The ore-deposits of the Shirataki mine are of the "Kieslager" type. The ores consist mainly of pyrite and chalcopyrite with small amounts of magnetite and hematite. Magnetite and hematite occur in layers of quartz schist inter-bedded conformably with bedded sulfide ore-deposits. The paragenesis of this quartz schist are as follows:
magnetite—pyralspite—calcite—quartz and hematite—pyralspite—hornblende
Hematite containing a seriate arrangement of very fine ilmenite is commonly found only along pyral-spite-quartz laminae of quartz schist. The texture between hematite and ilumenite is concluded to be an exsolution intergrowth, which was once contained in unmetamorphosed country rock, on the following grounds:
1) The unmixing temperature of hematite-ilmenite solid solution is estimated at above 900°C. The country rock of the Shirataki ore-deposits belongs to the Sanbagawa glaucophanic metamorphic zone, which is characterized by high pressure and low temperature. It seems that the country rock has. never reached a temperature higher than 300°C during metamorphism.
2) The exolution texture is not found in contact with hornblende. Furthermore, the seriate arrangement of ilmenite in hematite runs rarely into sphene accompanied by hornblende. These matters seems to show that iron was spent to form hornblende only in exsolution hematite bodies.
3) Such a mineral assemblage as hematite-ilmenite-quartz may be stable in the Sanbagawa metamorphic zone.
The quartz schist is conformable to the bedding of the country rock and layered sulfide ore-deposits, and seems to have been derived from such an original rock as chert. The writer reported ferruginous. chert with hematite-ilmenite exsolution grains in the Nebutoyama mine, which belongs to the "Kieslager" type in a weakly metamorphosed area. These exsolution grains between hematite-ilmenite probably were supplied from the effusive matter of submarine volcanism within the Chichibu geosyncline.
This conclusion agrees well with the recent theory that the "Kieslager" resulted syngenetically from submarine volcanism.

View full abstract

The present paper deals with the heavy minerals observed in the sandstones of the Takashima Coal Field (Paleogene), Kyushu, giving weight percentage, kinds of heavy minerals, and character of distribution frequencies. Statistical methods are used to check the differences in heavy mineral frequen ties in each formation.
1) The weight percentage of the heavy minerals is the least (0.1%±) in the sandstones of the Hashima formation, and is the most (1-5%±) in those of the Koyaki and Iojima formations.
2) The kinds of the heavy minerals are : zircon (colorless, purple, brown), garnet (colorless, brown, pink), tourmaline (brown, green, blue, gray, etc.), epidote, zoisite, titanite, rutile, muscovite, biotite (brown, reddish brown), a green mica-like mineral, iron opaques (magnetite, ilmenite, hematite, limonite, pyrite, etc.), green hornblende (rare), pyroxene (rare), monazite (rare), glauconite, anatase, a chlorite-like mineral, and carbonates.
3) The character of the heavy mineral frequencies in each formation is : Iojima formation (upper member) : epidote and titanite, rare; colorless zircon, purple zircon, garnet, tourmaline and rutile, abundant.
Iojima formation (lower member) : epidote and titanite, abundant ; colorless zircon and brown tourmaline, rare.
Okinoshima formation : epidote and titanite, rare ; colorless garnet, biotite and anatase, abundant.
Hashima formation : colorless zircon, very abundant ; purple zircon, rutile and iron opaques, abundant ; epidote and titanite, rare.
Futagojima formation (upper member) : epidote and titanite, rare ; colorless zircon, brown tourmaline and biotite, abundant.
Futagojima formation (lower member) : epidote and titanite, common; colorless garnet, abundant.
Koyaki formation : epidote and titanite, very abundant ; biotite, abundant ; glauconite, rare.

