1970 Volume 20 Issue 103 Pages 315-327
[Correction] Mining Geology Vol.21 No.105 (1971) pp.e1a-e1a
[Correction] Mining Geology Vol.20 No.104 (1970) pp.e1c-e1c
The black ore bearing formations at the Hokuroku black ore deposit district are the Ohkuzu formation and the Ohtaki formation. The pyroclastic rocks of these formations show generally perfect grading and sometimes partially double-grading. They are turbidite containing mud balls at the bottom of the sea.
Generally, pyroclastic rocks of the country rock of the Shakanai ore deposits consist of two groups, large-scale pyroclastic flow members and small-scale pyroclastic flow members. The latter was recognized only around the black ores and the volume is nearly same with that of the black ore deposit.
The large-scale pyroclastic flows were controlled by shapes of the foot wall rhyolite. They flowed from west to the Shakanai mine area and deposited thick sediments in the concave places of the foot wall rhyolite. The relation between these large-scale flow members and black ore deposits, however, cannot be directly recognized. On the other hand, in case of the small-scale flow members flowing just before the ore deposition, they control the form of ore deposits. Where there are small-scale muddy or sandy sediments showing graded bedding deposit over the ore bodies, the extent of the ore deposit appears to be inferred by the occurrence of the graded bedding.
No. 1 ore body of the Shakanai mine shows perfect graded bedding. The black ores of No. 1 ore body consist of 8 unit-bodies (lateral diameter of each unit body is 30 to 60 meters and the maximum thickness is 15 to 18 meters.). The unit bodies of the southwest accompany "the breccia-like black ores" showing graded bedding by the size and density of ore fragments and imbrication by angular ore fragments. In addition, sole marks can be seen at their base.
Judging from the observation mentioned above, it is concluded that "the breccia-like black ores" are parts of a unit ore body deposited with pyroclastic rocks by submarine turbidity currents.
