Back arc basins

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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

The chemistry of hydrothermal solutions from midoceanic ridges has been reasonably explained by the effect of buffering by alteration minerals (Seyfried, 1987 Bemdt et al., 1989). Therefore, it might be worth explaining the chemical composition of hydrothermal solutions from back-arc basins in terms of chemical equilibrium between hydrothermal solutions and alteration minerals. 

Submarine hydrothermal fluid end-member data observed at various plate boundary regions (end-member values are those obtained by extrapolation to an assumed value of zero magnesium, except for the values at site 2). BAB back-arc basin (Game, 1995)... 

The flK-f of plagioclase from MORB is generally lower than that of back-arc basin igneous rocks. Therefore, the higher m +/mY + of midoceanic ridge fluids compared with back-arc basin fluids could be explained in terms of Na-feldspar/K-feldspar/hydro-thermal fluids equilibrium. 

The differences in base metal concentrations in the two types of hydrothermal solution are unclear, probably because of scarcity of data. However, it seems obvious that wpe/tWMn ratio of midoceanic ridge hydrothermal solution is higher than back-arc hydrothermal solution (Table 2.15). This may be due to the differences in Fe and Mn contents of volcanic rocks at back-arc basin and midoceanic ridges and temperature of fluids. 

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),... 

Gena et al. (2001) reported advanced argillic alteration of basaltic andesite from the Desmos caldera, Manus back-arc basin which was caused by interaction of hot acid hydrothermal fluid originated from a mixing of magmatic gas and seawater. It is noteworthy that the acid alteration is found in back-arc basins (Manus, Kuroko area) but not in midoceanic ridges. 

Isotope data on hydrothermal solution from back-arc basins and midoceanic ridges are summarized in Table 2.15 (Gamo, 1995 Scott, 1997). 

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. 

As discussed in previous chapters, gold deposition in epithermal systems and back-arc basins (Kuroko deposits) occurs in relatively higher /oj conditions than base metal... 

Gena, K., Mizuta, T., Ishiyama, D. and Urabe, T. (2001) Acid-sulphate type alteration and mineralization in the Desmos Caldera, Manus Back-arc Basin, Papua New Guinea. Resource Geology, 51, 31 14. 

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