On the Iron-rich Banded Ore from the Toyoha Mine, Hokkaido, Japan

Abstract

The lead-zinc deposit of the Toyoha Mine consists of two different groups of epithermal veins, e.g., one is the earlier vein which is controlled by E-W fracture system (Tajima, Harima, Bizen etc.) and the other, the later one running to NW-SE trend (Soya, Hiyama etc.). The former is mainly composed of sphalerite, galena and a lot of quartz, and the latter is characterized by the presence of rhodochrosite and poverty of quartz.
The iron-rich banded ore occurs in Bizen-vein, especially in a limited area adjacent to the intersecting point with Hiyama-vein. The pretty stripe of the banded ore has the paragenesis of earthy hematite-wedge shaped hematite-magnetite-pyrite-quartz. There exists, however, nearby the banded ore, the semi-banded one with the paragenesis of earthy hematite-goethite. This one occurs in siderite-chlorite rock and we take it to be an embryo of the normal banded ore.
The formation temperature of these ores was determined by the homogenization temperature of liquid inclusions in quartz associated with the ore minerals.
Judging from the mode of occurrence and the nature of the ore it is considered that the banded ore might have been formed as the result of the diffusion metasomatism caused by the later mineralization. The metasomatism should be characterized by the movement of mobile components such as CO₂, O and S relating to the oxidation-reduction process of the hydrothermal solution.