Studies on the Molybdenum Deposits of the Daito District, Shimane Prefecture (1st report)

Abstract

In the Chugoku province, there are many tungsten and molybdenum deposits closely associated with the igneous activities ranging in age from late Mesozoic to early Tertiary. The molybdenum deposits are distributed especially in the northern part of this province, i.e. the San-in province. The molybdenum deposits of the Daito district are located about 20km southwest of Matsue city, Shimane prefecture. They are now the most productive deposits of molybdenum in Japan.
The present paper briefly discusses the relations of the molybdenum deposits of this district to the igneous intrusives and the origin of these deposits in the light of field observations as well as laboratory works.
The results obtained are as follows:
(1) The rocks occunrring in the Daito district are many intrusives of various rock types and related metamorphic rocks.
(2) The most important deposits (Honko and Hinotani deposits of the Daito mine, Kamitani, Honko and Namera deposits of the Seikyu mine, and Honko deposits of the Higashiyama mine) are all situated around the margin of the Rengeji biotite granite and/or near the contact between this granite and the Kawai hybrid rocks serving as the cap-rock of the granite. The deposits seem to be closely related to the Rengeji biotite granite.
On the other hand, the Minamiko deposits of the Seikyu mine are located in the Kawai hybrid rocks that are the cap-rock of the Ouchidani biotite granite ; the deposits are believed to be closely related to this granite.
(3) The molybdenum deposits are hydrothermal molybdenite quartz veins, 10 to 100cm thick, generally 100m long in the dip direction, and 250m long in the strike direction. Molybdenite and quartz are common essential vein minerals, and other minerals such as biotite, plagioclase, adularia, epidote, galena, sphalerite, pyrite, ankerite and calcite are also found as accessory minerals.
(4) The country rocks adjacent to the veins are rather strongly altered. The alteration zone commonly comprises two transitional zones, namely, sericitized zone (in places, silicified zone is recognized) and chloritized zone, from the inner side toward the outer side. In the sericitized zone, about 10cm wide, feldspar and biotite are markedly replaced by aggregates of sericite and quartz. In the chloritized zone, about 10cm or more wide, feldspar is slightly replaced by sericite and calcite, and biotite is completely converted into aggregates of chlorite and pyrite.