Native Arsenic at the Semenov-2 Seafloor Massive Sulfide Deposit (13°31′13 N, Mid-Atlantic Ridge): An Indicator of Low-Temperature, Reducing, and Acidic Formation Conditions

Abstract Native arsenic, which is an extremely rare seafloor mineral, was found in opal-rich massive sulfide samples of the basalt-associated Semenov-2 seafloor massive sulfide (SMS) deposit (13°31′13 N, Mid-Atlantic Ridge) located on top of an oceanic core complex, which exposes mantle rocks on the seafloor. Native arsenic occurs in interstitial opal as small (<10 µm) subhedral to euhedral crystals and forms three mineral assemblages. In assemblage 1, native arsenic overgrows Se-bearing (up to 1.39 wt.% Se) galena and contains S (1.14–5.07 wt.%), Cu (0.42–1.09 wt.%), and Zn (0.90–1.32 wt.%). In assemblage 2, native arsenic forms euhedral crystals often arranged in stellar or elongated aggregates, hosts fine (∼1 µm) inclusions of Au-bearing native silver (50.31 wt.% Au = 0.35 apfu) in the center of the crystals, and is locally overgrown by rare argyrodite, Ag8GeS6. This native arsenic contains S (0.29–1.65%), Cu (0.38–1.85 wt.%), and Zn (0.58–1.86 wt.%). In assemblage 3, native arsenic overgrows and pseudomorphically replaces As-bearing (up to 18.01 wt.% As) pyrite (in most cases) and rarely sphalerite and contains S (0.11–3.99 wt.%), Fe (0.28–1.64 wt.%), Sb (0.48–1.05 wt.%), and, locally, Cu (0.29–0.37 wt.%) and Zn (0.54–0.65 wt.%). Thermodynamic modeling of the formation of native arsenic in the Selektor software shows that it forms after the interaction of a relatively low-temperature (∼100–150 °C) reducing (Eh120°C = −0.29 V) acidic (pH120 °C = 3.98) hydrothermal fluid with previously formed As-bearing sulfide minerals (As-rich pyrite in our case) rather than after the direct rock/seawater interaction. The formation of native arsenic from the hydrothermal fluid occurred within pores in late opal without mixing with seawater that was favorable for conductive cooling and reducing conditions. Our findings showed a potential for the precipitation of diverse late low-temperature ore minerals after the crystallization of late opal, which is ubiquitous in SMS deposits.