Chemical and stable isotopic compositions (δD, δ¹⁸O, and δ³⁴S) of non-volcanic hot spring waters around the Miocene Kofu granitic complex surrounding the Kofu basin in the South Fossa Magna region of central Honshu, Japan, were analyzed in order to investigate water–rock interactions and to determine the origin and sulfur isotopic characteristics of their trace amounts of SO₄ ^2–ion.

All water samples from the granitic rocks were classified as Na–Alkalinity (Alk) type, whereas water samples from the volcanic rocks were classified as Na–Alk, Na–SO₄, Na–SO₄·Cl·Alk, and Ca–SO₄ types. The water in the samples originated from meteoric water, and the average recharge altitude of the samples ranged from 947 m to 1397 m based on the altitude effect of δ¹⁸O. The Na–Alk type waters from the granitic rocks were likely formed by the montmorillonization of plagioclase, cation exchange reaction of Na–montmorillonite, and calcite precipitation. Trace amounts of SO₄ ^2– ion of this type of water were derived from the oxidation of sulfide such as pyrite in granitic rocks or the roof sedimentary rocks of the Shimanto group, where H+ caused by the sulfide oxidation was consumed in the process of plagioclase weathering. SO₄ ^2–ion content in the Na–Alk type water from the granitic rocks reflected the δ³⁴S values of granitic and sedimentary rocks of the Shimanto group. Water samples from the ilmenite series area have negative values ranging from –15.1 to –4.6‰, whereas waters from the magnetite series area have positive δ³⁴S values ranging from +1.7 to +8.0‰.

The hot spring water quality of the Na–Alk, Na–SO₄, Na–SO₄·Cl·Alk, and Ca–SO₄ types from the volcanic rocks area were estimated to be controlled by anhydrite dissolution, plagioclase weathering, cation exchange reaction of Na–montmorillonite, and precipitation of calcite during the fluid flow and mixing process. Different concentrations of SO₄ ^2–ions determined for these waters have a wide range of δ³⁴S values ranging from –4.1 to +13.6‰, which is likely attributed to the dissolution of ³⁴S-rich and ³⁴S poor anhydrite. The ³⁴S-rich SO₄ ^2– ions were interpreted to be derived from sulfate in sulfuric acid, which arose from the disproportionation reaction of volcanic sulfur dioxide, whereas the ³⁴S-poor SO₄ ^2– ions were derived from the oxidation of ascending hydrogen sulfide in shallow ground waters during the active stage of past volcanism.