14:15 〜 14:30
[HRE11-02] モデル計算を用いた低硫化型浅熱水性金銀鉱床における鉱化の垂直分布及び富鉱部形成のシミュレーション
キーワード：鉱床学、地化学モデリング、低硫化型浅熱水性金銀鉱床
Boiling and mixing are considered to be the most important processes for mineralization in the low-sulfidation epithermal gold-silver deposits. Evidence of boiling is often found in the deposits such as the presence of lattice-bladed quartz and fluid inclusions showing boiling evidence based on coexistence of liquid-rich and vapor-rich inclusions. Even though boiling is likely to be more important for the formation of low-sulfidation epithermal gold-silver deposits than mixing, base metal-rich low-sulfidation epithermal gold-silver deposits tend to have less evidence of boiling in the ore vein texture as well as fluid inclusions, suggesting that mixing may have been important for the auriferous mineralization. Some low-sulfidation epithermal gold-silver deposits show vertical zoning of metals including gold, silver, copper, lead, and zinc (e.g., National district, USA, Apacheta deposit, Peru, and Hishikari deposit, Japan, Guanajuato district, Mexico), however effect of boiling and mixing for the vertical zoning of metal through mineralization have been scarcely debated. Therefore, we apply geochemical modeling for an open-isenthalpic boiling system as well as the mixing system, with different initial temperatures, carbon dioxide content, salinity, and total sulfur content for the low-sulfidation epithermal gold-silver deposits to numerically evaluate the effect of boiling and mixing phenomenon for the vertical zonation of gold-silver deposits.
Geochemical modeling of the mixing phenomenon suggests that mixing of hydrothermal fluid with steam-heated acid sulfate water is effective for depositing a significant amount of gold, but not for other metals such as silver, copper, lead, and zinc. Since the pH of the hydrothermal fluid decreases after the mixing with steam-heated acid sulfate water, it would not deposit a significant amount of metals except for gold. In contrast, mixing of hydrothermal fluid with carbon dioxide-rich steam-heated water is effective for depositing gold, silver, copper, lead, and zinc. Since the mixing of hydrothermal fluid with carbon dioxide-rich steam-heated water would decrease the temperature with a relatively constant pH (near neutral pH), mixing is effective for depositing gold, silver, copper, lead, and zinc. If the mixing with carbon dioxide-rich steam-heated water occurs from a deeper to a shallower depth, gold, silver, and zinc would be rich in the deeper zone, while lead and copper would be rich in the shallower zone.
Geochemical modeling of boiling phenomenon suggests that boiling is effective for depositing gold, silver, copper, lead, and zinc. Temperature, carbon dioxide content, salinity, and hydrogen sulfide content affect the amount of metals, vertical extent, depth of mineralization, and gold/silver ratio. Hydrothermal fluid with low temperature, low salinity, and high hydrogen sulfide content would form a deposit with a high gold/silver ratio, a narrow vertical interval of gold-silver rich zone, while fluid with high temperature, high salinity, low hydrogen sulfide content would form a deposit with low gold/silver ratio, wide vertical interval of gold-silver rich zone. In addition, gold and silver tend to deposit in the shallower zone above the deepest zone from the boiling initiation depth, while lead and zinc tend to deposit in the deeper zone, especially when the fluid has high lead and zinc content. In a summary, initial boiling temperature, carbon dioxide content, salinity, and total sulfur content in the hydrothermal fluid affect the vertical metal zoning and deposited gold/silver ratio. Our modeling of boiling phenomenon demonstrates similar characteristics with natural low-sulfidation epithermal gold-silver deposits rather than the modeling of mixing phenomenon, suggesting that boiling is likely to be more important for the formation of metal zoning in the low-sulfidation epithermal gold-silver deposits.
Geochemical modeling of the mixing phenomenon suggests that mixing of hydrothermal fluid with steam-heated acid sulfate water is effective for depositing a significant amount of gold, but not for other metals such as silver, copper, lead, and zinc. Since the pH of the hydrothermal fluid decreases after the mixing with steam-heated acid sulfate water, it would not deposit a significant amount of metals except for gold. In contrast, mixing of hydrothermal fluid with carbon dioxide-rich steam-heated water is effective for depositing gold, silver, copper, lead, and zinc. Since the mixing of hydrothermal fluid with carbon dioxide-rich steam-heated water would decrease the temperature with a relatively constant pH (near neutral pH), mixing is effective for depositing gold, silver, copper, lead, and zinc. If the mixing with carbon dioxide-rich steam-heated water occurs from a deeper to a shallower depth, gold, silver, and zinc would be rich in the deeper zone, while lead and copper would be rich in the shallower zone.
Geochemical modeling of boiling phenomenon suggests that boiling is effective for depositing gold, silver, copper, lead, and zinc. Temperature, carbon dioxide content, salinity, and hydrogen sulfide content affect the amount of metals, vertical extent, depth of mineralization, and gold/silver ratio. Hydrothermal fluid with low temperature, low salinity, and high hydrogen sulfide content would form a deposit with a high gold/silver ratio, a narrow vertical interval of gold-silver rich zone, while fluid with high temperature, high salinity, low hydrogen sulfide content would form a deposit with low gold/silver ratio, wide vertical interval of gold-silver rich zone. In addition, gold and silver tend to deposit in the shallower zone above the deepest zone from the boiling initiation depth, while lead and zinc tend to deposit in the deeper zone, especially when the fluid has high lead and zinc content. In a summary, initial boiling temperature, carbon dioxide content, salinity, and total sulfur content in the hydrothermal fluid affect the vertical metal zoning and deposited gold/silver ratio. Our modeling of boiling phenomenon demonstrates similar characteristics with natural low-sulfidation epithermal gold-silver deposits rather than the modeling of mixing phenomenon, suggesting that boiling is likely to be more important for the formation of metal zoning in the low-sulfidation epithermal gold-silver deposits.