Irish Ore Field Research Articles

A Distal, High-grade Irish-type Orebody: Petrographic, Sulfur Isotope, and Sulfide Chemistry of the Island Pod Zn-Pb Orebody, Lisheen, Ireland

AbstractIrish-type Zn-Pb deposits are important global sources of zinc, but despite a fundamental understanding of ore genesis within the Irish orefield, a detailed understanding of fluid migration and chemical evolution pathways related to sulfide and carbonate precipitation is lacking. We present the first petrographic, paragenetically constrained sulfur isotope and mineral chemistry study of mineralization at the Island Pod orebody, Lisheen deposit. The Island Pod orebody comprises high-grade mineralization that is less deformed than elsewhere in the Irish orefield. Consequently, studies of the Island Pod orebody and its mineralization provide information on the evolving nature of hydrothermal fluids involved in ore deposition.The Island Pod orebody consists almost exclusively of pyrite, sphalerite, and galena, with several stages of calcite and dolomite precipitation. Pre-ore, diagenetic pyrite is commonly overgrown by early main ore-stage pyrite, with both phases frequently replaced by main ore-stage sphalerite. In many cases, early main ore-stage pyrite is texturally zoned and exhibits chemical zoning patterns, reflecting that episodic influxes of hydrothermal fluids contained variable concentrations of As, Co, Ni, and Tl. The main ore stage was dominated by the formation of sphalerite and galena from mineralizing fluids that were depleted in these trace elements (e.g., As, Co, Tl) compared to the early main ore stage. Sulfur isotope analysis reveals four distinctive but slightly overlapping isotopic groupings, corresponding to different mineral and paragenetic stages: (1) δ34S values range from –47.7 to –30.7‰, associated with diagenetic pyrite; (2) δ34S values range from –34.3 to –14.7‰, related to early main ore-stage pyrite; (3) δ34S values range from –15.5 to +1.7‰, corresponding to main ore-stage sphalerite; and (4) δ34S values range from –11.1 to +17.4‰, associated with galena. Large variations in S isotope composition are common at intragrain and at other small spatial scales. The textures, paragenetic sequence, and ranges in δ34S values are consistent with hydrothermal sulfide deposition where the fluids containing bacteriogenic sulfide mixed with metal-bearing fluids. Replacement and remobilization from other Lisheen orebodies may have contributed to some of the higher sulfur isotope ratios observed in the Island Pod orebody.The excellent preservation of sulfide textures in the Island Pod orebody observed during this study demonstrates that it is an ideal location to study hydrothermal fluid evolution, including episodic fluid flow, mixing, precipitation, and compositional variations during the early main ore stage. In other Irish Zn-Pb orebodies, these early-ore textures are often obscured due to more complex dissolution and replacement processes, making interpretation of the early hydrothermal activity challenging. Consequently, the petrographic, mineral chemistry, and sulfur isotope studies of the Island Pod orebody presented here contribute to an enhanced understanding of ore-forming processes in similar deposits, where mineralization is often associated with more complex deformation or repeated pulses of hydrothermal activity.

Diatremes Act as Fluid Conduits for Zn-Pb Mineralization in the SW Irish Ore Field

Irish-type mineralization is commonly attributed to fault-controlled mixing of a seawater-derived, sulfur-rich fluid and basement-derived, metal-rich fluid. However, maar-diatreme volcanoes discovered in close spatial and temporal association with Zn-Pb mineralization at Stonepark in the Limerick basin (southwest Ireland) bring a new dimension to established geologic models and may increase the deposit-scale prospectivity in one of the world’s greatest Zn-Pb districts. Stonepark exhibits many incidences of dolomitic black matrix breccias with associated Zn-Pb mineralization, the latter typically occurring within 150 m of the diatremes. Highly negative δ 34 S pyrite values within country rock-dominated black matrix breccias (–12 to –34) are consistent with sulfide precipitation from bacteriogenic sulfur reduction in seawater-derived brines. However, δ 34 S values of Zn-Pb sulfides replacing black matrix breccias (–10 to 1) reflect multiple sulfur sources. Diatreme emplacement both greatly enhanced country rock fracture permeability and produced conduits that are filled with porous volcaniclastic material and extend down to basement rock types. Our δ 34 S data suggest that diatremes provide more efficient fluid pathways for basement-derived fluids. The diatremes introduce another potential sulfur source and facilitate a greater input of metal-rich basement-derived hydrothermal fluid into the system compared to other Irish-type deposits such as Navan and Lisheen, evidenced by Stonepark’s more positive modal δ34S value of –4. Irish-type deposits are traditionally thought to form in association with extensional basement faults and are considered unrelated to extensive Carboniferous magmatism. Our results indicate that a direct link exists between diatreme volcanism and Zn-Pb mineralization at Limerick, prompting a reevaluation of the traditional Irish-type ore formation model, in regions where mineralization is spatially associated with volcanic pipes.

