The Ballynoe barite deposit is a conformable, mineralized horizon of Lower Carboniferous age overlying a diastem and mass faunal extinction demarking the transition from a quiet water environment to one of dynamic sedimentation. The geometry of the barite orebody correlates with the palaeo-topography of the footwall, which acted as an important control over the lateral extent, thickness and nature of the mineralization. Sedimentary features within the barite horizon suggest that it was precipitated in the form of a cryptocrystalline mud which underwent major diagenetic modification resulting in extensive stylolitisation, recrystallisation and remobilization. There is abundant and compelling geological and isotopic evidence for early local exhalation from the presence of a hydrothermal vent fauna consisting of a delicately pyritized worm tubes hosted and haematized filaments of apparent microbial origin. The worm tubes are remarkably similar to examples from modern and ancient volcanic-hosted massive sulphide deposits, and the filamentous microfossils have similarities to modern Fe-oxidizing bacteria. Strontium in the barite has a 87Sr/86Sr ratio indistinguishable from sea-water between 350 and 344Ma whilst oxygen isotopes from barite and chert suggest a diagenetic origin in equilibrium such sea-water around 60-70°C. Fluid inclusion studies have shown that, in general, low temperature inclusions are very saline (20-25%) whilst at higher homogenization temperatures they are more dilute (9-12%).

This article discusses the results of fluid inclusions study in barite collected at the Jan Mayen vent field area (Troll Wall and Perle & Bruce) and Loki’s Castle vent field of the Mohns Ridge segment of the Arctic Mid-Ocean Ridge. Three mafic-hosted volcanogenic massive sulfide deposits were examined within the active vent fields that adequately correspond to geological settings of ultraslow-spreading ridges and PT-conditions. Hydrothermal sediments were investigated to determine the temperature and salinity of the fluids responsible for barite precipitation. The hydrothermal origin of the barite was confirmed by its morphology. Fluid inclusions are two-phase and homogenized into the liquid phase on heating at temperatures below 287ºC. The salt concentration in fluids trapped in inclusions is 2.6–4.4 wt.% NaCl eq. The crystallization temperatures varied from 276ºС to 119ºС and from 307ºС to 223ºС for the Jan Mayen and Loki’s Castle vent fields, respectively. The data obtained allowed us to confirm evidence of fluid phase separation in the hydrothermal systems and to expand our knowledge of the temperature and salinity of mineral fluids previously known from recent direct measurements during the cruises within the G.O. Sars research vessel. The fluid inclusions data obtained from barites emphasize the fluid features characteristic of volcanogenic massive sulfide deposits, the similarities and differences for the studied hydrothermal sites, and allow comparisons with similar products from other active hydrothermal systems.