U-Pb DATING OF APATITE FROM SILVERMINES DEPOSIT, IRELAND: A MODEL FOR HYDROTHERMAL ORE GENESIS

Carbonate-hosted sedimentary sulfide deposits are the world’s largest sources of the critical metals Zn and Ba, as well as a significant source of Pb (Leach et al., 2005; Burton, 2022). Determining the dominant processes that formed these deposits has historically been difficult due to the lack of geochronological constraints on mineralization. This lack of geochronological constraints on carbonate-hosted sedimentary sulfide deposits results from their relatively simplified mineralogy (e.g., pyrite, sphalerite, and galena) and the lack of genetically associated magmatism and thus a general paucity of mineral phases suitable for geochronology. The Irish Zn-Pb ore field, Europe’s largest Zn producer, is no exception. Even when age constraints exist, results have often been conflicting. The Silvermines deposit epitomizes this issue with disparate ages from sphalerite Rb-Sr geochronology (360 ± 5 Ma; Schneider et al., 2007), pyrite Re-Os geochronology (334.0 ± 6.1 Ma; Hnatyshin et al., 2015), and paleomagnetism (269 ± 4 Ma; Symons et al., 2007).

Determining which, if any, of these ages represents the true age of mineralization at Silvermines has important implications for genetic models of Irish-type deposits. Textural analyses of dolomitic breccias and sulfides have been interpreted to support an origin by sedimentary exhalation of hydrothermal fluids along the sea floor, indicating stratigraphic deposition from as early as ~352 Ma (Larter et al., 1981; Boyce et al., 1983, 2003; Andrew, 1986; Mullane and Kinnaird, 1998; Lee and Wilkinson, 2002). Such an early age was supported by sphalerite Rb-Sr ages of 360 ± 5 Ma (Schneider et al., 2007). However, other workers used textural analysis of sulfides to suggest an epigenetic replacement origin for much of the dolomitic breccia and sulfides (Hitzman and Beaty, 1996; Reed and Wallace, 2004). Subsequent Re-Os analyses of pyrite intergrown with sphalerite and galena at Silvermines yielded an age of 334.0 ± 6.1 Ma (Hnatyshin et al., 2015). A much later paleomagnetic age (269 ± 4 Ma) from the Ba-rich Magcobar pit of Silvermines has also been produced (Symons et al., 2007) and would be temporally associated with deeper burial and the Variscan orogeny.

Several Irish Zn-Pb deposits hosted within the Waulsortian Limestone Formation are associated with hydrothermal dolostone breccias colloquially known as “black matrix breccia” (Hitzman et al., 1992). Apatite has been recognized as a minor, very fine grained phase within the black matrix breccia at the Lisheen deposit (Redmond, 1997; Hitzman et al., 2002). Additionally, apatite has been recognized in black matrix breccia from both the Silvermines and Galmoy deposits, but occurrences were too fine grained for U-Pb analysis. Recent sampling of the black matrix breccia from the Magcobar open pit at Silvermines, however, has resulted in discovery of coarse-grained apatite suitable for U-Pb analysis. This study geochemically characterizes this coarse-grained apatite and presents the first U-Pb age for an Irish-type Zn-Pb deposit.

Sulfide deposits of the Irish Midlands are predominantly hosted by a sequence of transgressive Mississippian carbonates (Fig. 1A) and are spatially associated with a series of late Devonian-Early Carboniferous extensional basins and their associated fault systems (Hitzman and Beaty, 1996; Johnston et al., 1996; Kyne et al., 2019). The onset of Variscan compression at 314 ± 1 Ma (Quinn et al., 2005) resulted in the reactivation of both crustal-scale Caledonian faults within basement rocks along the Iapetus suture zone (Fig. 1A) and extensional fault systems in the overlying Devonian-Carboniferous sedimentary sequence. Sulfide deposits in the Irish Midlands are spatially associated with these extensional fault systems, which acted as conduits for deep-seated, metal-bearing hydrothermal fluids (Johnston et al., 1996; Hitzman, 1999). Within the early Carboniferous Waulsortian Limestone Formation, fluid infiltration resulted in the formation of black matrix breccia (Hitzman et al., 1992), which was originally termed “dolomite breccia” at the Silvermines deposit (Andrew, 1986). The black matrix breccia comprises clasts of undolomitized and dolomitized Waulsortian Limestone Formation hosted in a very fine grained dolomitic matrix with trace pyrite, quartz, barite, and apatite that was preferentially replaced by sulfide minerals, making it a key target for mineral exploration.

