Gold-silver alloy (electrum, native gold, and native silver) is the main Au and Ag mineral occurring in hydrothermal Au-Ag ore deposits. Recently, a great many analytical data on this mineral have been obtained by electron microprobe analyzer (e.g., Shikazono, 1986; Shikazono and Shimizu, 1987). These studies revealed that the Ag/Au ratio of this mineral from Au-Ag deposits in Japan and in the Korean Peninsula ranges widely, mostly from pure gold to pure silver. Shikazono and Shimizu (1986, 1987) clarified that this mineral from different deposits has different Ag contents. For instance, the Ag/Au ratio of this mineral from epithermal vein-type deposits in Japan is higher than that from hypo/meso-thermal vein-type deposits in Japan.

Several other types of ore deposits from which Au and Ag have been produced arc found in Japan. The main types include (1) epithermal-type deposits, (2) hypo/mesothermal vein-type deposits, (3) Kuroko-type deposits, (4) bedded cupriferous iron sulfide-typc deposits (or Besshi-type deposits), and (5) skarn-type deposits.

In the Korean Peninsula, main Au-Ag deposits are (1) epithermal vein-type deposits, (2) hypo/mesothermal vein-type deposits, (3) alaskite-type deposits, and (4) skarn-type deposits.

In this study, the modes of occurrence of the Au-Ag minerals from deposits in Japan, the Korean Peninsula and Taiwan and a great many analytical data on this mineral (more than 6500 data points) from more than 100 mines are summarizied, A presentation of what factors are important for controlling the compositional variations in the natural Au-Ag alloy is then made. Chemical composition of the Au-Ag alloy is used to limit the phyaicochemical environment of Au-Ag mineralization.

There are three names for natural Au-Ag alloy; native gold, electrum, and native silver. Definitions of electrum, native gold, and native silver have been differently used by different investigators, and they are ambiguous. For example, Ramdohr (1969) used electrum for Au-Ag alloy containing 30% to 45% Ag. Barton and Toulmin (1964) used electrum to describe Au-Ag alloy from pure gold to pure silver. Uytenbogaardt and Burke (1971) called Au-Ag alloy containing 25 weight % Ag or more Ag electrum. Boyle (1979) noted that electrum has been applied to gold containing 20% or more silver. In this paper, following Barton and Toulmin (1964), electrum is used for natural Au-Ag alloy from pure gold to pure silver, because electrum is the name popular to workers of ore deposits.