Origin and formational environment of Noda-Tamagawa manganese ore, northeast Japan: Constraints from isotopic studies

The Noda-Tamagawa mine, one of the biggest bedded manganese deposits in Japan, is located in the north of the Kitakami mountain range. The manganese ore (1-5 m in thickness) is conformably embedded in the cherty and shaly wall rocks, and is mineralogically zoned from the rhodonite ore of the wall rock side to the central ore constituted by pyrochroite-hausmannite passing by the intermediate tephroite ore. This zonal distribution of the manganese ore is the result of thermal metamorphism by the Cretaceous Tanohata granodiorite body. The presence of manganosite and pyrochroite in the metamorphosed ore suggests that the primary manganese mineral, before the metamorphism, was rhodochrosite. The sulfur and carbon content in the wall rocks and manganese ore were quantified, and the results display a pattern similar to that of Black Sea sediments. A series of sulfur isotopic data obtained from a similar to200m stratigraphic traverse including the manganese ore horizon shows that the sulfur isotopic composition (delta(34)S) varies between -42 and +10parts per thousand. The majority of values is around -25parts per thousand suggesting that the sulfides in the rocks of the Noda-Tamagawa mine were formed during diagenesis by the intervention of sulfur reducing bacteria. The rarely found manganese carbonate in hausmannite ore has light delta(13)C values ranging between -21parts per thousand and -3parts per thousand, confirming the sedimentary origin of this primary manganese ore. Thus, rhodochrosite (the most probable precursor of the manganese minerals of the Noda-Tamagawa deposits) was metamorphosed by the intrusion of the Tanohata granodiorite body to form the several manganese silicates and oxide minerals currently found in the Noda-Tamagawa site.