Coupling Mineralogy, Textures, Stable and Radiogenic Isotopes in Identifying Ore-Forming Processes in Irish-Type Carbonate-Hosted Zn–Pb Deposits

: 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 δ 34 S values, with later phases displaying positive δ 34 S values, indicating both bacterial sulfate reduction (BSR) and hydrothermal sulfur sources, respectively. However, maximum δ 34 S 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 mineralization hosted in the overlying Thin Bedded Unit (TBU) comprises sphalerite replacing framboidal pyrite, both exhibiting negative δ 34 S values ( − 37.4 to − 8.3‰). These features indicate a BSR source of sulfur for TBU mineralization, which may represent seafloor exhalation of mineralizing fluids that formed the Tara Deep orebody. The Island Pod orebody, at Lisheen, shows a mineralogical paragenetic sequence and δ 34 S values broadly similar to other Lisheen orebodies. However, the lack of minor Cu, Ni, and Sb minerals suggests a setting more distal to hydrothermal metal feeder zones than the other Lisheen orebodies. Pb isotope data indicate a very homogeneous Lower Palaeozoic Pb source for all Navan orebodies. Lower Palaeozoic metal sources are also inferred for Lisheen, but with variations both within and between orebodies. Carbon and oxygen isotopic variations at Navan and Lisheen appear to result from fluid-carbonate rock buffering. The emerging spectrum of mineralogical and isotopic variations define proximal to distal characteristics of Irish-type systems and will assist in developing geochemical vectoring tools for exploration. NIST NBS 981 ( n 36) for this method at UCD are Pb/ Pb and Pb/ Pb 36.6920. The homogeneity of the new Pb isotope analyses for Navan and Lisheen is consistent with published data for other Irish deposits, suggesting a local crustal reservoir dominated by Lower Palaeozoic rocks. The new high-precision analyses from several Navan orebodies do not confirm the large variations in previously published data, suggesting that the Pb isotope ratios of the metal-bearing fluid is much more homogeneous than previously considered. The identification of geochemical halos surrounding the Tara Deep deposit confirms the potential to develop vectors towards undiscovered mineralization.

The Irish Midlands are dominated by Lower Carboniferous limestones ( Figure 1). These rocks were deposited in tropical, typically shallow, seas and associated with sedimentary basins developed during crustal extension [26]. They represent a marine transgression over clastic sedimentary rocks of late Devonian to Tournasian age known as the Old Red Sandstone, a terrestrial/littoral red bed succession [26]. Irish-type deposits are developed primarily in Tournasian carbonate sedimentary rocks at two distinct stratigraphic levels. The Navan deposit, and some smaller deposits in the northeastern Irish Midlands, are hosted mainly by micrites in the basal Tournasian limestone sequence [11]. Further to the south and west, deposits including Lisheen, Galmoy and Silvermines are hosted in dolomitized Waulsortian limestones. In both cases, these consist of the stratigraphically oldest limestones in their respective regions [27]. The Old Red Sandstone of the Irish Midlands was deposited on an eroded basement of Ordovician to Silurian metasedimentary and metavolcanic rocks. These rocks were brought together, deformed and metamorphosed by the closure of the Iapetus ocean during the Caledonian orogeny [2,3]. They are in most places covered by Devonian and younger rocks and consequently poorly understood. In the southeast Midlands they were probably deposited on the microcontinent of Ganderia-Avalonia [29]. Combined seismic and lower crustal xenolith studies suggest that Precambrian crust may be absent beneath the Iapetus suture zone [30], which underlies much of the Irish Midlands.
The mineralogy and fine-grained textures of Irish-type deposits have proved a major obstacle to most isotopic dating attempts. Ages based on remnant magnetism [31][32][33][34] are contentious because of conflict with geological constraints. However, recent palaeomagnetic analyses of rocks distal to mineralization have yielded similar remnant magnetic directions to mineralized rocks; the ~310 Ma apparent polar wander path age is consistent with a magnetization event during the Variscan orogeny [31,32]. Consequently, ages based on palaeomagnetic data are probably unrelated to ore formation. Ore-stage pyrite from Lisheen has yielded a well constrained Re-Os isochron age of 346.6 ± 3 Ma, whilst a slightly less secure errorchron (MSWD = 19) age of 334.0 ± 6.1 Ma was obtained from Silvermines pyrite [35]. Observations of sulfide clasts in a Lower Visean conglomerate at Navan suggest that at least some mineralization here is of Lower Visean age [11], consistent with the Lisheen Re-Os date.

