ABSTRACT

The δ¹⁸O and δD values of water of crystallization of gypsum from the Kuroko type mineralization in Japan mostly fall in narrow ranges from -6 to -8 and from -75 to -90‰, respectively. While the mineralization has generally been accepted as of submarine hydrothermal origin, the present results indicate that the gypsum ores are in isotopic equilibrium with meteoric waters instead of sea water. The most plausible interpretation for this observation is that the primary submarine anhydrite or gypsum was hydrated or re-equilibrated with meteoric waters after the uplifting of the deposits. However, there is some evidence that meteoric hydrothermal solution might have been involved in certain stages of Kuroko mineralization at Shakanai. The δ¹⁸O and δD values of gypsum of the Tsutsumizawa deposit in Hanaoka, Northeast Honshu, fall on a line with a slope (δD/δ¹⁸O) of -5.3. This suggests that the deposit was formed by fractional hydration of an anhydrite deposit under a limited supply of water, of which isotopic ratios were similar to present-day meteoric waters. Gypsum ores of Noto and Osaka-Matsushiro, Southwest Honshu, are isotopically quite different from most of other Kuroko deposits. These gypsum ores seem to have been hydrated by hydrothermal waters which isotopically are significantly shifted from meteoric waters.