2010 Volume 60 Issue 3 Pages 153-164
High-indium ore deposits are associated within the rise tectonic zones of progressive rift-rise structures resulted from the late Cenozoic back-arc spreading of the northern Honshu arc. We examined the indium concentration mechanism around the Hokuroku district, which constitutes one of the typical rift-rise settings. The Tatsumata mine area in the SE wing of the Hokuroku rift comprises three segments of base-metal vein system which is thought to be xenothermal type related to gabbroic to dioritic stocks emplaced at ca. 17 Ma. Almost all the specimens taken from stock piles of the ores have high indium contents ranged from several tens to 400ppm, correlated with tin, bismuth and cobalt contents.
The vein system is composed of two different mineralization stages; chalcopyrite-pyrite veins with subordinate pyrrhotite, magnetite and rare sphalerite, and the following sphalerite-dominant veins with subordinate galena and a few manganese minerals. Indium is contained in the both stage mineralizations. It is concentrated only in the marginal part of the sphalerite crystal with chalcopyrite exsolution blebs, although it has no obvious correlation with Zn and Cu contents.
Sulfur isotopes indicate two different series of the hydrothermal activity. The first is characterized by relatively low δ34S values which exists in the gabbroic stocks, the lower unit silicified rocks and the early stage veins. This series was suggested to be formed dominantly from a magmatic hydrothermal solution with relatively reducing condition. Another series has relatively heavy δ34S values and occurs in the diorite porphyry stocks, the upper unit silicified zones and the later stage veins. This series was suggested to be formed by a relatively oxidized hydrothermal solution originated by mixing with sea water. Indium may have been released in the early stage hydrothermal fluid with tin, bismuth and cobalt and was involved in sphalerite crystal, probably replaced to iron, of the late stage mineralization.
In consideration of the stratigraphic position and the mineralization timing in the tectonic history of the northern Honshu arc, it was suggested that the mineralization environment changed from a terrestrial rise setting in which magmatic emanation was predominant from a deep seated magma chamber to a rift wing setting in which brine water was circulating along a fracture system formed by rapid rifting of the adjacent area. The drastic redox condition change of the ore-forming solution in conjunction with the rapid tectonic change should be important for the indium concentration.
