Shallow magma storage and degassing at Brothers volcano, Kermadec arc: implications for submarine hydrothermal processes and seafloor mineralisation

Magmas degas volatiles and metals into submarine arc-related hydrothermal systems, but the nature of this process and its role in seafloor massive sulfide (SMS) mineralisation are poorly understood, particularly in submarine arcs hosting Cu–Au-rich deposits. To address this, we studied glassy melt inclusions (MIs) and matrix glasses from Brothers submarine volcano in the Kermadec arc, where there is strong evidence for magmatic involvement in hydrothermal processes and Cu-Au-rich mineralisation. We present a new dataset of volatile contents (H2O, CO2, S, Cl), sulfur speciation, and trace element concentrations for these materials, including samples recovered from IODP drill cores. Primitive basaltic MIs from a nearby mafic ridge imply that the volatile contents and fO2 (~ FMQ + 1.5) of parental Brothers melts are within the normal range for arc magmas, but they contain high concentrations of some chalcophile metals (notably, Cu). Silicic (dacite/low-SiO2 rhyolite) MIs from the main Brothers edifice record degassing of an S-bearing vapour phase under oxidising conditions (fO2 ~FMQ + 2) at very shallow depths (~ 1–3 km below seafloor). These vapours are Cl- and metal-poor, but trigger acid sulfate alteration and likely provide a dilute source of trace metalloids, plus some Au and Cu. The Cl contents (max = 6480 μg g−1) of silicic Brothers MIs are among the highest known for any subalkaline arc magmas. Increasing Cl contents with differentiation indicate a progressive approach to brine saturation, inferred to be reached only at late stages of crystallisation when the magma is no longer eruptible. Such late, shallow degassing of Cl is traditionally viewed as unfavourable for metal extraction, contrasting with deeper and earlier degassing in subaerial porphyry systems. The unusually high Cu and Au grades of Brothers SMS mineralisation, however, imply that significant quantities of metal are mobilised into the hydrothermal system. We suggest that the adverse effects of late Cl degassing on metal extraction at Brothers are offset by (1) very shallow magma storage, providing circulating hydrothermal fluids direct access to dense metal-rich brines and magmatic sulfides, and (2) elevated metal contents in primitive parental Brothers melts, which enhance the overall magmatic metal budget. The Brothers example suggests that these factors may be more important controls on Cu–Au‑rich mineralisation in submarine arcs than in subaerial arcs built on thicker crust, where more favourable degassing regimes can develop. Ultimately, Brothers demonstrates that arc-related SMS-forming systems can be strongly magmatically influenced and share both commonalities and differences with Cu-Au mineralised subaerial systems.