西南北海道の稲倉石型マンガン鉱床産エレクトラムの化学組成

抄録

A number of epithermal gold-silver-lead-zinc rhodochrosite veins of Neogene age are distributed in southwestern Hokkaido, Japan. They are the principal vein-type manganese deposits in Japan and called the In-akuraishi-type manganese deposits. Gold and silver are important by-products in these ore deposits.
In this study electrum from five representative ore deposits of this type, namely, Imaiishizaki, Jokoku, Yakumo, Oe and Inakuraishi, has been analysed by electron microprobe analyzer.
The veins show banded structure, and electrum occurs in sulfide-rich bands alternating with rhodochrosite bands. It associates mainly with pyrite, sphalerite, galena, chalcopyrite, tetrahedrite, acanthite, polybasite, pyrargyrite, rhodochrosite, quartz and barite with a small amount of Ag-Pb-Sb-S minerals, stannite, canfieldite and argyrodite.
Rounded grains of electrum, 10 to 20 microns in diameter, are common. Various kinds of compositional heterogeneity (e.g., irregular, zoned, homogeneous etc.) are found in electrum grains.
Silver content of electrum ranges from 16.70 to 82.35 atomic percent as a whole. However, most electrum contains 55-75 atomic percent Ag. This mode value of Ag content is higher than those of Kuroko and epithermal gold-silver vein deposits in Japan. Both Ag content of electrum and FeS content of sphalerite increase in an order; Kuroko deposits<epithermal gold-silver quartz vein deposits<Inakuraishi-type manganese deposits<epithermal gold-silver bearing lead-zinc vein deposits.