Electron probe microanalysis data for several siderite specimens from Miocene epithermal vein-type deposits of Au-Ag type (Ohmori mine) and Pb-Zn type (Toyoha mine) in Japan reveal a remarkable contrast in zinc content. Siderite in the Ohmori deposit contains an appreciable amount of zinc, ranging from 0.8 to 5.8 in ZnO weight percent, whereas the zinc content in the Toyoha deposit is less than can be detected by present techniques.How the zinc content of siderite is affected by physicochemical variables such as temperature, oxygen fugacity, and pH has been evaluated theoretically. Applying the results to the analytical data obtained, it is concluded that Au-Ag mineralization took place under relatively high oxidation conditions in comparison with Pb-Zn mineralization. This view is strongly supported by the mode of distribution of iron in sphalerite in these types of mineralization.

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