Abstract

The assemblage and mode of occurrence of common gangue minerals from more than 70 Neogene vein-type deposits in Japan have been studied and summarized. The predominant gangue minerals vary with different deposit types: quartz, chalcedonic quartz, adularia, calcite, smectite, interstratified mica-smectite, interstratified chlorite-smectite, sericite, zeolites, and kaolinite in Au-Ag-rich deposits; chlorite, quartz, sericite, carbonates (calcite, rhodochrosite, siderite), and (magnetite) 1 in Pb-Zn-rich deposits; chlorite, sericite, siderite, hematite, magnetite, and (epidote) in Cu-rich deposits. Based on the gangue mineral assemblage, filling temperature, and preliminary analytical data on fluid inclusions, typical ranges of CO 2 fugacity (f (sub CO 2 ) ) and temperature for Au-Ag-rich, Pb-Zn-Mn-rich, and Cu-Pb-Zn-rich deposits were determined to be 10 (super -3) to 1 atm, 190 degrees to 250 degrees C; 10 (super -1) to 10 atm, 200 degrees to 250 degrees C; and 10 (super -1) to 10 3 atm, 200 degrees to 350 degrees C, respectively.This f (sub CO 2 ) - temperature relationship is similar to those of active geothermal systems. This similarity and that of thermochemical calculations for the stability of gangue minerals suggest that the f (sub CO 2 ) for vein-type mineralization in Japan is controlled by alteration minerals commonly occurring in the country rocks (e.g., kaolinite, plagioclase, epidote, K-mica, K-feldspar, and calcite).From a summary of host rocks for Neogene vein-type mineralization districts in Japan, it was found that many Cu-Pb-Zn-rich vein-type deposits occur in organic-rich sedimentary rocks such as shale and mudstone as well as in altered igneous rocks. In contrast, almost all the Au-Ag-rich vein-type deposits occur in altered volcanic rocks, but only a few in sedimentary rocks. Therefore, the difference in the host rocks may explain the relatively higher f (sub CO 2 ) for Cu-Pb-Zn-rich vein-type deposits and the lower f (sub CO 2 ) for Au-Ag-rich vein-type deposits.

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