The Ag/Au ratio of native gold and electrum and the geochemical environment of gold vein deposits in Japan

The chemical composition of native gold and electrum from auriferous vein and gold-silver vein deposits in Japan has been analyzed and summarized. The Ag/Au ratios of native gold and electrum from these two types of deposits are distinct, i.e., 10–20 Ag at % (auriferous vein) and 30–70 Ag at % (gold-silver vein). Thermochemical calculations suggest that the Ag/Au ratio of native gold and electrum should decrease with increasing chloride concentration and temperature. This is consistent with analytical results of native gold and electrum and fluid inclusion studies. Based on the Ag content of native gold and electrum, the Fe content of sphalerite, and the estimated temperatures, it is deduced that the sulfur activity for auriferous vein-type systems was lower than that of gold-silver vein-type systems.