Manganese ores of the Noda-Tamagawa mine are metamorphosed equivalents of bedded-type sedimentary manganese deposits. The ore lenses are typically zoned with a central pyrochroite-hausmannite zone, outer tephroite zone, and outermost rhodonite zone closest to the wall rock chert. The δ
O (VSMOW) values of 65 tephroite, 32 rhodonite and eight quartz samples from the manganese ores and chert are in the ranges of 9.9–20.0‰, 11.7–20.3‰ and 23.0–23.9‰, respectively. The overall order of
O-enrichment corresponds to the order of equilibrium fractionation among these minerals. However, the temperatures calculated from quartz-rhodonite and rhodonite-tephroite pairs from neighboring ore zones do not give temperatures consistent with those estimated from the mineral assemblages in the wall rocks. These data suggest that equilibrium was not established among neighboring mineral pairs, H
O, and CO
during metamorphism. The fluid/rock ratios during metamorphism were very low (≪1).