Sulfide ore-forming processes at the Longqi-1 hydrothermal field, Southwest Indian Ridge: Constraints from trace element and sulfur isotopic compositions

The Longqi-1 hydrothermal field (LQ-1 HF), the first active high-temperature vent field discovered along the ultraslow-spreading Southwest Indian Ridge (SWIR), provides a valuable setting for studying seafloor sulfide mineralisation. However, spatially resolved constraints on physicochemical gradients during chimney growth remain limited. This study presents mineralogical, trace element, and sulfur isotopic analyses of Cu-rich, Zn-Fe-rich, and Zn-rich chimneys from the LQ-1 HF, revealing systematic mineral zonation and metal enrichment across sulfide chimney walls. The Cu-rich chimney displays well-developed zonation, transitioning from anhydrite, pyrite, or marcasite with minor sphalerite at the exterior to massive chalcopyrite at the interior. The presence of bornite, covellite, and digenite suggests episodic seawater infiltration and oxidation during chimney evolution. Pyrite from outer zones is enriched in Mn, Mo, Tl, U, and V due to low-temperature precipitation from seawater mixing, while increasing Co in pyrite and Se in chalcopyrite toward the interior reflect higher temperatures and conditions favouring the stability of Cu-rich sulfides. In contrast, the Zn-Fe-rich and Zn-rich chimneys exhibit colloform to dendritic pyrite, marcasite, and sphalerite with minor chalcopyrite, consistent with lower-temperature, seawater-influenced precipitation conditions. Sulfur isotope gradients (δ34S: 4.5–9.9‰) reflect a mixed sulfur source from seawater sulfate and host rocks, with early-formed zones more seawater influenced. Elevated Sn and low Cd/Zn ratios are consistent with metal inputs from a heterogeneous lithological source, potentially involving ultramafic components and possibly facilitated by deep detachment faults. These results reveal the influence of host-rock composition, temperature, redox state, and seawater mixing on sulfide mineralisation at the detachment fault-controlled LQ-1 HF on the ultraslow-spreading SWIR.