On the Geology and the Pliocene Iron Sand Deposits of the Temmabayashi Mine, Aomori Prefecture, Japan

Abstract

The Temmabayashi mine is the only mine which is now worked underground, and producing monthlyabout 20, 000 tons of crude ores.
The Sokota Formation of the uppermost Pliocene age, which encloses the iron sand placers, lies on the Pliocene formations with a slight paraunconformity, and is covered unconformably by the Pleistocene sedimentary group.
As a result of sedimentological studies of the area, especially of underground observations and analyses of arill cores, the following features have been recognized.
1. The Sokota Formation consists of loose sandstone, conglomerate and fine-grained tuff of littoralfacies, and interfingers with the Shimizume Formation of shallow marine facies.
2. Besides unimportant placers in the underlying formation, the producing iron sand placers occurin three groups of ore beds, i. e., Tsurukodaira, Sokota and Jimba groups.
3. The Sokota group contains 13 ore beds which are arranged from SW to NE, successively imbrisating each others.
4. The largest ore bed develops as long as 900m. in strike side, and has a width of 400 m. in dipside.
5. Component minerals of the ores are magnetite, ilmenite, hematite, maghemite, limonite, pyrrhotite, etc., and gangiue minerals are quartz, feldspars, hypersthene, hornblende, apatite, etc., each being separate or locked complicatedly.
6. The total iron content of crude ores ranges from 22 to 55 percent, and that of magnetic concentraces is 51 to 60 percent.
7. According to facies changes of the overlying and underlying ormations, cyclic sedimentation of ironsand placers, and relations of ore beds to the enclosing formation, it may be concluded that the oredeposits were formed in a shallow lagoonal environment with clos erelations to wave motion, undercurrentflow and changing rate of subsidence.