Ashio deposit

The Ashio mine which is located in middle Honshu is one of the largest Cu ore producer in Japan until it was shut down in 1973 (Kanehira, 1991). The ore deposits 

The deposits are characterized by conspicuous metal zoning and polymetallic mineralization. From the centre to margin of the mine district, the following zonings are recognized Sn-W-Bi-Cu zone, Cu-As-Zn zone, and Zn-Pb-Cu-As zone (Nakamura, 1970). 

Main opaque minerals are chalcopyrite, cassiterite, stannite, arsenopyrite, bismuth-inite, pyrrhotite and sphalerite. The FeS content of sphalerite is high (about 18 mol% FeS). 

Dominant gangue minerals are quartz, sericite, kaolinite and siderite. The Fe content of chlorite is very high (Fig. 1.170). All these data suggest low /sj and /02 conditions. 

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The Fe "/Zn + ratio of coexisting stannoidite, sphalerite and tennantite-tetrahe-drite from the Tada, Omodani and Ohmidani deposits is low, compared with that from the other deposits such as the Ashio, Akenobe and Ikuno deposits (Fig. 1.182). The Tada, Omodani and Ohmidani deposits are characterized by polymetallic (Zn-Cu-Ag-Au Zn-dominated) mineralization, and tungsten is not recovered from these deposits. On the other hand, the Akenobe, Ikuno and Ashio deposits are characterized by polymetallic (Cu-An-Pb-Sn-W-Ag-Au-Bi) mineralization, and tungsten is recovered from these deposits. 

Shimazaki, H., Nakamura, T. and Kusakabe, M. (1993) Oxygen isotopic composition of ore fluids for the Ashio deposits. Mining Geology, 45, 269 (in Japanese). 

Fig. 1.31 Metal zoning in Osarizawa ore deposit (Saisei-hi, cross section, (b)) and Ashio deposit, cross section, (a) (Horikoshi 1977)...

Figure 1.18. Variation of Fe " /(Fe + Mg) and tetrahedral AI of chlorite from hydrothermal ore deposits Japanese Neogene Cu-Pb-Zn vein-type (open circle) and Kuroko deposits (solid circle). Localities 1 Ashio, 2 Yatani, 3 Toyoha, 4 Kishu, 5 Sayama, 6 Mikawa, 7 Furutobe, 8 Hanaoka, 9 Wanibuchi, 10 western Bergslagen (Shikazono and Kawahata, 1987).

Figure 1.83. Variation of Fe /(Fe + + Mg) and tetrahedral Al of chlorite from hydrothermal ore deposits Japanese Neogene Cu-Pb-Zn vein-type (open circle) and Kuroko deposits (solid circle). Localities 1 Ashio (Nakamura, 1960, 1963) 2 Yatani (Hattori, 1974) 3 Toyoha (Shikazono 1974a, Sawai, 1984) 4 Kishu (Shirozu, 1958) 5 Sayama (Shirozu, 1958) 6 Mikawa (Nagasawa, 1961) 7 Furutobe (Shirozu et al., 1975) 8 Hanaoka (Hayashi 1961, Hayashi and Oinuma, 1965 Tsuzuki and Honda, 1977 Shirozu et al., 1975) 9 Wanibuchi (Sakamoto and Sudo 1956, Iwao and Minato 1959, Katsumoto and Shirozu, 1973) 10 western Bergslagen (Baker et al., 1983) (Shikazono and Kawahata, 1987).

It is generally accepted that Kuroko deposits formed under the submarine environment, while polymetallic vein-type deposits in central and Northwest Japan (Ashio, Tsugu, Kishu, Obira, etc.) under the subaerial environment. 

In contrast, in Southwest Japan, polymetallic veins (so-called xenothermal-type deposits in the sense of Buddington (1935) or subvolcanie hydrothermal type in the sense of Cissartz (1928, 1965) and Schneiderhohn (1941, 1955) occur. Examples of these deposits are Ashio, Tsugu, Kishu and Obira. All these vein-type deposits have formed at middle Miocene age in western part of Tanakura Tectonic Line under subaerial environment. In these deposits, many base-metal elements (Sn, W, Cu, Pb, Zn) and small amounts of Au and Ag are concentrated. These deposits are associated with felsic volcanic and plutonic rocks along the Median Tectonic Line (MTL) or south of MTL. 

Figure 1.168. Geologic map of the Ashio mining area, showing the mineral zoning in the Ashio rhyolitic body and the location of the Kajika deposits (after Ashio mine, partly revised) (Nakamura, 1970).

Figure 1.170. Diagram showing the octahedral composition of chlorites from the subvolcanrc hydrothermal deposits, propylite, and Kuroko deposits in Japan (Nakamura, 1970). Chlorite occurring as a gangue mineral in the subvolcanic hydrothermal deposits Nos. 1, 2, 3 and 4 Chlorite from the Ashio copper mine. Nos. 5, 6, and 7 Chlorite from the Kishu mine. No. 8 Chlorite from the Arakawa mine. Nos. 9 and 10 Chlorite from the Ani mine. No. 11 Chlorite from the Osarizawa mine. Chlorite from the so-called propylite No. 12 Chlorite from the Yugashima mine. No. 13 Chlorite from the Budo mine. Chlorite from the Kuroko deposits No. 14 Chlorite from the Wanibuchi mine.

The chemical compositions of coexisting sphalerite and tennantite-tetrahedrite from the mines were determined. Except the Ashio polymetallic deposits, the other deposits have been formed at late Cretaceous related to felsic magmatism. 

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