17:15 〜 18:45
[HRE13-P10] Mineral textures and chemistry of the Utori vein group in the Osarizawa epithermal deposit, Akita, Japan
キーワード：Osarizawa deposit, ore petrography, textures of pyrite, mineral chemistry
The Osarizawa deposit in Akita is one of the largest vein-type Cu-Au deposits in Japan. The Osarizawa deposit was discovered in 708 and was mined until 1978 (Arribas and Mizuta, 2017). The deposit has approximately 560 veins (Arribas and Mizuta, 2017). In this study, we aim to present the vein mineralogy, textures, and chemistry of ore minerals in the Utori vein group of the deposit.
The main gangue mineral of the Utori vein group is quartz, which occurs mostly as comb and granular quartz with occasional feathery and zoned textures. Local cockade quartz is also found surrounding brecciated wall-rock fragments. Chlorite occurs abundantly in local areas, usually exhibiting acicular and fibrous textures. Some chlorite in the veins forms spherical aggregates near the wall-rock.
The most dominant ore minerals of the Utori vein group are pyrite and chalcopyrite. Chalcopyrite usually occurs as large euhedral to subhedral grains with abundant mineral inclusions of pyrite and quartz with sizes ranging from fine-grained to coarse-grained. Pyrite occurs mostly as fine-grained inclusions in chalcopyrite, or coarse-grained euhedral aggregates with fine-grained chalcopyrite blebs. Some pyrite aggregates show abundant triple junctions possibly caused by annealing. The most conspicuous texture of pyrite, however, is the ring-like texture formed by clusters of subhedral to euhedral pyrite, which is called atoll pyrite by England and Ostwald (1993). In the Utori vein group, the atoll pyrite is always surrounded by a chalcopyrite matrix but can surround chalcopyrite, other euhedral pyrite grains, and even voids. Another variation of the atoll pyrite is a group of subhedral pyrite with fibrous chalcopyrite exsolution arranged radially voids. Atoll pyrite with graphic texture was also found surrounding chalcopyrite and galena. Despite the significant variation of the atoll pyrite in the Utori vein group, all of them are interpreted to have been derived from the coagulation of originally dispersed hydrated iron sulfide to form framboid pyrite (England and Ostwald, 1993). The variations of atoll pyrite were instead attributed to different evolutionary paths such as addition of new mineral infillings, recrystallization, adsorption, and replacement by other minerals (England and Ostwald, 1993).
Other ore minerals in the Utori vein group include minor amounts of sphalerite and galena with local euhedral hematite that occurs as inclusions in quartz. Oxidized minerals were also found such as covellite replacing chalcopyrite along grain boundaries and fractures, and goethite that commonly fills the open spaces in the veins such as the surfaces of quartz or fractures inside chalcopyrite and pyrite grains.
References
Arribas, A., and Mizuta, T. (2017) Potential for Porphyry Copper Deposits in Northern Tohoku (or the Exploration Potential for Base and Precious Metal Deposits in Japan 2020). Resource Geology, 68, 144-163.
England, B.M., and Ostwald, J. (1993) Framboid-derived structures in some Tasman fold belt base-metal sulphide deposits, New South Wales, Australia. Ore Geology Reviews 7, 381–412.
The main gangue mineral of the Utori vein group is quartz, which occurs mostly as comb and granular quartz with occasional feathery and zoned textures. Local cockade quartz is also found surrounding brecciated wall-rock fragments. Chlorite occurs abundantly in local areas, usually exhibiting acicular and fibrous textures. Some chlorite in the veins forms spherical aggregates near the wall-rock.
The most dominant ore minerals of the Utori vein group are pyrite and chalcopyrite. Chalcopyrite usually occurs as large euhedral to subhedral grains with abundant mineral inclusions of pyrite and quartz with sizes ranging from fine-grained to coarse-grained. Pyrite occurs mostly as fine-grained inclusions in chalcopyrite, or coarse-grained euhedral aggregates with fine-grained chalcopyrite blebs. Some pyrite aggregates show abundant triple junctions possibly caused by annealing. The most conspicuous texture of pyrite, however, is the ring-like texture formed by clusters of subhedral to euhedral pyrite, which is called atoll pyrite by England and Ostwald (1993). In the Utori vein group, the atoll pyrite is always surrounded by a chalcopyrite matrix but can surround chalcopyrite, other euhedral pyrite grains, and even voids. Another variation of the atoll pyrite is a group of subhedral pyrite with fibrous chalcopyrite exsolution arranged radially voids. Atoll pyrite with graphic texture was also found surrounding chalcopyrite and galena. Despite the significant variation of the atoll pyrite in the Utori vein group, all of them are interpreted to have been derived from the coagulation of originally dispersed hydrated iron sulfide to form framboid pyrite (England and Ostwald, 1993). The variations of atoll pyrite were instead attributed to different evolutionary paths such as addition of new mineral infillings, recrystallization, adsorption, and replacement by other minerals (England and Ostwald, 1993).
Other ore minerals in the Utori vein group include minor amounts of sphalerite and galena with local euhedral hematite that occurs as inclusions in quartz. Oxidized minerals were also found such as covellite replacing chalcopyrite along grain boundaries and fractures, and goethite that commonly fills the open spaces in the veins such as the surfaces of quartz or fractures inside chalcopyrite and pyrite grains.
References
Arribas, A., and Mizuta, T. (2017) Potential for Porphyry Copper Deposits in Northern Tohoku (or the Exploration Potential for Base and Precious Metal Deposits in Japan 2020). Resource Geology, 68, 144-163.
England, B.M., and Ostwald, J. (1993) Framboid-derived structures in some Tasman fold belt base-metal sulphide deposits, New South Wales, Australia. Ore Geology Reviews 7, 381–412.