Samples and Methods
Samples from Tara Deep and Lisheen, that encompass many of the varied mineralization styles and host rocks, were collected from drill cores extracted by Boliden Tara Mines and Vedanta Resources Ltd., respectively. The mineralogical, textural, and paragenetic analyses were carried out by transmitted and reflected light microscopy and Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) at University College Dublin (Dublin, Ireland), the University of Glasgow (Glasgow, UK), the University of Huelva (Huelva, Spain) and Memorial University Newfoundland (St. John's, NL, Canada).
Sulfur isotope analyses were carried out on representative sulfides from the Tara Deep orebody and the overlying Thin Bedded Unit at Navan, and the Island Pod and the Main zone at Lisheen. Insitu laser ablation analyses of sulfide δ 34 S in polished sections, selected on the basis of the petrological study, were made using the established technique described by Kelley and Fallick [36] at the Scottish Universities Environmental Research Centre (SUERC). The polished sections were inserted into a sample chamber which was evacuated and subsequently filled with an excess of oxygen gas. A spot (~300 microns in diameter) of the sulfide was combusted using a Spectron Lasers 902QCWNd:YAGlaser (1W power). The released SO2 gas was purified in a vacuum line. Determination of the sulfur isotope composition of the purified SO2 gas (δ 66 SO2) was carried out using a VG SIRA II gas mass spectrometer. All sulfur isotope compositions were calculated relative to Cañon Diablo Troilite (CDT), and are reported in standard notation. Reproducibility based on complete duplicate analyses (including combustion) was better than 0.3 per mil. Experimentally determined sulfide mineral fractionation corrections for the SUERC laser and gas line system were calibrated for all base metal sulfides analyzed.
Carbonate isotope measurements (δ 13 C and δ 18 O) of calcite and dolomite phases were obtained from both the Lisheen and Navan deposits [37,38]. Carbonate phases were characterized by traditional petrography and Scanning Electron Microscopy at University College Dublin, and imaged using a CITL mk5 cold cathode-luminescence stage mounted on a Nikon 50i microscope at Imperial College London (London, UK). Sample powders were obtained using a handheld rotary drill. Clumped C-O isotope analysis was performed at the University of East Anglia (Norwich, UK). Carbon dioxide was produced by reacting 3-5 mg of carbonate powder with orthophosphoric acid in vacuo at 25 °C for a period of 12 h for calcite and 5 days for dolomite. Subsequent samples were reacted at a temperature of 90 °C for 20 min. The evolved CO2 was collected by cryo-distillation according to the methodology outlined by Dennis et al. [39]. Sample gases were analyzed for their isotope values on a custom-built MIRA (multiple isotope ratio analyzer) dual-inlet mass spectrometer [39]. The external precision for sample analysis is estimated as ±0.014‰.
Pb isotope analyses were carried out on representative galenas extracted using a handheld drill from Tara Deep, the Main Orebody and from Tatestown in the Navan area, and from all of the ore lenses at Lisheen. Approximately 5 mg of galena was dissolved by adding 2.5 mL of 6 mol/L HCl and 250 µL of 70% HNO3. The samples were dried down on a hotplate at ~120 °C, and re-dissolved in 19mL of deionized water and 1mL of 70% HNO3. Subsequently, an aliquot of 10 µL was taken, centrifuged, diluted with 2.5 mL of 3% HNO3, and spiked with 25 µL of thallium solution of known 203 Tl/ 205 Tl ratio to correct for mass bias fractionation. Sample solutions were analyzed using a Thermo-Scientific Neptune multi-collector inductively coupled plasma mass spectrometer at the National Centre for Isotope Geochemistry, University College Dublin (UCD

The Navan Deposit and Tara Deep Orebody
The Tara Deep orebody is located ~3 km SE of the South West Extension (SWEX) of the Navan deposit, at a depth of 1.2-1.9 km (Figure 2a). Tara Deep consists of a Zn-Pb semi-massive sulfide body, up to 60 m thick.
In the Navan area, the stratigraphic succession comprises the Tournasian basal limestone sequence, which forms the Navan Group, overlain by the Visean marine succession comprising the Fingal Group. The Navan Group consists of the footwall sequence of Red Beds, Laminated Beds, and Muddy Limestone (Figure 2b). These shallow water units, as well as the overlying Pale Beds, form allochthonous slide blocks which in turn are overlain by melange-like deposits of Shaley Pales, and/or Argillaceous Bioclastic Limestones. This allochthonous package is overlain by the Fingal Group, which comprises lenses of polymict submarine debris-flows overlain by a deep marine succession of interbedded, distal calc-turbidites, and shales known as the Upper Dark Limestone (UDL) ( Figure  2b). Tara Deep is located between two major extensional structures. The G Fault forms the northern boundary, while the southern limit is defined by a larger structure, the ENE-trending Navan Fault, which has a displacement of several kilometers. The structural terrace formed by the Navan and G faults is cut by the S fault, a NNW-striking, steep westerly dipping structure (Figure 2a).
At Tara Deep, mineralization occurs chiefly in two principal lithologies: 1. The Micrite Unit is the most important host, containing approximately 90% of the total resource [13], with stratabound mineralization occurring in fault-bounded blocks. Associated with a quiescent, sabkha-like depositional environment, the upper and lower micrites with an intervening calcitite can vary in thickness but are typically 50-60 m thick. Characteristic features include fine-grained, organic matter-rich, blue/dark grey, and abundant bird's-eye (fenestral) textures. The lower Micrite Unit is the most strongly mineralized and includes high-grade mineralization comprising replacive, massive sphalerite, galena, and marcasite ( Figure 3a). The upper Micrite Unit shows variable but lower grades. For example, fracture-infilling mineralization is typically observed as a series of sulfide veins (Figure 3b). Commonly the micrites host alternating layers of sphalerite and galena (Figure 3c).
2. Sedimentary breccias located near the hanging walls of the G and S faults, towards the NW of Tara Deep, host the remainder of the mineralization. The sedimentary breccias are composed chiefly of Pale Beds clasts with minor Waulsortian Limestone, replaced and cemented by sulfide (Figure 3d,e). As in the Navan deposit, the sedimentary breccias also contain detrital sulfide clasts, again suggesting a Lower Visean age for at least some of the mineralization.  The Tara Deep deposit is unconformably overlain by a deep marine succession, comprising the Visean basin fill, known as the Upper Dark Limestone (UDL) (Figure 2b). In the Tara Deep area, the lower UDL includes the Thin Bedded Unit (TBU), which comprises alternating sequences of dark lutites, siltstones, and bioclastic rocks (Figures 2b and 3f,g). Here, uneconomic seafloor sulfide mineralization has been identified above the Tara Deep deposit. It comprises 0.5-15 cm thick bedding-parallel layers of framboidal pyrite and minor disseminated sphalerite (Figure 3f). Mineralization is found extensively throughout the TBU, progressively increasing in abundance downwards. In addition, bioclastic layers within the TBU show a high degree of pyritization, including the replacement of fossils (Figure 3g).

