Trace-element distribution and ore-forming processes in Au–Ag-rich hydrothermal chimneys and mounds in the TA25 West vent field of the Tonga Arc

• Published in: Mineralium deposita 2023, Vol.58 (1), p.135-160
• Main Authors: Choi, Sun Ki, Pak, Sang Joon, Park, Jung-Woo, Kim, Hyun-Sub, Kim, Jonguk, Choi, Sang-Hoon
• Format: Article
• Language: English
• Subjects: Anhydrite; Barite; Boiling; Carbon dioxide; Chalcopyrite; Chemical analysis; Chemical precipitation; Crystallization; Distribution; Earth and Environmental Science; Earth Sciences; Enargite; Fluids; Geochemistry; Geology; Gold; Gypsum; High temperature; Hydrothermal fields; Hydrothermal plumes; Hydrothermal systems; Lava; Low temperature; Magma; Mercury; Metals; Mineral assemblages; Mineral Resources; Mineralization; Mineralogy; Minerals; Mounds; Nanoparticles; Petrography; Petrology; Plumes; Pyrite; Redox properties; Seawater; Selenium; Silver; Solid solutions; Sphalerite; Sulfates; Sulfides; Sulphides; Trace elements; Water analysis; Zincblende
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-022-01136-w

We report detailed mineralogy and geochemistry of hydrothermal mounds and chimneys in the TA25 West vent field (TA25 WVF), a newly discovered magmatic–hydrothermal system in the Tonga (Tofua) arc. Chimney samples are classified as sulfate- or sulfide-rich, based on major sulfide, sulfosalt, and sulfate minerals. The former type represents a simple mineral assemblage of predominance of anhydrite/gypsum + barite + pyrite, whereas the sulfide-rich chimneys show three different stages of mineralization with decreasing fluid temperature: sphalerite–pyrite dominated stage I, sphalerite–sulfosalts dominated stage II, and stage III is dominated by seawater alteration. Mound samples are characterized by sulfide assemblages and paragenesis similar to those of sulfide-rich samples, but abundant chalcopyrite indicates a relatively high-temperature mineralization. The chimney and mound samples are enriched in Au (average 9.2 ppm), Ag (297 ppm), As (1897 ppm), Sb (689 ppm), Hg (157 ppm), and Se (34.6 ppm). LA–ICP–MS and FE–TEM studies indicate that most of these elements occur in sulfides or sulfosalts in solid solution, although some occur as nanoparticles. This is mainly controlled by the combined effects of fluid conditions (temperature and redox state) and influx of ambient seawater. Petrography and trace-element compositions of sulfides and/or sulfosalts suggest that most concentrations of Au and Ag in the TA25 WVF result from the precipitation and/or adsorption of Au–Ag-bearing nanoparticles on rapidly crystallized sulfides, the substitution of Au and Ag in sulfide and/or sulfosalt minerals, and the saturation of Ag in hydrothermal fluids during late, relatively low-temperature mineralization (