岩 石鉱 物鉱 床学 会誌 60巻4号, 1968年 STUDIES ON THE GENESIS OF METALLIC MINERAL DEPOSITS OF SHIN JO AND YAMAGATA BASINS, NORTHEASTERN JAPAN (I) NORITSUGU OIZUMI Mining Division Prefectuval Governient of Yamagata The present structural set up of the Shinjo and Yamagata basins which are situated within the Inner Region, Northeastern Japan, is a result of repeated uplift and submergence coupled with igneous activity and sedimentation during the early stage of the Neogene. Those structur es formed as a result of uplift of the basement are the major faults and fissures parallel to the N-S trend of the present axis of the Basement Rise and the subordinate faults and fissures trending E-W and perpen dicular to that axis Those formed as a result of folding of the sedimentary rocks are the NNW-SSE trending structures of northern Shinjo Basin, the N-S trending structures of southern Shinjo Basin, the N-S trending structures of northern Yamagata Basin, and the NNE-SSW trending structures of southern Yamagata Basin. These structures include the fold axes and the faults and fissures parallel to them. In addition to these structures produced by folding of the sedimentary rocks, are the subordinate faults and fissures trending WNW and NE-SW to ENE. The crushed zones within these fractures were produced by an E-W lateral compression, which also produced exceedingly abundant E-W trending tension fractures, The NNE-SSE trending fissures adhere closely to these fractures caused by lateral compression. The individual ore deposits present within the two basins amount to a total of more than 300 ore deposits. These ore deposits include those of gold, silver , copper, lead, zinc, iron sulphide, and others. They are either of the massive type , or stock work type, or vein type of ore deposits. The general direction of distri bution of these ore deposits is N56•KW+1 degree. They are all hydro thermal in origin, and thevein type deposits amount to about 90% of their total number. Among these vein type deposits the most abundant strike direction is N 64•KE+2•K. The vein type deposits occur as ore fillings of fractures , which controlled the movement of ore solutions and localized the deposition of the ore minerals. Roughly speaking, the ore veins situated in the eastern part of both basins, western side of Ou Mountain Range , were formed mainly along those fissures produced by the uplift of the basement . Those from northern to northeastern Shinjo Basin were formed mainly in Manuscript received, July 4, 1968 Studies on the Genesis of Metalic mineral deposits 147 conjunction with the "Tertiary granite" intrusions. And those within southern Yarnagata Basin, were formed mainly along faults, bringing about their remarkable pinching and swelling character in this area. In the vincity of the border between the two basins, i.e., in the southern part of Shinjo Basin and in the northern part of Yamagata Basin, the ore veins are mostly within faults fissures sympathetic to the folds. The most abundant direction of ore veins and the rarest direction are perpend icular to each other in all the 3 "ore regions" defined for both basins. Judging from the ages igneous activity and geological structure, miner alization of these ore deposits, including the vein type deposits, the stock work deposits, and the massive deposits, occurred late during the Funakawa stage of sedimentation. This stage corresponds to the Furukuchi formation are situated within the Inner Region Northeartern Japan, of Shinjo Basin and the Ogureyama formation of the Yamagata Basin. INTRODUCTION The Tohoku Region, being considered the treasury of ore deposits of Japan, is one of the most favored areas for researches in economic geology. Because of these researches, the nature and shape of the ore deposits of this region have been clearly identified, and great advances in their development occurred in recent years. The expected commencement of exploitation of the so-called black ore deposits of Hokuroku District of Akita Prefecture will certainly make much use of studies on the genesis of ore deposits. The author spent more than 15 years in the geological survey of every kind of ore deposits in the Yamagata Prefecture, studying their nature and shape, and guiding the way for their development. Studies were made by many other persons, especially Funayama (1961) and Taguchi (1962), who made possible the great advances in the knowledge of the geological structures cf the Miocene sequence in this region. Studies made by T. Takeuchi (1961) concerning the igneous activities and the relationship between ore mineralization and the hollocrystalline rocks in the region yielded, many valuable informations. The K-A age determination of the granitic basement rocks conducted by Kawano (1966), gave us the knowledge of the ages and the distribution according to age of these rocks present within the region. Another important study is that made