THE GEOMETRY AND PETROGENESIS OF HYDROTHERMAL DOLOMITES AT NAVAN, IRELAND

The dolomites at Navan, Ireland, formed in Courceyan peritidal and sha llow-shelf limestones. The dolomite body has a plume-like geometry, cr oss-cutting both lithological boundaries and diagenetic barriers gener ated by sea-floor cementation and emergence. The dolomitizing fluids r ose parallel to major faults to diffuse laterally through the successi on, controlled by variations in permeability that reflect both facies variation on deposition and pre-dolomitization diagenesis. Cathodolumi nescent zones reveal three principal stages of dolomite emplacement, s eparated by dissolution surfaces, with each stage reflecting several c hanges in the character of dolomitizing solutions. The predominance of dull zones indicates burial rather than surface conditions. The dolom ites formed some time after burial in response to an areally limited h ydrothermally-driven flow. Isotope values (delta(18)O of -6.6 to -10.4 parts per thousand and delta(13)C to -0.2 to +2.5 parts per thousand PDB), and fluid inclusion data, suggest that these fluids had composit ions similar to those of Carboniferous seawater. However, these became hotter with time, with temperature increasing from 60 to 160 degrees C. The Navan dolomites are closely associated with Europe's largest zi nc-lead deposit. The distribution of the ores follows the same trend a s that of the dolomites and paragenetic relationships indicate that do lomitization and mineralization were temporally and genetically relate d.