Geological features of Ashio mountainland area
Detailed accounts of ore-forming manganese minerals
The geological, mineralogical, and geochemical data available up to the present on mananese ore deoisits investigated are summarized as follows:
1. Japanese bedded manganese ore deposits in older geosynclinal sediments are intimately associated with massive and thin-bedded cherts and basic volcanic rocks. The massive chert shows positive proportional prosperity to manganese ore beds, especially in thickness in many places. Manganese ore deposits in younger sediments are genetically connected with hot spring activities, including some active springs, and are characterized by the dominance of manganese dioxide minerals in the ores.
2. They are found in certain definite horizons and are always conformable with the country rocks. When folded, pinch-and-swell structure is very often developed. The extension of swollen parts is parallel to the trend of plunging of folding axis.
3. The original mineralogical constituents of manganese ores include rhodochrosite, “bementite”, hausmannite with or without manganosite, or manganese dioxide minerals. Some of them have spherulitic texture and low crystallinity.
4. In cases of contact metamorphism and metasomatism of these ores, there is consistent correspondence of mineral parageneses between the products derived from limestone, dolomite, and manganese carbonate rocks. The associations of newly-formed minerals are well interpreted by the metamorphic grades they suffered as indicated by the grades of metamorphic minerals in the country rocks. In some metamorphosed manganese ores, sedimentary textures are well preserved.
5. Similar to the previous case, the products in regionally metamorphosed manganese ores reflect the metamorphic grade of the metamorphosed country rocks.
6. The concentration of some heavy elements including uranium in pelitic rocks rich in carbonaceous material near manganese ore beds is known in many places. The elements concentrated therein include Sr, Ba, B, C, V, Mo, Co, Ni, Cu, Zn, As, Sb, and U. Also, some of them such as Sr, Ba, and Co are precipitated with manganese dioxides.
Many geological and geochemical evidences lead to the conclusion that Japanese bedded manganese ore deposits in older sedimentary rocks were syngenetically formed with the country rocks, including massive and thin-bedded cherts and basic volcanic rocks, which are genetically connected with basic volcanic activity taking place in the Chichibu geosyncline. Besides cherts and basic volcanic rocks the primary precipitates include rhodochrosite, “bementite”, and hausmannite with or without manganosite, and silicaous material represented by colloidal silica. Those in younger sedimentary rocks are the products of hot spring activity or related ones.
Most of those in older sedimentary rocks suffered later metamorphism of various grades sometimes overlapped by metasomatism. As the results, the products include very complex assemblages of manganese minerals to form so-called manganese skarn. In contrast with the case of limestone, the dissociation of carbonate of manganese takes place under lower temperature conditions. The pyrochroite ore from the Noda-Tamagawa mine, Iwate Prefecture is thought tobe the hydration product of manganosite formed by the thermal dissociation of rhodochrosite.
The heavy elements liberated as the result of basic volcanic activity and later concentrated into pelitic rocks near the manganese ore beds form such minerals as pentlandite, gersdorffite, and cobaltite when metamorphosed or veined by later granitic or hydrothermal materials.
The presence of primary di-, tri-, and tetravalent manganese minerals in ores indicates its mineralization under a fairly extensive range of Eh condition, which will be disclosed by future investigation.』