Geology and Ore Deposits of Seikyu Molybdenum Mine, Eastern Shimane Prefecture, Japan

Study on Molybdenum Deposits in Japan (2)

Abstract

The Seikyu mine, in eastern Shimane Prefecture is one of the most productive Japanese molybdenum mines, It is located in a part of the late Creataceous Chugoku batholith in the Inner zone of the Southwestern Japan. The area is a zone of various granitic rocks such as schistose hornfels (Hfs), schistose biotite hybrid (Hgs), biotite adamellite (Gad), granitic complex (Gc) consisting largely of leucocratic rocks, hornblende-biotite hybrid (Ghb), two-mica granite (Gm), aplite (Ga), and adamellite porphyry-porphyritic granite (Gp), Many molybdenum deposits are found between two large batholiths, of hornblende-biotite granodiorite (Gd) and biotite granite (Gb).
The main deposits called Seikyu-Honzan are distributed at the southeastern margin of the biotite adamellite, and embedded in the lower adamellite, in the granitic complex, and in the upper hornblende-biotite hybrid in descending order. The vertical ore zone is about 300 meters thick.
The deposits are composed of fourteen gently dipping mdybdenite-quartz veins, which can be grouped into the fallowing three systems : 1) N50-80°E, 10-20°SE; 2) N 50-80°W, 10-20°SW; and 3) N-S- N20°E, 10-20°E. There are no steeply dipping veins. The thickness of veins is wostly 5 to 20 cm. No distinct structural control is observed throughout the deposits.
Major constituents of the veins are quartz and molybdenite. The veins consist of small amounts of pyrite occurring all over the veins, in parts magnetite (hematite), sphalerite, galena, chalcopyrite, and secondary molybdenum minerals. In general the veins are surrounded by 30 cm wide zone of alteration such as silicification, sericitization, weak carbonatization. There are small amounts of potash feldspars, pale brown biotite, garnet, epidote, zeolites, clay minerals other than quartz, sericite, chlorite, calcite, dolomite, and siderite. This mineral assemblage puts the main molybdenum mineralization in a mesothermal stage.
Fault pattern of the Seikyu-Honzan deposits is of grid, indicated by WNW and NNE of steeply dipping faults and gently dipping ore veins. Movement along these fractures after the formation of the veins is rather small, less than a few meters, except two normal faults of WNW series-"Giant Fault" (drops 150 m) and "Sheared Zone" (drops 30 m). Sheets and dikes of andesite intrude along the ore veins and NNE faults.