Timing of Interplay between Hydrothermal and Surface Fluids in the Navan Zn + Pb Orebody, Ireland: Evidence from Metal Distribution Trends, Mineral Textures, and δ34S Analyses

摘要: Models of genesis for the Navan orebody are two distinct types. An early hypothesis that mesothermal (though nonmagmatic) deposition ore began when a supernatant seawater brine still had access to host sediments (during mid-Mississippian), has been challenged by recent suggestions favoring later (mid- late Mississippian Pennsylvanian) mineralization derived from cooler fluids traversing either Mississippian/Pennsylvanian basin or underlying basement south. These models, characterized here as Irish-type and Mississippi Valley-type respectively, interrogated their particular predictions with regard metal distributions sulfur isotope patterns associated various fault geometries. The basal 5 lens Zn + Pb deposit contains ~70 percent known tonnage ~90 Mt and, thus, is focus this examination. Lead distribution suggest migration metal-bearing was principally directed up mid-Mississippian, near vertical north-northeast, northeast, east-northeast minor normal faults. faults predate coeval major extensional, partly listric, which now control general disposition deposit. Only where these cross putative deep-seated northeast (Caledonoid) northwest structures they lead enrichments. A systematic δ 34 S survey in across five north-northeast– through east-northeast–trending enrichments, one east-northeast–trending, extensional adjacent trend, revealed positive values (1–18‰) galena, sphalerite, marcasite sampled within 3 m all on profile. Sulfides deep-seated, fluid generating have highlighted previous workers (Anderson et al., 1998). evidence reported strongly suggests rose fractures. Conversely, negative (–1 –26‰) were returned galena sphalerite more indicate locally bacteriogenic sulfide, reduced sulfate, dominated away Pyrite background level –29 ± 3.0 per mil However, pyrite low –34 2.7 recorded sample collected 1 fault. Thus, containing highly fractionated, sulfide also gravitated into at least occasion. There suggesting solutions periodically displaced sulfide-bearing Mineral petrography used contextualize sampling give qualitative indication degree chemical disequilibrium system. textures demonstrating comminution dissolution study that, coupled isotopic overprinting, force conclusion mineralizing first invaded lithologies during mid-Lower times, coincident active faulting. no reactivation sets encountered area post-Chadian tectonism, though major, reactivated time. Ore effected (reduced sulfate) reacting rising, fluids. Thus any model invoking an onset than does not stand scrutiny.