Abstract
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.
