Sources and enrichment processes of metal(loid)s in submarine back-arc hydrothermal systems: A combined Se and multiple S isotope approach

• Applicant: Dr. Manuel Keith
• Subject Area: Geology ; Mineralogy, Petrology and Geochemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 467339755

Hydrothermal systems associated with subduction zone volcanism host some of the world’s most important volcanogenic massive sulphide (VMS) deposits. Many of them formed in back-arc rifts, where seafloor massive sulphides occur as their modern analogues, where base (e.g., Cu, Zn, Pb), precious (e.g., Ag, Au) and rare metals (e.g., As, Se, Sb, Hg, Tl) may significantly be enriched compared to ancient VMS deposits that are currently mined on land. Many of these elements (e.g., Se, Sb, Tl) have a strong affinity to volatiles derived from magmas, but it is still unclear how magma degassing contributes to the metal budget of submarine hydrothermal mineralisation, which is particularly true for trace elements that are historically understudied. This project aims to quantify the geochemical fluxes in submarine back-arc hydrothermal systems by constraining the effect of (1) magmatic volatile influx, (2) variable host rock compositions and (3) metal fractionation in the hydrothermal upflow zone on the composition of active seafloor and ancient VMS mineralisation. This requires a continuous sample spectrum along hydrothermal upflow zones towards the seafloor, which can be achieved by combining drill core samples from modern oceanic crust with samples from ancient hydrothermal systems associated with VMS mineralisation. A cutting-edge analytical approach combining stable Se and multiple S isotopes with trace element micro-analysis will establish new fundamental concepts linking metal transport from source to sink in submarine hydrothermal environments. Preliminary results show that the claimed objectives are feasible, since seawater-derived Se does not influence the Se isotope signature of seafloor massive sulphides, which is a striking advantage compared to the commonly used S isotope system. Thus, Se isotopes have a high potential to pinpoint crustal metal sources even in seawater-dominated environments, as indicated by comparable Se isotope signatures between sulphide precipitates and the adjacent host rocks. In addition, preliminary evidence suggests that Se isotope fractionation is minor during high temperature fluid upflow, but it remains an open question, whether the contribution of magmatic volatiles can be tracked by Se isotopes according to the S isotope system. It is also unclear, whether hydrothermal and microbial reworking, causing metal remobilisation in active seafloor hydrothermal systems, also modifies related Se isotope signatures, which would have important implications for the interpretation of such data from ancient VMS deposits. These aspects are the focus of this continuation request, which will ultimately constrain, whether Se isotope systematics can trace different crustal metal sources in both active and ancient submarine hydrothermal environments.

• DFG Programme: Infrastructure Priority Programmes
• Subproject of: SPP 527: Bereich Infrastruktur - "International Ocean Discovery Program" (IODP)
• International Connection: Spain, USA
• Cooperation Partners: Dr. Stephan König; Dr. Andrew Martin