Epithermal vein deposits

Therefore, the wider time range from middle Miocene to present is considered below based on available age data on hydrothermal ore deposits (Kuroko deposits, epithermal vein deposits) and hydrothermal alteration in the mine areas in Northeast Japan. 

Before mentioning the characteristics of Kuroko and epithermal vein-type deposits in Japan, it is worthwhile to briefly describe the metallogeny, geology, geophysics, and tectonic situations of the Japanese Islands. 

Main hydrothermal ore deposit types of Neogene age that formed in and around the Japanese Islands are Kuroko deposits and epithermal vein-type deposits. This classification is based on the form of the deposits. 

Y sub-type (yellow ore type), and B sub-type (black ore type), according to Cu, Pb and Zn ratios (Fig. 1.9). However, the variation in the ratio is not wide, compared with epithermal vein-type deposits. Therefore, characteristic differences in each sub-type of Kuroko deposits are not discussed here. 

Major epithermal vein-type deposits in Japan are base-metal type and precious-metal type which are classified based on the ratios of base metals and Au and Ag which have been produced during the past (Table 1.2). 

Each deposit type is distributed in a different metallogenic province (Fig. 1.3) (Tat-sumi, 1970). Epithermal vein-type deposits occur in Miocene-Pliocene volcanic terrain. 

Compositional zoning in electrum grain is common (Shimazaki, 1974 Imai et al., 1981). The Ag content of rim of electrum grain is higher than that of core. Although Ag content varies widely, it is generally lower than that of epithermal vein-type deposits. 

Epithermal vein-type deposits can be divided into four types based on total metal produced and metal ratio base-metal type, precious-metal (Au, Ag) type, Sb-type and Hg-... 

The age of formation of epithermal vein-type deposits can be estimated from K-Ar ages of K-bearing minerals (adularia, sericite) in veins and in hydrothermal alteration zones nearby the veins. A large number of K-Ar age data have been accumulated since the work by Yamaoka and Ueda (1974) who reported K-Ar age data on adularia from Seigoshi Au-Ag (3.7 Ma) and Takadama Au-Ag deposits (8.4 Ma). Before their publication on the K-Ar ages of these deposits it was generally accepted that epithermal... 

Figure 1.62. Location of epithermal-type deposits in Japan (Shikazono and Shimizu, 1988a). 1 Green tuff and subaerial volcanic region of Tertiary/Quaternary ages, 2 Main Paleozoic/Mesozoic sedimentary terranes, 3 Main metamorphic terranes. TTL Tanakura tectonic line, ISTL Itoigawa-Shizuoka tectonic line, MTL Median tectonic line. Open circle epithermal Au-Ag vein-type deposits, solid circle epithermal base metal vein-type deposits, open triangle epithermal Au disseminated-type deposits.

The K-Ar age data are summarized in Figs. 1.64 and 1.65. It is obvious in these figures that (1) ages of formation of epithermal vein-type deposits vary widely from 15 to 1 Ma, but are mostly 6-1 Ma, (2) epithermal vein-type deposits have been formed... 

The differences in Zn/Fe ratio of tetrahedrite-tennantite in epithermal vein-type and Kuroko deposits and that of sphalerite in these deposits can be interpreted in terms of the following exchange reaction ... 

Among the epithermal vein-type deposits in Japan, four major types of hydrothermal alteration ean be diseriminated. They are (1) propylitie alteration, (2) potassic alteration, (3) intermediate argillic alteration, and (4) advaneed argillic alteration. The definitions of these types of alteration are mainly based on Meyer and Hemley (1967) and Rose and Burt (1979) who elassified the hydrothermal alteration in terms of alteration mineral assemblages. 

Meyer and Hemley, 1967). However, such lateral and concentric zonation has not been reported from the epithermal vein-type deposits in Japan. Montmorillonite-rich and silica-rich zones exist in the upper part of the Au-Ag veins such as the Seigoshi and Takadama (Nagasawaet al., 1981). 

In contrast to the hardly investigated lateral zonation around Japanese epithermal vein-type deposits, a few examples of vertical zonation are known. Potassic alteration grades upwards into intermediate argillic alteration in the wall rocks for the Toyoha (Okabe and Bamba, 1976), Ohe (Tsukada and Uno, 1980), Chitose (Hasegawa et al., 1981) and Kushikino (Imai, 1986). 

Numerous geochemical data (fluid inclusions, stable isotopes, minor elements) on the epithermal vein-type deposits in Japan are available and these data can be used to constrain geochemical environment of ore deposition (gas fugacity, temperature, chemical compositions of ore fluids, etc.) and origin of ore deposits. 

Salinities of inclusion fluids from epithermal vein-type deposits clearly indicate that the salinities of inclusion fluids from these types of deposits are distinctly different, that is, 20-2 NaCl equivalent wt% (base-metal vein-type deposits) and 0-3 wt% (Au-Ag vein-type deposits) (Shikazono, 1985b) (Table 1.13). Salinities of inclusion fluids from Kuroko deposits (0.5-5 wt% NaCl equivalent concentration) are between these two types of deposits. This kind of difference is observed in epithermal deposits in other countries (Hedenquist and Henley, 1985). 

Shikazono (1985b) summarized the assemblage and mode of occurrence of common gangue minerals from more than 70 Neogene epithermal vein-type deposits in Japan. 

Base metal-rich deposits including epithermal vein-type and Kuroko deposits occur in the Green tuff, while base metal-rich deposits are few in the Non-Green tuff region. 

Figure 1.115. 8 0-5 C of carbonates from epithermal vein-type deposits in Japan (Shikazono, 1999b).

