Recent exploration in the vicinity of the giant (>90 Mt) Navan orebody has resulted in the discov... more Recent exploration in the vicinity of the giant (>90 Mt) Navan orebody has resulted in the discovery of ore-grade mineralisation to the southwest of the deposit, much of which occurs within the Upper Pale Beds, a horizon that is only weakly mineralised above the main orebody. Within this new U lens, mineralisation preferentially occurs within bioclastic carbonate grainstones and calcareous quartz sandstones, and is dominated by sulphide replacement of the carbonate component of the host sequence. Much of the replacive mineralisation is spatially associated with hydrothermal cavities, which are filled by a variable mixture of brecciated replacement sulphide minerals, space-filling sulphide and gangue cements, and internal sediments. Mineralisation also occurs within veins and dissolution seams, and as disseminated sulphide minerals. Massive mineralisation is typically a complex, chaotic, combination of replacement, cavity, and fracture-filling sulphides. Fluid inclusion analyses of ore-stage saddle dolomite indicate temperatures at the time of sulphide precipitation of $90-150°C, with a maximum of 175°C. These temperatures are lower than those typically proposed for Irish-type deposits.
Abstract The Tara Deep Zn-Pb deposit, at Navan, Ireland, includes sub-economic pyrite-rich minera... more Abstract The Tara Deep Zn-Pb deposit, at Navan, Ireland, includes sub-economic pyrite-rich mineralization extending laterally for about 2 × 2 km within the overlying Lower Visean calc-turbidites, known as the ‘New-Thin Bedded Unit’. Here, we investigate the genesis of this pyritic mineralization and its links to the limestone-hosted Zn-Pb deposit lying 100 m below it. Four mineral assemblages have been identified: 1) in black shales, laminated pyrite comprises thin framboid-rich layers with minor interstitial sphalerite, both showing variable but low δ34S values ranging from −37.4 to 3.3‰; 2) in calcarenite and calcsiltite layers, pyritized fossils are widely distributed and occur chiefly as biodebris replaced by low δ34S pyrite (mean values of −13.2‰); 3) a replacive assemblage occurs as late remobilizations exhibiting both crosscutting and bedding-parallel styles, overprinting the early laminated pyrite. It comprises mostly of marcasite, with minor pyrite, sphalerite, chalcopyrite, galena, stibnite and Co-pentlandite, with high δ34S values up to 24.5‰; and 4) in black shales, bedded sulfide-rich cherts comprising microcrystalline quartz lenses exhibit δ18O mean values of 25.3‰. Cherts are rimmed by dolomite, associated with marcasite, pyrite, sphalerite, chalcopyrite, galena, siegenite and stibnite also with high δ34S values up to 44.2‰. In general terms, pyrite shows a relatively high Co/Ni ratio > 1 and sphalerite Zn/Cd ratios vary from 268 to 364. Textural analysis indicates overlapping of early-diagenetic and multi-phase hydrothermal sulfide mineralization. Development of laminated pyrite and pyritized calcarenites suggests that this mineralization was generated during early diagenesis, close to the seawater-sediment interface in oxygen-poor conditions under the influence of low-temperature hydrothermal fluid circulation. Later hydrothermal cherts and replacive sulfides suggest discharge of relatively warm hydrothermal fluids during early to mid-diagenesis, presumed to be linked to movements of nearby normal faults. Similarities in mineralogy and S isotope compositions suggest genetic links between the sub-economic pyritic mineralization and the underlying Tara Deep deposit, and consequently, that the former represents a geochemical halo with direct applicability in exploration for Zn-Pb deposits.
Irish-type deposits comprise carbonate-hosted sphalerite- and galena-rich lenses concentrated nea... more Irish-type deposits comprise carbonate-hosted sphalerite- and galena-rich lenses concentrated near normal faults. We present new data from the Tara Deep resource and overlying mineralization, at Navan, and the Island Pod deposit and associated Main zone orebodies, at Lisheen. Tara Deep mineralization predominantly replaces Tournasian micrites and subordinate Visean sedimentary breccias. The mineralization is mainly composed of sphalerite, galena, marcasite and pyrite. A range of Cu- and Sb-bearing minerals occur as minor phases. At Tara Deep, paragenetically early sulfides exhibit negative δ34S values, with later phases displaying positive δ34S values, indicating both bacterial sulfate reduction (BSR) and hydrothermal sulfur sources, respectively. However, maximum δ34S values are heavier (25‰) than in the Main Navan orebody (17‰). These mineralogical and isotopic features suggest that Tara Deep represents near-feeder mineralization relative to the Navan Main orebody. The subeconomic...
