2000 Volume 11 Issue 2 Pages 147-168
Lake Ohyunuma, which is an active explosion crater lake located at Noboribetsu Spa of southwest Hokkaido, Japan, has been investigated to elucidate the dynamics of clay formation processes under the acid hydrothermal conditions. The Ohyunuma lake water is characterized by distinct stratification of temperature and chemicals forming two layers. The upper layer water is relatively low-temperature (about 40°C), low-salinity, oxidized, sulfate-rich, chloride-poor acid (pH 2.4) solution, whereas the lower layer water is relatively high-temperature (max. 121°C), highsalinity, sulfide and chloride-rich acid (pH 2.7) solution. The distribution of minerals constituting the lake sediments is controlled by many factors such as location of discharge points, transportation by river waters, lake bottom topography, and flow rate and direction of lake water, in addition to in situ dissolution and precipitation processes of the minerals. Based on the lines of evidence such as calculations of saturation indexes and examinations by XRD, TEM, and AFM, the origin of the minerals was inferred as follows: elemental sulfur and pyrite are precipitated through the entire water column of Lake Ohyunuma involving both the upper and lower layer waters; the portions of them are also derived from the molten sulfur pool filled in the high temperature vent sites, associating with turbulent discharges. Alunite is precipitated in the lower layer waters and also formed locally in the upper layer waters by mixing of more acid stream and discharge waters which contain much more Al and sulfate ions. Smectite is of allogenic origin as it is mostly derived from the surrounding altered rocks, and the exchangeable cation composition deduced from the bulk sediment data is a function of temperature and solution composition. Kaolinite and halloysite may be initially translocated from the surrounding altered rocks and then they are undergoing dissolution during settling of the particles in the upper layer waters, but the dissolved materials are recrystallized as kaolinite in the lower layer waters. Most of silica minerals such as quartz, a-cristobalite, and tridymite are of allogenic origin. Diagenetic modification of the minerals with burial is not significant through the sediment column (0-67 cm depth) recovered in the present study.
