The bedded manganese and manganiferous iron deposits in this area are distinctly strata-bound. The manganese deposits are concentrated in a lower Permian formation of the North Kitakami belt and in a middle Jurassic formation of the Iwaizumi belt, while the manganiferous iron deposits are in a Paleozoic formation of the Hayachine belt. The manganese deposits occur abundantly in the alternation zone of slate and chert, but rarely in chert dominant zone, implying that the depositional environment of these deposits may have been a moderately deep sea-floor of eugeosynclinal basin. The deposits are conformable to the wall rocks rich in banded chert, and large-scale deposits are often underlain by massive chert which is thought to have been a product of hydrothermal activity. The manganiferous iron deposits, whose occurrence is limited to schalstein zone, are also conformable to the wall rocks which comprise mostly banded and massive cherts. The source of manganese and iron in the ores and the source of silica of the banded chert are thought to be in submarine basic volcanism as well as in leaching from land and submarine volcanic rocks. Bedded cherts associated with the manganese and manganiferous iron deposits may have been deposited from soluble silica in sea water and hydrothermal solution either by chemical or biochemical reactions.
Both the manganese and the manganiferous iron deposits in this area suffered thermal metamorphism related to an early Cretaceous granitic activity. Original mineral phases in unmetamorphosed manganese ores are considered to have been manganese dioxide minerals, rhodochrosite with or without hausmannite, jacobsite, bementite, penwithite etc. The metamorphosed equivalents show a systematic mineralogical change corrcsponding to the metamorphic grades of the country rocks. Four grades of metamorphism, the chlorite-, biotite-, cordieriteand potash feldspar zones, all of which are identifiable in the metamorphic mineral assemblages of interbedded pelitic and psamitic sediments, are reflected in the manganese ores by the existence of rhodochrosite ore, braunite ore, manganese silicate-rich ore and manganiferous amphibole and pyroxene-bearing silicate ore, respectively. Original mineral species of the manganiferous iron ores seem to have included ferric oxide hydrate, iron carbonate, manganese oxides, manganese carbonate, amorphous silica, some hydrosilicates etc., and they have been converted to hematite, magnetite, rhodochrosite, rhodonite etc. in the metamorphosed ores.
View full abstract