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.

This dataset comprises the geochemical and mineralogical analysis of six samples of massive sulphide deposit. Three samples were obtained from seafloor massive sulphide (SMS) systems obtained during RV Celtic Explorer cruise CE11009 (Mid-Atlantic Ridge 45° N, 2011) and RRS James Cook cruises JC082 (Mid-Cayman Spreading Centre, 2013) and JC138 (Mid-Atlantic Ridge at 26° N, 2016). Three samples were obtained from land-based volcanogenic massive sulphide (VMS) deposits on Wetar Island, Indonesia. It is thought these samples were obtained in 2002 at Kali Kuning and Lerokis Zones 4 and 5, but users should be aware there is little to no metadata about the Wetar Island sample origins. The sample analyses includes: (1) bulk geochemical analysis of sulphide samples by inductively coupled plasma mass spectrometry (ICP-MS) for bulk and effluent analysis and inductively coupled plasma orbital emission spectrometry (ICP-OES), (2) petrographic descriptions of samples by reflective microscopy, (3) geochemical analysis of seawater samples during experiments by ICP-MS, (4) mineralogical analyses (X-ray Diffraction) of sulphide samples, (5) mineralogical analyses by Scanning Electron Microscope and energy dispersive X-ray spectroscopy (SEM-EDS) on sulphide grains, (6) SEM backscattered electron (BSE) images of reacted sulphide grains, and (7) surface areas determined by Brunauuer-Emmett-Teller (BET) surface area analysis. All analyses were undertaken as part of a PhD project which aimed to enhance understanding of sulphide oxidation kinetics, the formation of Fe-oxyhydroxide from sulphide oxidation, its capacity to retain metals that would otherwise be lost to seawater in SMS systems, and the implications for potential seafloor mining operations. Analyses took place at the University of Southampton and the National Oceanography Centre Southampton, 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.

This dataset comprises a variety of sediment core data from the Semenov Hydrothermal Field region of the Mid-Atlantic Ridge, collected during RRS James Cook cruises JC224 (March - April 2022) and JC254 (October - November 2023). A total of 29 gravity cores, 9 mega-cores, and 25 push cores were successfully recovered. Analysis of the sediment core samples was conducted at the British Ocean Sediment Core Research Facility (BOSCORF), where the cores are held for long term storage, and included: imaging, spectrometry, geophysical and geochemical measurements of sediment and porewaters, radiocarbon dating, and stable isotope analysis. The main coring equipment used during both cruises was a gravity corer to collect sediment cores of up to 3 m in length. For JC224, a mega-core, with a USBL transponder attached to the frame, was used to obtain undisturbed cores from the upper 40 cm of the sediment surface. For JC254, push cores were also collected during several Remotely Operated Vehicle (ROV) dives using the Isis vehicle. The key objectives of the sediment coring were: (1) to reconstruct the history of the hydrothermal activity by dating plume fallout layers in the sedimentary record, and (2) to assess the role of the sediment cover in the preservation of seafloor massive sulphide (SMS) deposits in an ultra-slow spreading scenario. 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.