Role of boiling for the formation of vertical metal zoning and bonanza zone in low-sulfidation epithermal gold‑silver deposits: A geochemical modeling approach

Boiling and mixing are thought to be the most important processes for mineralization in the low-sulfidation epithermal gold‑silver deposits. Some low-sulfidation epithermal gold‑silver deposits show vertical zoning of metals including gold, silver, copper, lead, and zinc, but effect of boiling and mixing for the vertical zoning of metals through mineralization has been scarcely debated. Therefore, we applied geochemical modeling for an open-isenthalpic boiling system, with a hydrothermal fluid with different initial temperatures, carbon dioxide contents, salinities, and total sulfur contents to numerically evaluate the effect of boiling phenomenon for the vertical zoning in low-sulfidation epithermal gold‑silver deposits. Geochemical modeling of boiling phenomenon suggests that boiling is effective for depositing gold, silver, copper, lead, and zinc and that it could form a bonanza zone enriched in gold and silver in the shallow zone above the lead and zinc-rich zone. Temperature, carbon dioxide content, salinity, and hydrogen sulfide content affect the deposited amount of metals, vertical extent, depth of mineralization, and gold/silver ratio of the ore zone, although other factors such as permeability, gangue mineralogy, duration of mineralization, kinetics of mineral precipitation, change in porosity, thermal conductivity, and fractures of host rocks could affect mineralization as well. Our modeling of boiling phenomenon demonstrates similar characteristics with natural low-sulfidation epithermal gold‑silver deposits, suggesting that boiling is an important mechanism to cause vertical metal zoning in the low-sulfidation epithermal gold‑silver deposits, even though mixing could also be important to precipitate metals in some deposits.