We are very much indebted to Professors S. Takenouchi, T. Shoji, and M. Utada of the University of Tokyo for their readings of the original manuscript and their critical comments. Many samples used for the analyses were provided from the mineral collection of the Department of Petrology and Mineral Deposits, the University Museum, the University of Tokyo. Mining geologists of many Japanese Au-Ag mines (Konomai, Chitose, Koryu, Todoroki, Takadama, Sado, Yatani, Seigoshi, Nebazawa, Omidani, Sakoshi-Odomari, Kushikino, Hishikari, Iwato, Kasuga, Akeshi, and Gohaku) helped us in our underground surveys during the past 15 years. The sample containing electrum from the Nurukawa mine was provided through the courtesy of Mr. H. Hashiguchi of the Dowa Mining Company. The samples from the Holgol mine were collected by the late Emeritus Professor T. Watanabe of the University of Tokyo, and he permitted us to analyze the samples. The electron microprobe study was performed at the Ocean Research Institute, the University of Tokyo, and was supported in part by funds from Cooperative Program (Nos. 83111 and 85136) provided by Ocean Research Institute, the University of Tokyo. This study was partly financed by Grants for Fundamental Research from the Ministry of Education (No. 60540518, No. 61740464, No. 62540616, and No. 62740477).