The Lisheen Deposit and the Island Pod Orebody
The Lisheen deposit is composed of six separate stratabound orebodies, controlled by a network of normal faults: Main zone, Derryville, Bog Zone East, Bog Zone West, Bog Zone Central, and the Island Pod ( Figure 4a). The Lisheen deposit is located in the hanging walls of extensional faults. They form a series of left-stepping, northerly dipping, en echelon structures (Figure 4a) that exhibit a classic ramp-relay geometry [3,25]. Locally, orebodies are controlled by a complex system of relaying listric faults associated with hydrothermal breccia development [25].
Several phases of dolomitization occur in the Lisheen area [41]. The earliest is a fine-to mediumgrained, subhedral to euhedral grey dolomite that preferentially replaces micritic Waulsortian Limestone (D1a) [42]. The latest consists of recrystallized coarse white dolomite (D1b) that replaces coarser Waulsortian calcite crystals. Both dolomitization phases have been called the "regional dolomite" [42]. In the Lisheen deposit, the "regional dolomite" occurs in the footwalls of the main faults [43]. Mineralization at Lisheen is apparently related to a later hydrothermal dolomitization event towards the base of the Waulsortian limestones. This resulted in two main ore-related dolomites, which together with the sulfides, form the matrix of hydrothermal breccias and overprint the regional dolomite [10,24] At Lisheen, mineralization occurs as stratabound, massive sulfide lenses within the base of the BMB, and as semi-massive sulfides in the Lisduff Oolite associated with both the footwalls and hanging walls of the major faults ( Figure 4a). It mostly comprises pyrite, sphalerite, and galena. In addition, niccolite, tennantite, marcasite and chalcopyrite have been identified in close association to the feeder faults [20,24].

Tara Deep, Navan
The main mineralogy of the Tara Deep deposit is similar to other deposits in the Navan system [11], but with minor mineralogical differences. Mineralization consists of sphalerite, galena, marcasite and pyrite, while stibnite, tetrahedrite, and chalcopyrite are minor components ( Figure 5).
Pyrite is extensively distributed in the deposit, in both the Pale Beds and conglomerate-hosted mineralization. Pyrite shows a wide range of textures, including: Framboids, colloforms, recrystallized crystals, veinlets and fine-scale replacement of fossils. Two main pyrite generations have been recognized at Tara Deep. The first generation consists of framboidal pyrite partially replaced by colloform textures (Figure 5a) and fine-scale replacement of fossils ( Figure 5b). It occurs as scarce relicts unevenly dispersed throughout the deposit. The second generation of pyrite is a recrystallization ( Figure 5l Galena occurs in both the Pale Beds and conglomerate-hosted mineralization, and always shows a close relationship with sphalerite. It occurs mainly as skeletal aggregates, filling interstices and as euhedral coarse crystals. Two galena generations have been identified. The oldest generation appears mainly replacing primary pyrite textures ( Figure 5a) and as skeletal aggregates embedded in colloform pyrite (Figure 5g). The younger generation occurs as a late infilling in pyrite and as coarse crystals associated with the second generation of sphalerite ( Figure 5h). This galena generation contains inclusions of fine stibnite and tetrahedrite ( Figure 5i).
Marcasite is widely distributed throughout the deposit but is most abundant within the calcarenites overlying the micrite horizons, and in the mineralized breccias and conglomerates (Figure 3b-e). Marcasite occurs as veinlets, radial aggregates and with colloform texture. It is commonly observed partially replaced by the youngest pyrite generation ( Figure 5j) and is also associated with the second sphalerite and galena generations.
Stibnite is a minor but common component of the Tara Deep deposit. It occurs in the second generation of coarse galena crystals, as very fine inclusions associated with tetrahedrite ( Figure 5i) and as laths associated with the second generation of sphalerite ( Figure 5k).
Chalcopyrite also appears in small amounts in the mineralized breccias and conglomerates. It is usually intergrown with the youngest sphalerite generation and filling interstices in pyrite aggregates ( Figure 5l).

TBU Overlying Tara Deep, Navan
Pyrite and sphalerite comprise the main sulfide mineralogy of the uneconomic mineralization hosted in the overlying TBU at Tara Deep. Pyrite typically occurs as framboids forming 0.5-15 cm thick bedding-parallel layers which replace black mudstones (Figure 3f). These framboids have a very high degree of preservation ( Figure 5m), with recrystallization being poorly developed. Pyrite also appears as a microfossil replacement, mainly of radiolaria (Figures 3g and 5o). Sphalerite occurs interstitially to, and replacing, pyrite framboids ( Figure 5n).

