Searching for the 'next big thing': Examining the potential for new feeder zone mineralisation in the western Navan area, County Meath, Ireland

This study focuses on the Navan Deposit, County Meath, Ireland – a carbonate-hosted Zn-Pb sulphide deposit originally comprising 110Mt @ 8% Zn and 2% Pb, prior to depletion by mining. Navan belongs to the “Irish-type” deposit group – Zn-Pb deposits hosted in the Irish Ore Field that have characteristics of both MVT and SEDEX-style deposits, though cannot be easily be categorised as either. The Irish-types formed from mixing of two fluids – a metalliferous hydrothermal fluid, and a bacteriogenic-sulphide enriched brine. The hydrothermal fluid from Irish-types in the southern Irish Midlands yield a respective hydrothermal fluid homogenisation temperature and salinity range of 170-280oC and 12-18 wt.% NaCl. The Navan hydrothermal fluid was notably lower temperature at 100-140oC and had a salinity of 5-10 wt.% NaCl. Mixing of this moderate temperature fluid with a high salinity bacteriogenic-sulphur enriched brine (70-100oC, 20-25 wt.% NaCl) allowed high-grade Zn-Pb ore to precipitate along a shallow, extending sea basin at approximately 345 Ma (Lower Carboniferous). Navan ore has an average Zn/Pb ratio of 4, though low-grade, low Zn/Pb below 3 is observed in the hangingwall of the Randalstown Fault 200 metres west of the mine site – an end-Variscan reverse fault of at least several hundred metres throw and one kilometre dextral displacement. Low Zn/Pb has been previously noted in Navan and other Irish-type deposits as an indication of a nearby “feeder zone” – early-forming normal faults that allowed hydrothermal fluid upwelling. Hydrothermal fluids then mixed with downwelling bacteriogenic sulphide-enriched brines on, or close to the seafloor to produce high-grade Zn-Pb sulphide ore. Low Zn/Pb in western Navan, therefore, is of particular interest and may point to a new, unidentified feeder. In addition, Cpy- Qtz(-barite) veining has been identified in drillcore for the first time in the Navan area approximately 850 metres west of the Randalstown Fault, coinciding with the low Zn/Pb area. Notable chalcopyrite has been identified close to feeder-zone faults in other Irish-type deposits, though is absent at Navan – until this discovery. Mineralised samples were selected along four drillhole transects – The “A1 Transect” was selected trending southeast along the hangingwall of the Randalstown Fault, while the “A2 Transect” trends southeast along the Randalstown Fault footwall. The “B and C Transects” trend perpendicular and parallel to the Main Orebody, respectively. Sulphides from the Main Orebody and western Navan share a similar paragenesis, suggesting that both may have derived from the same hydrothermal fluid event. Sulphur isotope analysis constrains a δ34S range of -1.5 to 12.5‰ for Zn-Pb sulphides west of the Randalstown Fault. This range coincides with previous studies that have constrained the δ34S range for the hydrothermal fluid (-4 to 16‰) that supplied Zn-Pb to the Navan Main Orebody and is interpreted to represent hydrothermal fluid that migrated westward during early normal faulting. Fluid inclusion analyses constrain a mineralisation temperature and salinity range of 87-136.5oC, and 6.8-22.7 wt.% NaCl, respectively. This is broadly coincident with the hydrothermal fluid temperature and salinity range. Higher fluid salinity is interpreted to reflect mixing with limited downwelling seawater and/or in-situ pore water. Cpy-Qtz(-barite) veining in western Navan is hosted in a 10.5 metre thick stratal dolomite and formed from a separate mineralising fluid to the Navan Deposit. δ34S analysis of three chalcopyrites yield a range of 0.8-3.3‰, results consistent with a hydrothermal fluid origin. Fluid inclusion analysis of two-phase quartz-hosted inclusions yield a temperature range of 191-223oC, while monophase inclusions constrained a salinity range of 10.2-22.8 wt.% NaCl. The assemblage is both distinct from Navan and closer to higher temperature systems in the southern Irish Midlands – an observation supported by a mineralisation temperature range that is closer to the southern Irish-type deposit systems. The mineralising fluids were both higher temperature and more saline than the Navan hydrothermal fluid, potentially pointing to the feeder zone of a new, unidentified system.