Alabandite in the bedded manganese ore deposits in the Seta district, Gumma Prefecture.
1977 Volume 27 Issue 146 Pages 401-414
Details
1977 Volume 27 Issue 146 Pages 401-414
The modes of occurrence, associated mineral assemblages, and chemical compositions of alabandites from the bedded manganese ore deposits in Paleozoic formations of the Seta district, Gumma Prefecture, are described on the basis of microscopic observation and electron probe microanalysis. 22 samples were studied from five mines-Hagidaira, Rito, Hanawa, Showa, and Nakanoyama. From the results obtained, the conditions of formation of alabandite can be considered as follows.
1) Both wall rocks and ore minerals of the Hagidaira mine have been thermally metamorphosed by the intrusion of the Sori granodiorite, the former to biotite hornfels, while the latter are now composed mainly of rhodonite and tephroite, associated with pyrite, pyrrhotite, sphalerite, and chalcopyrite. The amount of alabandite in Hagidaira is significantly greater than in the other mines and its grain size is also larger. Furthermore, the FeS content of the alabandite varies considerably from 0.3 to a maximum of 10.0 mol.%.
2) The other four deposits, Rito, Hanawa, Showa, and Nakanoyama, occur in unmetamorphosed Paleozoic sediments. These ore deposits are composed mainly of fine-grained rhodochrosite, and small amounts of alleghanyite and pyroxmangite. Alabandite from these deposits is less in amount and its grain size is very small. The content of FeS in the alabandite varies only slightly over the range 0.2 to 2.0 mol.%.
3) The difference in the mode of occurrence and chemical composition of alabandite between the Hagidaira mine and the other four mines can be attributed to thermal metamorphism caused by the granodiorite. However, the variation of FeS content in alabandite in the orebody of the Hagidaira mine does not depend on temperature, but is due to changes in fs 2 produced by fo 2 and fCO2 variations between coexisting manganese oxide, silicate, and carbonate minerals.
4) On the other hand, it is considered that the formation of alabandite in the Rito, Hanawa, Showa, and Nakanoyama deposits is independent of thermal metamorphism.
