Gold and silver mineralization of black ore deposits in the Akita and Aomori area

In the Neogene Northeast Japan Arc, there are many Kuroko deposits formed in the middle Miocene. In particular, large-scale Kuroko deposits with an ore reserve of more than 10 million tons have been explored and studied for their economic potential, including the Uchinoutai deposit and the Matumine deposit. These deposits have been reported to yield yellow ore and black ore, that contain minerals such as bornite, Cu-S mineral, and enargite, which represent high-sulfidation environments. It is also known that these ores are associated with concentrations of gold and silver minerals such as stromeyerite, mckinstryite and electrum. On the other hand, the small-scale Kuroko deposits, which have an ore reserve of less than one million tons, produced gold and silver rich ores but ore production were limited. Many hydrothermal deposits have been discovered in the Okinawa Trough and the Izu-Bonin Arc, and the presence of gold and silver rich ores is also known in these deposits. In the Okinawa Trough, there are many presenting submarine hydrothermal deposits containing pyrrhotite and cubanite, which are estimated to have formed at low sulfur fugacity compared to the Kuroko deposits in the Hokuroku area.This study, we investigated a total of 27 deposits in the Akita-Aomori area, where Kuroko deposits of various types are deusely distributed, to disucus the gold and silver mineralization of the Kuroko deposits. Mineral description, microstructure description and mineral chemical analysis of the ores in each deposit were carried out. Based on the results, I the conducted research to elucidate the causes of the formation of gold and silver rich ores and classify the deposits, and to clarify the distribution of each type of Kuroko deposit. All the minerals in the Kuroko deposit were classified as skeletal minerals and pore-filling minerals. Pyrite, sphalerite and barite are often crystallized as skeletal minerals. Gold and silver minerals, bornite and enargite all occur as pore filling minerals. Chalcopyrite, tetrahedrite group minerals, galena, and quartz occur as skeletal and pore-filling minerals.The Kuroko deposit, 16 out of 17 deposits with an ore reserve of over 1 million tons, produced minerals such as bornite and Cu-S minerals and enargite under high sulfidation conditions. These minerals were confirmed in all the ore types such as siliceous ore, yellow ore, semi-black ore, black ore, barite ore, etc. in the studied Kuroko deposit. Among the high-sulfidation ores, bornite ores and Cu-S mineral-rich black ores are remarkably rich in gold and silver minerals, and the representative gold and silver minerals are stromeyerite, mckinstryite and electrum. Tetrahedrite (Ag), miscellaneous polybasite, and electrum-containing gold and silver rich ores without bornite and Cu-S minerals were also observed. On the other hand, there were no ores with significant concentrations of gold and silver minerals other than those containing enargite or black ore. Neither bornite nor enargite was found in the small-scale deposits. The c from these deposits were all ore with concentrations of polybasite, tetrahedrite (Ag) and electrum. The absence bornite and pyrrhotite suggests that they formed in an intermediate sulfidation environment. The tetrahedrite group minerals in the high sulfidation ores were tetrahedrite and tennantite. The tetrahedrite group minerals in the high-sulfidation ores are tetrahedrite and tennantite, both of which are poor in silver and rarely contain more than one atomic %. Most of the minerals are tennantite, and tetrahedrite is rare. On the other hand, tetrahedrite in intermediate sulfidation ores is rich in silver, and the silver content of tetrahedrite increases in proportion to the increase in Sb content. In addition, the ores rich in tetrahedrite (Ag) tend to be rich in electrum. The gold content of the electrum was 70-80 atomic % in high-sulfidation ores, while it was less than 60 atomic % in intermediate sulfidation ores. The gold content of the intermediate sulfide ores was less than 60 atomic % in the intermediate sulfide ores, and less than 50 atomic % in the ores coexisting with miscellaneous silver ores and pyrite-poor ores. The concentration of FeS in sphalerite as a skeletal mineral is less than 0.01-0.60 mol. % FeS for intermediate sulfidation ores and 0.01-0.20 mol. % FeS for high sulfidation ores. The FeS concentrations of pore filling sphalerite are 0.5-6.00 mol. % for intermediate sulfide ores and 0.1-0.60 mol. % for high sulfide ores. The range of FeS concentration in intermediate-sulfidation ores is larger than that in high-sulfidation ores.From the above observations, the Kuroko deposit can be classified into two types : ore formed in a high sulfidation environment and the other formed in an intermediate sulfidation environment. The large-scale Kuroko deposits ore, are a high to intermediate sulfidation type, while the small-scale deposit are intermediate sulfidation type. Gold and silver mineralization in the Kuroko deposit is caused by the circulation of hydrothermal fluids through a porous ore body consisting mainly of skeletal minerals formed on the seafloor.In summary high-sulfidation gold and silver rich ores are contain of stromeyerite, mckinstryite, and gold-rich electrum, with silver-poor tetrahedrite group minerals, bornite Cu-S minerals and galena. Such ores were produced in Fukazawa deposit, furutobe deposit, uchinotai deposit. Intermediate-sulfidation gold and silver rich ores are contain of tetrahedrite (Ag), polybasite, and silver-rich electrum, with chalcopyrite and galena. Such ores were produced in Mizusawa deposit, Tanosawa deposit, Shimotai deposit, Kowariswa deposit and Uchinotai deposit.All of the gold and silver rich ores are rich in voids until the end of mineralization, indicating that they formed in the upper part of the deposit where hydrothermal fluids cooled rapidly.博士（資源学）秋田大