The Origin of Magmas and Metals at the Submarine Brothers Volcano, Kermadec Arc, New Zealand

Abstract

Abstract International Ocean Discovery Program (IODP) Expedition 376 cored the submarine Brothers volcano of the Kermadec arc to provide insights into the third dimension and the evolution of the volcano and its associated ore-forming systems. We present new petrological and geochemical data on dacitic rocks drilled from Brothers as well as mafic rocks collected at two adjacent ridges. These data include major and trace element compositions of whole rocks, including many economically important metals and metalloids such as Cu, Ag, Pt, Au, Mo, As, Sb, Tl, and Bi, plus Sr-Nd-Pb isotope compositions as well as in situ analyses of glasses and minerals. We show that the basalts and basaltic andesites erupted at the volcanic ridges near Brothers represent potential mafic analogues to the dacites that make up Brothers volcano. Mantle melting and ore potential of the associated magmas are locally enhanced by raised mantle potential temperatures and a high flux of subducted components originating from the partially subducted Hikurangi Plateau. As a result, the parental melts at Brothers are enriched in ore metals and metalloids relative to mid-ocean ridge basalts (MORBs), and a high melt oxidation state (Δ log fO2 of +1.5 fayalite-magnetite-quartz [FMQ]) suppresses early sulfide saturation. However, solid sulfide crystallization occurs late during magma differentiation, with the result that the dacitic lavas at Brothers volcano are strongly depleted in Cu but only moderately depleted in Ag and Au. The dacites at Brothers thus have a high fertility for many metals and metalloids (e.g., As, Sb, Bi), and fluids exsolving from the cooling magma have a high ore-forming potential.