Magmatic controls on the ore-forming potential at the submarine Brothers volcano, Kermadec arc

The submarine Brothers arc volcano ~350 km NE of New Zealand is a unique site for studying active magmatic-hydrothermal ore formation. In 2018, IODP Expedition 376 cored the subsurface to provide insights into the third dimension and the evolution of the volcano and associated ore-forming system. We present new petrological and geochemical data on unaltered volcanic rocks from Brothers as well as from two adjacent volcanic ridges. Lavas erupted at these ridges represent potential mafic analogues to the dacites at Brothers. The volcano is located at the arc frontal end of a cross-arc volcanic chain that extends into the backarc. This cross-arc chain is assumed to be the surface expression of an anomalously warm zone in the mantle wedge and the devolatilization of the partially subducted Hikurangi Plateau enhance slab input. This increased flux facilitates the transfer of metals to the wedge and is efficiently liberating metals from residual sulfides in the mantle wedge. As a result, magmas erupted at Brothers volcano and the adjacent volcanic ridges are enriched in ore metals and metalloids. Whereas sulfide saturation is initially suppressed by a high melt oxidation state (Δ log fO2 of +1.5 FMQ), the dacitic lavas of Brothers volcano are depleted in chalcophile elements (e.g., Cu, Au), which limits its porphyry ore potential. This is due to late-stage sulfide segregation during magmatic differentiation. However, other metals and metalloids (e.g., Mo, As, Sb) are progressively enriched and ultimately transferred into the epithermal-hydrothermal system by fluids exsolving from the cooling magma.