In the Shirataki mine the main ore deposit follows the plunge of an asymmetrical overturned anticline and is over 4, 000 m long. Several ore shoots are highly folded, and are nearly concordant with the structure of the enclosing schists. The various kinds of ores in the ore bodies include a massive or banded pyritic ore, impregnated ore, chalcopyrite-rich ore and apophyses. Many massive or banded pyritic ores show intense intraformational folding. The constituent minerals are pyrite, chalcopyrite, sphalerite, bornite, chalcocite, covellite, native silver, stromeyerite, tetrahedrite (?), an undetermined mineral (Rosagrau Kuferglanz?), hematite, magnetite, ilmenite, guartz, a carbonate mineral, green hornblende, albite, chlorite, garnet and epidote.
Chalcopyrite, sphalerite, quartz, a carbonate mineral and chlorite show "pressure shadow" features around some pyrite and magnetite crystals in impregnated ores. In a few apophyses ores stromeyerite is found among chalcocite and native silver, and a microtexture of bornite and chalcocite shows lamellar intergrowth and mutual boundaries that are probably of simultaneous origin in metamorphic differentiation process.
The field and microscopic features of this deposit are reasonably explained by the theory of sulphide syngenesis, later modified by regional metamorphism.

抄録全体を表示

The ore-deposits of the Okuki and the Nebutoyama mine are composed mainly of copper and iron sulfides and are of the bedded type found within an older formation in Japan. The main ore minerals are pyrite and chalcopyrite. They often show various colloform textures.
Iron-titanium oxide minerals are commonly found associated with these sulfide minerals, though they are not abundant. Most of them rarely occur in the same layer with sulfide minerals. Magnetite, hematite and ilmenite are commonly present. These minerals show typical sedimentary occurrence .and are associated with fine magnetite grains and pyrite in colldform texture and are sometimes found with radiolarian fossils. However, exsolution texture between magnetite or hematite and ilmenite is commonly found associated with the textures suggesting their low temperature mineralization as previously described. Very small grains of chalcopyrite are generally closely associated with oxide minerals and, in some cases, in them. This seems to show that all of these metallic constituents were separated and crystallized from the mother solution almost simultaneously.
These textures have not been reported in many descriptions of similar ore-deposits in the highly metamorphosed area of Japan.
The writer concludes that the ore-deposits of the Okuki, the Nebutoyama and similar mines were derived from submarine volcanism in the Chichibu geosyncline. Some iron-titanium oxide and sulfide minerals were formed in the same high temperature conditions as in the pyroclastic rocks and. were then deposited in the bottom of the sea. However, the main sulfide minerals and perhaps some of the iron-titanium oxide minerals, precipitated from colloidal solution.

抄録全体を表示

以上述べた事柄を要約すればつぎのようである.
1)与内畑鉱山の鉱床は第三紀中新世上部に属すると思われる黒色泥岩層を母岩とする黒鉱式石膏鉱床である.
2)産出鉱物としては石膏のほかに少量の黄鉄鉱,黄銅鉱,閃亜鉛鉱および方鉛鉱の金属硫化鉱物,硬石膏,螢石,重品石および方解石などを随伴する.
3)上記産出鉱物の生成順序は第17図のようで,硬石膏,螢石,重晶石および方解石はしばしば石膏にて交代せられている.
4)石膏鉱体は塊状を呈し,その中央部は塊状石膏を主とする高品位部であるが,その周辺部は母岩の粘土分を混えて鉱染ないし網状石膏鉱体となり,さらにその外緑部は繊維石膏の細脈を伴う黒色粘土化帯に移化する.
5)黒色泥岩層中に挾有される凝灰質泥岩は前者に比して石膏の鉱化作用を蒙りがたく,しばしば部分的にcap rock的な役割をなし,その下盤に接して石膏鉱体を胚胎している.
6)母岩の変質は著しい粘土化作用と,都分的な珪化詐用とがあり,揃者は石膏の鉱化作用とほぼ時を同じくして行なわれ,後者はこれに先行するものと考えられる.
7)粘土化作用による化学的成分の変化は第2表のごとくで著しいMgOの増加と脱珪作用とが特徴的で,ごの結果苦土緑泥石およびモンモリオナイトの生成がみられる.
8)黒色泥岩はこれに挾有される凝灰質泥岩に比し人工鉱液に対し石膏を生成しやすく,かつまた,より容易に溶液の拡散を受けやすい.
9)本鉱床産石膏の大部分はCa¨およびSO₄′′を含む鉱液の物理的条件の変化にもとづく化学沈澱によると推察され,鉱体はある一定のhorizon付近,あるいはその当時の地表より一定の深度付近に胚胎したものと考えられる.

抄録全体を表示