The Silvermines deposit comprises several zones of massive sulfides (predominantly pyrite, sphalerite, and galena) as well as an adjacent deposit, primarily of barite, within the Magcobar zone (Andrew, 1986; Fig. 1B). These mineralized zones, located at the base of the Waulsortian Limestone Formation beneath and within black matrix breccia, dip gently northward within the Kilmastulla syncline (Fig. 1B). The black matrix breccia is more extensive than the well-mineralized zones and is spatially distributed along and within the hanging walls of a set of W-NW–trending extensional faults forming a linked normal fault array (Kyne et al., 2019).

Observations by Hitzman et al. (1995) show that mineralization and alteration at Silvermines were initiated by the formation of replacive silica-hematite along the base of the Waulsortian Limestone Formation, as well as within the upper portion of the underlying Ballysteen Formation near the Magcobar barite deposit. This appears to have been followed by development of the black matrix breccia at the base of the Waulsortian Limestone Formation throughout the deposit area. The Magcobar barite orebody forms a stratiform sheet at the base of the Waulsortian Limestone Formation beneath the breccia. There are no obvious crosscutting relationships between the barite mass and the black matrix breccia. Barite does, however, cut and brecciate earlier-formed silicahematite–altered zones. The barite orebody is cut by pyrite (± marcasite) veins, some of which contain trace sphalerite (± galena). The Silvermines base metal deposits form largely stratiform sheets at the base of the Waulsortian Limestone Formation, at the same stratigraphic position as the Magcobar barite orebody (Andrew, 1986). Sulfides form massive sulfide lenses as well as zones of partial replacement of portions of adjacent black matrix breccia (Hitzman and Beaty, 1996). Sulfides were also precipitated in the discordant orebodies associated with replacive, ferroan hydrothermal dolostone within the Ballysteen Formation along the G fault as well as in discordant veins cutting the upper Devonian Old Red Sandstone, with adjacent disseminated sulfides in the Shallee area south and west of the G fault (Andrew, 1986; Kyne et al., 2019). Sulfide zones are locally crosscut by late dolomite-sulfide veins and veinlets.

A representative surface sample of black matrix breccia was collected from the northern high wall of the Magcobar open pit (approx. N 52°, 47’, 38”; W 8°, 15’, 11”). The sample contained minor visible fine-grained pyrite and no visible barite or apatite. Two 60-μm-thick sections were prepared from the sample for petrographic and geochemical analysis, revealing the presence of coarse-grained (>100 μm) apatite.

To determine U-Pb dating suitability of the apatite, sample characterization through backscattered electron (BSE) and cathodoluminescence (CL) imaging was conducted using the Science Foundation Ireland Centre for Research in Applied Geosciences (iCRAG) Tescan TIGER MIRA field emission gun (FEG) scanning electron microscope (SEM) at the Centre for Micro-Analysis (CMA) at Trinity College Dublin (TCD), Ireland. The operating voltage was 20 kV, and a working distance of 20 mm was used. Spatially referenced areas of interest were recorded for further analysis by laser ablation.

Trace element analyses were conducted to geochemically characterize the apatite crystals. Apatite crystals were analyzed in situ via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in three analytical sessions: a preliminary run comprising 31 spots, a subsequent run comprising 88 spots, and a final run comprising 14 spots. Apatite U-Pb and trace element data were acquired simultaneously using a Photon Machines G2 193-nm ArF Excimer laser ablation system coupled to a Thermo iCAP-Q quadrupole (Q)-ICP-MS at the Department of Geology, TCD. Subsequent to the REE spot analyses, two representative apatite crystals (Fig. 2; App. Fig. A1) were selected and analyzed for further elemental map generation in order to characterize trace element abundances. Detailed analytical LA-ICP-MS procedures are provided in the supplemental material (App. Methods A1).

Apatite crystals are hosted in the black matrix breccia in a matrix comprising fine-grained, slightly ferroan dolomite (Fig. 2) with remnant Waulsortian Limestone Formation clasts exhibiting angular, dissolved edges. The apatite appears to have intergrown with the surrounding fine-grained matrix dolomite. Apatite within the black matrix breccia forms subhedral to euhedral, elongated crystals ranging from ~100 to ~400 μm in length (Fig. 2). Disseminated, generally fine-grained pyrite is observed throughout the breccia matrix and in places forms aggregates. Apatite grains adjacent to such pyrite aggregates are locally fractured or brecciated within a pyrite-rich dolomite cement, forming <10- to 50-μm fragments.

