Chemical compositon of chlorite with special reference to the iron vs. manganese variation, from some hydrothermal vein deposits, Japan

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Chlorite, which occurs commonly as a gangue mineral in four hydrothermal vein deposits, that is, Chitose, Yatani, Sado and Ohe, was quantitatively analyzed by X-ray microanalyser. Chlorite from Yatani, Sado, and Ohe, is Fe-rich, while chlorite from Chitose is Mg-rich. The chlorite geothermometer (WALSHE, 1986) applied to these vein chlorites gives temperatures equivalent to or slighly lower values than those obtained by fluid inclusion thermometry. They contain a large or small amount of manganese (maximum MnO content 17.5 wt. % at Sado deposit), and show Fe-Mn compositional variation at more or less constant MgO content in each vein. This is probably owing to their difference of time and space of chlorite formation. For example, the activity of Fe 2+ ion relative to that of Mn 2+ ion in hydrothermal solution decreases with temperature lowering at Chitose and Yatani deposits. While, the activity ratio increases with decreasing temperature at Sado and Ohe deposits. The exchange of Fe 2+ and Mn 2+ between chlorite and hydrothermal solution is a function of physicochemical parameters (e.g. temperature, oxygen fugacity, pH, total dissolved sulfur, and activity of Mn 2+ and Fe 2+ ions in aqueous solution). It is inferred that the compositional variation of chlorite in a vein might be influenced by the local or temporal changes of hydrothermal environments, such as oxidation-reduction state and/or temperature.
資源地質学会の論文