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

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 δ 18 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 to 20.0, 11.7 to 20.3, and 23.0 to 23.9 per mil, respectively. The overall order of 18 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 are not consistent with those estimated from the mineral assemblages in the wall rocks. The δ 18 O values for MnCO 3 , the most probable candidate of the precursor manganese mineral, were calculated from the oxygen isotope mass balance among manganese silicates, silica, and CO 2 to be 19.2 to 23.4 per mil. If the MnCO 3 initially precipitated from fluids of δ 18 O = 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 MnCO 3 having been precipitated by low-temperature submarine hydrothermal solutions.