In Fig. 1.43, 8 S and 8 0 of sulfates from epithermal vein-type deposits (Watanabe and Sakai, 1983) are plotted. These data show that 8 S (mostly from -f24%c to -t-37.8%o) and 8 0 of barite (0.1%oto - -18.7%o) from epithermal Au-Ag-Te vein-type deposits are higher than that of epithermal base-metal vein-type deposits (8 " S -t-16.0%o to -f24.6%c, 5 0 +2. %c to + 2A%c). 

Lead isotopic data on the epithermal deposits together with Kuroko deposits are plotted in Fig. 1.116 (Sato and Sasaki, 1973 Sato et al., 1973, 1981 Sato, 1975 Sasaki et al., 1982 Sasaki, 1987 Fehn et al., 1983). It is evident that lead isotopic compositions of epithermal vein ores are more scattered than Kuroko ores, although averaged values are similar to the Kuroko ores. This variation seems to be due to the difference in crustal materials underlying the ore deposits Lead isotopic compositions of different ore deposits which formed at different ages in the same district show the same values (Sasaki, 1974). 

As noted already, Ag content of electrum from the epithermal Au-Ag vein-type deposits is higher than that of the electrum from Kuroko deposits, and Ag/Au total production ratio of epithermal Au-Ag vein-type deposits (average 18) is lower than that of Kuroko deposits (average 76). Therefore, this relation is different from that found in the epithermal vein-type deposits. Ag/Au ratio of electrum may be controlled by the following reaction (Shikazono, 1981) ... 

It is thought from this reaction, that Au-rich electrum precipitates from ore fluids with high Cl concentration and low pH. Therefore, it is considered that different Cl concentration and pH are important factors causing different relationship between Ag content of electrum and Ag/Au total production ratio of Kuroko deposits and epithermal vein-type deposits. 

As noted already, epithermal vein-type deposits are classified primarily on the basis of their major ore-metals (Cu, Pb, Zn, Mn, Au and Ag) into the gold-silver-type and the base-metal-type. Major and accessory ore-metals from major vein-type deposits in Japan were examined in order to assess the possible differences in the metal ratios in these two types of deposits (Shikazono and Shimizu, 1992). Characteristic major ore-metals are Au, Ag, Te, Se and Cu for the Au-Ag deposits, and Pb, Zn, Mn, Cu and Ag for the base-metal deposits (Shikazono, 1986). Accessary metals are Cd, Hg, Tl, Sb and As for the Au-Ag deposits and In, Ga, Bi, As, Sb, W and Sn for the base-metal deposits (Table 1.22, Shikazono and Shimizu, 1992). Minerals containing Cu, Ag, Sb and As are common in both types of deposits. They are thus not included in Table 1.22. 

These correlations mean that the HSAB principle could be a useful approach to evaluate the geochemical behavior of metals and ligands in ore fluids responsible for the formation of the epithermal vein-type deposits. Among the ligands in the ore fluids, HS" and H2S are the most likely to form complexes with the metals concentrated in the gold-silver deposits (e.g., Au, Ag, Cu, Hg, Tl, Cd), whereas Cl prefers to form complexes with the metals concentrated in the base-metal deposits (e.g., Pb, Zn, Mn, Fe, Cu, and Sn) (Crerar et al., 1985). 

However, in contrast to these geologic and tectonic studies, very few studies on the relationship between tectonics and hydrothermal system in Neogene age have been carried out. Therefore, these studies are briefly summarized and then the relationship between geologic and tectonic evolution and evolution of hydrothermal system associated with the mineralizations (Kuroko deposits, epithermal veins) are considered below. 

Minor elements associated with the vein-type and Kuroko deposits are different. Characteristic minor elements concentrated to the ore deposits are Se, Te, Hg, As, Sb and Bi in the Au-Ag vein-type deposits, Ag, Bi, As, Sb, Sn, W and Mo in the base metal vein-type deposits, and Au, Ag, Sb, As, Mo and Bi in the Kuroko deposits. This difference in minor elements is consistent with that found in the other epithermal vein-type deposits in Japan (Shikazono and Shimizu, 1992). 

Uyeda and Kanamori (1979) divided mode of subduction into two types Mariana-type characterized steep subduction and Chilean-type characterized by gentle subduction estimated from the dip of Benioff-Wadati zones (Fig. 1.161). The geological phenomena associated with these subductions are shown in Fig. 1.162. It is inferred that the change in mode of subduction from Mariana-type to Chilean-type occurred at ca. 5 Ma in Northeast Honshu. Kuroko deposits are associated with Mariana-type, whereas epithermal vein-type deposits (particularly Au-Ag deposits) with Chilean-type. 

A frequency histogram of the Ag content of electrum from epithermal gold-silver vein-type deposits and the Tsugu deposit (Fig. 1.174) clearly indicates that the Au/Ag of electrum from the Tsugu deposit is higher than that from epithermal vein-type deposits. 

Hayashi, K., Maruyama, T. and Satoh, H. (2000b) Precipitation of gold in a low-sulfidation epithermal gold deposit Insights from a submillimeter scale oxygen isotope analysis of vein quartz. Econ. Geol, 96, 211-216. 

Izawa, E., Urashima, Y, Ibaraki, K., Suzuki, R., Yokoyama, T., Kawasaki, K., Koga, A. and Taguchi, S. (1990) The Hishikari gold deposit high-grade epithermal veins in Quaternary volcanics, southern Kyushu,... 

Shikazono, N. (1988b) Hydrothermal alteration associated with epithermal vein-type deposits in Japan a review. Mining Geology Special Issue, 12, 47-55. 

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