Magmatic-hydrothermal ore systems are our primary source of copper (Cu)a globally critical metal. Actively mineralising hydrothermal systems in modern arc and backarc environments are new potential sources of Cu as well as natural laboratories for understanding ore-forming processes. A fundamental problem is all massive sulfide deposits on the seafloor are associated with hydrothermal systems, but not all hydrothermal systems are mineralised and produce seafloor massive sulfide deposits. The

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... pose of this research is to understand why some arc-backarc hydrothermal systems produce Cu-rich seafloor massive sulfide deposits. Current hypotheses argue for direct magmatic contributions of Cu to hydrothermal systems suggesting an active role of magmatism in metal sourcing and enrichment, as interpreted for other styles of, and on-land examples of magmatic-hydrothermal ore systems. This study focuses on Solwara 1 Cu-Au seafloor massive sulfide deposit, located in the actively rifting East Manus Basin, Papua New Guinea. The project has taken a multi-scaled approach investigating regional tectonic controls as well as the local volcanic architecture and magma petrogenesis to improve our understanding of the drivers for seafloor massive sulfide formation. Regional analysis indicates the East Manus Basin exhibits three distinct phases of rifting, and where significant Cu-rich mineralisation is associated with the transition between Phase 1 and 2 of rifting. At the volcanic edifice scale, the andesitic to dacitic volcanic rocks of Suzette constitute the most Cu enriched samples in the East Manus Basin. These rocks can be separated into at least two magmas, a tholeiitic and a calc-alkalic suite. Least squares modelling suggests both fractional crystallisation and magma mixing can account for the compositions of the Suzette rocks. Copper sulfide globules discovered in the most evolved rocks are a critical piece of evidence that suggests that the magmas associated with Solwara 1 where sulfide undersaturated though much of their history, making them able to concentrate and supply significant Cu during magmatic degassing, likely associated with the rifting environment. Key findings of this study show that Solwara 1 Cu-Au seafloor massive sulfide deposit and the formation of other arc-backarc Cu-rich seafloor massive sulfide deposits require a III confluence of at least three factors: 1) magmatism focused at structural intersections within early rifting stages of arc crust, 2) low volume, fissure-fed effusive volcanism, and 3) fractional crystallisation of, and mixing between, sulfur-undersaturated calc-alkalic and tholeiitic magmas at mid to lower crustal depths with associated degassing. The presence of Cu-rich immiscible sulfide globules in volcanic rocks associated with seafloor massive sulfide is of exploration significance as it identifies Cu-and sulfur-enriched magmas that can be a vector for buried and undiscovered seafloor massive sulfide deposits.