Jade Umbsaar, PhD

J. Umbsaar¹, M. Anderson¹, D. Gregory¹, J. Jamieson²

¹Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada

²Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, Ontario, Canada

Seafloor massive sulfide (SMS) systems associated with arc volcanos are recognized as prospective targets for relatively mantle-incompatible elements (e.g., Zn, Pb, Ba, Co, Ge, Ga, W, Sn, In, As, Sb, etc.), and elements with an affinity for magmatic fluids (e.g., Au, Ag, Se, Bi, Hg). However, not all arc-hosted SMS systems are compositionally equivalent, and neither are the magmatic volatiles that contribute to those SMS systems. The VOLPA vent field is a previously uncharacterized SMS system located on the bi-modal felsic (boninite-dacite) Niua Volcanic Complex at the northern end of the Tofua Volcanic Arc, SW Pacific. Here, subduction occurs at full rates of 24 cm yr^-1. The rapidness of subduction, and age of the subducting slab result in “cold” serpentinite dehydration reactions that normally occur under the forearc being delayed, and instead occurring in the sub-arc. These dehydration reactions produce highly oxidizing fluids that have the capacity to contribute abundant large ion lithophile elements (Ba, Pb, Sr, Th), volatiles (H₂O, CO₂), and fluid-mobile oxyanions (As, Sb, W) to both the overlying volcanic and hydrothermal systems. These manifest as hydrothermal chimneys that are greatly enriched in barite and minerals indicative of a high sulfidation fluid (gratonite, semseyite, enargite, tennantite-tetrahedrite, anglesite, covellite, and proustite), which is consistent with previously established fingerprints of magmatic volatile flux into SMS systems; however, sulfur isotope signatures of sulfide minerals at VOLPA (δ³⁴S = -5.1–8.1 ‰) do not conform well to expected values of magmatic volatile flux (δ³⁴S < 0 ‰). The highly oxidizing conditions facilitated by the cold slab dehydration reactions produced a redox-controlled partitioning of heavier sulfur isotopes into the magmatic fluids, which is consistent with previously collected sulfur isotopes from both forearcs, and the adjacent Niua North vent field. Furthermore, Au is frequently associated with magmatic volatile flux, and despite the strong evidence of magmatic volatile flux to the VOLPA SMS system, the concentrations of Au are lower than the adjacent Niua South deposit, which has no indications of magmatic volatile flux. We demonstrate how Au abundances in these systems are temperature-dependent, and that magmatic volatiles do not necessarily function as a precondition for Au enrichment. The results of this work demonstrate how magmatic volatile compositions can vary as a function of subduction rate, and provide new insights into how we interpret magmatic volatile contributions to both modern and ancient hydrothermal systems.