Strontium and Y contents in the apatite are highly variable, ranging from ~300 to 2,300 and ~100 to 2,600 ppm, respectively (App. Table A1; Fig. 2). Europium contents range from 25 to 450 ppm. Elemental distribution maps show that the banding observed under CL within “Apatite 01” (Fig. 2B) is associated with increased U and Th concentrations. The spot ablation data show that the apatite has low light rare earth element (LREE) concentrations (e.g., at the tens to hundreds of ppm level; Fig. 3A); however, two samples do contain ~1,000 ppm ΣLREEs (Fig. 3A). The low LREEs are reflected in the steep upward slope of the first tetrad (La-Nd) in a chondrite-normalized REE + Y diagram (Masuda et al., 1987). The second tetrad (Sm-Gd) is marked by a positive Eu anomaly (Eu/Eu* = 3 – 4; McLennan, 1989). The third tetrad (Gd-Ho) is relatively flat and shows no Y anomalies. The last tetrad (Er-Lu) has a slight downward slope (Fig. 3A).

On a ΣLREE (La-Nd) versus Sr/Y plot, >98% of apatite analyses lie within the low-grade metamorphic to metasomatic field (LM), with the exception of two points, which fall into the mafic igneous (IM) category (Fig. 3B; O’Sullivan et al., 2020).

U-Pb analyses were conducted from 133 spots across 50 separate apatite crystals. The apatite U-Pb data (App. Table A1) lies along a mixing line (discordia) between common (or initial) Pb with a ²⁰⁷Pb/²⁰⁶Pb intercept at 0.8507 ± 0.0030, which is similar to the Pb isotope value of 0.8598 from Stacey and Kramers’ (1975) terrestrial Pb evolution model at c. 331 Ma; the lower intercept age of 331 ± 5.6 Ma is interpreted as the crystallization age (Fig. 3C).

The coarse-grained apatite analyzed in this study from the Magcobar deposit’s black matrix breccia is the first U-Pb age produced from the Irish Zn-Pb ore field. The U-Pb age produced in this study (331 ± 5.6 Ma) overlaps within error with a modern regression of the Re-Os age of pyrite-sphalerite-galena from Silvermines (334.0 ± 6.1 Ma; Hnatyshin et al., 2015), thus demonstrating that the Silvermines deposit is predominantly not a syngenetic mineralizing system. This is supported by 98% of the Silvermines apatite samples plotting within the low-grade metamorphic and metasomatic/hydrothermal apatite field (see O’Sullivan et al., 2020), thus comparable with hydrothermal sulfides.

Typical of hydrothermal apatite, the Silvermines apatite contains low LREE concentrations (Fig. 3A). No occurrence of cogenetic monazite or other LREE-rich mineral phases have been observed within the black matrix breccia sample, and these minerals have not been documented in any significant proportions within the Silvermines deposit or in other Irish Zn-Pb deposits. However, the Silvermines apatite has a very high abundance of Eu. Europium was likely inherited in an oxidized state from the precipitating fluid, which was potentially derived from the breakdown of plagioclase (Richardson et al., 2021) in the Paleozoic basement beneath the Silvermines deposit, the probable source of the metals and Ba (Everett et al., 1999). In such a case, the breakdown of plagioclase would also contribute Sr to the system (Richardson at al., 2021). As such, if the positive Eu anomaly observed in the REE distribution plot (Fig. 3A) was the result of plagioclase dissolution, Sr and the Eu anomaly would positively correlate. However, no correlation is observed, likely due to the apparent preferential uptake of Sr into the surrounding dolomite matrix (Fig. 2; App. Fig. A1).

The apatite in the Silvermines black matrix breccia was precipitated from a relatively LREE-depleted fluid that was oxidizing relative to the sulfur species present. This would have been a suitable environment to precipitate Ba out of solution. Though unambiguous paragenetic relationships indicating the relative age of black matrix breccia formation and massive barite precipitation have not been established, the presence of minor barite within the breccia matrix suggests at least some barite precipitation was synchronous with or may have postdated breccia formation.

These data allow a more detailed picture of the evolution of the Silvermines mineralizing system. It appears that oxidized, Fe-bearing, probably low-temperature fluids derived from basement-hosted fault zones initially moved outward from normal faults along the contact between the Waulsortian Limestone and the Ballysteen Formations, forming zones of silica-hematite alteration. This was followed by infiltration along the same contact by weakly acidic, low-salinity, perhaps higher-temperature fluids (Wilkinson and Hitzman, 2015), which facilitated the partial dissolution of limestone and resulted in dolomitization and formation of the black matrix breccia.

