On the Ore of the Yubaridake Manganese Mine, Hokkaido

Abstract

The ore deposits of the Yubaridake manganese mine were discovered in a blank part of the distribution area of this kind of ore deposits. Later, it was found that ore deposits of this kind are distributed sporadically along the median zone of Hokkaido.
The ore deposits of the Yubaridake mine are irregular flat masses or cylindrical in shape, occurring in diabase, schalstein (diabasic clastic rock) and chert of the Sorachi group which. is correlated with the Jurassic to the early Cretaceous. They consist of four ore bodies, two of which have been already mined.
The manganese minerals of the ore are braunite, rhodochrosite, alabandite, penwithite, rhodonite and manganese dioxide. A mineral closely resembling manganosite in external appearance was identified as alabandite as a result of various studies. This mineral was hitherto thought to be rare in the low temperature hydrothermal metasomatic ore deposits.
Both the country rock and the ore contain such microfossils as radiolaria belonging to Superfamily LIOSPHAERICAE, Superfamily ARCHIPILIICAE and Family STYLOSPHAERIDAE. The ore also shows the colloform texture, which seems to have been replaced by braunite and rhodochrosite, and the fossil-replacing texture in which the metasomatism might have advanced successively with the order of quartz, rhodochrosite and braunite. In some cases the fossil-replacing texture is weakly altered, so that the internal structure of fossil is distinct, and only quartz is the replacing mineral. In other cases the alteration is much more advanced so that the internal structure is indistinct, and the replacing minerals are rhodochrosite and braunite, with scarce quartz. In such cases, rhodochrosite remains in microfossils of a relatively large size, but in small fossils rhodochrosite is replaced by braunite. In fossils of an intermediate size, rhodochrosite is partly replaced by braunite. Thus, different degrees of metasomatic alteration are recognized.
Since the manganese ore deposits, inclusive of those of the Yubaridake mine, having the above characteristics, are distributed along the median zone of Hokkaido and they are similar in the mode of occurrence, these ore deposits may be sedimentary ore beds or they were formed under low temperature conditions in a shallow depth.
In the past it was considered that all of the ore deposits of this kind in Hokkaido were formed by metasomatism along the shear zones. However, the origin of some of the ore deposits should be reexamined from the syngenetic viewpoint.