Epithermal base-metal deposits

S values of epithermal base metal deposits are higher than those of the epithermal Au-Ag deposits and range mostly from - -3%c to -f-7%o (Fig. 1.111). Although most of 8 " S values for base-metal deposits lie in this range, 8- " S of composite sample of sulfides from the Motokura Cu-Pb-Zn deposits, Ohmori Cu-Ag deposits, Hosokura Pb-Zn deposits, Sasayama Cu-Pb-Zn deposits and Imai-lshizaki Cu-Pb-Zn deposits are low, that is, -1-0.1, -1-1.8, -1-2.2, —0.9 and —2.1%o, respectively (Shikazono, 1987b Shikazono and Shimizu, 1993). 

These data could be explained by the sulfur of barite from epithermal Au-Ag-Te deposits came both from volcanic gas (SO2) and marine sulfate, but that of epithermal base-metal deposits came from marine sulfate and oxidation of H2S. 

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. 

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.

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).

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).

Figure 1.89 shows typical range of /sj and temperature for epithermal base-metal vein-type and Au-Ag vein-type deposits. It is noteworthy that the ranges of /sj for epithermal Au-Ag, epithermal Au-bearing base-metal, and epithermal Au-free base-metal vein-type deposits are different, while temperatures are not different. 

As mentioned already, small amounts of electrum occur in epithermal base-metal vein-type deposits. Electrum is not observed in the epithermal base-metal vein-type deposits in which pyrrhotite occurs (e.g., Toyoha-Soya, Oizumi, and Hosokukura Pb-Zn deposits). However, electrum is found in epithermal base-metal vein-type deposits in which hematite is commonly observed (e.g., Osarizawa and Ani Cu-Pb-Zn deposits). This indicates that electrum precipitates in relatively high /s2 and /oj condition. 

The /02 of ore fluids responsible for the epithermal base-metal veins might have been in the predominance field of reduced sulfur species because (1) pyrrhotite is occasionally found in these deposits, (2) selenium content of argentite is very low and (3) H2S is dominant in the present-day epithermal base-metal fluids. Implication of selenium content of sulfides will be considered later. Barite is sometimes found in the late-stage of mineralization. Thus, it is likely that /oj of barite stage lies in the predominance field of oxidized sulfur species. 

If OTci- and pH are assumed to be 1-5 molal and lower than that for K-feldspar-K-mica-quartz equilibrium, respectively, EAu/EAg is estimated to be considerably lower than 0.1. Therefore, EAu/EAg of ore fluids for epithermal base-metal vein-type deposits is thought to be considerably lower than 0.1. 

SD and S O. 8D and of the ore fluids responsible for epithermal Au-Ag and base-metal vein-type deposits in Japan have been estimated from analyses of fluid inclusions (Hattori and Sakai, 1979) and minerals (Watanabe et al., 1976). These data are shown in Fig. 1.103. 8D values of ore fluids for epithermal Au-Ag vein-type deposits are similar to those of present-day meteoric water values. 8D values of epithermal ore fluids for base-metal vein-type deposits are slightly higher than those of epithermal Au-Ag vein-type deposits. This may be due to the boiling of epithermal base-metal ore fluids and involvement of seawater. 

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). 

Thus, a pH increase could be favorable for the deposition of Ag. This may imply that the Ag content of electrum is high when substantial amounts of CO2 loss occur. The CO2 concentration of ore fluids responsible for epithermal base-metal vein-type deposits... 

Figure 1.124. Ag/Au total production ratio from each mine and Ag content of electrum. Solid circle epithermal Au-Ag vein-type deposits. Open circle epithermal base metal vein-type deposits. Solid square hypo/mesothermal polymetallic vein-type deposits. Open. square epithermal Au disseminated-type deposits. I Tada, 2 Toyoha, 3 Omidani, 4 Innai, 5 Ikuno, Oe-Inakuraishi, 7 Nebazawa, 8 Kawazu, 9 Todoroki, 10 Yatani, 11 Seigoshi, 12 Sado, 13 Takeno, 14 , awaji, 15 Yugashima, 16 Takadama, 17 Handa, 18 Konomai, 19 Sakoshi-Odomari, 20 Toi, 21 Sanru, 22 Arakawa, 23 Taio, 24 Chitose, 25 Hokuryu, 26 Okuchi, 27 Fuke, 28 Yamagano, 29 Akeshi, 30 Kasuga (Shikazono, 1986).

Origin of sulfide sulfur of epithermal base-metal veins is thought to be same as that of Kuroko deposits because average 8 S value of base-metal vein-type deposits is - -4.7%o which is identical to that of Kuroko deposits (- -4.6%o) (Shikazono, 1987b). Namely, sulfide sulfur of base-metal veins came from igneous rocks, sulfate of trapped seawater in marine sedimentary rocks, calcium sulfate (anhydrite, gypsum) and pyrite. 8 S of sulfide sulfur of epithermal base-metal vein-type deposits can be explained by the interaction of seawater (or evolved seawater) with volcanic rocks. 

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). 

As for the mineralization at the middle Miocene age in the Northeast Japan, Kuroko and epithermal base-metal veins have been formed. No enrichment of Sn and W is found in these deposits. 

The ore fluids responsible for epithermal base-metal vein-type deposits were generated predominantly by meteoric water-rock interaction at elevated temperatures (200-350°C). Fossil seawater in marine sediments was also involved in the ore fluids responsible for this type of deposits. Epithermal precious metal ore fluids were generated by meteoric water-rock interaction at 150-250°C. Small amounts of seawater sulfate were involved in the ore fluids responsible for epithermal precious metal vein-type deposits occurring in Green tuff region (submarine volcanic and sedimentary rocks). 

Base-metal deposits, 428, 439 Epithermal, 396, 399 Massive sulphide, 403-405, 422 Elura, Australia, 417, 418, 431, 432... 

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).

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-... 

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). 

Filling temperature and NaCl eq. concentration of fluid inclusions from epithermal gold-silver and base-metal vein-type deposits (Shikazono and Shimizu, 1992)... 

The difference in selenium content of acanthite from the different types of ore deposits can, of course, also be explained by the difference in ESe/ES in the ore fluids i.e., the ore fluids responsible for the formation of epithermal Au-Ag vein-type deposits may have had a higher ESe/ES ratio than that for epithermal Pb-Zn vein-type deposits. It is likely that considerable amounts of sulfur were derived from marine rocks (Green tuff) and were incorporated into ore fluids for base-metal veins. 

Figure 1.107 shows the frequency of 8 C of carbonates from epithermal Au-Ag vein-type deposits and that from base-metal vein-type deposits. The carbonates are divided into two types type A and type B. Type A is characterized by (1) abundant occurrence in each deposit (2) coexistence with sulfide minerals and (3) large grain size. Main carbonate minerals are rhodochrosite and Mn calcite, whereas calcite is the main carbonate mineral for type B. Mn-carbonates of type A occur in Pb-Zn-Mn vein-type deposits. Type B is characterized by (1) poor amounts in each deposit (2) coexistence... 

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. 

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