Mineralogy, geochemistry, and S isotope investigation of seafloor massive sulfides from the Yunzang hydrothermal field, South Atlantic Ridge

Abstract

Research on the ore-forming model of modern seafloor hydrothermal sulfide deposits is of significant economic and scientific importance. The limited research on recently discovered seafloor sulfide deposits has resulted in a vague comprehension of water-rock interactions, metal origins, and the subsequent metal deposition process. The Yunzang hydrothermal field is located on the South Mid-Atlantic Ridge and represents a newly discovered seafloor hydrothermal sulfide deposit. However, the geological features, primary mineral assemblages of ores, and the source regions of metals in the hydrothermal field remain unclear. In this study, sulfides were analyzed using a TESCAN Integrated Mineral Analyser (TIMA) and in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to investigate trace element distributions and S isotope compositions. Analysis by TIMA shows that the Yunzang hydrothermal field is mainly composed of pyrite, chalcopyrite, and sphalerite. Two types of ore with distinct characteristics were identified, one rich in chalcopyrite and the other in sphalerite. Within these ores, two different types of pyrite were identified, namely colloform and euhedral pyrite. Colloform pyrite formed at low temperature in outer margins of the chimney is enriched in Mn, V, Ag, Tl and Cd. Euhedral pyrite formed at high temperature in inner zone of the chimney is enriched in Co, Se, Cu and Sn. The variation in trace elements within pyrite is primarily governed by the synergistic effects of fluid conditions (such as temperature and redox state) and the influx of ambient seawater that accompanies the chimney formation process. The higher concentrations of Mo and V in the pyrite found in the outer wall of the chimney indicate significant interactions between seawater and hydrothermal fluids. The lower Au/As ratio in pyrite proves that gold exists within the lattice of pyrite rather than in nano-particles. The lower concentrations of Sn and Bi in chalcopyrite suggest that ore-forming materials may primarily originate from leaching of the surrounding rocks, with a lack of magmatic volatile components. The Fe/Zn ratio in sphalerite indicates that the Yunzang hydrothermal field belongs to sediment-starved SMS, with the formation temperature of sphalerite ranging from 242 to 262 °C. The positive δ³⁴SV-CDT values of pyrite (1.0 ‰–3.7 ‰) suggest 83 %–96 % of S was derived from the leaching of basement basalts, 4 %–17 % from oxidation of seawater sulfate. Based on these analytical results, the growth history of chimney and the fluid source in the Yunzang hydrothermal field are proposed.