Geochemical characterization of the Gnitahei seafloor massive sulfide deposit, Arctic Mid-Ocean Ridge
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- Department of Earth Science 
The Gnitahei SMS deposit (72.75°N 3.81°E) is a newly discovered seafloor massive sulfide deposit located on the slow to ultraslow spreading Mohns Ridge in the Arctic Mid-Ocean Ridge system. The deposit was discovered by Norwegian Petroleum Directorate in 2019. During 2020 and 2021 research expeditions, extensive bathymetric mapping and numerous samples from the deposit were collected from ROV dives. This study brings new mineralogical, geochemical, stable isotope, and fluid inclusion data with the aim to give an insight into ore-forming processes responsible for the formation of the Gnitahei SMS deposit. Hosted within mafic lithology, the Gnitahei deposit is formed within an actively slumping block on the NW dominating fault, in which hydrofracturing and tectonic-induced mass wasting is responsible for the mineralization. The primary ore mineral assemblage consists predominantly of Fe- and Cu- rich sulfides associated with variable amounts of Au. The hydrothermally altered basalt is mainly composed of quartz, chlorite and clay minerals. Both the mineralization and associated hydrothermal alterations show a vertical zonation. The deeper part of the deposit is characterized by extensive Mg- chloritization, while silicification prevails in the upper portion of the system. The mineralization found in the deepest part of the deposit occurs as a network of tiny sulfide veinlets that evolve upward into a stockwork of massive sulfide veins. In contrast, the mineralization found in the uppermost part of the deposit often shows porous to colloform textural features. The mineralization is characterized by a vertical zonation in terms of the trace element compositions, with enrichment of Co, Cu, and Ni recorded in pyrite from the deeper part of the system compared to pyrite from the uppermost part of the deposit that rather exhibit an enrichment in Tl and Mo. The deposit is overlain by a silica cap rock precipitated on top of coalesced hydrothermal material and consolidated basalt breccias. Unusually high δ34S values (mean value of +14.3 ‰) indicate a high proportion (up to 73 %) of seawater derived reduced sulfur is involved in the formation of the mineralization. Trace element compositions combined with the heavy δ34S signature of the sulfide mineralization, suggests that hydrothermal fluids have reacted with ultramafic lithologies deep in the system. Fluid inclusion studies revealed that the ore-forming fluids were hot (~ 330 °C) and saline (~ 6.5 wt.% NaCl) aqueous solutions capable of transporting Cu, Fe, and Au in the form of their chloride complexes. Mixing of hot metal-bearing saline ascending hydrothermal fluids with cold seawater, has been recognized as a main mechanism for deposition of Au-bearing sulfide mineralization at the Gnitahei SMS deposit.
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