平瀬鉱山およびその周辺の地質鉱床
1973 年 23 巻 118 号 p. 119-136
詳細
1973 年 23 巻 118 号 p. 119-136
The oldest rocks of the Hirase mine area are various gneissose and granitic rocks of the so-called Hida Complex. They crop out in the northeastern and southwestern parts of this area. The Jurassic to Cretaceous Tedori Formation overlies these rocks uncomformably in the southwestern part of this area. The Cretaceous Nohi Rhyolites Group is most extensively distributed in this area. It is composed of acidic welded tuffs intercalated with rhyolite lava, its tuff and tuff breccia, and mudstone.
Granitic rocks, K-Ar ages of which are about 60 m.y., intrude the Nohi rhyolites. They crop out as several. stocks along the Sho-gawa (river) and are called "Shirakawa Granites." Fine-to medium-grained biotite granite and hornblende-biotite granodiorite are their major facies. The Shirakawa granitic rocks show contact aureoles in the surrounding Nohi rhyolites. Andalusite-bearing assemblage is seen around some of the plutons, such as Hatogaya and Hirase stocks. Dykes of quartz diorite porphyry, hornblende andesite, pyroxene andesite and basalt occur in these stocks mainly along fault zones.
Metamorphic rocks of the Hida complex thrust up to the Nohi rhyolites along the Morimo tectonic line. It strikes north-northwest. A similar fault zone passing through the Mihoro dum reservoir is called "Mihoro tectonic line." There are many faults of NW-trend diversing from the Mihoro tectonic line, some of which cut throngh the Hirase granitic stock.
There are many kinds of mineral deposits in the area, namely Cu-Pb-Zn veins in the Fcdori formation, Au-Ag quartz veins in the Nohi rhyolite, graphite deposits in the Hida complex and molybdenite-quartz veins in the Shirakawa stocks. Yet, only molybdenum deposits, those of the Hirase mine in particular, are productive. The Hirase mine is one of the most important molybdenite mines in Japan.
The Hirase deposits are composed of 29 molybdenite-quartz veins. The veins strike N-S to NNE and dip steeply west. Productive veins occur in the marginal part of the Hirase stock. The granitic rocks of this part are very heterogeneous and become homogenous toward the interior. The veins also become poor or thin out in the interior.
Molybdenite occurs along walls of quartz veins as fine-grained crystals or very coarse-grained cuhedral ones. The latter predominates in drusy parts of the quartz veins and accompanies coarse-grained calcite crystals in some places. Molybdenite seams occur in some parts of the altered Nohi rhyolites. Molybdenum grade of this ore is very low, 100 to 200 ppm, but the quantity is large. The amount of MoS2 in the altered rhyolites at Kitani (about 3 km north of the Hirase mine) is estimated about twice as much as the total historical production of the Hirase mine. Molybdenites in any mode of occurrence have been recognized in rocks between the Morimo and Mihoro tectonic lines.
