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OXYGEN ISOTOPE STUDY OF METAMORPHOSED MANGANESE DEPOSITS OF THE NODA-TAMAGAWA MINE, NORTHEAST JAPAN

Ken-ichiro Hayashi and Mohammed El Rhazi

Department of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan

Corresponding author: email,khayashi{at}mail.cc.tohoku.ac.jp

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 ¹⁸O (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 ¹⁸O 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 ¹⁸O = 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.