Back arc

These different sites of hydrothermal and ore-forming activity may have resulted from the mode of subduction of the Pacific Plate. Mariana-type subduction (characterized by a steep angle of subduction and back-arc basin formation Uyeda and Kanamori, 1979) during middle Miocene caused WNW-ESE extension, submarine hydrothermal activity, thick accumulation of bimodal (basaltic and dacitic) volcanic activity (Green tuff) and Kuroko-type formation (Shikazono and Shimizu, 1993). Plio-Pleistocene Chilean-type subduction (shallow-dipping subduction zone, E-W compression Uyeda and Kanamori, 1979) and oblique subduction of the Pacific Plate beneath the North American Plate led to uplift and expansion of land area, subaerial hydrothermal activity accompanied by meteoric water circulation, subaerial andesitic volcanic activity and formation of vein-type deposits. 

It is clear in Fig. 1.10 that the distribution of Kuroko deposits is restricted in a narrow zone in the Green tuff region which was called a Kuroko belt by Inoue (1969). This belt was formed by rapid subsidence under the extensional stress regime and is thought to have been a back-arc depression zone at middle Miocene age. The relationship between tectonic setting and formation of Kuroko deposits is discussed in section 1.5. 

Several different hypotheses on the tectonic setting of the Kuroko mine area have been proposed. They include volcanic front of island arc (T. Sato, 1974 Horikoshi, 1975a), rifting of island arc (Cathles, 1983a), back-arc depression (Fujioka, 1983 Uyeda, 1983), and back-arc basin. 

In recent years, many hydrothermal solution venting and sulfide-sulfate precipitations have been discovered on the seafloor of back-arc basins and island arcs (e.g., Ishibashi and Urabe, 1995) (section 2.3). Therefore, it is widely accepted that the most Kuroko deposits have formed at back-arc basin, related to the rapid opening of the Japan Sea (Horikoshi, 1990). 

The summary of the bulk chemical compositions (major elements, minor elements, rare earth elements), Sr/ Sr (Farrell et al., 1978 Farrell and Holland, 1983), microscopic observation, and chemistry of spinel of unaltered basalt clarifies the tectonic setting of Kuroko deposits. Based on the geochemical data on the selected basalt samples which suffered very weak alteration, it can be pointed out that the basalt that erupted almost contemporaneously with the Kuroko mineralization was BABB (back-arc basin basalt) with geochemical features of which are intermediate between Island arc tholeiite and N-type MORE. This clearly supports the theory that Kuroko deposits formed at back-arc basin at middle Miocene age. 

Two hypotheses of seafloor depth at the time of mineralization have been proposed based on foraminiferal data, ca. 3500 m (Guber and Ohmoto, 1978 Guber and Merrill, 1983) and 1500 m (Kitazato, 1979). Considering seafloor depth of present-day ore formation at back-arc basins and fluid inclusion data mentioned above, shallow seafloor depth hypothesis (Kitazato, 1979) seems more likely. If the pressm e-temperature condition of Kuroko ore fluids was close to the boiling curve, the depth could be estimated to be 1,000-1,500 m, which is similar to that for present-day back-arc mineralization such as Okinawa Trough. 

Precipitation of barite and quartz. Barite and quartz are the most common gangue minerals in the submarine hydrothermal ore deposits such as Kuroko deposits and back-arc basin deposits (e.g., Okinawa, Mariana deposits) (Halbach et al., 1989 Shikazono, 1994 Shikazono and Kusakabe, 1999). These minerals are also common in midoceanic ridge deposits. 

Barite is abundant in back-arc basin hydrothermal system such as Okinawa, Manus and Mariana (Shikazono and Kusakabe, 1999). 

Barite-silica chimney found in back-arc basin formed in the conditions similar to that of ferruginous chert and barite bed in the Kuroko deposits temperature is relatively low (ca. 150-100°C), and flow rate of fluids may be slow. 

Barite is abundant in the massive strata-bound ore bodies (black and barite ores) in Kuroko deposits and occurs in the ferruginous chert ore in Kuroko deposits, and chimneys in active deposits at back-arc basins. 

The above argument on the calculation of chemical composition of ore fluids, seawater-rock interaction experiments, and isotopic compositions of ore fluids clearly demonstrates that Kuroko ore fluids were generated by seawater-rock interaction at elevated temperatures. The chemistry of present-day hydrothermal solution venting from back-arc basins and midoceanic ridges (sections 2.3 and 2.4) also support this view. 

Figure 1.60. Variation of subsidence rate for syn-rift basins in the Uetsu district, northeast Honshu (Yamaji, 1990). The line of boxes shows the spatially averaged subsidence rate. The rate after 15 Ma is not clear because of uncertainty in paleobathymetry. However, the rate probably decreased to the order of 10-100 m/m.y. If the rate had been of the order of 1 km/m.y. after 15 Ma, the water depth of the inner arc region at 14 Ma would have been much deeper than modem, young, back-arc basins.

As already discussed, /02 of Kuroko ore fluids is considered to lie in the predominance field of reduced sulfur species from the following two reasons (1) Selenium content of sulfides is very low (Yamamoto, 1974) and (2) H2S is dominant in hydrothermal solution venting from back-arc basins (section 2.3) from which hydrothermal ore deposits being similar to Kuroko deposits form. 

