No abstract

No abstract

Genetic models for many Irish Lower Carboniferous Zn + Pb ore deposits invoke a dual supply of sulphide. The dominant source is from the bacteriogenic reduction of Lower Carboniferous seawater sulphate, but a significant, minor supply is derived from deep-seated sources. The δ 34 S range of the latter component varies among the deposits: from the lightest range of —15% to 0% at Keel, to the heaviest of –4% to +14.4% in the Navan/Tatestown area. We hypothesize that such sulphur is leached mainly from diagenetic sulphide minerals in the underlying Lower Palaeozoic sediments. 6”s of pyrite in the Silurian/Ordovician Moffat Shales (—17.1% to —0.6%), and of sphalerite and galena in Lower Palaeozoic-hosted veins at Salterstown (—8.7% to -4.5%) and Wanlockhead (—10.3% to —5.1%) are consistent with the hypothesis. Below the Navan ore deposit, Lower Palaeozoic shales containing minor diagenetic pyrite with a very wide range in δ 34 S have been found; δ 34 S is typically heavy (+16%) but extreme values up to +62% are encountered. The general enrichment in δ 34 S is in accord with the noticeably heavy isotopic composition of deep-seated sulphur in the Navan orebody and its Tatestown satellite. Our preliminary results therefore suggest that geographical variations in the δ 34 S range of Lower Palaeozoic diagenetic -write_. may have contributed to the isotopic variation in the deep-seated sulphur among the Irish deposits.

Models of genesis for the Navan orebody are of two distinct types. An early hypothesis that mesothermal (though nonmagmatic) deposition of ore began when a supernatant seawater brine still had access to the host sediments (during the early to mid-Mississippian), has been challenged by recent suggestions favoring a later (mid-to late Mississippian to Pennsylvanian) mineralization derived from cooler fluids traversing either the Mississippian/Pennsylvanian basin or the underlying basement from the south. These models, characterized here as Irish-type and Mississippi Valley-type respectively, are interrogated as to their particular predictions with regard to metal distributions and sulfur isotope patterns associated with various fault geometries. The basal 5 lens of the Navan Zn + Pb deposit contains ~70 percent of the known tonnage of the ~90 Mt orebody and, thus, is the focus of this examination. Lead distribution patterns suggest that migration of metalbearing fluids was principally directed up early to mid-Mississippian, near vertical north-northeast, northeast, and east-northeast minor normal faults. These faults predate or are coeval with the major extensional, partly listric, east-northeast faults which now control the general disposition of the deposit. Only where these major east-northeast faults cross putative deep-seated northeast (Caledonoid) and northwest structures are they associated with lead enrichments. A systematic δ 34 S survey in the 5 lens across five minor north-northeast-through to east-northeast-trending faults associated with distinct lead enrichments, and one east-northeast-trending, partly listric, major extensional fault adjacent to that trend, revealed positive δ 34 S values (1-18‰) for galena, sphalerite, and marcasite sampled within 3 m of all the faults on the profile. Sulfides with positive δ 34 S values associated with the deep-seated, metal-bearing fluid generating the Navan deposit have been highlighted by previous workers (Anderson et al., 1998). The evidence reported here strongly suggests that the metal-bearing fluids rose through all the fractures. Conversely, negative δ 34 S values (-1 to-26‰) were returned in galena and sphalerite sampled 3 m or more from these faults. These negative values indicate that locally derived bacteriogenic sulfide, reduced from seawater sulfate, dominated away from these faults. Pyrite δ 34 S values suggest a background level of-29 ± 3.0 per mil across the profile. However, pyrite δ 34 S values as low as-34 ± 2.7 per mil were recorded in one sample collected from within 1 m of a fault. Thus, fluids containing highly fractionated, bacteriogenic sulfide also gravitated into these faults on at least one occasion. There is also evidence suggesting that the metal-bearing solutions periodically displaced the locally derived bacteriogenic sulfide-bearing fluid in and near the faults. Mineral sulfide petrography is used to contextualize the sampling and to give a qualitative indication of the degree of chemical disequilibrium of the system. Mineral textu...

No abstract