The Akenobe Ore Deposits
Their Geologic Structure and Fracture Patterns
1973 Volume 23 Issue 118 Pages 137-151
Details
1973 Volume 23 Issue 118 Pages 137-151
The polymetallic ore deposits of the Akenobe Mine are of xenothermal type and are worked for copper, zinc, tin, and tungsten, owned by Mitsubishi Metal Corporation.
The mine area belongs to the Maizuru Folded Belt trending ENE-WSW, being composed mainly of the Permo-Carboniferous green rocks (basic lava and its tuff) at the lower part and slate. The green rocks crop out extensively at the eastern part of the Akenobe Fault, while at the western part black slate is predominant on the surface, which grades into the green rocks in the deeper part.
Igneous rocks in the area are composed of intrusive meta-gabbro and granite porphyry of the Triassic age (Yakuno complex). Granodiorite which belongs to the Miyazu granite of the Late-Cretaceous or Early Tertiary age also occur in the northeastern part of the area. The writers believe that ore deposits of the area are genetically related to this granite.
The geologic structure of the Paleozoic sedimentary rocks is characterized by two or three stages folding. The trend of the folding axis is roughly NE-SW, WNW-ESE and NW-SE. Dome structure has been recognized in the southern part of the mine. The fracture patterns including veins are characterized by NW-trending tension fractures dipping NE and also by NE-trending shears dipping NW in the eastern part of the mine.
In the western part the veins follow shear fractures parallel to the outer rim of the folded Palaeozoic rocks, making ring-shape arrangement. Many of these veins dip outwards, while those occurring in the inside of the ring structure dip inwards.
The fracture patterns formed by the latteral pressure that caused the fold structures are thought to be very complex. Based on the distribution of the lodes of the mining area, the writers guess that the vein fissures and faults were formed originally by the lateral compression in the older age, and then, some of them were removed by the stress due to upheaval of the granitic cryptobatholith in adjacent age of the mineral deposition.