The Tarz Pb-Zn deposit is one of the underground active mines in the Ravar-Kuhbanan area, located... more The Tarz Pb-Zn deposit is one of the underground active mines in the Ravar-Kuhbanan area, located in the North of Kerman province. In this mine, mineralization was observed in two distinct sulfide and carbonate divisions within the dolomitic-limestone host rock units of the Middle Triassic succession (Shotori Formation). Field observations show that mineralization occurred mainly along the faults as veins or veinlets and in lesser extent as massive textures. Galena, sphalerite, pyrite and chalcopyrite are the most important primary sulfide minerals in the Tarz Pb-Zn deposit. Secondary minerals associated with the Pb-Zn deposits such as smithsonite, hemimorphite, cerussite and anglesite were also identified in some of the investigated samples. The average concentrations of Pb and Zn were 19 and 24% in the studied samples, respectively. Other elements such as Cd, Ag, S, Sb, Te, Se, As and Cu also showed significant enrichment in the investigated samples. Evaluation of the obtained results using multivariate statistical methods can reveal the possible relationships between mineralogical phases and geochemical analysis. For example, the correlation between Pb, Ag, Tl and Sb can be related with the galena mineralization or the geochemical relationship between As, Bi, Cu, P, Fe, Co, and S is related with the pyrite and chalcopyrite mineralization. Statistical relationships also showed that Zn has only a weak geochemical association with Se, U and Mo. The strong correlation of Ca and Mg is also due to the host-rock mineralogy, which is mainly composed of course and fine crystal dolomites at the margin and far distances of ore veins, respectively.
... & Ashton 1982, 1985; Taylor 1984; Andrew et al. 1986). ... vol-canicl... more ... & Ashton 1982, 1985; Taylor 1984; Andrew et al. 1986). ... vol-caniclastic rocks (Morris 1984). The Moffat Shales are black, graptolitic shales with minor chert, ranging in age from Ordovician (Llandeillo) to Silurian (Llandovery) (Lapworth 1878; Peach & Horne 1899). ...
For the past quarter century the Navan mine has exploited Europe's premier Zn-Pb deposit. Its gen... more For the past quarter century the Navan mine has exploited Europe's premier Zn-Pb deposit. Its genesis is controversial, in part because it has not proved amenable to radiometric dating. The ore is epigenetic and is hosted in Lower Carboniferous platform carbonates of the early Courceyan Navan Group (~351 ± 4 Ma) and of the late Chadian-early Arundian boulder conglomerate (~338 ± 1 Ma). Paleomagnetic analyses were done on 282 specimens from 26 sites in the mine using alternating-field and thermal step demagnetization plus isothermal remanence analyses. Paleomagnetic fold tests on Navan Group ore and Arundian carbonate strata and conglomerate tests on mineralized and unmineralized clasts were negative, meaning that the clasts and adjacent rocks were magnetized at some time after clast deposition and that the magnetization postdates folding. Further, a contact test using a Tertiary dike that cut ore was positive, meaning that the remanence predates intrusion of the dike. Both the ore and host rocks retain a stable characteristic remanence direction in single-to pseudosingle-domain magnetite at D = 196.0°, I = 6.0°, (N = 21, α 95 = 2.6°, k = 147) that defines a pole position of 25.6°W, 31.8°S (δ p = 1.3°, δ m = 2.6°). Comparison of the pole to the rotated Laurentian and unrotated eastern Avalonian apparent polar wander paths yields a late Arundian to early Asbian age of 333 ± 4 Ma for the chemical remanent magnetization that is primary in the ore minerals and a secondary remagnetization in the host rocks. The result implies that the ore mineralization event followed deposition and tilting of the overlying Arundian-Holkerian Lucan Formation at Navan.
