Japanese islands

It is difficult to define the normal range of iodine intake in humans, and despite efforts to provide iodine supplementation in many geographic areas of the world, endemic iodine deficiency and attendant goiter remain a world health problem (147). Exposure to excess iodine may sometimes lead to the development of thyroid disease. This unusual type of iodide-induced goiter has been found, for example, in 10% of the population of a Japanese island where fishermen and their families consume large quantities of an iodine-rich seaweed and have an iodine intake as high as 200 mg/d (148). 

Miocene-Pliocene Hydrothermal Ore Deposits in and around the Japanese Islands... 

Before mentioning the characteristics of Kuroko and epithermal vein-type deposits in Japan, it is worthwhile to briefly describe the metallogeny, geology, geophysics, and tectonic situations of the Japanese Islands. 

Main hydrothermal ore deposit types of Neogene age that formed in and around the Japanese Islands are Kuroko deposits and epithermal vein-type deposits. This classification is based on the form of the deposits. 

Figure 1.5. Three island arc junctions in the Japanese Islands (Kubota, 1994).

Figure 1.6. Di.stribution and temporal and spatial relationship of late Cenozoic gold deposits in the Japanese Islands. 1 Quartz vein-type gold deposits with little to no base metals. 2 Gold-silver deposits with abundant base metals. 3 Distribution boundary of gold deposits formed during the Miocene. 4 Location of Plio-Pleistocene gold deposits at the actual island arc junctions. 5 Location of Plio-Pleistocene gold deposits in front of the actual island arc junctions. Numbers in the figure are K-Ar ages of epithermal Au-Ag veins (Kubota, 1994).

Most of epithermal precious-metal vein-type deposits in Japan can be classed as adularia-sericite-type, and low sulfidation-type. Very few hot spring-type deposits (quartz-alunite-type, high sulfidation-type) are found in the Japanese Islands. A summary of various characteristic features of adularia-sericite type (low sulfidation-type) is given mainly in section 1.4. 

A few examples of hot spring-type deposits occur in the Japanese Islands. The characteristics of this type of deposits are described briefly in section 2.7. 

Numerous studies on the geologic and tectonic evolution in and around the Japanese Islands from Miocene to present have been carried out (e.g., Kitamura, 1959 ... 

The type of volcanic activity in and around the Japanese Islands changed throughout Tertiary. In early Tertiary subaerial andesitic activity was intense. For example, in... 

From late Miocene to present, subaerial arc-volcanic activity (calc—alkali rocks, andesite, tholeiitic and high alumina basalt) started associated with uplift of the Japanese Islands. This volcanic activity is different from that at middle Miocene age. 

From late Miocene, uplift took place due to the collision of Pacific plate to North American plate under the Kuril arc and Japanese island arc. 

Horikoshi (1995) showed areal extension of volcanic belts since middle Miocene in the Japanese Islands. He suggested that Hg and Sb mineralization in outer zone of Southwest Japan and north Hokkaido (Kitami district) related to forearc igneous activities in trench side. 8 S values become heavier across the Northeast Japan arc from the trench to the back-arc side. He thought that the changes in tectonic character from forearc to arc environment controlled the 8 S values. 

Isozaki, Y. and Maruyama, S. (1991) Studies on orogeny based on plate tectonics in Japan and new geotectonic subdivision of the Japanese Islands. J. Geography, 100, 697-761 (in Japanese). 

Kobayashi, K. (1983) Spreading of the sea of Japan and drift of Japanese island arc a synthesis and speculation. Mining Geology Special Issue, 11, 23-36. 

Kubota, Y. (1994) Temporal and spatial relationship and significance of island arc junction on the late Cenozoic gold deposits in the Japanese Islands. Resource Geology, 44, 17-24 (in Japanese). 

Niitsuma, N. (1979) Development of geologic structure of Northeast Japanese Island arc. Kagaku (Science), 49, 36-43 (in Japanese). 

Niitsuma, N. and Akiba, F. (1984) Subduction of plate and tectonics of the Japanese Islands of Neogene age. Kaiyokagaku (Ocean Science), 163, 4-9 (in Japanese). 

Sasaki, A. (1987) Isotope systematics of ore leads from the Korean Peninsula and the Japanese Islands. Mining Geology, 37, 223-226 (in Japanese). 

