Quartz Simultaneously Precipitated with Kuroko Ores In the Uwamuki No.4 Deposit, Kosaka Mine

Abstract

Quartz is the only silica mineral in black, yellow and siliceous ores of the Uwamuki No.4 (Kuroko) deposit, Kosaka mine. Intimate textural relationship between quartz and sulfide minerals in Kuroko ores suggests that these minerals were precipitated simultaneously. Filling temperatures, T, of fluid inclusions in quartz from the siliceous ore (MARUTANI, 1977) can be fitted by;
T (°C)=289.7+0.2159y
where y represents vertical distance in meters from estimated sea bottom at the time of siliceous ore mineralization. The steep thermal gradient within siliceous orebody may be a consequence of mixing between ascending hydrothermal solution and down-circulating cool sea water. Net contamination of sea water is computed to be less than 25%.
Contents of quartz and barite in six black ores are measured. The ratios quartz/sulfides and barite/sulfides show excellent parallel relation. Mixing of cool and oxygenic sea water with discharged hydrothermal solution causes both decrease in temperature and introduction of sulfate ion. Consequently, quartz and barite precipitate simultaneously with sulfide minerals in black ore. Common occurrence of bipyramidal quartz in black ore indicates that the quartz grew up in open space. Besides, the grain size of quartz increases towards the margin of the black orebody. It will be ascribable to gradational slow cooling of the discharged hydrothermal solution on sea floor.