Sanjo Mine
Shimo, Ofuku village, Mine city, Yamaguchi pref., Japan
Occurrence
High-T hydrothermal skarn deposit
Skarn formed by the Late Cretaceous (85 Ma) diorite intruded in limestone of the Permian Guadalupian (260 Ma) Tsunemori Formation of the Akiyoshi Limestone Group, a member of the Akiyoshi accretionary complex. Formation of the high temperature hydrothermal deposit associated with skarn may be related to the Late Cretaceous (88 Ma) Sasanami Caldera Complex approximately 20 km apart from this locality. The diorite is S-type (Ilmenite-series). Approximately 2 km apart from the diorite, heat source. The Tsunemori Formation is mélange composed of a mudstone matix, basalt, and reef limestone of the Carboniferous to Permian Akiyoshi Limestone Group . The Tsunemori Formation is considered as trough-fill sediments, and the Akiyoshi Limestone Group as reef on the summit of the seamount in the Panthalassa (Panthalassic Ocean). Ages of limestone was determined by radiolarians.

Outcrop of oxidation zone of the high temperature hydrothermal deposit associated with skarn in the Tsunemori Formation. The banded structure from the upper left to the lower right is the bedding plane of the host. Black parts are magnetite and green spots are malachite. The host is aggregate of grossular and calcite with minor amount of quartz. Euhedral crystals of these minerals were recovered from the pocket at the left of the coin.
Reported Minerals
- Cuprotungstite
- Conichalcite
- Malachite
- Scheelite
- Chalcopyrite
- Chrysocolla
- Grossular
- Cu allophane
- Adularia
Mineral Assemblages
- Grossular - Scheelite - Cuprotungstite
- Grossular - Adularia
Localities
- Kagada (W-rich)
- Shin-Yaguki Mine (W-rich)
- Naganobori Mine (Co-rich)
- Sanjo Mine (W-rich)