Chemical mass transfer during hydrothermal alteration of carbonates: Controls of seafloor subsidence, sedimentation and Zn–Pb mineralization in the Irish Carboniferous

Petrographic and geochemical data were acquired to evaluate models for the formation of hydrothermal dolomite breccias that host zinc–lead orebodies in the Rathdowney Trend of the Irish orefield and assess element mobility during hydrothermal alteration of carbonates. Macroscopic and microscopic textures indicate that the breccias largely formed by a pseudobrecciation process involving dissolution and replacement of precursor Waulsortian dolomite of diagenetic origin during subseafloor hydrothermal fluid flow. Geochemical analyses show that, compared with the Waulsortian dolomite precursor, breccia “clasts” and “matrix” contain respectively higher concentrations of Al, Ti and light rare earth elements which are linearly correlated, indicative of mass loss during hydrothermal interaction. Bulk rock mass loss estimates average 33% indicating extensive dissolution of carbonate although petrographic evidence and limited clast rotation or translation rule out the development of significant open space. This supports a mechanism of alteration whereby hydrothermal fluids, migrating along grain boundaries, microcracks and solution seams, drove dissolution and replacement of the surrounding host rock with partial collapse of the transient microporosity produced. Progressive interaction led to the gradual isolation of domains (“clasts”) surrounded by replacement dolomite (“matrix”). Massive sulfide mineralization formed subsequently by stratabound replacement of the dolomite pseudobreccia by hydrothermal fluids preferentially focused within the more permeable and possibly more reactive lithology. This process involved further volume loss (30% of the replaced rock). Quantitative mass transfer analysis indicates that during hydrothermal interaction, net loss of Na, P, Mn, Sr and sometimes Cu, and net gain in Li, K, Fe as well as the ore-associated elements typically occurred. Marked fractionation of the REE is observed: Eu was added, implicating elevated temperature, acidic and reduced hydrothermal fluids; Ce was also added, possibly derived from remobilization of pre-sulfide oxide mineralization; the LREE were typically immobile or partially removed; and the HREE were preferentially mobilized and transported beyond the scale of sampling. Thus, the HREE, together with Na, P, Mn, Sr and possibly Cu, may form a distal chemical anomaly at the base of the Waulsortian Limestone Formation. The net addition of Li, K and Be in dolomite breccia matrix samples, combined with their increase in concentration due to dolomite dissolution, produced a broad anomaly of these elements extending for at least 1.5 km from the center of the Lisheen deposit. These various geochemical anomalies have potential utility in mineral exploration. A total volume loss of up to 68% at Lisheen is consistent with a marked thinning of the Waulsortian biomicrite facies above massive sulfide. The observed thickening of upper and supra-Waulsortian facies in the trough developed above this zone is interpreted to reflect infill of a seafloor depression up to ~ 90 m deep, developed in response to underlying hydrothermal dissolution. This model implies mineralization initiated during the latter stages of Waulsortian deposition in the late Courceyan/early Chadian at a depth of approximately 150 m below the seafloor.

A Review of Fluid Inclusion Constraints on Mineralization in the Irish Ore Field and Implications for the Genesis of Sediment-Hosted Zn-Pb Deposits

