1977 Volume 27 Issue 143 Pages 181-189
The Iwami zinc-lead-copper deposits are kuroko-type deposits in the "green tuff region" of the Southwest Japan. The ore deposits are divided into three ore zones: the stratiform kuroko ore zone at the top, the massive gypsum ore zone, and the stockwork ore zone extending downwards. Sulfide minerals in the stratiform and stockwork ore zones are sphalerite, pyrite, galena, and chalcopyrite, with minor luzonite and tetrahedrite.
The wall rocks of the stockwork ores consist of dacitic pyroclastics and small rhyolitic intrusives. They are intensively silicified in the high grade ore bodies, and are argillized in the low grade ore bodies. Hydrothermal alteration minerals in the wall rocks are quartz, Mg-chlorite (IIb), interstratified Mg-chlorite-saponite, sericite (1M), and interstratified sericite-montmorillonite (rich in sericite layer), sometimes with minor gypsum. Late calcite occurs in fissures and vugs.
The stratiform kuroko ores consist of the sulfide minerals with minor quartz and Mg-chlorite and rare sericite. Barite occurs in the upper part of the stratiform ore zone, and kaolinite is found sparsely in the lower part. Clay veins and a clay zone surrounding the gypsum ore body consist of sericite and interstratified sericite-montmorillonite with Mg-chlorite, interstratified Mg-chlorite-saponite, quartz and gypsum. Montmorillonite is found in the clay zone at the peripheral part of the stockwork ore zone.
The mineral assemblages and the data of fluid inclusions suggest that the alteration and mineralization in the Iwami ore deposits were brought by the hydrothermal solutions with ionic strength: 0.5 (18, 000 ppm C1), fO2: 10-35 atm, fs2: 10-12 atm, fCO2: 0.1-1 atm, and pH: 5.7 at 250°C.
