Spatial variations in submarine caldera-hosted hydrothermal systems: Insights from sulfide chemistry, Niuatahi caldera, Tonga rear-arc

Submarine “black smoker” systems and their associated seafloor massive sulfides (SMS) may represent economic resources for future generations. However, the processes leading to spatial variations in the mineralogical and chemical composition of subduction zone-related hydrothermal systems remain poorly constrained. The large submarine caldera of Niuatahi volcano hosts several active hydrothermal vent sites associated with faults at the caldera wall and with young post-caldera volcanic cones, venting vapor-rich and black smoker-type-fluids with temperatures up to 334 °C. We combine bulk sulfide chemistry with in-situ trace element data and S- and Pb isotopes of pyrite, sphalerite, chalcopyrite to decipher key ore-forming processes causing spatial variations in metal(loid) enrichment. We refer these spatial variations within the caldera to a continuum between magmatic fluid-dominated venting at the central cones (high Cu, As, Bi, Te, Au, Sb, δ34S = -10.6 - 2.7 ‰) compared to fluid-rock interaction and seawater mixing at the caldera wall (high Au, Ag, Cd, Pb, δ34S = -0.6 - 6.3 ‰). Lead isotopes of sulfide separates suggest a connected hydrothermal circulation cell and/or similar source rock compositions in the central part of the caldera compared to a discrete one at the caldera wall. We conclude that metal(loid)s from distinct sources (magmatic volatiles vs. host rock leaching) combined with hydrothermal fractionation (e.g., boiling) leads to spatial variations in economically relevant elements (e.g., Te, Au, Ag, Bi, Se, Co) in submarine caldera-hosted hydrothermal systems. This has important implications on exploration of fossil SMS or volcanogenic massive sulfide deposits on land.