Chemical compositions of rhodonite and tephroite from the Noda-Tamagawa mine, Iwate Prefecture

Abstract

Manganese deposits of the Noda-Tamagawa mine occur as a banded and folded stratiform in chert bed of the Akkagawa formation which belongs to Jurassic age, and were thermally metamorphosed by intrusion of Tanohata granite. In result, they generally became rhodonite (outside), tephroite (inside) and hausmannite (center) which formed ore zones with a banding structure as shown in Fig. 2.
Rhodonite and tephroite from each zone were quantitatively analyzed by EPMA to know their chemical compositions, as given in Tables 1, 2 and 3. According to these data, compositional ranges of rhodonite are 1.2-13.8 mole% FeSiO₃, 62.3-88.9 mole% MnSiO₃, 1.3-13.2 mole% MgSiO₃ and 2.3-17.8 mole% CaSiO₃, while those of tephroite are 0.5-11.5 mole% Fe₂SiO₄, 70.7-91.9 mole% Mn₂SiO₄, 6.2-21.2 mole% Mg₂SiO₄ and 0.0-1.8 mole% Ca₂SiO₄. The compositions of rhodonite and tephroite differ with not only localities, but also at a same place as shown in Figs. 4 and 5, respectively. They also change irregularly their compositions in the ore zones of both hanging and foot-wall sides at same place as shown in Fig. 6. Such compositional changes of rhodonite and tephroite are thought to reflect those of primary materials such as rhodochrosite and bementite etc. Although rhodonite and tephroite were formed by thermally metamorphic reaction between rhodochrosite and silica occurred primarily as hydrothermal sedimentary products due to postaction of submarine volcanism (WATANABE et al., 1970a, b), diffusion of atoms such as Fe, Mg, Ca and Si etc. in the ore bed by the metamorphism is considered to be slow. From the data of compositional changes of both minerals, the distance of the atomic diffusion is limited within a few centimeters.