Inakuraishi-type manganese deposits of Neogene age are the major vein-type manganese deposits in Japan. Imaiishizaki, Jokoku, Yakumo, Oe and Inakuraishi in southwestern Hokkaido are the representative ones of this type. The veins are composed mainly of rhodochrosite, sphalerite, galena, pyrite, chalcopyrite and quartz, with minor amounts of silver minerals of the ruby silver, polybasite and tetrahedrite groups.
In this study the ruby silver group minerals from the above deposits have been examined microscopically and analyzed by electron microprobe. Their ubiquitous deep red internal refrec-tion under the ore microscope reveals that the ruby silvers are composed simply of pyrargyrite-proustite series minerals. No finding of pyrostilpnite-xanthoconite series minerals suggests that a small amount of Cu (up to 1.07 wt.%) may stabilizes high-temperature form ruby silvers in the deposits. Their formation temperature estimated from the coexisting electrum-sphalerite-pyrite-argentite assemblage and fluid inclusion studies is about 160 to 260°C.
Electron microprobe analyses of pyrargyrite-proustite series minerals show heterogeneous distribution of Sb and As in individual grains. A clear oscillatory zoning with respect to the Sb and As contents is obvious in samples from the Inakuraishi and Yakumo deposits.
As/(Sb+As) ratio (in atomic percent) in the minerals varies considerably from deposit to deposit: 0.0-4.2% (Imaiishizaki), 3.1-30.3% (Oe), 1.3-79.7% (Jokoku), 3.2-99.5% (Inakuraishi) and 0.5-100.0% (Yakumo). The ranges indicate that a complete solid solution series exists between natural pyrargyrite and proustite. The average formula derived from 291 analyses is (Ag
3.01, Cu
0.02)3.03(Sb
0.53, As
0.45)0.98S
2.99.
In these deposits, As/(Sb + As) ratio in pyrargyrite-proustite series minerals is positively correlated with those in the tetrahedrite and polybasite group minerals. The pyrargyrite-proustite series minerals are characterized by relatively high content of As and absence of Se compared with those from Japanese epithermal gold-silver quartz vein deposits.
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