1988 Volume 38 Issue 209 Pages 233-246
Ores of the Shimokawa massive sulfide deposits are composed mainly of pyrite, pyrrhotite, chalcopyrite, sphalerite, and magnetite. Minor amounts of cobalt pentlandite, cobaltian mackinawite, cubanite, cobaltite, and galena are observed under the microscope. Gangue minerals are quartz, calcite, siderite, sericite, chlorite, amphibole, apatite, and carbonaceous materials. The assemblage of pyrite-pyrrhotite-magnetite-siderite is considered to have been formed under a condition close to 250°C, 101.7 atm fco2, 10-38 atm fo2, and 10-14 atm fs2. The pyrr-hotite-pyrite-cubanite-chalcopyrite assemblage has been modified by a small change in fs2 during the retrograde process from iss-pyrite-pyrrhotite assemblage formed above 300 or 350°C. The cobalt-iron partition temperatures between pyrrhotite and pyrite are 200-300°C for monoclinic pyrrhotite-pyrite and 270-350°C for hexagonal pyr-rhotite-pyrite. The gangue mineral composition containing siderite and calcite as the main constituents is consistent with the estimated submarine environment rich in carbonaceous materials where an adequate supply of elastics from adjacent continental areas was available.
