Oxygen isotope study of metamorphosed manganese deposits of the Noda-Tamagawa mine, northeast Japan.

Abstract

Manganese deposits of the Noda-Tamagawa mine are metamorphosed equivalents of bedded type. The ores are typically zoned with a central pyrochroite-hausmannite, outer tephroite, and the outermost rhodonite ores are closest to the wall rock chert. The δ18O(SMOW) values of 11 tephroite, 15 rhodonite and 5 quartz samples in the manganese ores and chert were determined to be in the ranges of 9.9-16.4‰, 11.3-19.6‰, and 22.3-23.8‰, respectively. The overall order of 18O enrichment corresponds to the order of equilibrium fractionation among these minerals. However, the isotope temperatures calculated from quartz-rhodonite and rhodonite-tephroite pairs from neighboring ore zones did not give temperatures consistent with those estimated from the mineral assemblage in the wall rocks. This discrepancy suggests that the amount of oxygen-bearing fluid, either H2O or CO2, was small during the metamorphism. The δ18O values for MnCO3, the most probable candidate of the primary manganese mineral, were calculated from the oxygen isotope mass balance among manganese silicates, silica mineral, and CO2 to be 17.0-20.0‰. If the MnCO3 initially precipitated from fluids of δ18O=0.0-2.5‰, the temperatures of precipitation can be calculated to be between 75° and 127°C. These temperatures suggest that the primary MnCO3 was precipitated by submarine hydrothermal solutions.