Stratigraphic Overturn and Deformation of Bedded Manganese Ore Deposits in the Hamayokokawa Mine, Nagano Prefecture

Abstract

The Hamayokokawa manganese mine which is located at 10km WSW of Tatsuno-machi, is one of the largest bedded manganese mine in Japan, producing annually more than 10, 000 tons of high grade manganese ore.
The geology of this area consists mainly of Palaeozoic chert, slate, sandstone etc. In the mapped area the Palaeozoic strata are classified into the following two formations :
1) Yokokawa Formation: this formation is apparently conformable with the underlying Narai formation and consists mainly of slate and thick chert members.
2) Narai Formation: this distributes at the northwestern side mainly consisting of muddy turbidite. The general trend shows strike of N 50° E and dip to 60° SE.
Judging from graded beddings observed in the Narai formation, the Palaeozoic strata of this area are overturned, so that the massive chert is the hanging wall at present.
The manganese ore deposits are, embedded in the chert at the boundary between the Narai formation and the Yokokawa formation. The ore deposits are arranged, from west to east, in such a way as low grade ore zone, high grade ore zone, broken ore zone and washed-out zone by turbidity current.
The high grade ore zone, which shows complicated folding pattern, is considered to be formed by turbidity current directly after ore deposition. In the northeastern part of the mining area the chert and the ore deposits are washed out by turbidite. The erosion gives the manganese ore an appearance of conglomerate and/or pebbles in turbidite.