STUDIES ON THE GENESIS OF METALLIC MINERAL DEPOSITS OF SHINJO AND YAMAGATA BASINS, NORTHEASTERN JANAN (I)

Abstract

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°W+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°E+2°. 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 conjunction with the “Tertiary granite” intrusions. And those within southern Yamagata 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.