Besshi-type deposits

Besshi-type deposits in comparison with Kuroko deposits and midoceanic ridge deposits... 

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. 

Paleozoic-Mesozoic volcanogenic stratiform Cu deposits in Japan which are generally metamorphosed have been called Besshi-type deposits (Kato, 1937) or bedded cupriferous iron sulfide deposits (Kanehira and Tatsumi, 1970). 

General descriptions of Besshi-type deposits have been given by Kanehira and Tatsumi (1970), Franklin et al. (1981) and Fox (1984). 

Besshi-type deposits in Japan are divided into Besshi-subtype and Hitachi-subtype based on geological, mineralogical and geochemical characteristics (Sato and Kase,... 

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. 

Sato and Kase (1996) summarized geological characteristics and inferred tectonic settings of the Besshi-type deposits as shown in Table 2.21. These geological characteristics are summarized below. 

Besshi-type deposits in Sanbagawa metamorphic terrain occur in the Minawa Formation which is composed of basic schist. Sometimes, they are associated with quartz schists. Probably, quartz has been originally formed from hydrothermal solution like siliceous ore in Kuroko deposits. Original rocks of basic schists are basaltic lava and hyaloclastics. Detailed geochemical investigation on the basic schists in the Sanbagawa... 

List of metallic minerals from the Besshi-type deposits (Kase and Yamamoto, 1988)... 

Pyrite of the Besshi-type deposits is characterized by high Co content ranging from n x 10 to n x 10 wt% (Itoh and Kanehira, 1967), which is quite a bit higher than that of Kuroko deposits. 

Kase and Horiuchi (1996) obtained a large number of analytical data on sphalerites from sixteen Besshi-type deposits, mainly at Besshi and its vicinity, Hitachi, and Shimokawa. They revealed that (1) the Mn/Zn and Co/Zn ratios of sphalerite may have markedly increased during contact metamorphism, while the Cd/Zn ratios remained unchanged (2) the Emco/ lwzn (2/n total dissolved concentration in ore fluids) and Emco/S/wzn ratios in the initial ore solutions responsible for the mineralizations at Besshi which was calculated ba.sed on the equilibrium fractionation model between hydrothermal solution and sphalerite and analytical data on sphalerites are quite similar to the ratios of hydrothermal solutions at EPR 21 °N (3) however, these ratios for the Hitachi solutions are very low and different from those of the Besshi-subtype solution. 

Recrystallization texture is also common for chalcopyrite and sphalerite. The occurrence of chalcopyrite and sphalerite filling the interstices between the pyrite cubes and chalcopyrite inclusion within them is considered to be due to the recrystallization of pyritic ores containing chalcopyrite and sphalerite (Yui, 1983). Yui (1983) suggested that these textures found in the Besshi-type deposits are useful in interpreting ore textures of the Kuroko ores, particularly their diagenetic recrystallization features because such textures are commonly observed in the Kuroko ores (Yui, 1983 Eldridge et al., 1983). 

Chalcopyrite disease in sphalerite which is common in Kuroko deposits has not been reported from the Besshi-type deposits. 

A large number of sulfur isotope data on the Besshi-type deposits are available, although the variation in individual deposit has not been studied well (Yamamoto et al., 1968, 1984a,b Kajiwara and Date, 1971 Miyake and Sasaki, 1980) (Fig. 2.52). The sulfur isotopic compositions of sulfides are different in different regions. 

Bulk chemical composition data of the Besshi-type deposits are summarized in Tables 2.23-2.25. 

Bulk chemical composition data of the Besshi-type deposits (Besshi), the seafloor sulfide deposits from the Mid-Atlantic Ridge at 23°N (MAR), the Galapagos Spreading Center at 86 W (GSC) and the East Pacific Rise at 21 N (EPR) (Kase and Yamamoto, 1988)... 

Fig. 2.53. Se/S atomic ratios of pyrite from the Besshi-type deposits, the deposits of the Fujimi and Fudotaki groups of the Hitachi mine and the Kuroko-type Iwami deposit (Kase and Yamamoto, 1988).

Cu, Pb, Zn, Fe) deposition in epithermal systems. Thus, it is interesting to summarize the gold distribution in Besshi-type deposits, considering the geochemical environment of gold deposition. 

It is noteworthy that bornite, chalcocite and tetrahedrite-tennantite which are common minerals in Kuroko deposits occur in gold bearing Besshi-type deposits. Although these minerals are considered to be secondary minerals, depositional environments of these minerals are characterized by higher /s, and foj conditions. It is also noteworthy that these deposits are rich in pyrite rather than pyrrhotite. Probably, Besshi-subtype deposits in Shikoku formed under the higher fo and /sj conditions than the deposits characterized by pyrrhotite (Maizuru, Hidaka, Kii, east Sanbagawa). Such typical Besshi-type deposits (Besshi-subtype deposits in Shikoku) are characterized by simple sulfide mineral assemblage (chalcopyrite, pyrite, small amounts of sphalerite). Inclusion of bornite in pyrite is also common in these deposits. 

Sato and Kase (1996) summarized lead i.sotope data of ores from 12 major Besshi-type deposits (11 Besshi subtypes and 1 Hitachi subtype) (Fig. 2.57). 

The geochemical environment of ore deposition for Besshi-type deposits is generally difficult to estimate because of the effect of metamorphism. 

Fig. 2.59. Metallogenic evolution of the Besshi-type deposits expressed on a log/s,-T diagram (Watanabe et al 1993).

Kase, K. (1986) Tellurian tennantite from the Besshi-type deposits in the Sambagawa metamorphic belt, Japan. Can. Mineral, 24, 399-404. 

Mizuta, T. (1988) Compositional homogenization of sphalerite in hydrothermal ore deposits of Japan by post-depositional diffusion processes with particular reference to metamorphosed Besshi-type deposits. Mining Geology, 38, 263-278. 

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. 

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