Many fluid inclusion studies have been carried out in the Irish Midlands basin ore field (Lower Carboniferous) since the earliest work by Ed Roedder in the late 1960s. Results show that, in the ore deposits, the total range in fluid salinity is 4 to 28 wt percent NaCl equiv but with the majority billing in the moderate-salinity range between 8 and 19 wt percent. This variability is interpreted in terms of mixing between moderate-salinity ore fluids and low-temperature brines during ore formation. The most northerly ore deposits of Navan and Abbey-town are, distinct in containing fluids of both lower and higher salinity than is typical of the Waulsortian-hosted deposits farther south (Tynagh, Silvermines, Lisheen, and Galmoy). Subeconomic prospects tend to display a narrower range, in salinity mostly at the lower end of the range observed in the ore deposits. In some prospects, and cm the margins of some ore deposits, evidence for dilution is observed, interpreted to reflect mixing between hydrothermal fluids and unmodified seawater. This process is inferred to be unfavorable for mineralization.Homogenization temperatures, a reasonable proxy for true trapping temperatures in the ore field, range from 70 degrees to 280 degrees C but with the majority falling between 130 degrees and 240 degrees C. There is no evidence for systematic stretching or leakage of inclusions related to the postentrapment heating implied by elevated thermal maturity indicators. The highest temperatures are observed in the Waulsortian-hosted systems, with peak temperatures of -280 degrees C supported by local, high-grade Cu and Ni mineralization. in the Navan and Abbeytown deposits, lower temperature fluids appear to have been more prevalent. The subeconomic prospects formed over essentially the same temperature range as the ore deposits (90 degrees-270 degrees C), with the exception of the morphologically and texturally distinct Mississippi Valley-type (MVT) systems in the region (e.g. Kinnitty, Harberton Bridge) that formed at lower temperatures (50 degrees-100 degrees C).Similar hydrothermal fluids to those recorded in both deposits and prospects are widely observed in dolomite (and sometimes calcite) cements within Courceyan-Arundian-age rocks, indicating that hydrothermal fluid activity occurred over an extremely large area (>30,000 km(2)) and probably over an extended time period. There is a broad regional division in fluid properties, suggesting that the northwestern and southeastern provinces, separated by the trace of the Iapetus suture zone, may represent partly decoupled, large-scale flow regimes. Up to three, low-temperature brine types are also recorded by cements in the host-rock sequence, indicating that a complex range of evaporation and fluid-rock interaction processes were ongoing in the shallow basin succession during the period of hydrothermal activity.Halogen data show that fluids involved in mineralization were originally seawater-derived brines, produced by evaporation to varying degrees. Relatively high temperature, basement-interacted hydrothermal fluids were derived from partially evaporated seawater (molar Cl/Br = 559-825). Their compositions can be explained by dolomitization in the Carboniferous succession prior to circulation to depth; alkali exchange, reduction, and metal-leaching from the lower Paleozoic basement; and mixing with low-temperature brines that locally penetrated the upper parts of the basement rock package. Fertile ore fluids appear to be characterized by higher delta O-18 (+7 to +9 parts per thousand), lower delta D (-25 to -45 parts per thousand) and much higher metal contents than otherwise similar fluids sampled in basement-hosted feeder veins distal to deposits. This may reflect highly efficient metal scavenging in deeper and/or higher temperature reaction zones that underlie the principal deposits. In the ore deposits, these fluids mixed with Br-enriched bittern brines (Cl/Br similar to 290) produced by evaporation of Carboniferous seawater past halite saturation. It is inferred that bittern brine generation occurred in the shallow marine shelf regions in the footwalls to the synsedimentary fault systems that controlled the localization of mineralization. These brines then migrated into hanging-wall depressions where they ponded within permeable sediments and became enriched in H2S via bacteriogenic sulfate reduction. The coincidence of structurally controlled, high-temperature reaction zones, brine-producing footwalls, and hanging-wall traps, with bacterial blooms above upwelling plumes of hydrothermal fluids, can be interpreted as a self-organizing system that locally converged on ore-forming conditions. Understanding the first-order structural control of the ore systems will therefore be critical for predicting new deposits.The Irish ore field presents arguably the best database available on the thermal and chemical characteristics of hydrothermal fluids involved in sediment-hosted ore genesis. The system shares much of the variety and complexity observed in other intracratonic basin-hosted Zn-Pb(Ba) ore districts. This includes the coexistence of contrasting styles of mineralization that are typically observed in the more distal and platform-marginal parts of the basinal environment. The thermal and chemical fluid heterogeneity observed is typical of modern intracratonic basin systems and should be expected in large paleohydrothermal systems where recharge of surface-derived fluids is involved.

Targeting Stratiform Zn-Pb-Ag Massive Sulfide Deposits in Ireland through Numerical Modeling of Coupled Deformation, Thermal Transport, and Fluid Flow

We explore three- and two-dimensional, regional- and local-scale numerical models for the development of Irish Zn-Pb-Ag mineralization. The calculations examined one of the proposed genetic models for the deposits in which mineralization was associated with mixing of two fluids of different salinities in the hanging walls of faults undergoing active deformation during mineralization. The numerical models involve coupling between volume changes arising from elastic-plastic deformation, single-phase fluid flow, and mixing of fluids with different salinities together with thermal transport both by conduction and by advection. We evaluated critical parameters that could optimize the rate of mineralization in three dimensions at the regional scale and in two dimensions at the local scale. Topographic relief is proposed as one driver of fluid flow. The models indicate a competing process, through convection of high-salinity fluids, mainly within faults and originating as partially evaporated seawater, which mix with lower salinity fluids driven by topographic relief. At the regional scale, although there was substantial fluid flow through the basinal sediments, considerable flow of basin-derived fluids also occurred through the basement, with subsequent focusing through faults. The effect was enhanced if the basement permeability was increased by fracturing. Basinal fluids (fluid 1, initially in the basin or introduced from meteoric sources in the adjacent highlands) were extensively exposed to the basement and became focused into both northward- and southward-dipping faults. A second, more saline fluid (fluid 2) was driven down into the rock column by convection; it had most exposure to the basinal sediments and, in some situations, to the basement before mixing with fluid 1. The positions of the fluid mixing sites involved a balance between topographically driven flow and convection in three dimensions at a regional scale. Changes in porosity and permeability induced by plastic deformation influenced the rates of mineralization at the local scale and local Darcy flow rates at mineralizing sites. Mineralization at the local scale resulted from the interplay of fault dip direction, displacement, deformation-induced dilation, seal thickness, and integrity. The modeling results are compatible with empirical data from the Irish ore field, suggesting that north-facing faults, dipping away from the inferred position of topographic highs, preferentially host economic deposits. There is nothing intrinsic in the modeling that constrains the timing of the mineralization relative to compressional or extensional events, although a body of research favors mineralization overlapping with extension, as at the Navan, Silvermines, and Tynagh deposits. In contrast, at the Lisheen deposit, mineralization postdates the main extensional fault offsets. We suggest that a switch to shortening is a possible driver for this mineralizing system.

