The Ballynoe Stratiform Barite Deposit, Silvermines, County Tipperary, Ireland

Abstract

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%).