Abstract

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, an outer tephroite zone, and an outermost rhodonite zone closest to the wall-rock chert. The δ18O (VSMOW) values of 65 tephroite, 32 rhodonite, and eight quartz samples from the manganese ores and chert are in the ranges of 9.9 to 20.0, 11.7 to 20.3, and 23.0 to 23.9 per mil, respectively. The overall order of 18O 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 are not consistent with those estimated from the mineral assemblages in the wall rocks.

The δ18O values for MnCO3, the most probable candidate of the precursor manganese mineral, were calculated from the oxygen isotope mass balance among manganese silicates, silica, and CO2 to be 19.2 to 23.4 per mil. If the MnCO3 initially precipitated from fluids of δ18O = 0.0 to 2.5 per mil, the temperatures of precipitation can be calculated to be between 62° and 111°C, consistent with the primary MnCO3 having been precipitated by low-temperature submarine hydrothermal solutions.

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