Midocean ridge

F17 Deposition of marine sulfate via thermal vent reactions at midocean ridges 43 ... 

Jurassic—Cretaceous Besshi-type and Mn-Fe strata-bound deposits are present in Hidaka, Hokkaido (Fig. 1.2). Geochemical data and geological evidence all point to a midoceanic ridge environment of ore formation. values of Shimokawa Besshi-type... 

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. 

Gamo (1995) revealed based on the chemical and isotopic compositions of hydrothermal fluids from midocean ridges that the precipitation of minerals and interaction... 

Some applications of the coupled fluid flow-reaction model were carried out to the ore-forming process (e.g., Lichtner and Biino, 1992). However, a few attempts to understand quantitatively the precipitations of minerals from flowing supersaturated fluids in the submarine hydrothermal systems have been done (Wells and Ghiorso, 1991). Wells and Ghiorso (1991) discussed the silica behavior in midoceanic ridge hydrothermal system below the seafloor using a coupled fluid flow-reaction model. 

Amorphous silica and barite precipitate simultaneously from white smoker in midoceanic ridge hydrothermal system (Edmond et al., 1979). It is inferred that amorphous silica precipitates in the chimney at a later stage than sulfides and sulfates (anhydrite and barite) which constitute chimneys from which black smoker is emerging. 

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. 

However, it cannot be decided at present which processes (degree of seawater-rock interaction or mixing ratio of seawater, igneous water and meteoric water) are important for the generation of Kuroko ore fluids solely from the isotopic studies. But experimental and theoretical considerations on seawater-volcanic rocks interaction and origin of hydrothermal solution at midoceanic ridges suggest that Kuroko ore fluids can be produced dominantly by seawater-volcanic rock interaction. 

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. 

Bowers, T.S. and Taylor, H.P. Jr. (1985) An integrated chemical and stable isotope model of the origin of midocean ridge hot spring systems. J. Geophys. Res., 90, 12583-12606. 

Bowers, T.S., Von Damm, K. and Edmond, J.M. (1985) Chemical evolution of midocean ridge hot springs. Geochim. Cosmochim. Acta, 49, 2239-2252. 

Hajash, A. (1975) Hydrothermal processes along midocean ridges An experimental investigation. Contrib. Mineral. Petrol, S3, 205-226. 

Mitra, A. (1994) Geochemical implications of rare earth element pattern in hydrothermal fluids from midocean ridges. Geochim. Cosmochim. Acta, 58, 5105-5113. 

Shikazono, N. and Kusakabe, M. (1999) Mineralogical characteristics and formation mechanism of sulfate-sulfide chimneys from Kuroko area, Mariana trough and midocean ridges. Resource Geology Special Issue, 20, 1-12. 

Wells, J.T. and Ghiorso, M.S. (1991) Coupled fluid flow and reaction in midocean ridge hydrothermal system The behaviour of silica. Geochim. Cosmochim. Acta, 55, 2467-2481. 

Wolery, T.J. (1978) Some chemical aspects of hydrothermal processes at midoceanic ridges — A theoretical study I, Basalt-seawater reaction and chemical cycling between the oceanic crust and the oceans. II, Calculation of chemical equilibrium between aqueous solutions and minerals. Ph.D. Thesis, Northwestern U. 

Chemical analytical data are summarized in Table 2.13 and Table 2.14. The back-arc deposits are characterized by higher Pb, Ba Ag, Au, As and Sb contents than midoceanic ridge deposits. This difference is due to different mineralogy which is described below. 

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. 

A few REE data on hydrothermal solutions are available (Fig. 2.34). Chondrite normalized REE patterns of hydrothermal solutions from Vienna Wood, Pacmanus and Desmos, Manus Basin exhibit positive Eu anomaly and LREE enrichment are similar to midoceanic ridge solution and Kuroko ore fluids. This positive Eu anomaly (Fig. 2.35) may have been caused by the selective leaking of Eu due to the interaction of an ascending hydrothermal solution and footwall volcanic rocks (Gena et al., 2001). It is interesting to note that altered basaltic andesite has a negative Eu anomaly and this feature is the same as that found in the Kuroko mine area (Shikazono, 1999). 

Fig. 2.33. H2Saq concentration.s as a function of temperature for hot spring fluids at midocean ridges as a function of redox. Assuming AMPC (anhydrite-magnetite-pyrite-calcite) and PPM (pyrite-pyrrhotite) buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations indicate temperatures of approximately 370-385°C. Solid star Okinawa. (Modified after Seyfried and Ding, 1995.)...

Comparison of back-arc deposits with midoceanic ridge deposits... 

Chemical compositions of major elements (alkali, alkali earth elements. Si) in back-arc and midoceanic ridge hydrothermal solutions are not so different (Table 2.15). This is thought to be due to the effect of water-rock interaction. For example, Berndt et al. (1989) have shown that mQ i+ of midoceanic ridge hydrothermal fluids is controlled by anorthite-epidote equilibrium (Fig. 2.37). Figure 2.37 shows that /Mca2+/m + of back-arc hydrothermal fluids is also controlled by this equilibrium. 

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. 

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). 

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. 

Kawahata and Shikazono (1988) summarized S S of sulfides from midoceanic ridge deposits and hydrothermally altered rocks (Fig. 2.42). They calculated the variations in 5 " S of H2S and sulfur content of hydrothermally altered basalt as a function of water/rock ratio (in wt. ratio) due to seawater-basalt interaction at hydrothermal condition (Fig. 2.43) and showed that these variations can be explained by water/rock ratio. The geologic environments such as country and host rocks may affect S S variation of sulfides. For example, it is cited that a significant component of the sulfide sulfur could... 

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. 

Li values of Mariana hydrothermal solution (—8.5%o) are similar to average value (—9%c) of midoceanic ridge hydrothermal solution (Elderfield and Schultz, 1996). This value can be explained by the constant mixing ratio of basaltic Li (8 Li = —4%o) and seawater Li (S Li = —32.3%o) (Elderfield and Schultz, 1996). 

Bulk compositions of midoceanic ridge deposits and back-arc deposits are summarized in Tables 2.16 and 2.17. It is clear that midoceanic ridge ores contain higher amounts of Fe, Mn, Zn, Co, Ni, Se and Pt but lower amounts of Au, Ag, Cu, Pb, Ba, As and Sb compared with back-arc deposits (Tables 2.18 and 2.19). 

Although mineralogy is different in different site, the more abundant minerals in back-arc deposits than in midoceanic ridge deposits are barite, anhydrite, electrum, As-minerals (realgar, orpiment), tetrahedrite-tennantite and galena. 

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. 

Chemical composition of selected mineral assemblages from midocean ridge sulfide deposits (Hannington et al.. 1995). N number of analyses... 

Midoceanic ridge deposits are divided into volcanic-type and sedimentary-type (Gamo, 1995) or sediment-starved type or sediment-covered type (Scott, 1997). Metals concentrated to two types are distinct. In the sulfide deposits at Escanaba Trough,... 

Chemical composition of Kuroko ore and MORB (midoceanic ridge basalt) (logarithmic unit in wt%) (Shikazono, 1988)... 

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