Abstract
Mineralogical and isotopic studies have been made on rocks and minerals from the exploratory drillhole 54-NK-1 (Depth: 700m) in the Nakao geothermal area, Gifu, Japan. The rocks, mainly composed of welded tuff, have been afftected to varying degrees by hydrothermal alteration. Sericite and chlorite are common in the altered rocks. Kaolinite and calcite also occur as hydrothermal alteration products. The δD values for kaolinites indicate that the hydrothermal water involved was derived from the local meteoric water. Assuming that calcites are in equilibrium with the water at present-day temperatures (up to 200°C), the δ18O values for calcites indicate that the water has undergone oxygen isotopic exchange with surrounding rocks, resulting in an 18O-enrichment of about 2 permil at a depth of 550-700m. This is interpreted to mean that a greater reservoir of hot water may exist at depth, and that the minimum water-rock weight ratio is about 2:1in the reservoir.