View full abstract

The problems of "Dolomitization and Ore Deposition" in the deposits of foreign countries, especially of the United States, have been discussed by many investigators. In our country, however, little attention has been paid to the hydrothermal dolomite-rock associated with metalliferous deposits.
In his research on the geology and ore deposits of the Akatani-Iide iron and lead-zinc field, Niigata Prefecture, the writer has been confronted with these problems. He was also able to study these problems during visits in 1952 to the Sennin iron mine, Iwate Prefecture, and to the Igashima fluorspar mine, Niigata Prefecture. The purposes of the papers are to present these topics and to point out their geologic significance. This paper describes the paragenetic relation of ores and altered rocks in each mine, and reaches the following conclusions:
1) These iron, lead-zinc, and fluorspar deposits under discussion are not contact-metasomatic but are hydrothermal-metasomatic deposits. The hydrothermal mineralizations that formed workable ores are thought to be intimately related to the acidic volcanism (liparite eruption and intrusion) of the middle Miocene; on the other hand, the contact-metasomatism is thought to be related to the acidic plutonism (granite intrusion) of the early Tertiary. Based on these views, the rocks in these deposits have undergone two distinctly separated periods of mineralization.
2) The iron deposits with specular hematite in our country have been long believed to be of the contact-metasomatic type. However, from the present-day views as mentioned above, such a genetic consideration must be wholly revised. The main hematite-mineralization in our country probably is included in the Neogene metallization that formed numerous epithermal deposits in the "Green Tuff Region".
3) In these deposits, the close association of ores with dolomitized limestone and/or "altered skarn" is very marked. For instance, in the Akatani mine some of the specular hematite orebodies are enveloped in dolomitized limestone. In a few places, however, the limestones in contact with the ores are definitely unaltered. The dolomitization of limestone involves a change in color, texture and porosity. The front of dolomitization in limestone is usually sharply defined, this tendency is especially conspicuous in dark-colored limestone.
4) In specular hematite deposits of the Akatani and the Sennin mine, the alteration of skarn is very intense over an extensive area, and no orebodies are found where the skarn is unaltered. In the Sennin mine, the selective metasomatism of "altered skarn" is always found.

View full abstract

The Motoyama deposits of the Kosaka Mine, Akita Prefecture, are one of the typical Kuroko deposits in Japan. The deposits which originally had a bedded form, are imbricated by the strike-slip thrusts which took place in close connection with the post-ore intrusion of the Akamori dacite.
The zonal arrangement of the ores in the deposits is considered to have resulted from the lateral transition of the ore during the deposition. Discussions of the period of the mineralization suggest a syngenetic origin for the deposits.

View full abstract

The Paleogene formation, scattered in the vicinity of Hokonoko, Kumamoto Prefecture, has been described in a few reports, but until now, its stratigraphy and geologic age had not been determined We have recognized, in this area the following two new formations in the Paleogene: the Kannondake formation and the Hokonoko formation.
The Kannondake formation contains coal seams and represents the upper half of the Paleogene in question. Some fossil marine mollusks obtained from this formation indicate the stratigraphic position of the formation. They are : Faunus nipponicus NAGAO, Melania (?) miikensis NAGAO, etc. Considering these fossils and rock facies, the Kannondake formation is equivalent to the coal-bearing Komenoyama. formation, which belongs to the lowermost part of the Ariake group.
The Hokonoko formation is the lower half of the Paleogene in question. The formation closely resembles the Akasaki formation, Amakusa stage, in all of the important points: non-marine, non-fossiliferous and purplish, and the formation stratigraphically underlies the coal-bearing formation.
The geologic structure of the formations in this area is very simple. The Paleogene is divided into segments and scattered in tiers along the valley walls by some 'graben'-faults trending NE.
These formations are distributed in the region between the Amakusa and the Asakura coal-field and are equivalent to the Komenoyama formation of the Miike coal-field. The distribution of the lowermost part of the Paleogene coal-bearing formation is presented as one of the proofs of the existence of a new coal-field under the Plain of Kumamoto.

View full abstract