Trace element and sulfur isotope study of sulfide chimneys from the mafic-hosted Daxi Vent Field: Implications on the genesis of seafloor massive sulfide deposit at a segment end

Abstract

The mafic-hosted Daxi Vent Field (DVF) on the Carlsberg Ridge hosts an actively forming massive sulfide deposit on the neovolcanic ridge at a non-transform offset (NTO). Here, we present mineralogy, geochemistry and in situ sulfur isotope analysis of the sulfide chimneys sample from the DVF, which can be classified into Fe-rich and Zn-rich types based on the major sulfide minerals. Our results show that the Fe-rich chimneys, primarily consisting of pyrrhotite and isocubanite, exhibit enrichment in Co, Se, and Ni, with heavy δ³⁴S in pyrite (up to 17.38 ‰). The highly saline fluids resulting from phase separation processes lead to the significant Co enrichment. The precipitation of pyrite in a relatively closed environment intensified by the Rayleigh fractionation, leading to significant heavy S isotopic composition. On the other hand, the Zn-rich chimneys are predominantly sphalerite, and enriched in Sn, Ag, As, Ga, Ge, and Cd. The fluid-seawater mixing processes contribute to the elevated Sn levels and the wide range of S isotopic compositions (δ³⁴S = 1.47–8.34 ‰) in sphalerite. The unusual Co and Sn enrichment as well as unique S isotopic composition in the DVF are linked to the thin ocean crust at NTOs, where the thin ocean crust may favor interaction of fluids circulating in the subsurface with ultramafic rocks, despite the absence of ultramafic lithologies on the seafloor. This study demonstrates the exploration potential for polymetallic resources in the NTO along slow-spreading mid-ocean ridges.