Hydrothermal Alteration at the Uku "Roseki" Mine, Yamaguchi Prefecture
1974 Volume 24 Issue 123 Pages 31-43
Details
1974 Volume 24 Issue 123 Pages 31-43
The Uku "Roseki" mine is located in the northeastern part of the Yamaguchi Prefecture. At present, it produces monthly about 1, 500 tons of andalusite-bearing "Roseki" ore for the refractories. The main constituent minerals of the ore are andalusite, pyrophyllite, kaolinite, and quartz, and the accessories are diaspore, corundum, and muscovite.
The geology around the mine consists mainly of the Fukuga formation of the Abu group and biotite granite intruded therein, both in late Cretaceous age. The Fukuga formation is divided into three members as follows:
Mitsugadake rhyolite member: ryholite lava, tuff and welded tuff.
Kiyodani member: tuffaceous sandstone, siltstone, shale, conglomerate and rhyolitic tuff.
Kanai andesite member: pyroxene andesite lava and tuffs.
Hydrothermally altered zone is widespread mainly in the Kiyodani member. The altered zone is divided into two zones; sericite zone and andalusite zone. The andalusite zone, furthermore, is subdivided into three, namely, andalusite-pyrophyllite-kaolinite subzone, quartz-andalusite-pyrophyllite subzone, and quartz-andalusite subzone, from center to margin of hydrothermally altered mass. Occurrence of the andalusite zone is concordant with the bedding plane of the Kiyodani member.
The mineralogical and chemical changes of the altered rocks have been investigated using optical, chemical, and X-ray diffraction methods. In comparison with the original composition of the tuffaceous sandstone, siltstone aria rhyolitic tuff of the Kiyodani member, the andalusite-pyrophyllite-kaolinite subzone and the quartz-andalusite-pyrophyllite subzone show addition of AL2O3 and H2O, while the quartz-andalusite subzone show addition of SiO2. Throughout the andalusite zone, there is remarkable reaching of MgO, CaO, Na2O and K2O. From the mineral assemblages of andalusite-corundum and andalusite-quartz, it is considered that the "Roseki" deposit of the Uku mine was formed under the condition of temperature and pressure higher than those of the Mitsuishi and the Shokozan, which can be regarded as the typical "Roseki" deposits in Japan.