Island Pod and Main Zone, Lisheen
Based on petrographic analysis of the Island Pod and Main zone orebodies, Lisheen mineralization consists mainly of pyrite, sphalerite, galena, and marcasite ( Figure 6). Minor chalcopyrite, arsenopyrite, tennantite, cobaltite, gersdorffite, jordanite, bournonite, boulangerite, ullmannite, tetrahedrite, bravoite, pararammelsbergite, and niccolite have been reported from the Main zone, Derryville, and Bog zones [10,12,20,45]. Pyrite is extensively distributed in all the orebodies at Lisheen. It occurs in a variety of forms, often in close association with sphalerite and galena. Marcasite is relatively rare by comparison and is common near E-W normal faults and feeder zones. Several pyrite generations and morphologies have been recognized at Lisheen [12,46]. Early framboidal pyrite is only locally observed in Lisheen, sometimes with sphalerite replacement. Pyrite is often noted as being replaced by sphalerite ( Figure  6a,e), with fracture infilling by galena (Figure 6g). Minor colloform and dendritic pyrite (Figure 6b) also occur. Texturally and chemically later zoned pyrites are common at Lisheen (Figure 6h) Sphalerite shows a wide range of textures including colloform, intergrowths with dolomite, replacing pyrite, and as euhedral aggregates. In the Island Pod it displays two main textures; colloform and non-colloform, crystalline sphalerite. Early sphalerite shows colloform texture ( Figure  6c) and is often intergrown with ore stage dolomite (Figure 6d). Later sphalerite has been identified as fine-grained euhedral aggregates cementing coarser euhedral pyrite crystals ( Figure 6h) and replacing early pyrite (Figure 6a). Several generations have been identified based on color and traceelement chemistry (Figure 6c).
Galena is widely dispersed throughout all the orebodies at Lisheen. Galena can occur as dendritic crystals, filling cavities, by replacement and veining. Galena has been identified from all parts of the mineralization at Lisheen, with early dendritic galena commonly observed in association with colloform sphalerite and/or infilling dolomite (Figure 6f). A later generation occurs as coarser crystals and infillings of open spaces in brecciated late pyrite (Figure 6g). This later galena has been reported associated with Pb sulfosalts [12].

Sulfur Isotope Composition
A total of 31 spot analyses of the sulfur isotope composition of pyrite, marcasite, sphalerite, and galena (Figures 7, 8 and Table A1) were carried out on samples from Tara Deep and the TBU, Navan (Figure 7b), and from the Island Pod and Main zone oolite-hosted mineralization, Lisheen ( Figure  8b).

Island Pod and Main Zone, Lisheen
Previous S isotope analysis of samples from Derryville, Main zone, and Bog zone orebodies at Lisheen (n = 119) reveal δ 34 S values ranging from −44.1 to +11.8‰ (average of -11.6‰). Barite ranges from +14.3 to +18.1 δ 34 S [12] (Figure 8a). One clear peak in the δ 34 S distribution pattern is evident (-18 to +6‰). However, the total range for sulfide minerals (−44 to +12‰) is broadly similar to the Navan Main orebody, except that the spectrum appears to be lower by about 5‰, including both the lowest values (pyrite) and the highest values (barite).
δ 34 S values of pyrite, sphalerite, and galena from the Island Pod and Main zone oolite-hosted mineralization vary from −42.3 to +9‰ (Table A1 and Figure 8b) and fall within the range previously reported for Derryville, Main zone, and Bog zone (Figure 8a Pyrite shows the widest range, with δ 34 S values of −42.3 to −7.4‰ (mean of −25.3‰; n = 9; s.d. = 11.9). Based on the textures and paragenesis, early pyrite shows a lighter S isotope compositions, while slightly later pyrite, which may be brecciated (Figure 6g and 8(b4)) and is often seen as euhedral zoned crystals (Figures 6h and 8(b3)), exhibits relatively high δ 34 S values.

C-O Isotope Composition
Carbon and oxygen isotope data from carbonate phases in Irish-type deposits have been used to constrain fluid processes associated with ore formation [43,48,49].

Navan
At Navan, C and O isotope data have been reported by Braithwaite [48], Pearce [49] and Ashton et al. [11]. In addition, 14 new analyses are presented from late carbonate veins (Figure 9a and Table  A2) from NW of the Randalstown Fault, which contain coarse honeyblende sphalerite, most likely remobilized from the SWEX orebody. Ashton et al. [11] provide a review of the carbonate paragenesis and C-O isotope data of the whole Navan deposit, summarized briefly here.
Dolomitization at Navan was not as extensive as at Lisheen, with much of the host stratigraphy remaining un-dolomitized. A plume-shaped body of dolomitization in the NW portion of the Main Orebody [48] extends in an ENE direction with stratal fingers along sandy and silty grainstone lithologies [11]. Based on fluid inclusion and C-O isotopic data it was interpreted as hydrothermal in origin, most likely reflecting upwelling basinal brines that became hotter (~60 to 160 °C) with time [48]. The plume also connects with stratiform dolomite layers which locally form the hanging wall to some ore lenses and thus predate mineralization [11]. Following early dolomitization, sulfide deposition occurred synchronously with further calcite and dolomite precipitation [49].
The ranges of C and O isotope ratios for Navan carbonates (Figure 9a) are in most respects similar to those for Lisheen. Most analyses of early dolomite phases at Navan (e.g., 5 lens dolomite, massive dolomite) lie close to the host stratigraphy δ 13 C and δ 18 O values (i.e., Pale Beds Group), except that they extend to lower δ 18 O. Early dolomitization at Navan is marked by slightly lower δ 13 C (−1 to 3‰) than at Lisheen (2-3‰). This is particularly true for samples of "massive dolomite" from the core of the plume. Syn-mineralization carbonate phases at Navan show considerable scatter across the several stages identified (Figure 9a) with comparable δ 18 O values to early dolomites, but extending to negative δ 13 C values. Post-ore late calcite veins from Navan show the largest range of δ 18 O (from 15 to 26‰) with restricted δ 13 C (−2.6 to 1.8‰).

