Geology and Ore Deposits of the Ohe Mine, Hokkaido (1)

On the Vein Structures of the Senzai Vein

Abstract

The Ohe Mine is one of the many manganese mines situated in southwestern Hokkaido.
Geologically, the Ohe Mine area is composed of igneous rocks overlain by Neogene Tertiary sedimentary beds. The former include quartz diorite, porphyrite and basalt; the latter are various green tuffs and felsitic rocks.
The geologic structure is characterized by folds, faults and basaltic dykes which show the same trend as the veins.
The ore deposits consist of many parallel veins which strike about N50°-60°W. The Senzai vein is representative. It pinches at places where it crosses the fault and the boundary of the country rocks, and ore shoots were formed in the upper part of the quartz diorite. This may be due to the fissure system and epithermal conditions. The Senzai vein exhibits successive mineralization of manganese and sulphide minerals. It is divided into three stages and various ore zones as follows;
Stage of mineralization
Ore zone
[I] Sericitization·silicification·quartz vein (1)
…………Sulphide ore (1)
[II] Manganese ore…………Sulphide ore (2)
[III] Quartz vein (2) + manganese ore (2)
The ore zones are formed in units. The lengths of their strike and dip sides are about 30m, and they show typical echelon arrangement.
The constituents of the manganese ore are rhodochrosite, a little rhodonite and siderite. The sulphide ores are pyrite, zincblende, galena and a little chalcopyrite, tetrahedrite and hematite.
It is considered that the above. mentioned minerals are respectively characterized by the various stages of mineralization, especially siderite which forms a solid solution with rhodocrosite and increases in relative amonmts in the later stage of mineralization. Considerable amounts of native gold are found only in the sulphide ore of the latest stage.