ABSTRACT

Carbon and oxygen isotope abundances were determined for hydrothermal rhodochrosites from Oe, Inakuraishi and Jokoku mines. The isotopic ratios of pale pink rhodochrosites from Oe and Inakuraishi are in the narrow ranges from -7.5 to -9.5‰ in δ¹³C_PDB and 2.0 to 7.0‰ in δ¹⁸O_SMOW. δ¹⁸O values of compact brown rhodochrosites from Oe and of large red rhodochrosite crystals from Inakuraishi spread in a range higher than that of pale pink ones. The isotopic ratios of the Jokoku rhodochrosites are in the ranges from -2.5 to -7.5 ‰ in δ¹³C and from 7.0 to 15.0 ‰ in δ¹⁸O. Pale pink rhodochrosites from Oe and Inakuraishi were derived from homogeneous hydrothermal solutions which were probably meteoric in origin. Some amounts of compact brown rhodochrosites from Oe and of large red rhodochrosite crystals from Inakuraishi were precipitated possibly from remaining hydrothermal solutions which might have exchanged oxygen isotopes with host silicates. The higher δ¹³C and δ¹⁸O values of the Jokoku rhodochrosites are probably due to contamination with limestone. Homogeneous distribution of the isotopic ratios suggests that limestone was dissolved somewhere and the solution was brought to Jokoku mine.