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 (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 , 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. The minor folds and faults developed within this basin are highly disturbed. The axis of the Basement Rise, in this basin, trends N-S in the north and gradually turns NNE-SSW in the south. The axes of folds of the Neogene sedimentary rocks trend somewhat obliquely to this trend of the Basement axis.

A. Faults and Folds

1) Shinjo Basin•\The major faults and folds within this basin are: the NW-SE trending Daisenzan anticline of Daisenzan area in Okachi pass in the north; the NNW-SSE trending Okawa anticline, which includes the Arisawayama Dome; and the broad N-S trending Inohana anticline of Dewa Hilly Land in the west. The synclinal structures corresponding to these anticlines are from east to west, namely: Hitane, Mizunashi-Omori and Kabuyama. West of this group of synclines is the Kabutovama-Okawairi syncline, which starts from Kabutoyama along the eastern slope of Gassan Volcano. The Arisawayama Dome was produced by the intrusion of the so called Tertiary granitic rocks (Taguchi, 1962). The broad Kokuzodake anticline, in the southern part of the basin, trends N-S in the neighborhood of northern Mogami Rives, NNW-SSE in its southern extension, and make up a semi-domal structure shown by the dips along the banks of . The syncline corresponding to this anticline contains two or three crenulations. The faults related to those structures include the Aozawa fault extending from Tachiyazawa in the south to Masuda in the north; and the Oashizawa and Nishikawa faults in the east. These faults all trend N-S. In the southeastern and southern part of the basin the axial planes of the folds all dip westerly; with steep dips in the west and gentle dips in the east. As a result of super-folding in the western side, corresponding reverse faults are numerous in this 152 Noritsugu Oizumi Studies on the Genesis of Metallic mineral deposits 153 portion. The major fold structures within the basin are the Hanezawa anticline and Alagarikawa anticline, along the northern flank of Mogami River. Parallel to these anticlines, along the southern flnaks of the Mogami River, are the Kuraoka anticline, Kodaira anticline, and others. In this side of Mogami River the folds are paralleled by major faults, a particular example of which is the Matsuyama fault. The major structure in the southeastern part of the basin is the Semi anticline, gently dipping towards the south and passing through Semi Hotspring. The western flnak of the anticline dips rather steeply, while the eastern finak is almost horizontal. Nor thwest of City are present the Nokurozawa anticline and Yubunezawa anticline both trending N-S. In the southeastern part of this city is the Obanazawa Ichinono faults northeast of Obanazawa City. The faults and folds flanking the northern and southern portion of Mogami River in Shinjo Basin are clearly different in trend. Those situated along the north side of Mogami River trend N-S while those

Fig. 2. Strrctural map of the area studied. Anticlines (= A. ) and Synclines (= S. ) Shinjo Basin 1. Daisenzan A., 2. Arisawayama Dome., 3. Okawa A., 4. Tno hanadake A., 5. Hitane S., 6.Mizunashi-Omori-Kabuyama S., 7.Kabutoyama-Okawairi S., 8. Kokuzo A., 9. Hanezawa A., 10. Magarikawa A., 11. Izumida S., 12. Kuraoka A., 13. Kodaira A., 14. Semi A., 15. Nogurozawa A., 16. Yubumezaw A., 17. Obanazawa S., Yamagata Basin 18.Kaishuzawa A., 19. Kuromoriyama A., 20. Ushirozawa A., 21. Nitta A., 22. Tsukinuno A., 23. Kokusube A. 24. Oya S., 25. Negota A. 26. Okuribashi A., 27. Kamimatsuyama S., 28. Yoshizawa A., 29. Mizubayashi S., 30. Yoshino A. Faults (=F.) Shinjo Bsin 1. Aozawa F., 2. Oashizawa F., 3. Nishikawa F., 4, Matsu yama F., 5. Kyodanbara F., 6. Ichinono F., Yamagata Basin 7. Yotsuyagawa F., 8. Mazawagawa F., 9. Takahi F., 10. Tateki F., 11. Yamanobe F., 12. Nanatsumori F., 13. Kaneyama F., 14. Arato F., 15. Mizubayashi F., 16. Kuromori F., 17. Kane yama F., 18. Akayama F., 19. F., 20. Furuyashiki F., 21, Tateyama F., 22,. Bobara F., 23. Hozawa F., 24, Nii yama F, 154 Noritsugu Oizumi in the southern side trend NNW-SSE. In eastern Shinjo Basin their trend is NNE-SSW in the north, and NW-SE in the south. This difference in structural trends, with the Mogami River acting as the border line, was confimred by the result of gravity measurement. The negative gravity anomalies in the central part of the basin, are divided into two in southern Shinjo City and northern Oishida Town, indicating a sharp E-W movement. From this measurement also, it was made possible to infer that a fault exist beneath the Pleistocene

