Pyrite trace element proxies for magmatic volatile influx in submarine subduction-related hydrothermal systems

Abstract

Listen

Translate article

Seafloor massive sulfides represent modern analogues to ancient volcanogenic massive sulfide deposits, which can be particularly enriched in volatile and precious metals (e.g., Te, Au, Ag, Cu, Bi, Se) in subduction-related systems. However, it remains unclear whether the influx of magmatic volatiles has a systematic control on the metal endowment of submarine hydrothermal mineralization on the plate-tectonic scale. Using a novel microanalytical approach based on the coupling of SIMS δ34S with trace element LA-ICP-MS on a scale of ~25 µm in pyrite from 11 submarine hydrothermal systems, we could demonstrate for the first time that the Te, As, and Sb contents and the ratios of these elements vary systematically with the δ34S composition of hydrothermal pyrite and native S. In contrast to trace element concentrations, Te/As and Te/Sb show a more significant correlation with δ34S in pyrite, indicating that element ratios provide a more robust record of metal sourcing. On this basis, we define a quantitative trace element threshold of high Te/As (>0.004) and Te/Sb (>0.6) ratios in pyrite that can be used to identify the influx of magmatic volatiles to submarine subduction-related hydrothermal systems independent of δ34S isotope measurements. Two-component fluid mixing simulations further suggest that even small amounts (<0.5 to ~5%) of magmatic volatile influx drastically change the Te/As (and Te/Sb) ratio of the modelled fluid, but only slightly modify its δ34S composition. Hence, Te/As and Te/Sb ratios are more sensitive in recording the influx of magmatic volatiles into submarine hydrothermal systems than S isotope systematics, which are typically influenced by seawater-derived S leading to ambiguous δ34S signatures. We conclude that Te/As and Te/Sb systematics in pyrite provide a robust proxy to evaluate the metal sources in submarine hydrothermal systems from the grain to plate-tectonic scale.