The changes in stress fields, and intensities of igneous and hydrothermal activities seem to correlate to oscillatory motion of the Pacific plate (Jackson s episodes) (Jackson et al., 1975 Jackson and Shaw, 1975) (Masuda, 1984). Masuda (1984) and Takeuchi (1987) pointed out that the oscillatory motion of Pacific plate during the least 42 Ma correlates with magmatism, the intensity of tectonism, the change of stress field and the history of sedimentary basin in arc-trench system (Fig. 1.147). The above arguments also suggest that the mineralizations in arc and back-arc systems relate to the oscillatory motion of the Pacific plate. 

The above-mentioned changes in paleogeography, volcanism, crustal movement (subsidence and uplift), and stress field clearly demonstrate that these features of back-arc volcanism in early-middle Miocene are quite different from those of Island arc volcanism in late Miocene to present. According to Yoshida and Yamada (2001), the age of change in volcanism from back-arc type to Island arc type in Northeast Honshu was 12.7 Ma and this age corresponds to the age of Kuroko formation. 

Hydrothermal solution venting from midocean ridges and back-arc basins has positive Eu anomaly (Klinkhammer et al., 1983 Miehard et al., 1983 Mitra, 1994 Shikazono, 1999a) (Fig. 1.158). Therefore, the positive Eu anomaly of the sedimentary rocks is thought to be due to a contribution of hydrothermal solution. In order to know the contribution of hydrothermal solution the positive Eu anomaly of seawater (Eu/Eujg gjgj.) is useful. 

This kind of temporal and spatial relationship between epithermal Au vein-type mineralization and back-arc mineralization are found also in the Izu-Bonin area. Seafloor... 

As mentioned above, formation of back-arc basins and marginal seas may be important for the formation of Kuroko and vein-type deposits, although genetic relationship between Kuroko formation and opening of the Japan Sea is not clear. For example, Horikoshi (1977) insists that vein-type deposits in Northeast Hokkaido did not form without the opening of Ohotsuku back-arc basin. 

In section 2.3 and in Chapter 3, it is shown that the formation of back-arc basins take important role for the mineralization (back-arc deposits (Kuroko deposits), epithermal Au veins) and global geochemical cycle. Thus, it must be worth considering the formation mechanism of back-arc basins. 

The origin of the back-arc basins has been investigated considerably (Karig, 1971 Sleep and Tok.soz, 1971 Uyeda and Kanamori, 1979 Tamaki and Honza, 1991 Uyeda, 1991 Tamaki, 1995) and various explanations for the origin have been proposed. 

Tamaki and Honza (1991) summarized the previously proposed models and the currently plausible models of back-arc spreading (Fig. 1.163). 

Model I shown in Fig. 1.163 is a slab-induced upwelling model. Upwelling generated along the down going slab-mantle boundary (Karig, 1971) or by secondary convection that is introduced by the slab (Sleep and Toksoz, 1971) causes back-arc formation (Karig, 1971). 

The active back-arc extension in the Okinawa Trough and the Taupo Depression in New Zealand can be explained by the injection model. 

Model 3 is a plate kinematic model. The retreat of a back-arc plate forms a back-arc basin (Dewey, 1980). 

Model 4 is also a plate kinematic model. The retreat of a fore arc plate forms a back-arc basin. This model seems attractive. Jackson et al. (1975) found the periodicities of rotational motions of the Pacific plate. When the direction of the Pacific plate changed and obliquely subducted, the compressional force of oceanic plate to continental plate decreases. That means that the retreat of fore arc plate occurs. 

In any model, back-arc basins form under the extensional stress regime and are associated by Mariana-type subduction by Uyeda and Kanamori (1979) rather than Chilean-type subduction. 

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. 

Halbach, P., Nakamura, K., Wahsner, M., Lange, J., Sakai, H., Kaselitz, L., Hansen, R.-D., Yamano, M., Post, J., Prause, B., Seifent, R., Michaelis, W., Teichmann, R, Kinoshita, M., Marten, A., Ishibashi, J., Czerwinski, S. and Blum, N. (1989) Probable modern analogue of Kuroko type massive sulfide deposits in the Okinawa Trough back-arc basin. Nature, 333, 496-499. 

Ishibashi, J. and Urabe, T. (1995) Hydrotheimal activity related to arc-back arc magmatism in the Western Pacific. In Taylor, B. (ed.), Backarc Basins Tectonics and Magnetism, Plenum Publ, pp. 451—496. 

Shikazono, N. (1994) Precipitation mechanisms of barite in back-arc basins. Geochim. Cosmochim. Acta, 58, 2203-2213. 

Shikazono, N. (1999a) Rare earth element geochemistry of Kuroko ores and altered rocks implication for evolution of submarine geothermal system at back-arc basin. Resource Geology Special Issue, 20,... 

Shikazono, N., Utada, M. and Shimizu, M. (1995) Mineralogical and geochemical characteristics of hydrothermal alteration of basalt in the Kuroko mine area, Japan Implications for the evolution of back arc basin hydrothermal system. Applied Geochemistry, 10, 621-642. 

Tatsumi, Y., Maruyama, S. and Nohda, S. (1990) Mechanism of back arc opening in the Japan sea role of asthenospheric injection. Tectonophysics, 181, 299-306. 

Tsuchiya, N. (1989) Submarine basalt volcanism of Miocene Aosawa formation in the Akita-Yamagata oil field basin, back-arc region of Northeast Japan. Geol. Soc. Japan. Mem., 32, 399-408. 

Se-type) that have been formed at extensional environment which is similar to back-arc environment. 

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. 

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