There are remarkable differences of stratigraphic sequence, geologic structure and lithofacies between northern and southern parts of the Pombetsu thrust fault. They maybe considered to have resulted from the crustal movement which brought about the geologic structure of the Sorachi Coal Field and the existence of the Minenobu barrier. The structure which was caused by a compressive force from the NOTE-SSW direction took the form of NE-SW thrusts accompanied simultaneously by horizontal displacement to the SWW direction in the southern part of the Sorachi Coal Field, i. e., Minenobu barrier.
The primary structure of the Sorachi Coal Field, which was folded by a compressive force from NNE, rotated clock-wise. (latter force.) The western part of the Sorachi Coal Field (the plain side) slided to NE, and simultaneously the: eastern part (the mountain side) moved to SW. And then, the southern part moved to SW as it was. thrust upon the Minenobu barrier. Furthermore, the fact that the Yayoi fault was thrust up due to a compressive force from SE has. been verified by the geological data obtained through the surface and underground survey of the Pombetsu. Coal Mine. Consequently, the age of the crustal movement which formed the geologic structures is clearly different between the Sorachi Coal Eield and the Yubari Coal Field. The author emphasizes that the structure of the Yubari Coal Field was formed after the completion of the structure of the Sorachi Coal Field.

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There are many field evidences suggesting that the limonite deposits of the Akita iron mine were formed by sedimentation of hydrous ferric oxides separated from iron-containing spring waters. The ferruginous precipitates adsorbed various substances from the spring waters, being amorphous when formed. We have, however, no information on the environmental conditions of the precipitation and the mode of crystallization of the precipitates. It is of particular importance from a geochemical point of view to define the temperature and the pH of the solution from which the precipitates formed, and to elucidate the processes of crystallization after the sedimentation of the precipitates.
The mineralogical and chemical features of the limonite ores are much suggestive of the genesis of the deposits. The ores, composed mainly of goethite and jarosite, have low crystallinity, which may provide an evidence that they were formed at ordinary temperatures. The chemical compositions of the ores are shown in Table 1. Considering the results of investigations on aging of hydrous ferric oxides and on coprecipitation of anions with insoluble hydrous oxides, the present author concludes that the precipitation and crystallization of hydrous ferric oxides took place at ordinary temperatures and at low pH, presumably at pH 2 to 3.
In consequence, we may summarize the formation of the limonite deposits as follows : acidic spring waters, containing ferrous iron, phosphates and sulfates, precipitated hydrous ferric oxides during their flow on the ground surface. A certain microbiological process accelerated the oxidation of ferrous iron. The accumulation and the subsequent crystallization of the precipitates containing phosphates and sulfates have formed finally the limonite deposits composed of goethite, jarosite and some iron phosphate minerals.

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Iron oxide, sulphide, and carbonate minerals are very widespread in ore deposits. Water and carbon dioxide are probably main constituents of ore-forming fluids. Magmatic ore deposits are often enclosed in graphite-bearing or carbonaceous limestone and/or pelitic rocks.
Stability relations among the above iron minerals and oxygen fugacity of the water-carbon dioxide system were determined by calculation in a temperature-oxygen fugacity diagram.
Because of common presence of pyrite and magnetite in magmatic ore deposits, the ore-forming fluid appears to have a lower oxygen fugacity than that of pure water or pure carbon dioxide or a mixture of them. Such a fugacity might have been produced by assimilation of so little amount of graphite as scarcely detected by ordinary chemical methods. Oxygen fugacity of such a fluid is approximately a divariant function of temperature and amount of graphite assimilated by the fluid independently of total pressure of the fluid.
Some common assemblages of iron minerals in magmatic ore deposits such as hematite-siderite (B-E in Fig. 4), pseudomorph of magnetite after hematite-pyrite-siderite without pyrrhotite (B-C-F in Fig. 4), zonal distribution of magnetite and pyrrhotite (C-D in Fig. 4), magnetite-pyrite and/or pyrrhotite without siderite (D-G in Fig. 4), are easily explained by the variance in the amount of graphite assimilates by ore-forming fluids. The zonal distribution of magnetite and pyrrhotite which is common in contact metasomatic deposits is hardly explained by the lowering of temperature alone.

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