Drilling has been carried out extensively in the Hokuroku area of Akita Prefecture since 1910 with the purpose of locating Kuroko deposits. Traditional drilling methods could not yield satisfactory results in all aspects such as drilling depth, core recovery, and so on. The major reasons for this dissapointing performance were the altered green tuff formation and the equipment and techniques not suited to the lithology of the formation.
Akema Drilling company established in 1929, has carried out drilling extensively in this area and at the same time has expended.every effort to improve the performance of the operation. The major improvement and innovation were in equipment and the techniques of drilling, of which the important items are as follows.
1. Application of large drilling machines and tri-cone bits.
2. Use of diamond bits and the improvement of the matrix, bortz size, and water way.
3. Application of wire-line method.
4. Use of chrome-mud.
5. Manufacture and use of over-sized bits and rods.
6. Improvement and use of derricks.
Because of the above development, now, the company can drill to depth of more than 1000 m with almost 100 per cent core recovery in these altered geological units. This has contributed greatly to the prospecting for Kuroko deposits.

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The Ponbetsu Fault extends east to west direction along the axis of the Minenobu Mountains. On the south side of the Ponbetsu Fault, the Ikushunbetsu Formation (a part of the upper Ishikari Group) directly overlies the Cretaceous Series with unconformity. In the area on the north side of the fault, rock facies of the lower Ishikari Group, from the Noborikawa to the Bibai formations, represents a marginal nature of the sedimentary basin. The Minenobu Mountains gradually uplifted in the late Cretaceous period and grew to the Minenobu Barrier during the deposition of the Ishikari Group (i.e. Paleogene).
Several derivative thrusts composing the Primary Fault Series can be observed on the south side of the Ponbetsu Fault, with E-W trend parallel to the Ponbetsu Fault. By these faults the Ikushunbetsu and Poronai formations are overlapped. Both formations are intensely disturbed, overturned or steeply inclined, forming an imbricated structure.
In the eastern part of the Poronai district, several faults with N-S or NE strike and almost vertical dip and the other faults of E-W trend with horizontal displacement separate the area into several blocks. These faults belong to the Secondary Fault Series. The underground study revealed that the E-W trending Yayoi Fault continues to the N-S trending Nakanosawa Fault.
In the area north of the Ponbetsu Fault, the Bibai Fault extends southeastwards and converges with the Makita Fault. There are a few echelon faults trending in NNW-SSE direction between the Bibai and Ponbetsu faults. On the western foot of the Minenobu Mountains, the Ishikari Group thrust up on the Pliocene Takikawa Formation along the Shimizu Fault.
Judging from the configuration and features of the faults described above, the Sorachi Coal-field was at first compressed westwards by lateral pressure in ENE direction. The Bibai Fault was developed in tensional manner when the lateral pressure was released. The echelon faults on the north side of the Ponbetsu Fault were formed by the pressure resulting in the Secondary Fault Series. The Shimizu Fault was also formed with this compression.
The structural development of the Ishikari Coal-field can be divided into two tectonic phases, Pre-Takikawa stage and Post-Takikawa stage. Structure of the Pre-Takikawa stage is characterized by the folding caused by lateral compression in ENE direction, as seen in the Sorachi Coal-field. Tensional fractures were produced at right angle to. the direction of this compression. The Bibai and other ENE-trending faults were grown as a result of later release from the compression. Because of the Minenobu barrier, a resistant body throughout the Pre-Takikawa stage, the Ponbetsu fault was formed at the southern end of the Sorachi Coal-field. Geologic movement of the Post-Takikawa stage was essentially controlled by lateral pressure from SE. The complicated structure of the Yubari Coal-field was thus formed.

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The authors studied the volcanic rocks in the region of the Hanaoka Mine by means of microscopical examination and X-ray diffraction, and concluded that the earlier one is dacite and the latter one genetically related to formation of the black ore deposits is rhyolite.
Chemical analyses of the dacites near the Matsumine and Fukazawa ore deposits clearly revealed increase of Mg and K, and decrease of Na from fresh core toward altered periphery in the dacite bodies. Consequently the authors suggest that the addition of Mg and K were performed during the mineralization of the black ore deposits. The phenomena on Mg and K can be seen up to the point about 1, 000 m apart from Fukazawa ore deposits. It seems probable to apply the phenomena to the exploration of the black ore deposits.

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[in Japanese]

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Argentian pentlandite has been found in the copper sulphide ores from the Nippo and Shinyama ore deposits of the Kamaishi mine, Iwate Prefecture, Japan. The mineral occurs as minute grains ranging from 20 to 300 μ across, and is closely associated with chalcopyrite, cubanite, cobalt- and nickel-bearing mackinawite and pyrrhotite. Under the reflected light, it is reddish brown in colour and is isotropic between crossed nicols.
The mineralogical properties of the mineral from the Nippo ore deposit may be summarized as follows. The Vickers microhardness numbers: 192-227 kg/mm² for a 50g load, the average chemical composition as determined by electron microprobe method: Ag 19.7, Cu 0.1, Fe 37.0, Ni 12.9, S 30.1, total 99.8 (wt. percent), and, the corresponding structural formula: (Ag_1.53Fe_5.62Ni_1.87)_9.02S_8.00, Z=4(S=8.00). The mineral is characterized by the highest silver content reported so far. The X-ray powder data correspond with those for argentian pentlandite from the Vuonos mine, Finland and are in harmony with a face-centered cubic structure, with a=10.59 Å.
According to the record of the Ohmine mine, the average silver content is 15 ppm in crude ore and 215 ppm in copper concentrate. In so far as the present writers' ore microscopy and electron microprobe study on the Nippo ores are concerned, only a few particles of electrum were recognized as silver-bearing mineral. Probably, the present argentian pentlandite might be the main carrier of silver in the copper sulphide ores.

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