The Navan Zn-Pb deposit in Ireland is hosted by a lower Carboniferous, carbonate-dominated sedime... more The Navan Zn-Pb deposit in Ireland is hosted by a lower Carboniferous, carbonate-dominated sedimentary sequence. In excess of 97 percent of the ore is hosted by a Courceyan, shallow-water succession termed the Pale Beds. The ore occurs as complex strata-bound, tabular lenses often dislocated by faulting and truncated by a submarine erosional slide structure. Above this surface (unconformity) lies the economically minor, but genetically important Conglomerate Group ore hosted by a complex debris flow termed the Boulder Conglomerate. Timing constraints on the mineralization can be defined by the relationship to the erosion surface and style of mineralization, suggesting a late Chadian or early Arundian age (•345 Ma). The bulk of the ore formed below the erosion surface in the semilithified to lithified Pale Beds ore. The mineralogy of the economic ores is relatively simple, being dominated by sphalerite and galena in a 5/1 ratio. Pyrite and marcasite are present in subordinate amounts in the majority of the Pale Beds ore but dominate the Conglomerate Group ore and the Pale Beds ore immediately below the erosion surface, particularly lens 2-1. Gangue minerals are dominated by calcite, dolomite, and barite. The morphology of the mineralization reflects subsea-floor emplacement. The orebody occurs as numerous strata-bound horizons, ranging from intense, high-grade massive sulfides to lower grade sulfide layers separated by barren limestone. In all cases, there is strong evidence for tectonic disruption during ore deposition. Sulfides were dominantly deposited as open-space growth and replacement of host rock. Open-space textures include dendritic-skeletal, stalactitic, internal sediment, geopetal, and coarse-bladed forms. The general absence of coarse, euhedral, cavity lining textures or substantial collapse brecciation suggests that large cavities were not present prior to mineralization. Replacement textures range from delicate pseudomorphs of biodasts to more destructive granular styles. Sulfide sulfur isotope analyses exhibit two dominant groupings of 6a4s from around-23 to-5 and 0 to 15 per mil, respectively, and a third minor grouping around-32 to-28 per mil. At least two sources of sulfur are invoked. Values around-23 to-15 per mil are considered to be the result of bacteriogenic reduction of lower Carboniferous seawater sulfate (•20%o), with a characteristic fractionation around 35 to 45 per mil away from sulfate; this is the most common range of 634S in the deposit. Values around 8 to 15 per mil are interpreted to be hydrothermal sulfide transported with the metal-bearing fluid. Mixing of these two components is inferred from the isotopic data and trends in the mineral paragenesis. Barite has a mean of 6a4S = 21 _+ 2 per mil, which falls within the range generally recognized for lower Carboniferous seawater sulfate. We consider that contemporaneous seawater sulfate was the dominant source of sulfur in the barite at Navan, implying that seawater accessed the Pale Beds at the time of mineralization.
We report sulfur and lead isotope analyses of mine concentrates from Navan, the largest zinc depo... more We report sulfur and lead isotope analyses of mine concentrates from Navan, the largest zinc deposit in Ireland. These samples, each representing up to a million tonnes of ore, show very limited isotopic variation, with mean δ 34 S =-13.6 ± 2 per mil (n = 20), and galena concentrate mean 206 Pb/ 204 Pb = 18.19 ± 0.03 (n = 7). Calculations suggest =90 percent of the Navan sulfides were derived through bacteriogenic reduction of Mississippian seawater sulfate, whereas metals were acquired from a local, orogenic crustal source beneath the orebody. Enhanced bacterial activity was fundamental to ore deposition at Navan: no bacteria, no giant ore deposit.
Models of genesis for the Navan orebody are of two distinct types. An early hypothesis that mesot... more 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 textures demonstrating comminution and dissolution are revealed by this study that, when coupled with evidence of isotopic overprinting, force the conclusion that mineralizing fluids first invaded the host lithologies during mid-Lower Mississippian times, coincident with active faulting. There is no evidence of reactivation of the minor fault sets encountered in the study area during post-Chadian tectonism, though the major, partly listric, east-northeast extensional faults were reactivated at that time. Ore deposition was effected by bacteriogenic sulfide (reduced from Mississippian seawater sulfate) reacting with rising, metal-bearing mesothermal fluids. Thus any model invoking an onset of mineralization later than the mid-Lower Mississippian does not stand up to this scrutiny.
