Microchemical and sulfur isotope variation in pyrite-marcasite: A proxy for low-temperature ore-forming processes in submarine hydrothermal systems

Pyrite-marcasite assemblages, formed at steep physicochemical gradients in submarine hydrothermal systems, serve as valuable records of fluid evolution and ore-forming processes. However, gaps remain in our understanding of the detailed pathways of pyrite-marcasite formation and their dynamics of trace element enrichment in seafloor massive sulfide deposits. Here, we utilize in-situ trace element and sulfur isotope analysis to systematically explore the microchemical variations within pyrite-marcasite assemblages from the basalt-hosted Xunmei and Niaochao vent fields on the slow- and fast-spreading ridges, respectively. Diverse pyrite-marcasite pairs were identified across sulfide chimneys based on grain size, texture, and mineral sequence, each displaying distinct microchemical characteristics. Colloform marcasite-pyrite intergrowths, formed at the outermost chimney walls, exhibit metal enrichment and isotopically light sulfur in marcasite (δ³⁴S values as low as 0.72 ‰) compared to pyrite, attributed to high surface-to-volume ratios and sulfur isotope exchange during the rapid crystallization of marcasite. In the transition layers, the assemblage is formed through the replacement of subhedral pyrite with anhedral marcasite, resulting in the redistribution of metals (e.g., As, Ag, Au, Co, Pb) and the creation of metal-rich zones. Variations in sulfur isotopes within this pair are linked to the reduction of anhydrite by hot fluids. In contrast, the dense pyrite-marcasite assemblages, formed through recrystallization processes, show metal depletion. Their distinctly high δ³⁴S values (mean: 4.61 ‰, n = 11) are caused by sulfate reduction in the subseafloor hydrothermal stockwork. During seawater alteration, the marcasite pseudomorphs after pyrrhotite, cemented by anhedral pyrite in the middle layer, are enriched in hydrothermal metals such as Sb, Co, Zn, Cu, and Au, likely due to the adsorption reaction. Meanwhile, the secondary marcasite exhibits elevated δ³⁴S values (up to 6.41 ‰) as a result of anhydrite reduction via pyrrhotite oxidation. These microchemical patterns of pyrite-marcasite assemblages provide critical insights into the redistribution and re-enrichment of metals during low-temperature mineralization and replacement in submarine hydrothermal systems.