Lisheen
Published C and O isotope data from the Lisheen deposit [43,50] and 20 new analyses are presented (Table A2, Figure 9b). The following broad carbonate sequence is well established [45,50]. As already mentioned, early "regional dolomitization" of the host stratigraphy comprises two phases, D1a and D1b, that replace micritic components of the host Waulsortian Limestone (Figure 6j). Mineralization at Lisheen is related to a later hydrothermal dolomitization resulting in two main orerelated dolomites: D2-BMB, which hosts most of the economic mineralization (Figure 4h), and D3-WMB (Figure 4i). A number of ore-stage carbonate phases are commonly intergrown with sulfides [24,45]. Following the main period of mineralization, the following carbonate generations have been recognized (in order): White ferroan dolomite (D4), pink saddle dolomite (D5), and late calcite (C6).
Available C and O isotope data for the Lisheen carbonate phases are shown in Figure 9b. The undolomitized Waulsortian Limestone Formation across Ireland (e.g., Tynagh, Togher, Ballycudduhy) is characterized by δ 18 O values of 22.2 to 28.4‰ and δ 13 C values of 2.9 to 4.1‰, typical of Carboniferous marine carbonates. Samples of early pale grey replacive regional dolomite (D1a) at Lisheen yield slightly lower δ 18 O values (mean 23.9‰; range 22.2-26.0‰). Subsequent coarser white regional dolomite (D1b) yields still lower values (mean 22.3‰; range 19.5-25.1‰). However, the appearance of BMB (D2) is marked by a return to high δ 18 O values (similar to D1a), whereas white dolomite of the WMBs (D3) is more similar to D1b. Post-ore carbonates are marked by a range of δ 18 O values, which are predominantly low and comparable to WMBs. Significant variation in δ 13 C at Lisheen only occurs in post-ore phases, which are marked by negative values (to -6.5‰).  (Table A2) are shown as larger symbols; small symbols include historical data associated with mineralization; and shaded fields represent historical data associated with both dolomitized and un-dolomitized Pale Beds to improve the clarity of the figure [11,43,[48][49][50]].

Navan
Twelve high-precision additional analyses from Tara Deep Pale Beds and Conglomerate-hosted mineralization, Tatestown, and the Navan Main Orebody have been obtained for comparison with  existing data from the Navan system ( Figure 10,

Lisheen
Although values are broadly similar to those obtained from Navan, 207 Pb/ 204 Pb and 206 Pb/ 204 Pb ratios are slightly higher, and the degree of homogeneity is striking. However, with the improved precision of 37 new analyses (Table A3, Figure 10), subtle isotopic differences have now been noted between the various ore lenses (i.e., Main zone, Derryville, Bog zone, Island Pod). This variation is within error of previously published analyses [20,24], with the most variation and lowest average Pb isotope ratios in the largest lenses [38]. In general terms, galena Pb isotope analyses exhibit consistent composition and mean 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb ratios of 18.1938, 15.5941, and 38.0630, respectively.