Recent exploration in the vicinity of the giant (>90 Mt) Navan orebody has resulted in the discov... more Recent exploration in the vicinity of the giant (>90 Mt) Navan orebody has resulted in the discovery of ore-grade mineralisation to the southwest of the deposit, much of which occurs within the Upper Pale Beds, a horizon that is only weakly mineralised above the main orebody. Within this new U lens, mineralisation preferentially occurs within bioclastic carbonate grainstones and calcareous quartz sandstones, and is dominated by sulphide replacement of the carbonate component of the host sequence. Much of the replacive mineralisation is spatially associated with hydrothermal cavities, which are filled by a variable mixture of brecciated replacement sulphide minerals, space-filling sulphide and gangue cements, and internal sediments. Mineralisation also occurs within veins and dissolution seams, and as disseminated sulphide minerals. Massive mineralisation is typically a complex, chaotic, combination of replacement, cavity, and fracture-filling sulphides. Fluid inclusion analyses of ore-stage saddle dolomite indicate temperatures at the time of sulphide precipitation of $90-150°C, with a maximum of 175°C. These temperatures are lower than those typically proposed for Irish-type deposits.
Abstract The Tara Deep Zn-Pb deposit, at Navan, Ireland, includes sub-economic pyrite-rich minera... more Abstract The Tara Deep Zn-Pb deposit, at Navan, Ireland, includes sub-economic pyrite-rich mineralization extending laterally for about 2 × 2 km within the overlying Lower Visean calc-turbidites, known as the ‘New-Thin Bedded Unit’. Here, we investigate the genesis of this pyritic mineralization and its links to the limestone-hosted Zn-Pb deposit lying 100 m below it. Four mineral assemblages have been identified: 1) in black shales, laminated pyrite comprises thin framboid-rich layers with minor interstitial sphalerite, both showing variable but low δ34S values ranging from −37.4 to 3.3‰; 2) in calcarenite and calcsiltite layers, pyritized fossils are widely distributed and occur chiefly as biodebris replaced by low δ34S pyrite (mean values of −13.2‰); 3) a replacive assemblage occurs as late remobilizations exhibiting both crosscutting and bedding-parallel styles, overprinting the early laminated pyrite. It comprises mostly of marcasite, with minor pyrite, sphalerite, chalcopyrite, galena, stibnite and Co-pentlandite, with high δ34S values up to 24.5‰; and 4) in black shales, bedded sulfide-rich cherts comprising microcrystalline quartz lenses exhibit δ18O mean values of 25.3‰. Cherts are rimmed by dolomite, associated with marcasite, pyrite, sphalerite, chalcopyrite, galena, siegenite and stibnite also with high δ34S values up to 44.2‰. In general terms, pyrite shows a relatively high Co/Ni ratio > 1 and sphalerite Zn/Cd ratios vary from 268 to 364. Textural analysis indicates overlapping of early-diagenetic and multi-phase hydrothermal sulfide mineralization. Development of laminated pyrite and pyritized calcarenites suggests that this mineralization was generated during early diagenesis, close to the seawater-sediment interface in oxygen-poor conditions under the influence of low-temperature hydrothermal fluid circulation. Later hydrothermal cherts and replacive sulfides suggest discharge of relatively warm hydrothermal fluids during early to mid-diagenesis, presumed to be linked to movements of nearby normal faults. Similarities in mineralogy and S isotope compositions suggest genetic links between the sub-economic pyritic mineralization and the underlying Tara Deep deposit, and consequently, that the former represents a geochemical halo with direct applicability in exploration for Zn-Pb deposits.
Irish-type deposits comprise carbonate-hosted sphalerite- and galena-rich lenses concentrated nea... more Irish-type deposits comprise carbonate-hosted sphalerite- and galena-rich lenses concentrated near normal faults. We present new data from the Tara Deep resource and overlying mineralization, at Navan, and the Island Pod deposit and associated Main zone orebodies, at Lisheen. Tara Deep mineralization predominantly replaces Tournasian micrites and subordinate Visean sedimentary breccias. The mineralization is mainly composed of sphalerite, galena, marcasite and pyrite. A range of Cu- and Sb-bearing minerals occur as minor phases. At Tara Deep, paragenetically early sulfides exhibit negative δ34S values, with later phases displaying positive δ34S values, indicating both bacterial sulfate reduction (BSR) and hydrothermal sulfur sources, respectively. However, maximum δ34S values are heavier (25‰) than in the Main Navan orebody (17‰). These mineralogical and isotopic features suggest that Tara Deep represents near-feeder mineralization relative to the Navan Main orebody. The subeconomic...
