Pyrite Records of Episodic Venting and Metal Enrichment at the Xunmei Hydrothermal Field, South Mid-Atlantic Ridge

The hydrothermal system serves as a critical conduit for heat transfer, typically evolving from low-temperature diffuse venting to high-temperature focused venting, or vice versa, which governs the enrichment of trace metals and their spatial distribution in pyrite. At the Xunmei hydrothermal field (26°S) on the South Mid-Atlantic Ridge, hosted in N-MORB, morphologically different pyrites provide a continuous record of the complete hydrothermal fluid evolution. This study utilized these varied pyrites to reveal the evolution of ore-forming fluids and the (re)distribution of metals driven by multi-stage episodic hydrothermal activities. Petrographic analysis identifies two mineralization stages, i.e., chimney growth dominated by high-temperature focused venting, and subsequent sulfide mound formation overprinted by late-stage diffuse venting fluids. Coupled in-situ analyses of trace elements and sulfur isotopic compositions of morphologically distinct pyrites indicate that chimney formation involved seawater mixing, magmatic degassing, and ascent of chlorine-rich magmatic fluid, with the magmatic fluid being the predominant ore-forming fluid. Thermodynamic conditions gradually stabilized, and the overgrowth of sulfides by amorphous silica suggests subsequent system cooling. Melt inclusions within plagioclase phenocrysts confirm magmatic degassing, while metallic minerals on inclusion bubble walls and residual metallic minerals in the melt phase demonstrate that ore-forming metals preferentially partition into the gas phase during magmatic immiscibility. Sulfide mound development resulted from chimney collapse, internal fluid recirculation, seawater infiltration, and overprinting by diffuse fluids. Metal enrichment in pyrites correlates with specific mineralization processes. Seawater mixing enriches Tl, V, and Mo. Magmatic degassing is associated with anomalous enrichment of Te, Au, and Cu. High-temperature magmatic influx elevates Se and Co concentrations, further enhanced by internal fluid circulation. Seawater-sulfide interaction induces a galvanic effect, leading to the removal of Zn, Ga, and Cd from the hydrothermal system. This study systematically elucidates the metallogenic mechanisms driven by multi-stage episodic fluid evolution at the Xunmei hydrothermal field, confirms the direct contribution of magmatic fluids to mineralization, and provides theoretical support for prospecting and resource evaluation of hydrothermal systems on slow-spreading mid-ocean ridges.