Vein-type deposits

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. 

Precious metal vein-type deposits include enrichments in Au, Ag, Hg, Te, Se, Sb, As, S, and Bi. Base metal vein-type deposits contain Pb, Zn, Mn, Ag and Cu, whereas Kuroko deposits are enriched in Cu, Pb, Au, Ag and Ba. 

These different sites of hydrothermal and ore-forming activity may have resulted from the mode of subduction of the Pacific Plate. Mariana-type subduction (characterized by a steep angle of subduction and back-arc basin formation Uyeda and Kanamori, 1979) during middle Miocene caused WNW-ESE extension, submarine hydrothermal activity, thick accumulation of bimodal (basaltic and dacitic) volcanic activity (Green tuff) and Kuroko-type formation (Shikazono and Shimizu, 1993). Plio-Pleistocene Chilean-type subduction (shallow-dipping subduction zone, E-W compression Uyeda and Kanamori, 1979) and oblique subduction of the Pacific Plate beneath the North American Plate led to uplift and expansion of land area, subaerial hydrothermal activity accompanied by meteoric water circulation, subaerial andesitic volcanic activity and formation of vein-type deposits. 

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. 

Kuroko-type deposit Vein-type deposits Sulfur deposits... 

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. 

Kuroko deposits are strata-bound and massive in form (Fig. 1.7) and syngenetically formed on the seafloor and/or sub-seafloor environment. Vein-type deposits are fissure-filling and epigenetically formed (Fig. 1.8). 

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). 

Base-metal vein-type deposits may be divided into Pb-Zn-type and Cu-type (Otsu and Harada, 1963). However, this sub-classification is not considered here for simplicity of discussion. 

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. 

During the Miocene age, polymetallic vein-type (xenothermal-type, subvolcanic-type) and gold-quartz vein-type (mesothermal-hypothermal-type) mineralizations occurred mainly in middle to western part of Japan. They are described in section 1.6.1. In section 1.6.2, Hg and Sb vein-type deposits are described. 

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. 

Polymetallic vein-type deposits occur in middle Miocene volcanic terrain in central and western Japan. 

Figure 1.16. Chemical composition of tetrahedrite-tennantite (Shikazono and Kouda, 1979). A Au-Ag vein-type deposits, B Kuroko deposits, C Taishu-Shigekuma Pb-Zn vein-type deposits, D Skam deposits (Kamioka).

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. 

Figure 1.43. Relationship between 5 0 and values of barite from some Kuroko and vein-type deposits. Abbreviations are HN, Hirano NK, Nagaki AK, Akakura YU, Yuno.sawa SD, Sado KY, Katsuyama TN, Teine OS, Osarizawa MZ, Mitsuzawa HT, Hata OE, Oe AI, Akaishi KN, Kohinata MT, Miyatamata KZ, Karuizawa FN, Funauchi KO, Koyama IK, Inakuraishi and OG, Ogoya. K.S.R. indicates Kuroko sulfate region (Watanabe and Sakai, 1983).

Lead isotopic data on Kuroko deposits, vein-type deposits in Honshu and volcanic rocks are summarized and plotted in Fig. 1.44 (Fehn et al., 1983). 

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. 

Many Cu-Pb-Zn vein-type deposits are hosted by organic sedimentary rocks such as shale and mud.stone but almost all Au-Ag deposits occur in altered volcanic rocks. This difference in the host rocks affects the chemical features of ore fluids (/02. /s2> /CO2) (section 1.4.4). 

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... 

Figure 1.63. Distribution of vein-type deposits in a part of the inner belt of Northeast Japan. A the sedimentary members of the Onnagawa stage, B the sedimentary members of the Nishikurosawa stage (Nakamura and Hunahashi, 1970).

The ore deposits can be classed into two types based on the types of associated metals Au-Ag rich deposits (Type A) from which An and Ag are produced as main products, and base metal (Cu, Pb, Zn, Mn, (Sn), (W), (Bi), (Mo), (Sb)) rich deposits (Type B) from which Au and Ag are recovered as byproducts. The deposits are associated with felsic and intermediate volcanic rocks but generally not with felsic plutonic rocks. In Japan Au-Ag deposits associated with granitic rocks (e.g., Au-Ag vein-type deposits in Kitakami) occur commonly. However, these plutonic-type deposits are not described here. 

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

Base metal vein-type deposits (Type 1-B) ... 

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). 

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

FeCOs) of Mn-carbonates varies widely in a range of 10 -10 mole fraction. Siderite from Au-Ag vein-type deposits (Ohmori) contains appreciable amounts of zinc (0.8-5.8 wt% as ZnO) (Shikazono, 1977b). 

---