Metal sources
In the past three decades it has become increasingly apparent that individual Irish-type deposits display remarkably homogeneous Pb isotope compositions, which vary systematically across Ireland. Ratios of 206   be contoured to reflect variations in basement sources from which metals were derived [54,55]. Galena from Precambrian and Lower Palaeozoic mesothermal gold, volcanogenic massive sulfide and SEDEX occurrences in NW Ireland are characterized by a µ value of 9.2 (reflecting Laurentianaffinity crust). Similar Lower Palaeozoic-hosted galena occurrences in SE Ireland plot along a trend with a µ value of ~10, typical of Ganderian-Avalonian basement. A source of Pb from the Old Red Sandstone has been ruled out by Everett et al. [54] on isotopic grounds. Within the Longford-Down Terrane that runs across the centre of Ireland, data from both Carboniferous-hosted Zn-Pb deposits and Lower Palaeozoic basement veins (their purported feeders) form linear arrays between these end-member curves [55], suggesting mixed Pb sources. The majority of Pb for Irish base metal deposits was most likely derived from lower Palaeozoic Ganderian-Avalonian basement with probably a minor Laurentian component [54][55][56]. The lack of Pb isotope variation in new data for Lisheen (Table A3, Figure 10) is consistent with existing published results [21,24] and with the regional variation exhibited by the other Irish deposits [55]. This suggests a relatively wellhomogenized crustal reservoir and a common Pb source at Lisheen (Figure 10). Subtle Pb isotope variations between ore lenses may reflect contamination upon ascent, and is the focus of ongoing research. Navan is unusual in the Irish Midlands as large isotopic differences have been noted between the different ore lenses ( Figure 10). Mills et al. [52] proposed in their study of the Main Orebody, that while the Pb isotope composition was relatively homogeneous in the lower ore lenses (3 to 5), increasingly unradiogenic signatures occur in the upper lenses (1 and 2) ( Figure 10). This was suggested to represent a tapping of progressively deeper sections of the underlying crust [52]. New analyses of samples from the Main Orebody (Table A3) have not confirmed any such variation between the ore lenses, with all values clustered near those from the SWEX epigenetic halo [53], Tara Deep Pale Beds and Conglomerate-hosted mineralization, Tatestown occurrence, and the Navan Main Orebody concentrates (Table A3 and Figure 10). The significant scatter of ratios in the [52] dataset may be a consequence of instrument mass fractionation during TIMS analysis. We propose that Pb isotope variations at Navan are likely to be subtle as at Lisheen, though this has yet to be tested by detailed analyses.
Regarding the transport mechanism for metals in Navan and Lisheen, this process may be comparable with those reported by Bischoff et al. [57], who demonstrated by laboratory tests that heated brines can leach considerable amount of metals from Lower Palaeozoic greywackes to form Irish-type orebodies. This is consistent with fluid inclusion data reviewed by Wilkinson et al. [58], indicating that metal-bearing fluids could be derived from partially evaporated seawater. Following the convection cell model of Russell [59], Irish-type deposits have been proposed to be the result of brines that descended into the Lower Palaeozoic basement, where they leached metals and were heated, subsequently rising along normal faults below the developing deposits [13]. In addition, reactivation of normal faults, related to on-going extensional activity, together with cyclicity in sulfide deposition, indicate a superposition of mineralizing fluid pulses at both Navan and Lisheen [11,25].
By analogy with other SEDEX ore deposits, elevated concentrations of some metals and metalloids (Cu, Co, Ni, As, and Sb) within the deposits can be associated spatially with feeder zones; these elements are typically more abundant close to vent systems [60]. At Lisheen (Figure 4a), Cu, Co, Ni, and As are more abundant within the oolite-hosted mineralization closer to the major structures [20,24,25]. The association of these elements with these major EW faults supports the hypothesis that they were the pathways for hydrothermal metal transport during ore formation. This is consistent with strong leaching of the basement rocks observed close to the main fault zones at Lisheen [10]. The regional metal distribution across the different orebodies at Lisheen suggests that the Island Pod, which has especially low Cu, Co, Ni, and As, is a distal orebody to the main feeder(s). This is also consistent with low trace metal signatures (e.g., Cd, Co) in Island Pod sphalerite [46].
The Tara Deep deposit shows higher Cu and Sb concentrations as stibnite, tetrahedrite, and chalcopyrite (Figure 5i,k,l) than the Navan Main Orebody and SWEX. This metal distribution pattern suggests that Tara Deep is situated proximal to a significant feeder region within the Navan system, likely the S fault (Figure 2a). In Tara Deep, the circulation of metal-bearing fluids continued during the deposition of the Lower Visean sedimentary breccias (Figures 2b and 4e) and the immediately overlying TBU (Figures 2b and 4f). Significant amounts of marcasite in the mineralized conglomerate ( Figure 4e) and the large amount of laminated pyrite, with low Zn and Pb concentrations, within the TBU (Figure 4f) suggest that TBU mineralizing fluids were progressively exhausted in Zn and Pb.

Sulfur Sources
Barite δ 34 S values reported for both Lisheen (mean of 16‰) and Navan (mean of 21‰) [6,24] are comparable to Lower Carboniferous seawater sulfate. This suggests that contemporaneous seawater was the source of sulfate for barite at Navan and Lisheen.
In Irish-type deposits, the distribution of S isotope compositions in sulfides is interpreted as the mixing of two different sulfur sources. Negative δ 34 S values in sulfides have been classically interpreted as the result of bacteriogenic reduction of Lower Carboniferous seawater sulfate in an open system [3,9,17,18,47,61]. For example, early framboidal and colloform pyrite in both Navan (Figure 5l,f) and Lisheen (Figure 6a) have δ 34 S values as light as -40 and -44.1‰, respectively, representing a fractionation of ~60‰ from seawater sulfate. This extreme fractionation can only be explained by bacterial sulfate reduction (BSR) [61,62].
Available sphalerite and galena δ 34 S data from Tara Deep suggest a comparable bimodal distribution pattern to the Navan Main Orebody (Figure 7a,b). The two main peaks at -10 and 10‰ suggest two separate sources of sulfur. Early colloform and euhedral aggregates of sphalerite ( Figure  5d-f), alongside early skeletal galena, and galena replacing pyrite (Figures 5a,g, and 7(b2)), display negative δ 34 S values. The correlation of these negative δ 34 S values with rapid crystallization textures (Figure 7b) suggests that mixing of two fluids, one bearing metals and the other carrying bacteriogenic sulfide, was the local trigger for sulfide deposition [3]. The group with positive δ 34 S values (10‰) is associated with late interstitial sphalerite and galena with stibnite exsolutions (Figure  5h,i,k, and 7(b5)). These textures and associated S isotope results have been related to periods of intense hydrothermal activity, where sulfide precipitation occurs through changing P-T conditions, creating supersaturated conditions and reducing the buoyancy of the metal-bearing, hydrothermal fluid [11,18]. In contrast to the Navan Main Orebody, pyrite in Tara Deep displays a bimodal δ 34 S distribution (Figure 7b), similar to sphalerite and galena. This pattern suggests again two different sulfur sources, BSR associated with framboidal and colloform pyrite (Figure 5a,f) and thermal reduction related to recrystallized pyrite (Figure 5j). However, marcasite in Tara Deep displays an exclusively positive range of δ 34 S values, suggesting that marcasite precipitation occurs only in periods of intense hydrothermal activity. It is also worth noting the trend toward higher δ 34 S values in Tara Deep (up to 25‰) compared to the Main Navan Orebody (up to 17.5‰) (Figure 7a,b). This, in conjunction with the higher abundance of Cu and Sb minerals at Tara Deep (Figure 5i,l), suggests that there were more intense pulses of hydrothermal fluid at Tara Deep.
The uneconomic TBU mineralization, that overlies the main Tara Deep deposit, hosts both laminated framboidal pyrite and associated sphalerite (Figure 5m,n). These textures reveal negative δ 34 S values, suggesting a BSR origin for sulfide, possibly during early diagenesis.
At Lisheen, the predominance of negative δ 34 S values in all the generations of sphalerite and galena (main peak at -14‰) supports a BSR sulfur source for the main mineralization stages ( Figure  8a,b), as previously proposed [24]. However, some higher δ 34 S values (up to 12‰), relating to paragenetically late textures, such as replacive galena (Figure 6g,h), suggest a minor hydrothermal source for sulfur. In addition, spatial variations in S isotope compositions throughout the Lisheen orebodies can also be associated with feeder zones. In that sense, the lower δ 34 S values from the Island Pod compared to the Main zone support the notion that the Island Pod is a distal orebody to the main Lisheen feeder.

