Mineralization of Fe/Mn-precipitates From Hot Springs in the Sanbe Volcanic Area, Japan

Mt. Sanbe is an active volcano located in western Japan. At southwest slope of Mt.Sanbe, hot springs are discharged with flow rate of 2000 liter per minute, which are accomapnied by iron and manganese precipitates. We studied chemical composition of hot spring waters by IC, ICP-AES and AA analysis, and of precipitates by XRF analysis. Water chemistry of the main hot spring (t= 30-39 °C) is Na-Cl type and contains significant iron (= 5.6-7.5 mg/l) and manganese (= 1.1-1.6 mg/l). Around the main spring discharge, both ferrihydrites (contains 67-76% of Fe) and manganese oxides (contains 71-79% of Mn) are precipitated. About 30 to 100m east from the main hot spring, black precipitates mainly consists of manganese oxides (contains 5-83% of Mn) are distributed in the area of 90 m × 45 m. Cold spring waters (t= 12-20 °C) in this area also contains Mn (0.1-0.6 mg/l), but their Fe concentration was under detection limit. Relative chemical comoposition of the cold spring waters are similar to that of the main hot spring, which implies dilution with groundwater. Total bacterial count measured by DAPI staining was 16,000-37,000 cells/ml in these waters. In order to estimate microbial activity, we conducted manganese oxidizing experiments. The oxidation rate was estiamted as 4-5 nM per hour for the experiment in which 5 g of fresh manganese wad was added to 200ml of the 0.20 μm filtered hot spring water. This rapid oxidation rate would suggest that microbial activity plays important role in manganese precipitation from the Sanbe hot/cold spring waters.