Epithermal-type

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.

This mechanism as a main cause for epithermal-type Au deposition is supported by sulfur isotopic data on sulfides. Shikazono and Shimazaki (1985) determined sulfur isotopic compositions of sulfide minerals from the Zn-Pb and Au-Ag veins of the Yatani deposits which occur in the Green tuff region. The values for Zn-Pb veins and Au-Ag veins are ca. +0.5%o to -f4.5%o and ca. -l-3%o to - -6%c, respectively (Fig. 1.126). This difference in of Zn-Pb veins and Au-Ag veins is difficult to explain by the equilibrium isotopic fractionation between aqueous reduced sulfur species and oxidized sulfur species at the site of ore deposition. The non-equilibrium rapid mixing of H2S-rich fluid (deep fluid) with SO -rich acid fluid (shallow fluid) is the most likely process for the cause of this difference (Fig. 1.127). This fluids mixing can also explain the higher oxidation state of Au-Ag ore fluid and lower oxidation state of Zn-Pb ore fluid. Deposition of gold occurs by this mechanism but not by oxidation of H2S-rich fluid. 

Sudo, T. (1954) Types of clay minerals closely a.s.sociated with metalliferous ores of the epithermal type. Sci. Repts. Tokyo Kyoiku Daigaku, Sen C, 3, 173-197. 

Fe-minerals and sulfides are common in hydrothermal ore deposits. If we know these mineral species, chemical compositions of minerals and temperature, fo2-pH and Eh-pH conditions can be restricted. As an example of the estimates of foj and pH, vein-type deposits in Japan are taken into account. The vein-type deposits considered here include precious metal (Au, Ag) and base metal (Cu, Pb, Zn, Fe, Mn) deposits and are classed as epithermal-type (Shikazono 2003). The oxidation-reductimi conditirm can be estimated based on mineral assemblages and chemical compositions of minerals. Sphalerite is the most conunon sulfide mineral in these deposits. It contains iron and is zinc-iron solid solutimi (Zni xFexS). The relationship among FeS content of sphalerite, foj, pH and temperature is obtained from the reaction. 

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. 

Large epithermal gold vein-type deposits occur at major arc-arc junctions (Figs. 1.5 and 1.6) specifically, Chishima (Kurile)-Northeast Honshu, Northeast Honshu-Izu-Bonin and Southwest Honshu-Ryukyu. This may result from hydrothermal activities and mineralizations caused by intense volcanism at the arc-arc junctions. 

Gold-rich silica precipitates at the Osorezan volcano, which is located in the most northern part of Honshu, have features very similar to epithermal Te-bearing gold vein-type deposits of the Plio-Pleistocene. 

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). 

Figure 1.6. Di.stribution and temporal and spatial relationship of late Cenozoic gold deposits in the Japanese Islands. 1 Quartz vein-type gold deposits with little to no base metals. 2 Gold-silver deposits with abundant base metals. 3 Distribution boundary of gold deposits formed during the Miocene. 4 Location of Plio-Pleistocene gold deposits at the actual island arc junctions. 5 Location of Plio-Pleistocene gold deposits in front of the actual island arc junctions. Numbers in the figure are K-Ar ages of epithermal Au-Ag veins (Kubota, 1994).

Recently, epithermal gold deposits were divided into several types based on gangue minerals, and physicochemical environment of ore deposition (pH, H2S concentration of ore fluids). They are hot spring-type (Silberman, 1982 Berger, 1983a Berger and Eimon,... 

Most of epithermal precious-metal vein-type deposits in Japan can be classed as adularia-sericite-type, and low sulfidation-type. Very few hot spring-type deposits (quartz-alunite-type, high sulfidation-type) are found in the Japanese Islands. A summary of various characteristic features of adularia-sericite type (low sulfidation-type) is given mainly in section 1.4. 

Shikazono et al. (1990) divided epithermal precious-metal vein-type deposits into Te-bearing and Se-bearing deposits. As will be considered later, Te-bearing deposits are regarded as intermediate-type of adularia-sericite-type and hot spring-type. The distinction between these two types of deposits is discussed in section 1.4. 

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-... 

Epithermal base-metal vein-type deposits are distributed in the Green tuff region (Southwest Hokkaido, Northeast Honshu) (Fig. 1.62). The distribution area of this type of deposits is nearly same as that of Kuroko deposits. For example, large deposits (Osarizawa Cu-(Au) Ani Cu-Au Hosokura Pb-Zn deposits) occur in Northeast Honshu, but are more widely distributed in the Green tuff region than Kuroko 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... 

Epithermal base-metal vein-type deposits are characterized by the abundant occurrence of sulfides (chalcopyrite, pyrite, sphalerite, galena), and a scarcity of Au-... 

A large number of analytical data on chemical composition of sphalerite are available (Shikazono, 1974a Watanabe and Soeda, 1981). The FeS content of sphalerite from epithermal base-metal vein-type deposits varies widely mostly from 1 to 20 mol% (Fig. 1.68). 

Figure 1.68. Iron content of sphalerite from Kuroko, epithermal Au-Ag vein-type and epithermal base metal vein-type deposits (Shikazono, 1977a).

The Ag content of electrum from epithermal Au-Ag vein-type deposits is mostly in a range of 40-70 atomic% (Fig. 1.69). 

Very few data on the chemical composition of electrum from epithermal base-metal vein-type deposits are available. However, it is evident that the Ag content varies widely (Fig. 1.71). The Ag content of electrum from the Osarizawa and Okuyama Cu deposits is low N g (Ag atomic%) = 8.6-17.7), while the Ag content of electrum from Pb-Zn-Mn deposits (Toyoha, Oe, Inakuraishi, and Imai-Ishizaki) is high NAg = 60-80). Motomura (1986) reported that the Ag content of electrum from these deposits is higher than that from epithermal Au-Ag vein-type deposits. The geochemical implication of the Ag content of electrum is discussed in section 1.4.4. 

Chemical compositions of tetrahedrite-tennantite from epithermal base-metal vein-type deposits are characterized by (1) wide compositional variations, and (2) higher Zn and Sb contents and Ag and lower Fe, As, and Cu contents, compared with Kuroko deposits (Shikazono and Kouda, 1979). 

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 ... 

Figure 1.71. Frequency histogram for the Ag content of electrum from epithermal base metal vein-type deposits in Japan (Shikazono and Shimizu, 1988a).

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. 

The area of the potassic alteration is not wide, compared with the propylitically altered area. The width of potassic alteration zone away from the vein is generally within several tens of meters (ca. 50 m) (Shikazono and Aoki, 1981 Imai, 1986). The potassic alteration is usually found in the intermediate vicinity of the vein in the epithermal deposits in Japan. Thus it is evident that this type of alteration occurs genetically related to the ore deposition. 

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). 

In the Izu Peninsula, located in the middle part of Honshu, more than 20 epithermal Au-Ag vein-type deposits have been mined. Large Au-Ag mines are located in the western part of the peninsula. The Seigoshi mine is the largest one. The country... 

Figure 1.73. Distribution of epithermal Au-Ag vein-type deposits, propylitic and advanced argillic alterations and intrusive rocks of diorite prophyry (Shikazono, 1985a).

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