Kuroko and hydrocarbon deposits from northern Honshu, Japan: A possible common hydrothermal/magmatic origin?

Abstract

Northern Honshu is the most important area for mineral and oil resources in Japan. Many kuroko, deposits and oil and gas fields are distributed in two belts along the northeast Japan arc, the kuroko metal-belt on the Pacific side and the oil-belt on the Sea of Japan side. The kuroko deposits are located mainly in the Green Tuff strata which formed as a result of submarine volcanism during the late Miocene and Pliocene. Most of the source rocks of the oil and gas deposits formed at the same time as the kuroko deposits and some of them are located in reservoirs of hydrothermally-altered volcanic rocks in the Green Tuff region. There is general agreement that the kuroko deposits formed as a result of submarine hydrothermal and magmatic activity whereas almost all petroleum geologists and geochemists consider that hydrocarbon deposits were generated independently of such activity. Since the discovery of hydrothermally-generated petroleum in the Guaymas Basin, Gulf of California, however, it is clear that petroleum can be formed almost instantaneously in terrestrial and submarine hydrothermal areas. The paleo-northeastern Sea of Japan is therefore considered to be a potential area for hydrothermal petroleum generation because thick organic-rich sediments overlie an active submarine volcanic area. Several lines of geological and geochemical evidence suggest the possibility of hydrothermally-enhanced maturation of organic matter and the contribution of magmatic activity to the formation of these deposits. Although most of the oil and gas in northern Honshu has been generated conventionally as a consequence of the high geothermal gradients there, it appears that some of the oil and gas fields may have formed as a result of extensive hydrothermal and magmatic activity during the late Miocene to Pliocene. Because of the much steeper angle of the faults in the vicinity of the Hokuroku basin than in the Akita basin, the magmatic contribution to the kuroko, mineralization would have been far greater than to the oil and gas deposits of the Niigata and Akita basins. We therefore propose a strong relationship between metal and oil and gas generation in northern Honshu based on the structure and tectonics of the northern Honshu arc-backarc system.