Okinawa Trough

Miyashita (1995) proposed that the epithermal mineralizations in Hokusatsu district (Kyushu) are related to strata volcanoes in the volcanic depression and not to caldera formation, hy compiling the data on the ages of mineralization and volcanic activities, gravity and geology. He considered that the Hokusatsu volcanic depression zone is an extension of the Okinawa Trough. This volcanic depression started from 5-6 Ma (Kamata and Watanabe, 1985), where bimodal volcanism is found. 

Considering the above argument it is likely that the major epithermal mineralizations (Se-type and low sulfidation-type) are generally related to bimodal volcanism and not to calc-alkaline volcanism even in north and south Hokkaido and south Kyushu where andesite is dominant in the mine area, although it is thought some deposits (e.g., Takadama in northeast Honshu) formed closely related to the caldera volcanism (Seki, 1993). 

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. 

Honma et al. (1991) have shown that the Okinawa Trough basalts have significantly high K, Rb and Sr contents and D/H, 0/ 0 and Sr/ Sr ratios than N-Morb have and these are due to generation of magma from normal-type mantle peridotite modified by component from the subducted slab and crustal contamination. 

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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. 

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

Figure 1.164. Distribution of Cenozoic basalts and active rift systems in the northeast China region. Arrows indicate the horizontal convective current in the upper mantle associated with the upwelling of the asthenosphere beneath the region. A Baikal Rift B Shanxi Graben C Tancheng-Lujiang Fault D Okinawa Trough (Tatsumi et al., 1990).

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. 

Halbach et al. (1997) reported lead isotope data on volcanic rocks, sediments and ores from the hydrothermal JADE field in the Okinawa Trough and pointed out that lead isotopic compositions of Okinawa JADE ores are very similar to Kuroko ores (Fig. 2.31) and both sediments and volcanic rocks contributed comparable amounts of lead to the deposit. 

Fig. 2.30. Location and tectonic setting of Okinawa Trough. The solid circles mark the hydrothermal fields listed in Table 2.12. The broken lines show contours of 1000 m water depth. The triangles indicate Quaternary volcanoes (Ishibashi and Urabe, 1995).

Mica, kaolins (kaolinite and halloysite), Mg-rich chlorite, talc, montmorillonite and chlorite/montmorillonite are abundant in the host epiclastic sediments and pumiceous tuffs at Jade in the Okinawa Trough (Marumo and Hattori, 1997). 

No. Host Axial Seamount CASM 14 B North Fiji Basin 24 B Mariana Trough at 18 N 11 A Valu Fa, Lou Basin 47 B, A, D Eastern Manus Basin 26 D Jade Okinawa Trough 17 R... 

Site No. Name of site 1 Okinawa Trough JADE 2 Okinawa Trough CLAM 3 Okinawa Trough South Ensei 4 Izu-Bonin Suiyo SM. 5 Mid-Mariana Trough Alice Springs 6 South Mariana Trough Forecast vent 7 Manus Basin Vienna Woods 8 North Fiji Basin 9 Lau Basin Vai Lili... 

Among the back-arc deposits, the features of the Okinawa Trough deposits are similar to those of Kuroko deposits, but North Fiji and Mariana deposits are different. For instance, gold, silver, zinc, lead and arsenic are rich in the Okinawa Myojin-Knoll Caldera and some Kuroko deposits but poor in the North Fiji and Mariana deposits. 

These differences are considered to be attributed to the dilferences in compositions of rocks and alteration minerals interacted with circulating seawater or modified seawater at elevated temperatures. For example, high K and Li concentrations in the hydrothermal solution in the Mid-Okinawa Trough baek-arc basin (Jade site) are due to the interaction of hydrothermal solution with acidic volcanic rocks (Sakai et al., 1990). It is evident that the chemical compositions of hydrothermal solution are largely alfected by water-rock interaction at elevated temperatures. 

Okinawa Trough and at Escanaba Trough are even higher than that of seawater (0.7092), strongly suggesting the contribution of sedimentary material with high Sr/ Sr ratios (Gamo, 1995). 

Although the data are not plentiful, it is clear that the hydrothermal solutions of sediment-hosted ridges and back-arc basin covered by sediment (Okinawa Trough) contain high amounts of ammonium (2.8-13.6 p.molal) (Scott, 1997). This means that ammonium was derived by thermal maturation reaction of organic matter in sediments by the following reactions (Gamo et al., 1991),... 