The Tarz Pb-Zn deposit is one of the underground active mines in the Ravar-Kuhbanan area, located... more The Tarz Pb-Zn deposit is one of the underground active mines in the Ravar-Kuhbanan area, located in the North of Kerman province. In this mine, mineralization was observed in two distinct sulfide and carbonate divisions within the dolomitic-limestone host rock units of the Middle Triassic succession (Shotori Formation). Field observations show that mineralization occurred mainly along the faults as veins or veinlets and in lesser extent as massive textures. Galena, sphalerite, pyrite and chalcopyrite are the most important primary sulfide minerals in the Tarz Pb-Zn deposit. Secondary minerals associated with the Pb-Zn deposits such as smithsonite, hemimorphite, cerussite and anglesite were also identified in some of the investigated samples. The average concentrations of Pb and Zn were 19 and 24% in the studied samples, respectively. Other elements such as Cd, Ag, S, Sb, Te, Se, As and Cu also showed significant enrichment in the investigated samples. Evaluation of the obtained results using multivariate statistical methods can reveal the possible relationships between mineralogical phases and geochemical analysis. For example, the correlation between Pb, Ag, Tl and Sb can be related with the galena mineralization or the geochemical relationship between As, Bi, Cu, P, Fe, Co, and S is related with the pyrite and chalcopyrite mineralization. Statistical relationships also showed that Zn has only a weak geochemical association with Se, U and Mo. The strong correlation of Ca and Mg is also due to the host-rock mineralogy, which is mainly composed of course and fine crystal dolomites at the margin and far distances of ore veins, respectively.
... & Ashton 1982, 1985; Taylor 1984; Andrew et al. 1986). ... vol-canicl... more ... & Ashton 1982, 1985; Taylor 1984; Andrew et al. 1986). ... vol-caniclastic rocks (Morris 1984). The Moffat Shales are black, graptolitic shales with minor chert, ranging in age from Ordovician (Llandeillo) to Silurian (Llandovery) (Lapworth 1878; Peach & Horne 1899). ...
For the past quarter century the Navan mine has exploited Europe's premier Zn-Pb deposit. Its gen... more For the past quarter century the Navan mine has exploited Europe's premier Zn-Pb deposit. Its genesis is controversial, in part because it has not proved amenable to radiometric dating. The ore is epigenetic and is hosted in Lower Carboniferous platform carbonates of the early Courceyan Navan Group (~351 ± 4 Ma) and of the late Chadian-early Arundian boulder conglomerate (~338 ± 1 Ma). Paleomagnetic analyses were done on 282 specimens from 26 sites in the mine using alternating-field and thermal step demagnetization plus isothermal remanence analyses. Paleomagnetic fold tests on Navan Group ore and Arundian carbonate strata and conglomerate tests on mineralized and unmineralized clasts were negative, meaning that the clasts and adjacent rocks were magnetized at some time after clast deposition and that the magnetization postdates folding. Further, a contact test using a Tertiary dike that cut ore was positive, meaning that the remanence predates intrusion of the dike. Both the ore and host rocks retain a stable characteristic remanence direction in single-to pseudosingle-domain magnetite at D = 196.0°, I = 6.0°, (N = 21, α 95 = 2.6°, k = 147) that defines a pole position of 25.6°W, 31.8°S (δ p = 1.3°, δ m = 2.6°). Comparison of the pole to the rotated Laurentian and unrotated eastern Avalonian apparent polar wander paths yields a late Arundian to early Asbian age of 333 ± 4 Ma for the chemical remanent magnetization that is primary in the ore minerals and a secondary remagnetization in the host rocks. The result implies that the ore mineralization event followed deposition and tilting of the overlying Arundian-Holkerian Lucan Formation at Navan.