(Petroleum Resources Development Co., 1961). 2) Yamagata Basin•\In the Yamagata Basin broad syn clinal and anticlinal structures such as existing in Shinjo Basin, do not occur. Faults are also small scale, but they are numerous in number and criss-crossing one another. In the northern part of the basin, are present the Kaishuzawa anticline and the Kuromoriyama anticline both trending N-S. Major faults are the Yotsuyagawa fault along the Yotsuya River, the Mazawa fault along the Mazawa River, and the Takahi fault passing through Kaishu Village and extending northward. All of these faults trend N-S. In between these faults are abundant minor faults which criss-cross one another. In the western part of this basin are the Nitta anticline, Tsuki uno anticline, and Kokusube anticline; all trending N-S. The structures situated from Volcano to Aterazawa, trend NNE-SSW, and they are namely: the Oya syncline, Nekota anticline, and Okuribashi anticline. The largest fault is the Tateki fault; and in the eastern side of Shirataka Volcano , are the Yamanobe fault and the Nanatsumori faultn crossing perpendicularly the major faults.

The southern part of the basin is a highly disturbed region. In this area are the Mizubayashi syncline and the Yoshino anticline , both trending NNE-SSW. Parallel to these folds are numerous other smaller synclines and anticlines. The major faults are the Mizu bayashi and the Kuromori faults. These faults also trend NNE-SSW. Aside from these faults are the Kaneyama and the Arato faults, both crossing the NNE-SSW faults obliquely.

In the southeastern part of the basin , are the NNE-SSW trend ing Akayama and Furuyashiki Faults. Folding in this part is only minor.

In the eastern part of the basin , no prominent fold structures are found, but numerous faults are present . These faults include the

Bobara and Niiyama faults trending NNE-SSW , and the Tateya ?? a and Hozawa faults crossing the first two perpendicularly . Aside Studies on the Genesis of Metallic mineral deposits 155 from these faults another N-S trending fault is inferred connecting Yamagata City and Tendo City. This is suggested by the approximately 500 meter difference in depth of the Basement in the eastern and western sides, as shown by the result of gravity measurements. From similar measurements, it was found out that the northern side of the E-W flowing River in the central part of the basin gave high gravity value while the southern side gave low gravity value. Thus, it is inferred that a fault runs parallel to this river surving as a hinge line to the southern and western flanks (Geological Survey of Japan, 1961). B. Fissures The fissures within the two basins are classified according to their origin. One class includes that caused by the uplift of the Basement and another class includes those caused by the folding of the sedimentary rocks. Those caused by the uplift of the Basement, whether within the sedimentary rocks or within the Basement rocks, are related to the axis of the Basement Rise which trends N-S to NNE-SSW. Those parallel to this trend are exceedingly abundant and those perpendicular to this trend, running along the E-W direction, are next in abundance. Examples of the N-S trending ore-filled fissures are : the Teng uzawa vein of Kamuro Mine, northeastern Shinjo Basin; the Akataki and Tsuchiuchi veins of Raidaki Mine, eastern Shinjo Basin; the 7 veins of Chiyogawawa Mine of eastern Yarnagata Basin ; and the Dainan vein of southeastern Yamagata Basin. Examples of the E-W trending ore-filled fissures are: the Kosahi vein of Kamuro Mine, northeastern Shinjo Basin; the Maehi vein of Daiyama Mine, eastern Shinjo Basin; the Nakahi and the Honpi of Tengumori Mine, eastern Shinjo Basin; the Fudokaki and Ogihira veins of northern Yamagata Basin; the Himesawa and Umenoki veins of eastern Yamagata Basin; and the Nishizawa and Asahi veins of Azuma Mine, southern Yamagata Basin. These groups of fissures within the granitic Basement rocks cross each other perpendicularly. Among the fissures within the granitic mass of Asahi Mountainland, as measured along the 5,050 meter long tunnel in Sonooka, Asahi-mura, Higashitagawa-gun, the NNE-SSW trend is the most abundant, giving 33.8%, and the E-W trend is next, giving 19.4%. These two direc tions are perpendicular to one another. Aside from these two direc tions are the abundant NE-SW fissures situated within the intervals of the E-W and NNE-SSW fissure systems. The NW-SE direction 156 Noritsugu Oizumi fissures are practically absent. The two mainfissure systems resulting from the rise of the Base ment, strongly affect the sedimentary rocks directly overlying the Basement. Within these sedimentary rocks are weak zones, fissure systems, and fissures similar to those of the Basement. Examples of the N-S trending system within the sedimentary rocks but produced by the uplift of the Basement are; the Namari zawa vein of Marukura Mine, and the Maehi vein of Nakanomata Mine of northern Shinjo Basin; the Kurikoma, Komagatake and Shirouma veins of Suginoiri Mine, eastern Shinjo Basin; the Higashi No. 1 of Mitate Mine, northern Yamagata Basin; the Furuyashiki and Gando veins, eastt rn Yamagata; and the Shiregane and Minamizawa veins of western Yamagata Basin. These veins are all situated within sedimentary rocks directly in contact with granitic rocks. Examples of the E-W trending fissures system are the Marukura vein of northern Shinjo Basin; the Wasabizawa of Sugisawa Mine, the Joto vein of Yanagihara Mine, northeastern Shinjo Basin; the Tengu and Ryujin veins of Ryujin Mine; the Azuma and Tatekura veins of Tatekura Mine, and the several number veins of Akakura Mine, all in eastern Shinjo Basin; the Jigokuzawa, Senj uin and Ichinosawa veins of Horai Mine, the No. 1 and the No. 3 veins of Omoshiroyama Mine, and the Utsunosawa and Kotezawa veins of Gando Mine all of eastern Yamagata Basin; the Azuma vein of Azuma Mine, southeastern Yamagata Basin; and the Niijiku and Mansaku veins of southern Yamaga Basin. All of these vein filled fissures are situated within the lowermost formations of the sedimentary sequence which is directly in contact with the Basement . These fissures within the sedimentary rocks run through the Basement in general, and a part of them may be observed in the mine tunnels. As shown by the result of stereographic projection of the ore vein systems, the most abundant of the fissures, related to the folding of sedimentary rocks are the N60-70E trending fissures; and next to these in abundance are the E-W fissures. Hence, the relatively large-scale fissures within the sedimentary rocks , are those trending along these directions . However, the fissure systems related to the folding of these sedimentary rocks are those N-S to NNW-SSE trending fissures parallel to the axes of the folds , and th ose perpendiulcar to this trend , which are E-W to ENE-WSW. Considering minor details, the minor fissures and veinlets within these fissure zones, generally trend E-W. These minor fissures in Studies on the Genesis of Metallic mineral deposits 157 both basins, are inferred to be tension cracks resulting from lateral compression along the E-W direction (Billings, 1962). For example, in the massive ore deposits, as in the Yoshino Ore District, every ore body has the NNE-SSW fracture system as their major structural control. This fracture system comprises 27% of the total number of fissures, and the WNW system, which is per pcndicular to this trend, comprises 17•“. In the Kaninomata deposit of Ohcri Mine, the trend of the fold axes is N-S, but in the bedded deposits, the majority of the vertical veinlets in the second ary enrichment zone are those trending E-W. These E-W trending fissures are produced by tension generated by lateral compression along the E-W direction. (McKinstry, 1942). In the case of the stockwork deposits, as in the Fukufune, Daro, Daiichi, Shuyama and Koyama ore deposits, the most abundant direction of the net work of fissures are those trending E-W. Those trending NE-SW crossing the E-W fissures 30 to 60 degrees, are also abundant. Associated with these major fissure systems are many others, making up the fissure systems very complex, but these other fissures differ with the kind of country rocks in which they are developed