S S data on H2S and sulfides from Okinawa Trough (Okinawa Izena) show a high S S value (- -8%o) (Sakai, H. pers. conun cited in Ishibashi andUrabe, 1995). 8 " S values from other districts are similar to those of midoceanic ridges -i-2.1%o to 4-3. l%o, Mariana Trough (Kusakabe et al., 1990) -f0.3%o to -t-2.2%o, Minami Ensei Knoll (Nedachi et al., 1992 4-2.l%o to 4-2.8%o, Kita-Bayonnaise caldera (lizasa et al., 1992) 4-0.9% to 4-1.2% (Kaikita caldera) (vein part) (Ishibashi and Urabe, 1995). S " S values from the Okinawa (Izena) sulfides are higher than any of S S data from midoceanic ridge sulfides and H2S of hydrothermal solutions. 

B values of hydrothermal solution from back-arc basin are lower than those from midoceanic ridge hydrothermal solution (Gamo, 1995). 8"B values of Okinawa Trough hydrothermal solution are particularly low (—5%o to — 10%c) (Ishikawa and Nakamura, 1993), suggesting a contribution of sedimentary boron to hydrothermal solution. 

The iron content of sphalerite from back-arc deposits is lower than midoceanic ridge deposits. Pyrrhotite and wurtzite are not common in back-arc deposits, although they were identified from Iheya Ridge, Middle Okinawa Trough, and Mariana. 

N Axial Seamount CASM 14 N. Fiji Basin 24 Mariana Trough at 18°N 11 Valu Fa, Lau Basin 47 Eastern Manus Basin 26 Jade, Okinawa Trough 17... 

Gamo, T., Sakai, H., Ishibashi, J., Oomori, T., Chiba, H., Shitashima, K., Nakashima, K., Tanaka, Y. and Masuda, H. (1991) Growth mechanism of the hydrothermal mounds at the CLAM site. Mid Okinawa Trough, inferred from their morphological, mineralogical, and chemical characteristics. Proc. JAMSTEC Symp. Deep Sea Res., 1, 113-184 (in Japanese). 

Ishizuka, H., Kawanobe, Y. and Sakai, H. (1990) Petrology and geochemistry of volcanic rocks dredged from the Okinawa Trough and active back-arc basin. Geochem. J., 24, 75-92. 

Marumo, K. and Hattori, K.H. (1997) Seafloor hydrothermal clay alteration at Jade in the back-arc Okinawa Trough Mineralogy, geochemistry and isotope characteri.stics. Geochim. Cosmochim. Acta, 63, 2785-2804. 

Nedachi, M., Veno, H., Ossaka, J., Nogami, H., Hashimoto, J., Fujikura, K. and Miura, T. (1992) Hydrothermal ore deposits on the Minami-Ensei Knoll of the Okinawa Trough - Mineral assemblages. In Proc. JAMSTEC Symp. Deep Sea Res., 8, 95-106 (in Japanese). 

Yamano, M., Uyeda, S., Foucher, J.P. and Sibuet, J.C. (1981) Heat flow anomaly in the middle Okinawa Trough. Tectonophy.sic.s, 159, 307-318. 

As mentioned already in Chapter 2, submarine volcanism occurs not only at midoceanic ridges but also at subduction-related tectonic settings such as the Shikoku and Daito Basins, Farce Vela Basins, and Mariana Trough, Okinawa Trough and Izu Bonin Arc (e.g.. Wood et al., 1980 Dick, 1982 Delaney and Boyle, 1986). 

An experimental study at 350°C on the interaction between NaCl solution and graywacke which occurs widely in island arc geologic setting indicates that the final solution contains (0.6-0.7) ppm As (Bischoff et al., 1981). Analytical data on As concentration of hydrothermal solution at back-arc basins are few. Arsenic concentration of hydrothermal solution at Lau Basin is 6.0-8.2 ppm (Foquet et al., 1991). We can also estimate As concentration of hydrothermal solution based on the solubility data on orpiment and realgar because these As-bearing minerals are common in back-arc basin deposits (e.g., Okinawa Trough, Kuroko deposits). 

During the last three decades, many hydrothermal deposits have been discovered at midoceanic ridges, back-arc basins and subaerial active geothermal systems. Characteristic features of back-arc deposits at the western Pacific region (e.g., Okinawa Trough, Izu Ogasawara, North Fiji and Mariana deposits) are very similar to those of Kuroko deposits. 

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