum contents of epidote from geothermal systems vary widely, ranging from about 4 to 19 wt % for Fe₂O₃ and from 18 to 30 wt % for Al₂O₃. It was found that the iron content of epidote has roughly a positive correlation to the Fe₂O₃ content of original rocks, indicating that the Fe₂O₃ content of original rocks affects largely the variations in iron content of epidote. This correlation is consistent with the results of theoretical calculations on the water/rock interaction under the hydrothermal conditions which show that the epidote is formed under the rock-dominated conditions. Epidote associated with hematite has high iron content, while that associated with prehnite, pyrite and pyrrhotite has low iron content. These facts indicate that CO₂ and O₂ fugacities are also important factors controlling the compositional variations in epidote.