Shikazono, N., Shimizu, M., Inoue, A. and Utada, M. (eds.) (1999) The Japanese Island Arc Its Hydrothermal and Igneous Activity. Resource Geology Special Issue, 20. 

Tsunakawa, H. and Takeuchi, A. (1986) Paleo-stress field and igneous activity of Japanese Island. In Taira, A. 

Ueda, A. and Sakai, H. (1984) Sulfur isotope study of Quaternary volcanic rocks from the Japanese islands arc. Geochim. Cosmochim. Acta, 48, 1837-1846. 

Fig. 2.23 shows the distributions of major geothermal systems and epithermal gold deposits of Japanese Islands. It is interesting to note that their distributions are similar and they are distributed close to the volcanic front. 

Submarine metal precipitation at back-arc basins around the Japanese islands... 

Recently, several submarine hydrothermal sites have been discovered from the seafloor of back-arc depression zones and volcanic fronts near the Japanese Islands (Okinawa Trough and Izu-Bonin) (Fig. 2.29). The studies on these areas are described below. 

It was shown in previous chapters that intense hydrothermal activities occurred in the Neogene age in and around the Japanese Islands under the submarine and subaerial environments. In this chapter the influence of these hydrothermal activities on the seawater chemistry, and the global geochemical cycle are considered. 

Bulk rock chemistry of hydrothermally altered midoceanic ridge basalt has been well studied and used to estimate the geochemical mass balances of oceans today (Wolery and Sleep, 1976 Humphris and Thompson, 1978 Mottl, 1983). In contrast, very few analytical data on hydrothermally altered volcanic rocks that recently erupted at back-arc basins are available. However, a large number of analytical data have been accumulated on the hydrothermally altered Miocene volcanic rocks from the Green tuff region in the Japanese Islands which are inferred to have erupted in a back-arc tectonic setting (section 1.5.3). 

Tsunakawa, H. and Takeuchi, A. (1986) Paleo-stress field and igneous activity of Japanese Island. In Taira, A. and Nakamura, K. (eds.). Formation of the Japanese Island. Iwanamishoten, pp. 201-208 (in Japanese). 

Climate change in the vicinity of the Japanese Islands during the Neogene has been... 

Based on the data on Neogene shallow marine molluscs of the Japanese Islands, the paleoclimate was reconstructed by Ogasawara (1994). After the middle Miocene climatic optimum at 16 Ma, a gradual cooling commenced at about 14—13 Ma. 

The change in temperature near the Japanese Islands mentioned above seems to be consistent with the global temperature change (Ogasawara, 1994), as well as with changes... 

During the middle Miocene, Kuroko deposits, polymetallic vein-type deposits, gold-quartz vein-type deposits and Sb and Hg vein-type deposits were formed (see sections 1.3 and 1.6). Many vein-type deposits were formed not only in and nearby the Japanese Islands, but also at middle Miocene in northwest USA (Basin and Range Lipman, 1982), and elsewhere in the circum-Pacific regions (e.g., Peru). It is probable that large amounts of CO2 effused into the atmosphere from hydrothermal solution associated with this widespread mineralization and volcanic gas from subduction zones, causing an increase in temperature. 

During middle to late Miocene (15-6 Ma), very little vein-type (Fig. 4.7) mineralization occurred in the Japanese Islands. During 5 Ma to present many vein-type deposits formed. 

This metallogenic epoch in and near the Japanese Islands correlates well with Jackson s episode (Masuda, 1984). Jackson s episode means the change in the direction of Pacific plate, which was estimated from the array and age of Hawaiian islands by Jackson etal. (1975) (Fig. 4.8). 

Ogasawara, K. (1994) Neogene paleogeography and marine climate of the Japanese Islands based on shallow-marine molluscs. Paleogeogr. Paleoclimatol. Paleoecol., 1 8, 335-351. 

Yamanoi, T. (1993) Vegetation in Neogene age in Japanese Islands. Monthly Earth, 16, 181-190 (in Japanese). 

In and nearby the Japanese Islands, which are situated close to plate boundaries (Pacific, Philippine, North American and Asian plates), many hydrothermal ore deposits were formed during the Miocene-present eras. Major deposit types are Kuroko and epithermal vein-types. Epithermal vein-type deposits are classified into precious (Au, Ag)-types and base-metal (Cu, Pb, Zn, Fe, Mn, Ag)-types. 

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