Barite in the approximately 513 Ma Lemarchant volcanogenic massive sulphide (VMS) deposit, Newfoundland, consists of granular and bladed barite intimately associated with mineralization. Regardless of type, the composition of barite is homogeneous at bulk rock and mineral scale containing predominantly Ba, S, and Sr, with minor Ca and Na. The barite has homogeneous sulphur isotope compositions (δ34Smean = 27‰), similar to Cambrian seawater sulphate (25–35‰) and Sr isotope compositions (87Sr/86Sr = 0.706905 to 0.707485). These results are consistent with barite having formed from fluid–fluid mixing between Cambrian seawater and VMS-related hydrothermal fluids. The 87Sr/86Sr values in the barite are lower than mid-Cambrian seawater, which suggests that some of the Sr was derived from the underlying Neoproterozoic basement. Fluid inclusions in bladed barite are low-salinity, CO2-rich inclusions with homogenization temperatures between 245°–250 °C, and average salinity of 1.2 wt.% NaCl equivalent. Estimated minimum trapping pressures of between 1.7 to 2.0 kbars were calculated from aqueous–carbonic fluid inclusion assemblages. The fluid inclusion results reflect regional metamorphic reequilibration during younger Silurian regional metamorphism, rather than primary fluid signatures, despite the preservation of primary barite and fluid inclusion textures. These results illustrate that barite in VMS deposits records the physicochemical processes associated with VMS formation and the sources of fluids in ancient VMS deposits, as well as seawater sulphate and basement isotopic compositions. The results herein are not only relevant for the Lemarchant deposit but also for other barite-rich VMS deposits globally.

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