Hypo/mesothermal Au vein-type deposits in Japan

Usually, the Ag content is low (Fig. 3-F). Especially, the Ag content of electrum from deposits occurring in metamorphic rocks (Suwa, Hashidate, and Kinkei) is very low. The Ag content of electrum from deposits occurring in sedimentary rocks (Koma, Yamizo, and Kitakami districts) is also low, but higher than that from the deposits in metamorphic terrane. The former deposits are characterized by a paucity ofsulfide minerals. The Ag content of electrum from the Kohoku deposit, which occurs in acidic volcanic rocks, varies widely (N_Ag=14.1 to 61.7) (Nedachi, 1974). The Ag content of electrum in the deposits (e.g., Kohoku and Saigane) in which sulfide minerals commonly occur is relatively high.

Hypo/mesothermal polymetallic vein-type deposits in Japan

Although numbers of analysis are limited, the Ag content varies widely (Fig. 3-G). The Ag content for the Tsugu and Ashiyasu deposits is low (N_Ag=10.4 to 19.3), but for the other deposits (e.g., Ikuno), it is relatively high (N_Ag=22.3 to 83.0). Native silver occurs in the Ikuno deposit, but not in the others.

Hypo/mesothermal vein-type deposits in the Korean Peninsula

The Ag content of electrum from deposits of this type is widely variable, ranging from 40 to 100 atomic % (Fig. 3-H). As summarized in Table 8, the Ag content of electrumfrom the sulfide-rich deposits is high, but the Ag content of electrum from the sulfide-poor deposits is relatively low. Park et al. (1985) and Shelton (1986) suggested that the Ag content of electrum from ore deposits associated with Jurassic granitic rocks is lower than that from ore deposits associated with Cretaceous granitic rocks.

Kuroko-type deposits

The Ag content of electrum from deposits of this type varies widely, from 4.7 to 89.4 atomic % (Fig. 3-I). Electrum with low Ag content occurs in siliceous and yellow ores. For instance, electrum in siliceous ore from. the Nurukawa deposit contains about 20 Ag atomic %. Electrum with high Ag content occurs in black ore. Native silver is found in borniterich black ore (Matsukuma and Yui, 1979; Matsukuma, 1985).

Bedded cupriferous iron sulfide-type deposits

Data on electrum from deposits of this type are available only from. Shimokawa (Maeda et al., 1981). The Ag content ranges from 22.2 to 71.6 atomic % (Fig. 3-J). Maeda et al. (1981) clarified that the Ag content of electrum from this deposit is similar to that from the Kuroko-type deposits.

Skarn-type deposits in Japan

The Ag content of electrum varies widely ranging from 6.7 to 77.0 atomic % (Fig. 3-K). The Ag content for the Kamaishi Cu-Fe deposit is low (6.7 to 8.2 atomic %). On the other hand, the Ag content of electrum from the Tsumo Cu-Zn-Pb-Fe deposit is widely variable (N_Ag=13.9 to 69.2). Native silver is reported from the Tsumo-Maruyama deposit (Sugaki et al.,1981).

Skarn-type deposits in the Korean Peninsula

Electrum from the Holgol deposit has been analyzed. The Ag content ranges from 38.4 to 67.6 atomic % (Fig. 3-L). It increases from core to rim in a grain.

Compositional zoning in a grain

The variation range of Ag content in a grain of electrum from the sulfide-rich deposits is wide, while it is very narrow for the sulfide-poor deposits. For instance, compositional zoning in a grain of electrum from epithermal Au-Ag vein-type deposits is commonly recognized (e.g., Yamaoka and Nedachi, 1978; Shikazono, 1985b). The Ag content of electrum increases gradually from core to rim of a grain. An increase in Au content toward the rim of a grain was reported from the Arakawa deposit (Motomura et al., 1981) and the Nebazawa deposit (Nakayama and Enjoji, 1985). However, this pattern is not common. Compositional zoning is not observed for electrum in Te-bearing epithermal Au-Ag vein-type deposits (Fuke and Takeno). It has been pointed out that the variation range of Ag content in a grain is positively correlated with the grain size (Shikazono, 1985b). This relationship and compositional range in a grain and regularity of compositional zoning observed in a large grain support that the composition of electrum is retained during the postdcpositional period. The value of Au-Ag interdiffusion coefficient experimentally determined (Czamanske et al., 1973), the temperature of formation (200°C to 300°C) and the age of epithermal Au-Ag vein-type deposits (about 15 to 1 Ma) also suggest that the postdepositional change of Au and Ag contents of electrum is negligible (Shikazono, 1985b).

Shimazaki and Sato (1968), Shimazaki et al. (1969), Shimazaki (1974), and Imai et al. (1981) indicated that there exists a high Ag content zone along the margin of the grain of electrum from Kuroko-type deposits. The width of this zone is very narrow (1 to 3µm). This type of zoning is also found in electrum from the Shimokawa-bedded cupriferous iron sulfide-type deposit (Maeda et al., 1981). The Ag content of the inner part of a grain is homogeneous. This zoning pattern seems to be different from that for electrum from the epithermal Au-Ag vein-type deposits.

It is reported that the Ag content at the rim of an electrum grain from the Tsumo skarn-type deposit is higher than at the core (Sugaki et al., 1981).

Usually, compositional zoning is not observed for Au-rich electrum from the deposits of other types (hypo/mesothermal Au vein-type, alaskite-type, and epithermal Au disseminated-type). However, a slight increase in Ag content of electrum from hypo/mesothermal Au-rich vein-type deposits in Japan was observed.

As summarized in Fig. 4 and Table 8, the Ag content of electrum from the sulfide-rich deposits is generally high and varies widely. On the other hand, the Ag content for the sulfide-poor deposits is very low and does not vary widely.

2. Contents of Other Elements

Cu content:

Generally, the Cu content of electrum is less than 0.1 weight %). However, some electrum contains more than 0.1 weight % Cu, Cuprian electrum (Cu content up to 47.3 atomic %) from the Omine skarn-type deposit has been found (Yamaoka, 1981).

Hg content:

Generally, Hg cannot be detected by an electron microprobe analyzer. High Hg content (0.1-7.7 atomc %) of electrum from the Tsugu deposit in Japan has been found (Shikazono and Shimizu, 1988). The Hg content is correlated inversely with the Au content. Regular compositional zoning of the Hg content is not observed in a grain. High. Hg content parts are sporadically distributcd in the grain.

Sb content:

The Sb content was detected only in electrum from the Sakoshi-Odomari deposit (Urashima et al., 1981b). Urashima et al. (1981b) found that the Sb content correlates inversely with the Au content, and the highest Sb content is 2.4 weight %.

Ni and Te contents:

The Ni and Te contents have been reported from the Tsumo skarn-type and Takrno epithemial vein-type deposits (Sugaki et al., 1981; Soeda and Watanabe, 1981). However, the contents are very low, and carrying out more detailed studies on these minor elements is desirable.