Synsedimentary mineralization at Ballynoe barite deposit, near Silvermines, Co. Tipperary, Ireland

Mineralization at Ballynoe is hosted by Tournaisian limestones and comprise s mainly barite with pyrite. Eight stratiform units with a combined thickne ss of up to 31 m constitute the orebody, and individual barite units are up to 5.6 m thick. Each unit has distinct mineralogical, textural and sedimen tological characteristics. Between 1963 and 1993 more than 5 000 000 t of b arite of sp. gr. 4.23 was extracted from the pit. The barite is dominantly fine-grained, although locally it has been recrystallized to a coarser-grai ned texture. Pyrite occurs as both disseminations and intercalations within the barite, in chimneys on the footwall, as stock-work veins and joint-con trolled veins, and forms a cap up to 2 m thick to the barite orebody. Sedim entary features of the mineralization include bedding, syn-depositional bre ccias, load and slump structures, debris flow clasts and rip-up clasts. The orebody is conformable with sedimentary rocks in its immediate footwall an d hanging-wall. Thermometric studies show that two fluids were involved in mineralization. One originated within the crust and had a moderate temperature (180-225 deg rees C) and low salinity (8-15 equiv. wt% NaCl). Points of exhalation of th is fluid are marked on the footwall surface by pyrite hydrothermal chimneys . The other fluid originated in the local depositional basin and had a lowe r temperature (68-110 degrees C) and higher salinity (20-25 equiv. wt% NaCl ). Homogenization temperatures of these inclusions increase from the base o f the barite units upwards. Barite and pyrite were apparently deposited as sediments on the sea-floor. The most probable depositional model is the sedimentary-exhalative convecti on cell model. This is consistent with available sulphur isotope data, whic h indicate that Lower Carboniferous seawater sulphate was the dominant sour ce of sulphide sulphur (by bacteriogenic reduction) and barite-forming sulp hate.