群馬県根羽沢鉱山産銀黒鉱中にみられる金銀鉱物の共生関係と化学組成

The gold-silver deposit of the Nebazawa mine, Gunma Prefecture, is of a Neogene epithermal vein-type, and is hosted in the Katashina-gawa rhyolitic rocks probably of Paleogene age. Paragenetic and compositional variations of Au-Ag minerals have been investigated for the ginguro ores from the Manzai No. 2 and No. 3 veins and the Hon-pi vein, which are the major veins in this deposit.<BR>Microscopic observation reveals that there is a regularity in sequence of mineral deposition throughout the ginguro ores studied. The suggested general sequence of mineral deposition in the ginguro ores is as follows: (1) pyrite, marcasite and arsenopyrite, (2) chalcopyrite, sphalerite and galena, (3) Au-Ag minerals, and (4) covellite. Despite the similarity of the general sequence of mineral deposition, the observed Au-Ag mineral assemblage significantly varies with time and space, and can be classified into the following four types: (1) Au-Ag-Sb-As-S type (electrum, pyrargyrite, proustite, minerals of polybasite-aresenopolybasite series, miargyrite and freibergite), (2) Ag-S-Se type (native silver, argentite and selenian argentite) and (3) Au-Ag type (electrum and native silver) in the Manzai No. 2 and No. 3 veins, and (4) Au-Ag-Sb-S-Se type (electrum, native silver, pyrargyrite, polybasite and freibergite) in the Hon-pi vein. Electron microprobe studies for the Ag sulfosalt minerals also reveal that there is a significant difference in their Sb/(Sb + As) and Se/(S + Se) ratios between the Au-Ag-Sb-As-S type and the Au-Ag-Sb-S-Se type. A possibility is discussed that the observed paragenetic and compositional variations resulted from local and/or temporal changes in some physicochemical parameters (especially in oxidation potential and concentrations of CO2 and Cl) during the mineralizations.