Abstract
Epithermal vein-type manganese deposits of the Jokoku mine occur in Pre-Tertiary sedimentary and Tertiary acidic pyroclastic rocks of southwestern Hokkaido, Japan. Dolomite, which commonly occurs in the deposits, can be divided into two types; (1) fine-grained anhedral metasomatic dolomite in the Pre-Neogene limestone and (2) fine-grained dolomite of veins resulting from Neogene hydrothermal activity. Magnesium-bearing kutnahorite occurs in both the metasomatic dolomite and the dolomite veins. Kutnahorite in the metasomatic dolomite is occurred as a reaction rim between dolomite and rhodochrosite. Kutnahorite in the dolomite veins is euhedral and coexists with ferromanganoan dolomite. Formation temperature of the minerals associated with kutnahorite in metasomatic dolomite is estimated to be about 200°C based on the filling temperature of fluid inclusions etc. Based upon the chemical compositions and mode of occurrences of kutnahorite in metasomatic dolomite at the Jokoku mine, it is suggested that the hydrothermal solutions having low (mMg2+/mMn2+)aq would replace dolomite.
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