Discrete samples of Fe-oxyhydroxide and seafloor massive sulphide (SMS) were collected from the 13°30'N ocean core complex (OCC) region of the Mid-Atlantic Ridge where a number of seafloor massive sulphide deposits are known. This is also known as the Semenov Hydrothermal Field.

Metal concentrations in sediment; Stable isotopes in sediment; Lithology; Mineralogical composition

Cameras; sediment grabs; fluorescence microscopes; inductively-coupled plasma mass spectrometers; X-ray diffractometers; rock dredges

This dataset comprises images, geochemical composition, isotopic ratio, and X-ray diffraction (XRD) data from samples of Fe-oxyhydroxide (FeOOH) and seafloor massive sulphide (SMS) collected during the RRS James Cook cruise JC224 (March - April 2022) to the 13°30' N ocean core complex (OCC) region of the Mid-Atlantic Ridge, also known as the Semenov Hydrothermal Field. A total of 23 Robotic Underwater Vehicle (RUV) dives were completed with the HyBIS RUV (Murton et al., 2012) in addition to 6 rock dredges setup with a chain bag dredge and a steel bucket dredge behind it. Sampling was focused on the previously described massive sulphide-hosting areas: Semenov-1, -2, -4 and -5 (Escartin et al., 2017), as well as one dedicated dive to a near-circular feature protruding on the southern slope of the OCC - termed the "bulge". Analysis of the 42 FeOOH and SMS samples occurred onshore at the National Oceanography Centre (NOC) and the University of Southampton and the samples are held for long-term storage at NOC Southampton and Cardiff University. A range of analytical methods were used to collect the data, including: inductively-coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), and reflective light microphotography. The key aims of these analyses were: (1) to improve understanding regarding the formation of Fe-oxyhydroxide samples at SMS systems, and (2) to determine the potential of Fe-oxyhydroxide as an additional resource at SMS systems. The data were collected as part of a PhD project funded under the Natural Environment Research Council (NERC) project Ultramafic-hosted mineral Resource Assessment (ULTRA) grant, NE/S004068/1. This collection consists only of the data collected under the PhD project and not all data associated with the ULTRA project.

Escartin, J., Mevel, S., Peterson, D., et al. (2017) Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20′N and 13°30′N, Mid Atlantic Ridge), Geochemistry, Geophysics, Geosystems; Murton, B.J., Hunerbach, V., and Garrard, J. (2012) HyBIS: A new concept in versatile, 6000-m rated robotic underwater vehicles, Roberts G.N. and Sutton R.

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