C-O Isotopes
The C-O isotope variations at Lisheen can be explained primarily through processes of fluidrock buffering [50]. Although the exact cause for early regional dolomitization is not known, measured δ 18 O values are consistent with fluid of high δ 18 O and temperatures of formation (~150-200 °C), confirmed by clumped C-O isotope analyses [37,38]. It is perhaps no surprise that δ 18 O values for subsequent black hydrothermal dolomite from BMBs are similar to that recorded by corroded clasts of the fine-grained regional dolomite that it contains (Figure 9a). Dolomite in WMBs most likely formed as hydrothermal infills to collapse breccia associated with significant dissolution of the underlying Waulsortian limestones [45]. Measured carbonate mineral δ 18 O values are a function of high but varying temperatures and fluid δ 18 O values buffered by surrounding carbonate rocks [50]. Post-ore carbonate phases formed at low temperatures, typically < 40-125 °C [28,41]. Basement veins at Lisheen have yielded low δ 18 O values (16.7-18.4‰) and strongly negative δ 13 C values (−5.1 to −5.7) ‰) [50]. These results are consistent with fluids that have not been buffered by Carboniferous limestones.
At Navan, as at Lisheen, dolomitization of the host stratigraphy is marked by a reduction in δ 18 O (Figure 9b). Published δ 18 O values for different dolomite lenses are similar with the notable exception of parts of the 5-lens dolomite. Ore stage carbonates from across the deposit show considerable scatter in δ 18 O reflecting a combination of variations in temperature, local rock buffering (by carbonates with different δ 18 O values), the presence of in situ pore waters, and/or varying fluid/rock ratios [11]. Late calcite veins (~80-110 °C) that contain remobilized sulfides are marked by extremely variable δ 18 O (16-26‰), suggesting local rock buffering.

Controls on Metal Deposition
Similar to the Navan Main Orebody, the permeability of the host rock at Tara Deep exerted an important control on the mineralizing processes. In that sense, owing to its low compressive strength and high calcite content, the micrite was susceptible to fracturing and replacement. The high density of mineralized minor fractures in the Micrite Unit (Figures 2b and 3a-c) suggests that secondary permeability was developed during extension and hydraulic fracturing by pressurized hydrothermal fluids [11]. The circulation of acidic mineralizing fluids through these hydraulic fractures resulted in dissolution of carbonates leading to enhanced porosity/permeability and the buffering of the mineralizing fluids [9]. In addition, the presence of detrital sulfide clasts in debris flows above the Lower Visean erosional surface, that are later encased by mineralization, suggests that metalliferous fluids entered the system prior to the formation of those sedimentary breccias.
Uneconomic mineralization within the overlaying TBU is hosted by dark mudstone layers (Figures 2b and 3f). Here, the close relationship between organic matter and laminated framboidal pyrite suggests that this style of mineralization was generated by the replacement of soft sediments during early diagenesis and close to the seawater-sediment interface. The presence of laminated pyrite at these higher stratigraphic levels suggests that the mineralization process continued over a longer period at Tara Deep.
Compared to Navan, the Lisheen deposit is hosted at higher stratigraphic levels within the Tournasian sequence (Figure 4b). Mineralization mainly replaces hydrothermal dolomite breccias developed in the Waulsortian mud banks (Figure 3h,i) and to a lesser extent, the Lisduff Oolite Unit of the Ballysteen Limestone Fomation (Figure 4b). At Lisheen, regional dolomitization of the Waulsortian prior to the sulfide mineralization event resulted in a significant increase in porosity and permeability. According to Hitzman et al. [10] the development of this secondary permeability exerted an important control on the mineralizing processes. In that sense, hydrothermal mineralizing fluids circulated throughout the dolomitized Waulsortian rocks, resulting in the brecciation and sulfide deposition (Figure 3h,i). Textures suggest that in the first stages, sulfides replaced the BMB, followed by the replacement of the regional dolomite clasts in later mineralization stages. As proposed for Navan, the circulation of acidic mineralizing fluids resulted in the dissolution of carbonates and precipitation of sulfides, primarily by reaction of bacteriogenic sulfide with metalbearing fluids.
At both Navan and Lisheen, the clear association between major normal faults and metal distribution indicates strong structural control [10,11,18,25]. For example, Tara Deep is situated in the hanging wall of the ENE-trending G fault and the footwall of the Navan fault (Figure 2a). The structural terrace formed by the G and Navan faults is cut by the NNW-striking S fault (Figure 2a) and mineralization in the NW zone of Tara Deep seems to be focused in the area where the S Fault encroaches on the G Fault to the north, and the Navan Fault to the south [13]. Likewise, Lisheen is associated with relay-ramp zones along a complex fault system (Figure 3a). The location of the orebodies, metal distribution, and the related hydrothermal alteration with proximity to likely feeder zones is evidence that hydrothermal fluids were able to circulate along EW faults and minor faulting in the relay zones [10].

