Midoceanic ridge deposits

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. 

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. 

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

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... 

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. 

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,... 

The characteristic features of Besshi-type deposits are similar to those of midoceanic ridge deposits and back-arc deposits. The comparison among these features is given below. 

Sato and Kase (1996) summarized these characteristic features and divided Besshi-subtype into Group A (sediment-barren type) and Group B (sediment-covered type) (Table 2.21). These characteristic features of these Besshi-type deposits in Japan, mainly focusing on the Besshi-subtype and comparing of these features with those of Kuroko and midoceanic ridge deposits are described below. 

In Chapter 2, a geochemical, geological and mineralogical summary of active subaerial and submarine back-arc basin hydrothermal systems and mineralizations is given. The characteristic features of above-fossil and active subaerial and submarine hydrothermal systems are compared with fossil hydrothermal systems (epithermal vein-type and Kuroko deposits), and the causes for the differences in the characteristic features are considered. Characteristic features of Paleozoic-Mesozoic volcanogenic stratiform Cu deposits (Besshi-type deposits) are compared with those of midoceanic ridge deposits and Kuroko deposits. 

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... 

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

Back-arc deposits are generally similar to sediment-covered deposits at midoceanic ridge. For example, Pb, Ba and As are enriched in these deposits. 

Recently, chimney-like ores have been described from Kuroko deposits (Matsuku-ma, 1989 Shimazaki and Horikoshi, 1990 Shikazono and Kusakabe, 1999). The formation mechanism of chimney from the hydrothermal ore deposits at midoceanic ridges was clarified. Thus, these studies constrain the formation mechanism of Kuroko ore deposits. 

The characteristic features of chimney-like ores and chimney from Kuroko deposits and Mariana Trough are summarized and mineralogical and geochemical characteristics of Kuroko and Mariana chimneys and those of the midoceanic ridge chimneys are compared below. 

Average bulk compositions of samples from seafloor sulfide deposits at sediment-starved midocean ridges in host basalts... 

It is shown in Fig. 2.57 that the lead isotopic variation of the Besshi-subtype is similar to that of midoceanic ridge basalt, suggesting the lead in the Besshi-subtype was derived from mantle. The data from the Shimokawa, and Yanahara deposits (Group B) are slightly more radiogenic than Group A, suggesting that crustal lead was involved in the formation of the Shimokawa deposit, and lead isotopic values for the Shimokawa and Yanahara plot between MORB and Cretaceous-Tertiary deposits in Japan (Kuroko, skarn, vein-type deposits). 

Hg concentration in hydrothermal solution from back-arc basins and midoceanic ridges has not been determined. Experimental study on graywacke-water interaction suggests that the hydrothermal solution interacted with graywacke contains n x 10 ppm Hg (Bischoff et al., 1981). Cinnabar and metacinnabar are not common but were reported from several Kuroko deposits (Urabe, 1974). From the solubility data on cinnabar and metacinnabar (Barnes and Czamanske, 1967), we can place a limit on the Hg concentration of ore fluids to be n x 10 ppm. Using n x 10 ppm concentration and seawater cycling rate at back-arc basins, hydrothermal Hg flux from back-arc... 

The average chemical compositions of Kuroko ores and those of back-arc deposits suggest that Hg, As, Sb, T1 and Ba are concentrated to the ore fluids responsible for the Kuroko and back-arc deposits, suggesting that these fluxes from back-arc basins are high compared with midoceanic ridge fluxes. 

Barne.s, H.L. and Czamanske, G.K. (1967) Solubilities and transport of ore minerals. In Barnes, H.L. (ed.). Geochemistry of Hydrothermal Ore Deposits. New York Holt, Rinehart and Win.ston, pp. 334-381. Berndt, M.E., Seyfried, W.E. Jr. and Janeckey, D.R. (1989) Plagiocla.se and epidote buffering of cation ratios in midocean ridge hydrothermal fluids Experimental results in and near the supercritical region. Geochim. Cosmochim. Acta, 53, 2283-2300. 

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. 

Fluxes of volatile elements (CO2, S, As) and other elements (Hg, Mn, Ba) due to hydrothermal activities at back-arc basins were calculated. Probably the hydrothermal flux of minor elements concentrated in Kuroko deposits (Sb, Tl, etc.) is large compared with those from midoceanic ridges. CO2 flux from back-arc basins is estimated to be large compared with that from midoceanic ridges. 

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