ZINC ISOTOPE VARIATION IN HYDROTHERMAL SYSTEMS: PRELIMINARY EVIDENCE FROM THE IRISH MIDLANDS ORE FIELD

Little is known about the range or controls on the zinc isotope composition of terrestrial materials and no systematic studies have been carried out on ore-forming systems. We have obtained zinc isotope data from 19 sphalerite samples, formed over a range of well-constrained precipitation conditions, from the Irish Zn-Pb ore field. The results reveal variation in δ 66Zn (where δ 66Zn = [(66Zn/64Zn)sample/(66Zn/64Zn)standard– 1] × 1000), from –0.17 to 1.33 per mil relative to the Lyon JMC 3-0749L zinc standard. This variation is significant compared to the external reproducibility (±0.12‰, 2 σ ), and the data show very good mass-dependency with δ 67Zn and δ 68Zn values. Thus, natural variations in the zinc isotope composition of these ore minerals can be resolved. Our results span the entire range of δ 66Zn values measured on terrestrial geologic samples to date. The data suggest that variations in the primary source rock composition or precipitation temperature are unlikely to be important controls on the zinc isotope composition of sphalerite in the ore field. We suggest that the variation is most likely due to a kinetic fractionation involving the preferential incorporation of light zinc isotopes in sphalerite precipitated rapidly under disequilibrium conditions. However, we cannot rule out the possibility of mixing of zinc derived from two isotopically distinct sources. The significant variation in zinc isotope compositions we have observed in the Irish ore field confirms that such fractionations can provide new insights into mineralizing processes in the Earth’s crust.

Ore-Forming Processes in Irish-Type Carbonate-Hosted Zn-Pb Deposits: Evidencefrom Mineralogy, Chemistry, and Isotopic Composition of Sulfides at the LisheenMine

Lisheen is a strata-bound zinc-lead deposit formed during the Mississippian by replacement of hydrothermally dolomitized, grossly stratiform breccia bodies located near the base of the Carboniferous Waulsortian Limestone. It represents one of a number of carbonate-hosted massive sulfide ore deposits in the Irish ore field that, due to several unique features, have been classified as Irish type. Disseminated pyrite occurs in preore dolomite and around the margins of preore dolomite clasts within dolomite breccias. Early fine-grained sphalerite-pyrite mineralization occurs as infill of intergranular dolomite porosity. Locally, massive to semimassive iron sulfide is observed, mainly comprising pyrite with lesser marcasite. A complex polymetallic sulfide assemblage typifies the main ore stage, dominated by fine-grained disseminated, massive or colloform sphalerite and galena, with minor pyrite, chalcopyrite, arsenopyrite, tennantite, nickel- and cobalt-bearing minerals. Silver occurs in solid solution in tennantite, galena, and sphalerite. Dolomite and barite dominate the gangue, with lesser calcite. Main-stage mineralization involved the progressive replacement of preexisting iron sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts. Coarse crystalline sphalerite and euhedral galena crystals are generally restricted to fracture-fill mineralization or vugs within main ore-stage assemblages where they occur with euhedral dolomite and calcite. Barite intergrown with main ore-stage sulfides has δ 34S values of 14.3 to 18.1 per mil, consistent with the derivation of sulfate from coeval Carboniferous seawater. The δ 34S values for sulfides range from –44.1 to +11.8 per mil, with a mean value of –13.7 per mil, typical of the Irish ore deposits. The dominant low δ 34S signature is considered to be the result of bacterial reduction of coeval seawater sulfate. Extremely low δ 34S values, in the range of –38 to –44 per mil, are only observed in preore disseminated pyrite; such extreme fractionations are thought to be due to low bacterial sulfate reduction rates coupled with oxidative cycles in near-sea floor pore waters. Main ore-stage sulfides have δ 34S values in the range of –4 to –18 per mil, with a mode of –10 per mil, consistent with a typical bacterial fractionation from coeval seawater sulfate. Isotopic equilibrium between cogenetic sulfides is not observed. The bacteriogenic sulfur component was probably transported from bacterial colonies fringing the ore system by low-temperature brines. The δ 34S values of late ore-stage sulfides mainly range from –20.2 to +12.0 per mil, with the majority having relatively high values (mean = –3.0 ± 8.5‰, 1 σ ) interpreted as being due to the presence of a hydrothermal sulfur component, leached from the lower Paleozoic basement. For galena and sphalerite there is a general increase in δ 34S values with depth in the system, with time, and with proximity to east-west– and northwest-trending faults. These relationships suggest that input of hydrothermal sulfur from depth via fractures became increasingly important. Hydrothermal sulfur appears to be more important at Lisheen than the other major Irish deposits. Galena lead isotope analyses gave average 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.183, 15.594, and 38.080, respectively. These data do not correlate with ore-stage, galena texture or δ 34S. The results are comparable to previous data from Lisheen and from Silvermines, 35 km to the west, implying a common lead source in the lower Paleozoic basement. The textural, mineral, chemical, and isotopic evidence suggests that main-stage ore was precipitated as a consequence of rapid supersaturation, caused by fluid mixing within the permeable dolomite breccias. This process involved relatively high temperature (ca. 240°C), metal-bearing solutions derived from a basement-equilibrated fluid reservoir (carrying Zn, Pb, Fe, Cd) and shallow, saline (ca. 25 wt % NaCl equiv) formation waters rich in bacteriogenic H2S. Minor metals (Cu, As, Ni, Co) are thought to have been stripped from the footwall Old Red Sandstone during hydrothermal alteration around fault conduits. The availability of abundant seawater sulfate, operation of open-system bacterial sulfate reduction, and episodic availability of free oxygen imply that ore formation cannot have occurred at significant depth below the paleosea floor. Cessation of mineralization was due to a cut-off of the sulfur-rich brine supply, possibly by deposition of impermeable hanging-wall sediments. This process of ore formation is consistent with evidence from the other economic Irish-type deposits in the ore field.

