Understanding the fluid pathways that control the Navan ore body

This work is focused on carbonate-hosted base metal deposits in the Irish midlands with emphasis on the Navan ore deposit, County Meath, Ireland. The Irish ore deposits were created by the mixing of two fluids, a metal-bearing fluid and a sulfur-rich brine. Herein I aim to further the understanding of the creation, movement, and mixing of these two fluids and how they created the giant Zn and Pb deposit at Navan, as well as how post-ore genesis fluids are recorded in the rocks around Navan. The first chapter contains a summary of current knowledge and views of the deposit, local lithologies, structures, and mineralization. The second chapter is original work that examines how metal distribution patterns and 3D meshes of the paleo-surfaces can yield insights into the movement of mineralizing fluid during ore genesis. This work builds on previous work over the many years the mine has been operating. This new work shows the spatial variability in Pb and Zn concentrations and ratios and interprets these values with respect to vertical and horizontal fluid flow. It also builds on the work of others to interrogate the extent to which a major paleo-erosion event and surface has affected the mineralization found above and below this surface. This has significant bearing for the future of exploration in the area. The third chapter is original work that contains new noble gas data from Navan and deposits across Ireland that elucidate the temperature and tectonic setting that drove the metal bearing fluid that made the Irish midlands so well-endowed in base metals. Sulfides from every major carbonate-hosted base metal mine in Ireland were crushed to release noble gases trapped in fluid inclusions, which had retained 3He/4He signatures from the time of mineralization ca. 350 Ma. These 3He/4He ratios indicate a small but clear contribution of mantle-derived 3He, which reveals that mineralization occurred during an extensional event that introduced heat from the mantle. The fourth chapter is original work based on new 40Ar/39Ar geochronological results that constrain the timing of a later fluid flow event caused by the Variscan compression that inverted the local basin. This inversion event created large wrench and reverse faults and has greatly complicated the local lithology and metal extraction. The timing of this inversion event was interrogated by analyzing the 40Ar/39Ar systematics of disturbed feldspars along a large reverse fault. The 293 ± 3 Ma minimum age produced represent the first radiometric age of the Variscan compressional event in central Ireland and confirms the long held assumption that these faults are related to this large scale tectonic event. The fifth and final chapter is a combination of original and recently published work from others. It focuses on a newly discovered area of mineralization several km to the south of Navan. Mineralization, fluid inclusions, and the structural setting of this new area are evaluated and compared to ‘typical’ Navan mineralization. The new area was created by hotter hydrothermal fluid and did not mix with the surface fluid as effectively as the main deposit