Beyond vapours and brines: physical sulfide transport as a missing link in metal transfer at Brothers submarine volcano, with comparison to the Hellenic arc

Recent experimental and theoretical studies show that although deep sulfide-rich cumulates may sequester metals at depth, subsequent sulfide resorption during decompression and oxidation, together with physical flotation and transport of sulfide droplets, can strongly influence Cu–Au mobility in evolving magmatic systems and ultimately their ore potential. Despite its importance for metal redistribution between deep magmatic reservoirs, transient storage during ascent, and shallow ore-forming environments, the physical behaviour of magmatic sulfides during magma ascent and degassing remains poorly constrained.

Here we present new petrographic and geochemical observations from the active Brothers submarine volcano, focusing on sulfide- and vesicle-rich enclaves that provide direct evidence for sulfide–volatile interaction during shallow magma evolution. Optical microscopy, EPMA sulfide chemistry, and quantitative mineral mapping (QEMSCAN) indicate that most Brothers samples host compositionally similar arc-type magmatic sulfides (dominantly pyrrhotite ± minor chalcopyrite). However, cone lavas are markedly more sulfide-rich (0.004–0.02 area %, with groundmass sulfides 500 μm to 1.5 mm in size) than caldera lavas (≤0.003 area %, with sulfide inclusions <50 μm). Rare and diverse, previously undocumented sulfide-rich enclaves (10–30 area %) occur within highly vitrophyric (>90% glass) host lavas at hydrothermally active (NWC) and extinct (EC) Caldera sites, as well as within the Cone lavas. The latter, Cone-hosted enclaves represent earlier, more evolved (rhyolitic) melts relative to their dacitic hosts and locally preserve comparatively intact sulfide textures. In contrast, most Caldera-hosted enclaves display pervasive sulfide dissolution, resorption embayments, and replacement by Fe–Ti oxides, indicating efficient sulfide destabilisation and enhanced metal release during shallow degassing. These sulfide–vesicle–oxide textures closely resemble compound sulfide–vapour droplets described from Nea Kameni and Kolumbo in the Hellenic arc, suggesting a broader relevance of such phases in submarine magmatic–hydrothermal systems. An exception is a single cumulate enclave from the NWC containing exclusively Cu-rich magmatic sulfides (chalcopyrite + bornite; ~0.003 area %), confirming the presence of deep, cryptic sulfide-rich cumulates capable of scavenging significant metal contents at depth. Ongoing in situ LA-ICP-MS mapping of sulfides and their replacement products will further constrain the behaviour of Au, PGE, Se, and Te during these processes.

Taken together, these results provide the first direct petrographic evidence linking deep Cu-rich cumulates and transient sulfide-rich enclaves to volatile exsolution and late-stage degassing at Brothers submarine volcano. In a system dominated by late, shallow degassing, as independently constrained by melt inclusions and apatite, sulfide resorption and physical transport can dominate metal redistribution during magma ascent, with sulfide–vesicle compound phases acting as efficient transient carriers linking deep magmatic processes to shallow SMS mineralisation and offering insights relevant to porphyry-type systems.