Intracratonic crustal seawater circulation and the genesis of subseafloor zinc-lead mineralization in the Irish orefield

We have determined the chemical composition of ∼350-m.y.-old solutions extracted from fluid inclusions, and strontium isotopic compositions of hydrothermal minerals from the Irish zinc-lead orefield. These data show that ore-forming fluids were derived from evaporated seawater and acquired metals by deep circulation within fractures in continental crust. Mineralization occurred in the near-seafloor environment when these solutions returned to the surface via thermohaline convection and mixed with brines rich in H 2 S produced by bacterial reduction of seawater sulfate. The results indicate that deep penetration of seawater or evaporated seawater into the continental crust can occur in rift zones or extending passive margins and that this process can generate large volumes of base metal ore-forming solutions. Our results are inconsistent with topographic flow models for mineralization in the district, and support deep convection models for ore formation. The widespread development of evaporitic brines on the Laurussian continental margin under late Paleozoic greenhouse conditions is likely to have been critical for generating numerous accumulations of base metals in sedimentary basins at this time.

On diagenesis, dolomitisation and mineralisation in the Irish Zn-Pb orefield

Marine calcite cementation and lithification of Carboniferous carbonate sediments hosting Zn-Pb mineralisation in the Irish orefield occurred at or near the seafloor. A relatively early, fine-grained, grey replacive dolomite, preferentially developed in micrite, is widely developed in the Waulsortian Limestone Formation, the main host to mineralisation, and is pervasive in the southeastern Midlands in proximity to the Leinster Massif. This dolomite formed after the first four main stages of calcite cementation but probably also developed within tens of metres of the seafloor as evidenced by incorporation of clasts of dolomite in intraformational sedimentary breccias. Later, coarse-grained white dolomite preferentially replaced coarser components of the Waulsortian Limestone and infilled residual vuggy porosity. Whilst some of this coarse dolomite may be related to the fine replacive dolomite event, a common spatial association with fault zones, coupled with primary fluid inclusion data, suggest that a significant proportion of this phase precipitated during the onset of fault-controlled subsidence and widespread hydrothermal circulation within the Irish Midlands area. Fluids up to ~250 °C and 10–15 wt% NaCl equivalent, sourced from a Lower Palaeozoic basement-equilibrated fluid reservoir, infiltrated the carbonate sequence via faults and fractures. The more localised development of dolomite-cemented breccias (white matrix breccias) that are frequently associated spatially with mineralisation was a consequence of the increased focusing of these hydrothermal fluids. Ore formation was broadly synchronous with development of the white dolomite breccias but only happened where mixing occurred between the hydrothermal ore-fluids and localised, near-surface reservoirs of low-temperature, H2S-rich brine. In the Waulsortian, this process led to the precipitation of a distinctive black dolomite that forms a broad halo to massive sulphides. Although ore-stage sulphides postdate significant diagenesis of the host rocks, and often display "epigenetic" textures, the fact that much of the cementation occurred soon after carbonate deposition means that mineralisation does not have to have formed after significant burial. In fact, the occurrence of clasts of hydrothermal dolomite and sulphides in intraformational debris-flow breccias is only consistent with mineralising processes occurring in the near-seafloor environment, relatively soon after host-rock deposition. The regional development of a distinctive pink dolomite associated with faults and fractures was a post-ore event, and is considered to mark a regional brine migration linked to the onset of the Variscan orogeny. The development of this new tectonic and flow regime may have been responsible for the cessation of economic mineralisation in Ireland.

