The Fuke mine, situated at about 10 km north of Okuchi city in Kagoshima Prefecture, is one of the gold-silver ore deposits of the epithermal vein type in the Green-tuff region. The ore deposit is considered to be an intermediate type between the gold-silver vein and the base metal vein.
The high grade ore in the mine is named "Tozi-kin" in which can be visually recognized the part where fine-grained golds are concentrated. As for the electrum in this ore, mode of occurrence and chemical composition have been investigated by a microscope and EPMA. The results are as follows : 1. The Tozi-kin ore often shows a characteristic banded arrangement from the wall rock to the inner of the ore; Fe-Mg chlorite, drusy quartz with pyrite and hematite, fine-grained quartz with banded sulfides and electrum, and green clay mainly composed of chlorite-saponite mixed-layer.
2. Paragenetic sequence of vein minerals may be divided into four stages; First stage: Drusy quartz accompanied by pyrite and hematite. Second stage: Fine-grained quartz characterized by sulfides and electrum. Third stage: Altered minerals. Fourth stage: Barren quartz associated with calcite. 3. Observed ore minerals are electrum, galena, sphalerite, chalcopyrite, pyrite, hematite, hessite, and an unkown Ag-Au telluride.
4. Electrum precipitated after the precipitation of sulfides in the second stage.
5. The electrum filling intergranular cavities of quartz is irregular and angular in shape, whereas that in sulfides varies from granular to cylindrical, and often amoebic.
6. The grain size of electrum is generally less than 50 microns in diameter, but it attains 200 microns in some cases.
7. The chemical composition of electrum from the Fuke-honpi vein shows a very little fluctuation among grains as well as in a grain, with an average chemical composition Ag 19.1 wt.%, Au 80.6 wt.%.
Some of these properties of electrum such as the homogenity of composition and the relatively coarse grains, are considerably different from those of electrum in the Kuroko ore.
