Deep-sea drilling of the 13°30′ N oceanic core complex: Assessing links between fluid flow, metal enrichment and seafloor massive sulfide deposit formation near Semenov-1

Deep-sea drilling is a key tool in assessing the economic viability of seafloor massive sulfide (SMS) deposits. Drilling can be used to understand links between fluid flow, faulting and mineralization. Of particular interest on the seafloor due to their relative enrichment in Ni and Co, is an underexplored sub-class of SMS deposits that are associated with or hosted within ultramafic lithologies. We present initial results from sub-surface drilling near the Semenov-1 SMS deposit located on the 13°30′ N oceanic core complex on the Mid-Atlantic Ridge. Drilling recovered samples from up to 20.7 m below seafloor (mbsf). In the recovered core samples, a transition from chlorite and clay-rich breccias in the upper 7.5 mbsf to serpentinite, magnesite, dolomite and talc bearing rocks with locally mylonitic fabrics was observed. The contact between mafic and ultramafic lithologies marks the transition from brittle to ductile deformation regimes. Pyrite and marcasite occur throughout the drilled interval, commonly as disseminations. Pyrrhotite and isocubanite only occurred near the seafloor (∼2 mbsf) and are associated with native Au grains. Pyrite at 7.5 mbsf was notably enriched in Ni, whereas deeper pyrite contained higher Se. The average δ34S value for all sulfide minerals is 0.0 ± 5 ‰ (1σ, n = 114 spots) indicating sulfur was largely contributed from the leaching of igneous host rocks, and that magmatic volatile degassing is not an important process contributing to metal enrichment at Semenov-1. The source of sulfur or processes affecting isotopic fractionation also varied over time leading to large variations of ∼15 ‰ across individual pyrite grains. We interpret the drilled section as representing a cross-section through a basaltic veneer to the underlying upper detachment fault surface providing evidence for the complex nature of hydrothermal systems associated with core complexes. •Shallow seafloor drilling intersects the upper surface of an oceanic detachment fault.•A transition from mafic to ultramafic lithologies occurs at 7.5 m below the seafloor.•Dolomite, magnesite, talc and chrysotile are the major alteration phases.•The detachment surface is characterized by elevated Ni and Cr contents.