Implications for Exploration
The economic regions of Irish-type Zn-Pb deposits are becoming increasingly well understood. However, much less attention has so far been paid to the exploration value, for vectoring purposes, of sub-economic sulfide halos surrounding the deposits. Laminated pyrite hosted in the TBU overlying the Tara Deep deposit suggests that hydrothermal fluids that formed Tara Deep were able to reach the seafloor, with the potential for forming sedimentary-exhalative (SEDEX) mineralization [63]. Nevertheless, linkages between these halos and the Navan mineralization remain poorly understood. Understanding these links has the potential to dramatically enhance mineral exploration strategies for deeply buried deposits, as well as to strengthen genetic models of both epigenetic and SEDEX mineralization.
Ongoing research into exploration methods for vectoring towards Irish-type and related SEDEX deposits include the study of mineralogical and textural patterns and associated chemical and isotopic zonation, where possible, by means of micro-analytical methodologies. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) has been extensively applied in the evaluation of trace elements in sulfides, mainly in pyrite, and applied to exploration [64]. Sulfide trace element compositions, especially of pyrite, can be used to constrain sources of metals, fluid pathways and mixing sites as well as acting as a vectoring tool [64].

Conclusions
The data presented in this study suggests that the Tara Deep and Island Pod deposits closely resemble the Navan and Lisheen deposits, respectively. Common features include: Tectonic setting, main mineralogy, nature of host rock, and mineralization affinity. The main differences are centered on the geochemistry, dealing essentially with metal distributions, such as enrichment in Sb and Cu. At Tara Deep, elevated Sb and Cu suggest proximity to a major feeder system; while depletion in Cu, Co, and Ni in the Island Pod, in comparison to Lisheen, suggest a distal relationship to the feeder system.
Although the major mineralogy of Tara Deep and Island Pod deposits are similar to those of the Navan and Lisheen orebodies, respectively, there are differences regarding minor mineralogy. In Tara Deep, stibnite, tetrahedrite, and chalcopyrite host Sb and Cu enrichments and are interpreted as resulting from proximity to a feeder zone. In contrast, the lack of chalcopyrite and Ni-sulfosalts in the Island Pod, in comparison to Lisheen, suggest a position distal to major hydrothermal feeders.
Regarding S isotope compositions, both Tara Deep and the Island Pod show similar patterns to Navan and Lisheen, respectively. However, in detail, Tara Deep has slightly heavier δ 34 S values relative to Navan, suggesting either a larger input of hydrothermal H2S, or a reduced level of BSR. In contrast, the Island Pod shows lighter δ 34 S values than Lisheen, implying an increased BSR S source, or decreased hydrothermal fluid circulation.
C and O isotope data at Lisheen reveal a relatively simple fluid evolution, whereas the situation at Navan is far more complex. At Lisheen, carbonate δ 18 O is dominated by fluid-rock buffering by early regional dolomite at a range of temperatures. More negative δ 13 C values for post-ore phases are due to interaction with inorganic matter. Basement veins at Lisheen have yielded low δ 18 O values and strongly negative δ 13 C values consistent with fluids that have not been buffered by Carboniferous limestones. At Navan, carbonate phases across the system show extremely variable δ 18 O and δ 13 C values, reflecting a combination of variable temperature of carbonate formation, local host rock buffering, the presence of in situ pore waters, and/or varying fluid/rock ratios.
The homogeneity of the new Pb isotope analyses for Navan and Lisheen is consistent with published data for other Irish deposits, suggesting a local crustal reservoir dominated by Lower Palaeozoic rocks. The new high-precision analyses from several Navan orebodies do not confirm the large variations in previously published data, suggesting that the Pb isotope ratios of the metalbearing fluid is much more homogeneous than previously considered.
The identification of geochemical halos surrounding the Tara Deep deposit confirms the potential to develop vectors towards undiscovered mineralization. and information on the Navan and Lisheen deposits respectively; Paul Dennis and Alina Marca for assistance with C-O isotope analysis, and David Van Acken and Mick Murphy for Pb isotope analysis; and three anonymous referees for their reviews which have greatly helped to improve this paper.

Conflicts of Interest:
The authors declare no conflict of interest.