Abstract
The Koryu gold-silver mine is located approximately at 141°19' of east longitude and 42°51' of north latitude in southwestern Hokkaido. The ore deposits are of epithermal gold-silver-quartz vein type, and they are composed of the No. 1-No. 7 veins hosted by the Miocene black mudstone. In this study, we have determined homogenization and ice melting temperatures of fluid inclusions in quartz, and chemical compositions of some ore minerals by microprobe analyses for the interpretation of mechanism of gold-silver deposition in the two veins (No. 1 and No. 3). Three stages of mineralization are discriminated on the basis of brecciation and cross-cutting relations in the vein, and gold-silver deposition occurred predominantly during the middle stage. The ores ordinarily exhibit banded structure composed of quartz, ±adularia, ±clay minerals, ±carbonates, and gold-silver and other metallic minerals (ginguro). The ore minerals are electrum (44-74 atom.%Ag), aguilarite (Se/(S+Se) atomic ratio : 0.25-0.51), pyrargyrite-proustite, pearceite-polybasite, miargyrite, Ag-bearing tetrahedrite (22-24 wt.%Ag) , galena, sphalerite (0.6-1.2 mole% FeS), chalcopyrite and pyrite etc. Ag content of electrum tends to decrease slightly toward the upper levels in the No. 3 vein, whereas FeS content of sphalerite is invariable between the upper and lower levels. Fluid inclusions observed in quartz of the middle stage are two phase and liquidrich type. Homogenization temperatures for the No. 1 vein are mainly 220°-270°C with high frequency in the range 250°-260°C. The temperatures for the No. 3 vein are about the same as those for the No. 1 vein, but show higher values (250°-280°C) at 30 mL. Ice melting temperatures of inclusion fluids for the No. 1 and No. 3 veins are from −0.4°C to −1.3°C, and from −0.5°C to −1.4°C, respectively. Spatial variations of these homogenization and ice melting temperatures, together with the data previously reported, would indicate that the relatively CO2-rich ore fluids had boiled on the way to form ore shoots.
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