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[HRE12-P02] The Gold and Silver mineralization of layered yellow ore in Furutobe mine.
Keywords:Kuroko, Gold and silver, Hydrothermal, Sea water
An important ore was discovered in the Furutobe deposit to represent the gold and silver mineralization of the kuroko type deposit. In this study, we interpret the formation of this ore and the precipitation process of gold and silver. This ore has a layered structure, and characteristic mineral assemblages are identified in each layer. These layers are classified into five types of mineral assemblages A to E, identified by microscopic and macroscopic observation.
Bornite is the main component of A zone and chalcocite, silver-bearing tetrahedrite, stromeyerite, chalcopyrite, galena, sphalerite, and barite are accessory components. This zone includes barite lens. Pyrite and bornite are main components of B zone and chalcopyrite, galena, tetrahedrite are associated. This zone have lens-shaped chalcopyrite. Pyrite, bornite and barite are main components of C zone and accessory components are galena, sphalerite, tetrahedrite, and electrum. The amount of bornite decreases away from B zone in this zone. Pyrite, chalcopyrite and barite are main components of D zone and sphalerite, galena, and tetrahedrite are accessory components. Bornite is rare in this zone. Zone E consists of barite, pyrite and chalcopyrite.
Barite and pyrite increase from A zone to E zone. Bornite and Cu-S minerals are more abundant on the A band side, and chalcopyrite, galena and sphalerite are abundant on the E band side.
Mineral paragenesis in this ore are determined as that pyrite is the earliest, followed by copper minerals, sphalerite, and galena. Although chalcopyrite and bornite occur in different layers, their paranegentic stages are similar. Chalcopyrite and bornite either fill the pyrite grain boundaries or substitute pyrite and other minerals. Electrum in A zone occurs as small droplets in silver-bearing tetrahedrite and chalcocite. Electrum in C zone occurs along the grain boundaries and in voids of chalcopyrite and pyrite.
The occurrence of ore minerals indicates that this ore formation started by a large amount of pyrite precipitation. The reason why pyrite is small in amount on the A zone side is that it is substituted by bornite and chalcopyrite. Bornite and Cu-S minerals are abundant on A zone side, and chalcopyrite is more abundant on E zone side. This indicates that A zone side ore was formed at higher temperature and higher sulfur fugacity than that of E zonee. From the above, this ore was originally void-rich, mainly composed of pyrite, and the void-rich layers in the ore becames the hydrothermal flow path. In the center of the hydrothermal path (zone A and B), a solution of relatively high temperature and high sulfidation flowed and precipitated bornite, chalcocite, and stromeyerite. In outer C zone, seawater was mixed to create a medium-temperature, high-to-medium sulfidation solution, which precipitated bornite, chalcopyrite, and tetrahedrite. Finally, due to rapid cooling and decreaseed sulfur fugacity of the solution led the precipitation of electrum to fill the grain boundaries of the previously crystallized minerals. More involvement of sea water formed barite, sphalerite and galena at the latest in the ore.
Bornite is the main component of A zone and chalcocite, silver-bearing tetrahedrite, stromeyerite, chalcopyrite, galena, sphalerite, and barite are accessory components. This zone includes barite lens. Pyrite and bornite are main components of B zone and chalcopyrite, galena, tetrahedrite are associated. This zone have lens-shaped chalcopyrite. Pyrite, bornite and barite are main components of C zone and accessory components are galena, sphalerite, tetrahedrite, and electrum. The amount of bornite decreases away from B zone in this zone. Pyrite, chalcopyrite and barite are main components of D zone and sphalerite, galena, and tetrahedrite are accessory components. Bornite is rare in this zone. Zone E consists of barite, pyrite and chalcopyrite.
Barite and pyrite increase from A zone to E zone. Bornite and Cu-S minerals are more abundant on the A band side, and chalcopyrite, galena and sphalerite are abundant on the E band side.
Mineral paragenesis in this ore are determined as that pyrite is the earliest, followed by copper minerals, sphalerite, and galena. Although chalcopyrite and bornite occur in different layers, their paranegentic stages are similar. Chalcopyrite and bornite either fill the pyrite grain boundaries or substitute pyrite and other minerals. Electrum in A zone occurs as small droplets in silver-bearing tetrahedrite and chalcocite. Electrum in C zone occurs along the grain boundaries and in voids of chalcopyrite and pyrite.
The occurrence of ore minerals indicates that this ore formation started by a large amount of pyrite precipitation. The reason why pyrite is small in amount on the A zone side is that it is substituted by bornite and chalcopyrite. Bornite and Cu-S minerals are abundant on A zone side, and chalcopyrite is more abundant on E zone side. This indicates that A zone side ore was formed at higher temperature and higher sulfur fugacity than that of E zonee. From the above, this ore was originally void-rich, mainly composed of pyrite, and the void-rich layers in the ore becames the hydrothermal flow path. In the center of the hydrothermal path (zone A and B), a solution of relatively high temperature and high sulfidation flowed and precipitated bornite, chalcocite, and stromeyerite. In outer C zone, seawater was mixed to create a medium-temperature, high-to-medium sulfidation solution, which precipitated bornite, chalcopyrite, and tetrahedrite. Finally, due to rapid cooling and decreaseed sulfur fugacity of the solution led the precipitation of electrum to fill the grain boundaries of the previously crystallized minerals. More involvement of sea water formed barite, sphalerite and galena at the latest in the ore.