Searching for the “next big thing”: Examining the potential for new feeder zone mineralisation in the western Navan area, County Meath, Ireland
[摘要] This study focuses on the Navan Deposit, County Meath, Ireland – a carbonate-hosted Zn-Pbsulphide 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 thathave characteristics of both MVT and SEDEX-style deposits, though cannot be easily becategorised as either. The Irish-types formed from mixing of two fluids – a metalliferoushydrothermal fluid, and a bacteriogenic-sulphide enriched brine. The hydrothermal fluid fromIrish-types in the southern Irish Midlands yield a respective hydrothermal fluid homogenisationtemperature and salinity range of 170-280oC and 12-18 wt.% NaCl. The Navan hydrothermal fluidwas notably lower temperature at 100-140oC and had a salinity of 5-10 wt.% NaCl. Mixing of thismoderate 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 basinat 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 inthe hangingwall of the Randalstown Fault 200 metres west of the mine site – an end-Variscanreverse 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 anearby “feeder zone” – early-forming normal faults that allowed hydrothermal fluid upwelling.Hydrothermal fluids then mixed with downwelling bacteriogenic sulphide-enriched brines on, orclose 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 areaapproximately 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” wasselected trending southeast along the hangingwall of the Randalstown Fault, while the “A2Transect” trends southeast along the Randalstown Fault footwall. The “B and C Transects” trendperpendicular and parallel to the Main Orebody, respectively. Sulphides from the Main Orebodyand western Navan share a similar paragenesis, suggesting that both may have derived from thesame 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 thathave constrained the δ34S range for the hydrothermal fluid (-4 to 16‰) that supplied Zn-Pb to theNavan Main Orebody and is interpreted to represent hydrothermal fluid that migrated westwardduring early normal faulting. Fluid inclusion analyses constrain a mineralisation temperature andsalinity range of 87-136.5oC, and 6.8-22.7 wt.% NaCl, respectively. This is broadly coincident withthe hydrothermal fluid temperature and salinity range. Higher fluid salinity is interpreted toreflect 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 andformed from a separate mineralising fluid to the Navan Deposit. δ34S analysis of threechalcopyrites yield a range of 0.8-3.3‰, results consistent with a hydrothermal fluid origin. Fluidinclusion 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 assemblageis both distinct from Navan and closer to higher temperature systems in the southern IrishMidlands – an observation supported by a mineralisation temperature range that is closer to thesouthern Irish-type deposit systems. The mineralising fluids were both higher temperature andmore saline than the Navan hydrothermal fluid, potentially pointing to the feeder zone of a new,unidentified system.
[发布日期] [发布机构] University:University of Glasgow;Department:Scottish Universities Environmental Research Centre
[效力级别] [学科分类]
[关键词] Mining, exploration geology, Irish-type, Mississippi Valley-Type, sedimentary-exhalative. [时效性]