A NEW FOSSIL VENT BIOTA IN THE BALLYNOE BARITE DEPOSIT,SILVERMINES, IRELAND: EVIDENCE FOR INTRACRATONIC SEA-FLOOR HYDROTHERMALACTIVITY ABOUT 352 Ma

Considerable controversy exists as to the timing of the important Mississippian carbonate-hosted Irish-type Zn + Pb ± Ba ± Ag deposits. The Silvermines deposits have been defined as an end member of this style in that they have been interpreted to display textures indicative of sea-floor deposition. One of the strongest arguments in favor of this interpretation was the report of a hydrothermal vent field, including pyritic chimneys in the Ballynoe open-pit barite deposit. This paper adds to that body of evidence by describing a hydrothermal vent fauna from the same vent field, consisting of a delicately pyritized worm tube hosted by massive pyrite and hematized filaments of apparent microbial origin. The worm tube is remarkably similar to fossil worm tubes from modern and ancient volcanic-hosted massive sulfide deposits, and the filamentous microfossils have similarities to modern Fe-oxidizing bacteria. We have found no correlation between the worm tube and normal Mississippian fossils such as crinoids, whose replacement by pyrite in the immediately underlying Ballynoe footwall destroys original morphology. The sulfur isotope composition of the worm tube and host pyrite is essentially identical to that of the vent field pyrite and the main sulfide ore stage of Silvermines sulfides, all having a mean value about –20 per mil, indicating an open-system bacteriogenic sulfide source. These discoveries provide additional evidence for the exhalative nature of parts of the Silvermines orebodies, and imply that mineralization had begun in the Irish ore field by the late Tournaisian (~352 Ma).

CEMENTATION, HYDROTHERMAL ALTERATION, AND Zn-Pb MINERALIZATION OF CARBONATE BRECCIAS IN THE IRISH MIDLANDS: TEXTURAL EVIDENCE FROM THE COOLEEN ZONE, NEAR SILVERMINES, COUNTY TIPPERARY--A DISCUSSION

Sir: The origin of mineralization-associated breccias in the Irish Carboniferous is crucial to an understanding of Irish-type Zn-Pb deposits. One of the key lines of evidence that has been used to support a synsedimentary component to the Irish-type deposits has been the presence of mineralized clasts within the breccias, which were interpreted as being of sedimentary origin (Andrew, 1986; Ashton et al., 1986, 1992). However, if these breccias are, instead, of hydrothermal origin, then arguments for a synsedimentary mineralization component are invalidated (e.g., Peace and Wallace, 2000; Reed and Wallace, 2001). In this context Lee and Wilkinson (2002) have made a timely and important contribution to this problem of breccia origin. Lee and Wilkinson (2002) have advanced a case for mineralized sedimentary breccias at the Cooleen zone, Silvermines, suggesting that the black matrix breccias may have originally been debris flow breccias. We have no disagreement with Lee and Wilkinson’s basic observations, but suggest that these observations better support a hydrothermal origin for the brec-cias, as has previously been suggested for black matrix brec-cias elsewhere in Ireland (e.g., Hitzman et al., 1992). Specifically, we suggest that the breccias are the result of hydrothermal karsting that was directly associated with sulfide mineralization. Hydrothermal breccias are associated with mineralization at many localities throughout the Irish Midlands, and it is important that these breccias are not confused with genuine sedimentary breccias. The morphology and paragenesis of the breccias described by Lee and Wilkinson (2002) have a number of features in common with epigenetic hydrothermal breccias described from elsewhere in the Irish ore field, and from localities outside Ireland. Mineralized breccias from Ireland have been described from Galmoy, Lisheen, Silvermines, and …

Source or Sink? An Assessment of the Role of the Old Red Sandstone inthe Genesis of the Irish Zn-Pb Deposits