Fluids responsible for the formation of the Magcobar orebody would have had to have been strongly reducing in order to transport significant quantities (>100 ppm) of Ba (Cooke et al., 2000). This hydrothermal fluid was likely derived from partially evaporated seawater that had been modified through crustal interaction, leading to elevated concentrations of Ba, derived from alteration of potassium feldspar, and P (Wilkinson et al., 2011, and references therein). Sulfur isotope analyses demonstrate that these hydrothermal fluids mixed with normal Carboniferous seawater to precipitate barite (Coomer and Robinson, 1976; Samson and Russell, 1987) and presumably also the coarse-grained apatite.

Sulfides at Silvermines crosscut barite and replace black matrix breccia, indicating that basement-derived, metal-rich fluids continued to move up along the fault system and subsequently mixed with a fluid containing reduced sulfur that effectively halted the precipitation of barite and phosphate and allowed for sulfide mineralization. Numerous sulfur isotope studies throughout Ireland have demonstrated that this fluid represents Carboniferous seawater that has undergone biogenic sulfate reduction (Wilkinson and Hitzman, 2015, and references therein).

The U-Pb date reported here for apatite in the black matrix breccia indicates that the temporal separation between apatite precipitation and subsequent sulfide-bearing fluid infiltration at 334.0 ± 6.1 Ma (Hnatyshin et al., 2015) is indistinguishable, occurring within the uncertainty of the U-Pb age of 331 ± 5.6 Ma from this study. These ages, coupled with petrographic observations by ourselves and others (Reed and Wallace, 2004), suggest sulfide mineralization followed shortly after the development of the black matrix breccia.

Although the findings in this study neither confirm nor exclude the potential contribution of minor sea-floor exhalation at Silvermines, this young age strongly indicates that the bulk of ore mineralization at the Silvermines deposit occurred well after the interpreted age of exhalative ore deposition for the deposit (e.g., Boyce et al., 1983, 2003; Lee and Wilkinson, 2002). Furthermore, although Rb-Sr ages produced by Schneider et al. (2007) potentially fall in line with an exhalative explanation (360 ± 5 Ma), the available data set is unfortunately incomplete without proper analytical details, thus making it impossible to scrutinize. This fact, coupled with the age produced by Schneider et al. (2007; which is arguably older than the host rocks themselves) as well as no supplemental imaging to show the hosts of Rb-Sr in the samples, makes this data set intriguing but not useful until a study replicates the results using modern techniques.

Lastly, given the apparently low salinity of the initial hydrothermal fluids (Wilkinson and Hitzman, 2015), relatively high temperatures (>200°C; Wilkinson and Hitzman, 2015) would have been necessary for significant base metal transportation (Cooke et al., 2000). As such, a heat engine such as tectonism or volcanism would have been necessary to facilitate the development of the Silvermines deposits. Whereas the ages obtained from both apatite (this study) and pyrite (Hnatyshin et al., 2015) predate the onset of the Variscan orogeny by ~15 to 20 m.y., they do correspond to the approximate emplacement of the nearby Knockseefin basanites from the Limerick Igneous Suite (Somerville et al., 1992), as well as younger Rb-Sr alteration ages (ca. 332 and 335 Ma) calculated from altered Knockroe basalts (Slezak et al., 2022), suggesting that the igneous event responsible for eruption of these volcanic rocks may have driven, or contributed a heat source for, mineralization at Silvermines. Finally, the early Permian paleomagnetic age for Silvermines (269 ± 4 Ma in Symons et al., 2007), which is in line with other paleomagnetic ages from Irish deposits (Pannalal et al., 2008), suggests a late- to post-Variscan heating event overprinted mineralization at Silvermines deposit and affected virtually the entire Irish orefield.

This study is supported by iCRAG and funded by Science Foundation Ireland, the EU Regional Development Fund and industry partners, as well as SFI research grant number 16/RP/3849.

Nicholas Andrew Vafeas has a background in base metal exploration geology. After completing his M.Sc. and Ph.D on the Kalahari Manganese Field at the University of Johannesburg, South Africa, Nicholas continued his postdoctoral research at University College Dublin, Ireland, where much of his focus was placed on geochemical vectoring for critical metal deposits. In addition to his role as Policy Lead for the Geothermal Association of Ireland, Nicholas has consulted for the Irish Parliamentary Library Service on geothermal energy. He currently holds a fellowship position at Science Foundation Ireland within their Research Policy Division.