(Mckinstry, 1942). For example, in igneous rocks such as andesite and liparite, the fissures are at randam, but in the sedimentary rocks, the fissure are remarkably regular in their direction. In general, fissures are more highly developed within shale and rnudstone, especially when they are highly silicified, than in the tuff

(Lindgren, 1933). C. Relationships of Faults, Fissures, Hot Springs and Volcanoes Geologically and structi rally, hot springs and mineral springs, are naturally closely related to volcanic activity. Within the Yamagata Prefecture, the large and small hot and mineral springs, amounting to a total of 240, are situated in localities of so varied geologic setting. They are controlled by the exceedingly abundant structures in the Shinjo and Yamagata Basin. These include faults and folds, especially the N-S to N20W or N20E anticlines (Ichimura, 1963). The faults and intrusive rock bodies perpendicular and oblique to the main structural lines also serve to control the appearance of these springs (Tokairin, 1963). For example, the Semi Hotspring occurs within a network of fissures in a liparite dike perpendicularly traversing the E-W flowing Ogumi River. The Akakura Hotspring east of Semi Hctspring, along the same river occurs within the junction between the N30W system of fissures and the N30E fault. The Yudonosan Hotspring, 158 Noritsugu Oizumi

Table 2. Showing the Geology and Temperature Distinction of Hot Springs

along the slope of Gassan Volcano, is developed along the junction between a N-S trending fault and a liparite dike perpendicularly cutting the fault. The Hijiori Hotsp ?? g occurs along a hparite dike. As shown in the above table 86% of the high temperature and cold mineral springs occur within Neogene Tertiary rocks. About half of these are situated in the lower formations of this epoch. They are mostly related to Tertiary Neogene liparite and dacite intrusions. In part, they directly appear along the fissures of these igneous bodies. Notably, most of the area in which these springs occur are also zo-les of mineralization. The development of these springs is therefore clearly related to the Neogene igneous activity and to metallic mineralization (Cloos, 1939). The occurrence of these springs as well as that volcanoes are clearly controlled by geologic structures. Volcanoes do nct of course occur spontaneously at random anywhere. but they appear only in some particularly limited area. They are mostly distributed along straight lines (Yagi, 1957). Within the Shinjo and Yamagata Basins, many volcanic eruptions occurred during the Neogene; and they were naturally controlled by geologic structures. Mineraliza tion accompanied these eruptions, and many ore deposits are situated within the vicinity of volcanoes. The tectonic conditions which controlled the appearance of these volcanoes are thus , believed to be the same conditions which controlled deposition of ore minerals and the distribution of hot springs . (Cloos, 1939). (to be continued)