The Navan Zn-Pb deposit in Ireland is hosted by a lower Carboniferous, carbonate-dominated sedime... more The Navan Zn-Pb deposit in Ireland is hosted by a lower Carboniferous, carbonate-dominated sedimentary sequence. In excess of 97 percent of the ore is hosted by a Courceyan, shallow-water succession termed the Pale Beds. The ore occurs as complex strata-bound, tabular lenses often dislocated by faulting and truncated by a submarine erosional slide structure. Above this surface (unconformity) lies the economically minor, but genetically important Conglomerate Group ore hosted by a complex debris flow termed the Boulder Conglomerate. Timing constraints on the mineralization can be defined by the relationship to the erosion surface and style of mineralization, suggesting a late Chadian or early Arundian age (•345 Ma). The bulk of the ore formed below the erosion surface in the semilithified to lithified Pale Beds ore. The mineralogy of the economic ores is relatively simple, being dominated by sphalerite and galena in a 5/1 ratio. Pyrite and marcasite are present in subordinate amounts in the majority of the Pale Beds ore but dominate the Conglomerate Group ore and the Pale Beds ore immediately below the erosion surface, particularly lens 2-1. Gangue minerals are dominated by calcite, dolomite, and barite. The morphology of the mineralization reflects subsea-floor emplacement. The orebody occurs as numerous strata-bound horizons, ranging from intense, high-grade massive sulfides to lower grade sulfide layers separated by barren limestone. In all cases, there is strong evidence for tectonic disruption during ore deposition. Sulfides were dominantly deposited as open-space growth and replacement of host rock. Open-space textures include dendritic-skeletal, stalactitic, internal sediment, geopetal, and coarse-bladed forms. The general absence of coarse, euhedral, cavity lining textures or substantial collapse brecciation suggests that large cavities were not present prior to mineralization. Replacement textures range from delicate pseudomorphs of biodasts to more destructive granular styles. Sulfide sulfur isotope analyses exhibit two dominant groupings of 6a4s from around-23 to-5 and 0 to 15 per mil, respectively, and a third minor grouping around-32 to-28 per mil. At least two sources of sulfur are invoked. Values around-23 to-15 per mil are considered to be the result of bacteriogenic reduction of lower Carboniferous seawater sulfate (•20%o), with a characteristic fractionation around 35 to 45 per mil away from sulfate; this is the most common range of 634S in the deposit. Values around 8 to 15 per mil are interpreted to be hydrothermal sulfide transported with the metal-bearing fluid. Mixing of these two components is inferred from the isotopic data and trends in the mineral paragenesis. Barite has a mean of 6a4S = 21 _+ 2 per mil, which falls within the range generally recognized for lower Carboniferous seawater sulfate. We consider that contemporaneous seawater sulfate was the dominant source of sulfur in the barite at Navan, implying that seawater accessed the Pale Beds at the time of mineralization.
We report sulfur and lead isotope analyses of mine concentrates from Navan, the largest zinc depo... more We report sulfur and lead isotope analyses of mine concentrates from Navan, the largest zinc deposit in Ireland. These samples, each representing up to a million tonnes of ore, show very limited isotopic variation, with mean δ 34 S =-13.6 ± 2 per mil (n = 20), and galena concentrate mean 206 Pb/ 204 Pb = 18.19 ± 0.03 (n = 7). Calculations suggest =90 percent of the Navan sulfides were derived through bacteriogenic reduction of Mississippian seawater sulfate, whereas metals were acquired from a local, orogenic crustal source beneath the orebody. Enhanced bacterial activity was fundamental to ore deposition at Navan: no bacteria, no giant ore deposit.
Models of genesis for the Navan orebody are of two distinct types. An early hypothesis that mesot... more 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 textures demonstrating comminution and dissolution are revealed by this study that, when coupled with evidence of isotopic overprinting, force the conclusion that mineralizing fluids first invaded the host lithologies during mid-Lower Mississippian times, coincident with active faulting. There is no evidence of reactivation of the minor fault sets encountered in the study area during post-Chadian tectonism, though the major, partly listric, east-northeast extensional faults were reactivated at that time. Ore deposition was effected by bacteriogenic sulfide (reduced from Mississippian seawater sulfate) reacting with rising, metal-bearing mesothermal fluids. Thus any model invoking an onset of mineralization later than the mid-Lower Mississippian does not stand up to this scrutiny.
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