Introduction
Geological features of Ashio mountainland area
Genetical classification
Detailed accounts of ore-forming manganese minerals
『Genesis
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.』
References