The Zn-Pb (-Ba) deposits of central Ireland are hosted by Lower Carboniferous Waulsortian limestone and Navan Group carbonate sediments. The deposits are located near the margins of subbasins in close association with extensional faults. The source of metals and the flow pathways utilized by the hydrothermal fluids responsible for ore genesis have long been a matter of conjecture. The Devonian to Lower Carboniferous Old Red Sandstone has been suggested as a possible regional aquifer and metals source. Therefore, a petrographic and lead isotope study of this unit was conducted to assess its putative role in ore formation. Hydrothermally unaltered Old Red Sandstone has 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranging from 18.71 to 22.45, 15.52 to 15.86, and 38.57 to 42.71, respectively. The sedimentological characteristics of the red beds, coupled with results of isotopic modeling, support the conclusion that the lead isotope signature of the unit is predominantly controlled by radiogenic Caledonian granite detritus that has an Avalonian-like character. The modeled lead isotope composition of the Old Red Sandstone at the time of the Lower Carboniferous ore-forming event is too radiogenic for it to have acted as the main source of lead for the Irish ore field. Examination of drill core, outcrop, and underground exposures reveals that hydrothermal alteration within the Old Red Sandstone forms a halo around faults, indicating that fluid flow in the unit was predominantly structurally focused. These hydrothermally altered sediments have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.21 to 21.77, 15.54 to 15.91, and 38.02 to 41.57, respectively. Unlike the unaltered sediments, several altered samples have lead isotope compositions similar to galena from the Lower Carboniferous deposits. Petrographic examination reveals that these samples contain trace hydrothermal sulfides that are probably responsible for the orelike signature. Therefore, it appears that, at least locally, the Old Red Sandstone functioned as a sink for lead during the Lower Carboniferous mineralizing event. The conclusions from this study eliminate the Old Red Sandstone as the main source of lead in the ore field and suggest that deposit lead is derived primarily from lower Paleozoic rocks (± Precambrian crystalline basement). In view of these results, regional gravity-driven fluid flow models calling upon an Old Red Sandstone-focused flow regime and metals source can be discounted. Rather, deep circulation of fluids through the fractured basement underlying central Ireland plays an integral role in the evolution of the ore-forming system.

CEMENTATION, HYDROTHERMAL ALTERATION, AND Zn-Pb MINERALIZATION OF CARBONATE BRECCIAS IN THE IRISH MIDLANDS: TEXTURAL EVIDENCE FROM THE COOLEEN ZONE, NEAR SILVERMINES, COUNTY TIPPERARY

Sir: We are delighted to respond to the comments of Reed and Wallace on our paper on the origin of breccia-hosted base-metal mineralization in the Irish ore field (Lee and Wilkinson, 2002). Reed and Wallace’s thesis is that the mineralized breccias we have described within the Mississippian Waulsortian Limestone Formation at Cooleen are not of sedimentary origin and therefore, by extension, that synsedimen-tary breccias do not exist elsewhere in the ore field. If correct, one of a number of lines of evidence that have been used to infer a near–sea-floor origin for at least a component of sulfide mineralization in Ireland—that of reworked clasts of sulfide within sedimentary breccias—would be invalidated. Contrary to the suggestion of Reed and Wallace, the existence of sedimentary breccias spatially associated with mineralization in the Waulsortian is universally accepted in the literature. Mine-based investigations that include underground observations as well as drill core data and sedimentological studies have concluded that the stratiform, wedge-shaped breccia bodies commonly developed within the Waulsortian Limestone facies are sedimentary debris flows of Courceyan age (Boyce et al., 1983; Taylor, 1984; Andrew, 1986; Clifford et al., 1986; Somerville et al., 1992). Submarine fault scarp sedimentary breccias are common in sediment-hosted Zn-Pb deposits (e.g., Goodfellow et al., 1993) and display the same characteristics as the breccias at Cooleen. Hitzman and Beaty (1996, p. 125) also recognized that synsedimentary breccias are present within the Waulsortian at both Silvermines and Lisheen but went on to state that textural and field relationships clearly show that the principal episode of brecciation postdated regional burial dolomiti-zation, inferring a hydrothermal origin for the ore-hosting breccias. While we would not agree with this conclusion, particularly in relation to the Silvermines area, we accept that the ore-hosting breccias at Lisheen, Galmoy, and Silvermines have been …

A mineralogical and fluid inclusion study of the Harberton Bridge Fe–Zn–Pb deposit, County Kildare, Ireland

The Harberton Bridge deposit is located 2 km NW of the Lower Palaeozoic Kildare inlier in east central Ireland and represents one of a number of small, breccia-hosted Fe–Zn–Pb deposits that occur in the Kildare district. Widely regarded as an example of Mississippi Valley-type mineralization, the Harberton Bridge deposit provides important clues regarding the origin of the carbonate-hosted Zn–Pb deposits of the Irish orefield as a whole. Mineralization was studied in core using cathodoluminescence, transmitted and reflected light microscopy and fluid inclusion microthermometry. Fluid inclusions from calcite associated with sphalerite–galena–marcasite mineralization have homogenization temperatures in the range 45–69°C and salinities mainly between 12 and 17wt% NaCl equivalent. The data obtained suggest that the Harberton Bridge deposit formed from a single, low temperature (<80°C), moderate salinity fluid, considered to be a cooler version of the higher temperature fluids involved in mineralization at all of the economic Irish deposits. The inferred common fluid origin with the major deposits confirms the position of the Kildare district as representative of an end-member of mineralization styles in the Irish orefield. The differences in style are attributed to a late timing of mineralization, absence of abundant biogenic H 2 S and the involvement of a relatively shallowly basement-circulated, low temperature orefluid.

