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 characteristic features of base-metal and precious-metal types are summarized below and those of Sb- and Hg-type deposits are described in section 1.7. 

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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. 

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... 

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... 

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. 

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). 

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). 

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. 

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

Shikazono, N. (1989) Oxygen and carbon isotopic compositions of carbonates from the Neogene epithermal vein-type deposits of Japan Implication for evolution of terrestrial geothermal activity. Chem. Geol, 76, 239-247. 

Comparison of active geothermal systems with epithermal vein-type deposits... 

Close similarities between epithermal vein-type deposits and active geothermal systems have been cited by various authors (e.g.. White, 1955, 1981 Henley and Ellis, 1983 Shikazono, 1985a,b Izawa and Aoki, 1991). 

In this section (2.2), geochemical, mineralogical and geological characteristics of epithermal vein-type deposits summarized in section 1.4 will be compared with subaerial active geothermal systems associated with base metal and Au-Ag mineralizations mentioned in sections 2.1.1 and 2.1.2. 

It has been pointed out by Giggenbach (1981) on the basis of thermochemical calculations that epidote occurs at higher temperatures of at least more than 240°C, and K-feldspar occurs at restricted temperatures, i.e. below ca. 250°C, in active geothermal systems. These theoretical results seem to be consistent with those observed in epithermal vein-type deposits in Japan. 

This spatial difference is consistent with the distribution of hydrothermal deposits of middle Miocene (Kuroko and epithermal vein-type deposits in Japan). As mentioned in section 1.3, Kuroko deposits formed under the submarine environment, while polymetallic... 

This submarine vs. subaerial hypothesis for the origin of the two types of deposits (Kuroko deposits, epithermal vein-type deposits) can reasonably explain the difference in metals enriched into the deposits by HSAB (hard-soft acids and bases) principle proposed by Pearson (1963) (Shikazono and Shimizu, 1992). Relatively hard elements (base metal elements such as Cu, Pb, Zn, Mn, Fe) are extracted by chloride-rich fluids of seawater origin, while soft elements (Au, Ag, Hg, Tl, etc.) are not. Hard elements tend to form chloro complexes in the chloride-rich fluid, while soft elements form the complexes in H2S-rich and chloride-poor fluids. Cl in ore fluids is thought to have been derived from seawater trapped in the submarine volcanic and sedimentary rocks. 

In and nearby the Japanese Islands, which are situated close to plate boundaries (Pacific, Philippine, North American and Asian plates), many hydrothermal ore deposits were formed during the Miocene-present eras. Major deposit types are Kuroko and epithermal vein-types. Epithermal vein-type deposits are classified into precious (Au, Ag)-types and base-metal (Cu, Pb, Zn, Fe, Mn, Ag)-types. 

It is suggested that the mode of subduction of the Pacific Plate since the middle Miocene age related to Jackson s episode, hence oscillation of direction of lateral movement of Pacific plate. Synchronized igneous and hydrothermal activities and Jackson s episode indicate that the formations and characteristics of hydrothermal ore deposits (Kuroko and epithermal vein-type deposits) are largely influenced by plate tectonics (mode of subduction, direction of plate movement, etc.). For example, sulfur isotopic composition of sulfides is not controlled by /o and pH, but by of... 

Representative types of Neogene deposits include Kuroko and epithermal vein-type deposits. 

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