by the Geological Survey of Japan (1961), which determined the structural set up of the basement in this region from gravity measurements. With the results of these many researches, as reference, the author studied the over-all structural set up of the Yamagata and 148 Noritsugu Oizumi Shinjo Basins, and thus, was able to determine the geologic structures which controlled and localized the deposition of the metallic ore deposits of the two basins. The result o such study is being presented in this paper. GEOLOGICAL STRUCTURES The Shinjo Basin and Yamagata Basin areas are situated in east of the Mizuho Fossa Magna, which divides the majer structural zone, called the Honshu arc, into an eastern part and a western part. This arc is distinctly characterized by uplift, folding and igneous processes operating in combination with one another. Towards the north, these two basins are aligned with the Yokote and Takanosu Basins of Akita Prefecture, the Tsugaru plain of Aomori Prefecture and Watarijima Peninsula of Hokkaido at the extreme north. Towards the south, are aligned the Yonezawa Basin, and the Aizu Basin of Fukushima Prefecture. This N-S alignment turns southwestward from southern Niigata Prefecture to Nagano Prefecture. The eastern boundary of this Tertiary sedimentary basin of "Inner Tohoku Region" is flanked by the Kitakami Mountainland and Abukuma Mountainland. Its western boundary is flanked by the wide continental shelf of Japan Sea. (Ikebe, 1966). This alignment of basins is parall led by the alignment of Neogene Volcanoes. To the east is the line of the Nasu Volcanic Zone, which makes up the On Mountain Range. To the west is the Chokai Volcanic Zone which makes up the. Dewa Hilly land. This series of velocanic zones and basins trend N-S in the northern part of Tohoku Region, NNE-SSW in the central part, and gradually turns NE-SW in the southern part of this region . These trends correspond to that of the Kitakami-Abukuma alignment of Mountain Lands. The basement rocks of these two Mountainlands served as wall of resistance during the folding of the Neogene Sedimentary sequence. This super-folding process resulted in the formation of numerous folds paralleled by equally numerous faults . (Takahashi, 1939). The Basement, made up mainly of granitic rocks , of inner Tohoku Region are distributed in these zones, namely; the eastern granitic zone of Northeastern Japan, extending from Kitakami Mountainland to southwestern Hokkaido; the central granitic zone of Northeastern Japan extending from Abukuma Mountainland , th rough Ou Mountain Range and Tsugaru Peninsula; and the western 149 Fig. 1. Geological map of the area studied. 1 : Quaternary, 2 : Pliocene, 3 : Nakawatari and Noguchi Formations (Shinjo Basin), Oga Formation (Yamagata Basin), 4 : Furukuchi and Kusanagi Formations (Shinjo Basin), Ogureyama and Taro For mations (Yamagata Basin), 5 : Okawa and Mazawa gawa Formations 6 : Nozoki Formation, 7 : Neo volcanic andesite, 8 : Granitic rock 150 Noritsugu Oizuuli Table 1. Correlation Table of the Stratigrasphic units whithin the area Studied. granitic zone of Northeastern Japan extending from Northern Kanto District and northward to Uetsu Mountainland. These three granitic zones trend NW-SE in the northern part of Tohoku Region, NNW SSE in the central part of the same region, and from this part swerve N-S, and finally turn NNE-SSW (Kawano, 1967). Attend ant to the uplift of these Basement zones are numerous faults and fissures. The Shinjo and Yamagata Basins situated in the middle of N-S trending line of basins, are underlain by granitic Basement rocks, Neogene sedimentary rocks, and Neogene volcanic rocks. They are characterized by a highly complicated structural set up. In particular, faults and fissures resulting from the rise of the Basement and folding of the sedimentary rocks disturbed and complicated the tectonic setting of the region. Originally the longer axis of the Shinjo Basin trended N-S, but later on, its southern and northern parts, rotated towards the E-W. Thus, the present trend of the axis of this basin is NNW-SSE. The axis of the Basement Rise on the northern part of this basin trends NNW-SSE, while that of the southern part trends N-S. The axis of Studies on the Genesis of Metallic mineral deposits 151 this rise and those of the folds are somewhat oblique to one another. The synclines and anticlines produced during this folding are more numerous within the Shinjo Basin than within the Yamagata Basin and their trends are relatively regular. On the other hand, because of the fact that the Basement gran itic rocks are widely exposed in the Yamagata Basin, the structures produced by the uplift the Basement are more pronounced in the Yamagata Basin than in the Shinjo Basin. Also, because of numerous Neogene volcanic eruptions in the Yamagata Basin, broad synclines, anticlines and major faults are not developed within it.