A high-temperature hydrothermal origin for black dolomite matrix breccias in the Irish Zn-Pb orefield

AbstractWe make the first report of fluid inclusion data from a black dolomite matrix breccia from the Irish orefield which confirm a relatively high-temperature hydrothermal origin. Breccia matrix dolomite formed from NaCl-dominated fluids with temperatures primarily in the range 150–220°C and salinities of 13–20 wt.% NaCl equivalent, comparable with inferred ore-stage fluids at the Silvermines, Tynagh, Lisheen and Navan deposits. The spread in salinity is considered to be due to mixing of a moderate salinity (~12 wt.%), higher temperature fluid, probably derived from the Lower Palaeozoic metasedimentary basement, with a low temperature brine. Non-stoichiometric, black dolomites are therefore considered to be a product of relatively high-temperature hydrothermal processes involving the principal orefluid in Ireland, and thus their occurrence is believed to be a prime exploration indicator.Earlier, coarse white dolomite, texturally similar to regional dolomite cements commonly observed in the Irish Midlands, precipitated from fluids in the temperature range 140–200°C and salinities of 10.3–13.5 wt.% NaCl equivalent. This is comparable with the higher temperature, moderate salinity end-member inferred for the black dolomite. These data call into question a uniquely low-temperature burial origin for coarse white dolomite cements in Ireland. Paragenetically late low-Mg calcites were formed as a result of an influx of relatively low salinity fluids.

Fracture-controlled fluid flow in the Lower Palaeozoic basement rocks of Ireland: implications for the genesis of Irish-type Zn-Pb deposits

Abstract A fluid inclusion study has been carried out to test whether circulation of fluids within fractured very low grade metasedimentary basement rocks was an important process in the genesis of the carbonate-hosted base metal deposits of Ireland. In the Silvermines district, Silurian greywackes and mudrocks are host to several different cross-cutting vein types which are locally abundant in east-east-northeast and northwest-north-northwest trending, high angle, brittle fault zones. Structural analysis indicates that the vein orientations are consistent with formation under a northeast-southwest dextral trans-tensional regime, controlled locally by reactivation of pre-existing faults. Three vein types have been identified: (1) early hematitic calcite-quartz ± pyrite; (2) quartz-calcite ± sphalerite, galena, chalcopyrite, pyrite, barite; (3) ankerite-ferroan dolomite-quartz ± sphalerite, pyrite. Vein types 2 and 3 are associated with weak to locally intense sericite-chlorite-carbonate alteration. Primary fluid inclusion data from vein Types 2 and 3 show homogenization temperature-salinity characteristics (123–238°C, 9.7–20.6 wt% NaCl equivalent) overlapping significantly with the deposit data. In addition, primary inclusions hosted by quartz and sphalerite commonly contain CO 2 as indicated by the formation of gas clathrate and, rarely, solid CO 2 on cooling. Bulk fluid inclusion analyses on quartz vein samples show that the fluid composition is comparable with experimental data on fluids equilibrated with greywackes at temperatures of 200–350°C. Alkali geothermometry gives temperatures (158–219°C) not significantly different to homogenization temperatures suggesting that the veins formed under low fluid pressures. New inclusion data from Navan brown sphalerite confirm the importance of a relatively high temperature, moderate salinity (187–220°C, 15–18 wt% NaCl equivalent) mineralizing fluid as observed at Tynagh, Silvermines, Lisheen and in the Lower Palaeozoic basement. In these inclusions, gas clathrate has been identified. This is the first reported occurrence of this phase in fluid inclusions from the Irish base metal deposits. These data provide convincing evidence for the regional flow of high temperature fluids, similar to the Lower Carboniferous ore-forming fluids, within the Lower Palaeozoic metasedimentary basement. Systematic regional variations in homogenization temperature-salinity characteristics imply variable infiltration of local, low temperature, surface-derived fluids, consistent with a density driven convection model. In combination with existing isotopic data and thermal constraints, the new data strongly suggest that deep circulation of fluids was a fundamental process in the genesis of the Irish deposits. Therefore, purely topographically driven fluid flow models solely utilizing the Old Red Sandstone as a regional aquifer are inappropriate for the Irish orefield.