The Submarine West Rota Caldera Sulfide Mineral Deposit, Southern Mariana Arc: Exposed roots of a < 1 Ma hydrothermal system

We sampled a suite of fresh, altered, and variably mineralized volcanic rocks from West Rota Volcano (WRV) in the Mariana arc. The WRV is an extinct volcano with a 6 × 10 km-wide caldera, the largest in the Mariana arc portion of the Izu-Bonin-Mariana arc system. Samples recovered during dredging and remotely operated vehicle (ROV) dives range from fresh to highly hydrothermally altered rocks (high S and high loss on ignition values) across a compositional range from basalt to dacite. Alteration is characterized by silicification, formation of white mica, and chlorite in some samples, with intense bleaching of rocks. Altered rocks show elevated S (up to 16.9 wt.%), Cu (up to 14.1 wt.%), and various trace metals and metalloids including As, Ag, Bi, Mo, Sb, Se, Te, W, and Pb. We interpret this alteration and enrichment to fluids degassed from an underlying magma chamber interacting in the subsurface with circulating seawater, i.e. magmatic-hydrothermal alteration. Magmatic contributions to alteration are consistent with generally negative δ34SVCDT values that range from −15.4 to 2.3 ‰, more negative than is typical for a sediment-free mid-ocean ridge hydrothermal system and consistent with SO2 disproportionation. We hypothesize that the Cu enrichment in some samples, the intensity and mineralogy of magmatic alteration, and enrichment in magmatic-volatile elements suggests that the samples are from the roots, or stockwork zone, of a seafloor massive sulphide deposit. We suggest that the upper portions of what would have been a massive sulphide deposit (or deposits) were removed by subsequent caldera-forming eruptions and gravity processes.