ABSTRACT

Chemical and isotopic data are summarized for fluids from newly drilled wells in the Sumikawa geothermal system, northern Honshu, Japan, and the geochemical characteristics of the reservoir are discussed. The fluids are mainly of the Na-Cl type with a near neutral pH, and measured temperatures in wells often exceed 300°C. The total dissolved salt concentration is low (<3000 mg/kg), less than other high temperature geothermal systems in Japan. The chemical and isotopic compositions of the fluids vary from well to well. One well (S-2) produces both acid sulfate-chloride and weakly acid pH fluids from two different feed zones, with these fluids possibly related to a local magmatic component. The Sumikawa geothermal waters are diluted by a relatively shallow heated (∼200°C) water that contains sulfate plus bicarbonate but nil chloride. The hydrogen and oxygen isotopic compositions of the total fluid indicate that the fluids are of meteoric origin, with an oxygen isotope shift of less than 2‰. The SO₄ in the meteoric-dominated neutral pH waters comes from sulfur in rocks, which is essentially a mixture of supergene and fossil marine SO₄. On the basis of the N₂, Ar and He values in the vapor fraction, the chemical and isotopic variation of the fluids appears to be due to different degrees of minor magmatic contribution of volatiles to the hydrothermal fluid. Despite the meteoric-dominant source of most water, the magmatic contribution of volatiles results in fluids that are relatively immature and out of chemical equilibrium with the host rocks and alteration minerals.