Chemical exchanges between the lithosphere-ocean-atmosphere are focused at the active plate boundaries and are important in the global geochemical balance. Eventually, they may influence climate and impact on human activities. Hydrothermal circulation at these boundaries is the engine that transfers chemicals between the lithosphere and ocean. However, the structure of lithospheric contribution (hydrothermal leaching of solid rocks vs. direct mantle degassing) to the ocean chemistry is poorly understood. Insight into this problem (sources of metals and location of ore deposits) becomes increasingly important with the current interest in mining seafloor deposits and has high societal relevance. Unique seafloor hydrothermal processes in back-arc basins (BAB) produce a wide range of vent fluid chemistry and mineral deposits, and are considered as critically important for the genesis of volcanogenic massive sulfide deposits. While it has been suggested that input of magmatic fluids plays a crucial role in the chemistry of the BAB hydrothermal systems, its contribution as source of both transition and precious metals, has been a matter of debate. We propose a research project aimed at providing new constraints to define the relative role of metal-rich magmatic fluids and sub-seafloor metal precipitation/remobilization in BAB mineralization. We will achieve this goal studying non-traditional isotope (Cd-Zn-Sb) systematic of seafloor hydrothermal systems (substrate rock-fluid-deposit) in a spectrum of seafloor hydrothermal fields (from basalt- to dacite-hosted) in 2 BAB (Manus, Lau). This project is backed-up by results from previous studies that suggest contribution of magmatic volatiles, and that the available sample set is ideal for testing our hypotheses. It is expected that insight gained from Cd-Zn-Sb isotope systematic has great potential to improve our understanding of fundamental sub-seafloor processes responsible for metallogenesis in BAB hydrothermal systems.