Subseafloor processes in ultramafic-influenced volcanic-hosted hydrothermal systems: An insight from the Gnitahei massive sulfide mineralization, Arctic Mid-Ocean Ridges

The extinct Gnitahei seafloor massive sulfide (SMS) deposit was discovered in 2019 on the ultraslow spreading Mohns Ridge (Arctic Mid-Ocean Ridge System, 72.8°N, 3.81°E, 3022–2760 mbsl). It provides a unique opportunity to study subseafloor hydrothermal processes without drilling, as tectonic uplift and ongoing mass wasting expose the deeper parts of the hydrothermal system. The deposit is hosted by hydrothermally altered basalts within the footwall of a NE–SW striking normal fault, in proximity to the active Fåvne Vent Field. Driven by extensional tectonics and facilitated by the thin oceanic crust along the ultraslow-spreading Mohns Ridge, hydrothermal circulation likely extends deep into the serpentinized upper mantle. Hydrothermal alteration exhibits vertical zonation. Chloritization accompanied by remobilization of silica, LILEs, and REEs dominate the deeper parts of the system. In contrast, silicification associated with an enrichment in LILEs and REEs characterizes the upper part of the Gnitahei SMS deposit. The paleosurface is clearly marked by a layer of red jasper. The sulfide mineralization is dominated by pyrite and marcasite, with euhedral pyrite prevailing in deeper zones and colloform pyrite being more abundant near the paleosurface. Native gold occurs mostly as inclusions in pyrite. Trace-element data obtained from pyrite, including elevated Ni and Co concentrations and a high Au/Ag ratios, are consistent with fluid interaction with underlying ultramafic rocks. Sulfur isotope composition of pyrite reveals a strong 34 S enrichment (bulk δ 34 S VCDT = 11.1–15.9‰; in-situ δ 34 S VCDT = 10.8–17.3‰), indicating a significant contribution of reduced seawater sulfate, which constitutes ~50% of the total sulfur according to a two-component mixing model. The high influx of reduced sulfate is attributed to reducing conditions driven by serpentinization of ultramafic lithologies. Under these conditions, seawater sulfate can be reduced prior to anhydrite deposition, allowing isotopically heavy sulfur to infiltrate the hydrothermal system. Depth-dependent decreases in δ 34 S values likely reflect greater contributions of reduced seawater sulfate in shallower zones and/or enhanced fractionation at lower temperatures.