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

Abstract We report detailed mineralogy and geochemistry of hydrothermal mounds chimneys in the TA25 West vent field (TA25 WVF), a newly discovered magmatic–hydrothermal system Tonga (Tofua) arc. Chimney samples are classified as sulfate- or sulfide-rich, based on major sulfide, sulfosalt, sulfate minerals. The former type represents simple mineral assemblage predominance anhydrite/gypsum + barite pyrite, whereas sulfide-rich show three different stages mineralization with decreasing fluid temperature: sphalerite–pyrite dominated stage I, sphalerite–sulfosalts II, III is by seawater alteration. Mound characterized sulfide assemblages paragenesis similar to those samples, but abundant chalcopyrite indicates relatively high-temperature mineralization. chimney mound enriched Au (average 9.2 ppm), Ag (297 As (1897 Sb (689 Hg (157 Se (34.6 ppm). LA–ICP–MS FE–TEM studies indicate that most these elements occur sulfides sulfosalts solid solution, although some nanoparticles. This mainly controlled combined effects conditions (temperature redox state) influx ambient seawater. Petrography trace-element compositions and/or suggest concentrations WVF result from precipitation adsorption Au–Ag-bearing nanoparticles rapidly crystallized sulfides, substitution sulfosalt minerals, saturation fluids during late, low-temperature (< 150 °C). maximum measured temperature (242 °C) venting calculated formation temperatures sphalerite (229–267 ℃) below boiling at depths (966–1096 m) WVF, suggesting had little effect Au–Ag-rich WVF. presence enargite–tetrahedrite–tennantite assemblages, high magma-derived (e.g., Au, Ag, As, Sb, Hg, Se), low δ 34 S values (2.1 4.3‰) minerals relative host rocks, distribution CO 2 -rich plumes (500 1000 ppm) submarine influenced magmatic contribution an arc setting. Our results likely play important role supplying various metals, including Subsequently, rapid crystallization induced fluid-seawater mixing significantly contributes