2024 Volume 74 Issue 1 Pages 25-36
The Nakaosaka iron ore mine in Shimonita, Gunma Prefecture, Japan, is a geosite in the Shimonita Geopark and an industrial heritage site where iron ore (high-grade magnetite) was produced during the early Meiji era in Japan. The Nakaosaka iron ore mine is located to the north of the Oogitano–Iwayama tectonic line (the Median Tectonic Line of Japan). The iron ore deposit is composed of small but high-grade magnetite ore bodies, which are embedded in the Nanjai Formation and the Name granite. The Nanjai Formation can be correlated with the Jurassic Tanba–Mino accretionary complex, and the 70 Ma Name granite can be correlated with the Cretaceous Ryoke I-type granite.
The magnetite ore bodies are lenticular or vein shaped with wide alteration zones composed of actinolite or hydrobiotite + chlorite with carbonate stockwork veinlets. The magnetite has Si contents of <3 wt.% and often contains euhedral Clapatite micro inclusions. Only minor sulfides (mainly pyrrhotite) are associated with the magnetite ore suggesting a low sulfidation state. Composite löllingite and arsenopyrite grains occur alongside the magnetite, and the arsenopyrite has high As contents (~36 mol.%). This suggests a higher temperature than typical hydrothermal deposits. Furthermore, the magnetite geothermometer yields temperatures of >500 ˚C. Apatite and chlorite associated with the magnetite have Cl contents of <1 wt.%. The altered zones are not associated with acidic alteration index minerals such as alunite or kaolinite.
Around the Nakaosaka iron ore mine, magnetite with apatite occurs in a neutral to alkaline alteration zone that experienced high temperatures (>500 ˚C) close to a large-scale tectonic line. These features strongly suggest that the magnetite ore deposit is an iron oxide-apatite (IOA, Kiruna)-type deposit.
Key words: Iron-oxide type ore deposit, IOA-IOCG type ore deposit, Kiruna type ore deposit, Magnetite, Apatite, Nakaosaka iron ore mine, Shimonita Geopark
