On the Geology and Ore Deposits of the Shimokawa Mine, Especially of the Nakanosawa Area

Abstract

The Nakanosawa area, situated in the southern vicinity of the Shimokawa mine, is composed of the pre-Cretaceous black slates, various kinds of diabasic rocks, serpentinite and gabbro. General geological setting of the area is quite same as that of the producing area of the mine.
One of conspicuous features of the area is the broad shear zone sub-parallel to the western main diabasic mass that contains abundant intercalated slates. This shear zone, containing also lenticular bodies of diabasic rocks and gabbros, has a width of several hundred meters merging undisturbed slate zone to the east.
Three mineralized zones were detected within the shear zone. The main mineralized zone runs only on the hanging wall of the main diabasic mass, and corresponds to the horizon of champion ore bodies of the mine. The intermediate mineralized zone has only several ore stringers. The eastern mineralized zone is characterized by disseminations of granular pyrite, whose cracks are filled by pyrrhotite.
By recent exploration drillings of this area small cupriferous pyrite ore bodies are discovered in the deeper part of the main mineralized zone. This warrants further prospection in this area.
The occurrences of diabasic rocks and the intercalated slates are very complex. However, the coarse-grained nature, existence of spilitic facies with pillow structure, whitish coloration by abundant carbonate veinlets, and assemblage of the sulphide minerals may be usefull guide to distinguish the main diabasic mass and its hanging-wall main mineralized zone from the other diabasic rocks and mineralized zones.