Rare Element Enrichment in Lithium Pegmatite Exomorphic Halos and Implications for Exploration: Evidence from the Leinster Albite-Spodumene Pegmatite Belt, Southeast Ireland

Pegmatitic deposits of critical metals (e.g., Li, Ta, Be) are becoming increasingly significant, with growing interest in understanding metal enrichment processes and potential vectors to aid the discovery of new resources. In southeast Ireland, the Leinster pegmatite belt comprises several largely concealed Li-Cs-Ta albite-spodumene-type pegmatites. We carried out detailed mineralogical characterization and whole-rock geochemical analyses of six drill cores intersecting pegmatite bodies and their country rocks. Exomorphic halos 2–6 m thick, enriched in Li, Rb, Be, B, Cs, Sn and Ta, are identified in both mica schists and granitic rocks adjacent to spodumene pegmatites. Metasomatism in wall rocks visible to the naked eye is restricted to a few tens of centimeters, suggesting country rock permeability plays a key role in the dispersion of these fluids. We propose that halos result from the discharge of rare element-rich residual fluids exsolved near the end of pegmatite crystallization. Halo geochemistry reflects the internal evolution of the crystallizing pegmatite system, with residual fluid rich in incompatible elements accumulated by geochemical fractionation (Be, B, Cs, Sn, Ta) and by auto-metasomatic resorption of spodumene and K-feldspar (Li, Rb). The possibility of identifying rare-element enrichment trends by analysis of bedrock, stream sediments and soils brings opportunities for mineral exploration strategies in Ireland and for similar albite-spodumene pegmatites worldwide

Genetic Links between Irish-type Zn-Pb Deposits and Related Geochemical Halos

A broad geochemical dispersion halo has been identified with a direct link to the underlying Tara Deep deposit at Navan, Ireland. In situ laser S isotope analyses have been performed on petrographically well characterized samples from the halo. Four mineral assemblages have been identified. 1) In black shales, laminated pyrite comprising thin layers of framboidal low-delta S-34 pyrite with minor interstitial sphalerite. 2) Pyritized calcarenites are widely distributed and occur chiefly as biodebris replaced by low-delta S-34 pyrite. 3) A replacive assemblage occurs as late remobilizations exhibiting both crosscutting and bedding-parallel styles, overprinting the early laminated pyrite. It comprises mostly marcasite, with minor pyrite, sphalerite, chalcopyrite, galena, stibnite, arsenopyrite and pentlandite, with high delta S-34 values. 4) Hydrothermal cherts comprise thick microcrystalline quartz bands rimmed by dolomite, associated with marcasite, pyrite, sphalerite, chalcopyrite, galena, Ni-sulfosalts and stibnite with high delta S-34 values. These results indicate overlapping diagenetic and multi-phase hydrothermal sulfide mineralization. First, laminated pyrite and pyritized calcarenites suggests a bacterial origin within sediments during early diagenesis. Later, hydrothermal chert and replacive sulfides suggest hydrothermal exhalation during early-mid diagenesis. Similarities in mineralogy and S isotope compositions suggest genetic links between the halo and the underlying Tara Deep deposit

Genetic Links between Irish-type Zn-Pb Deposits and Related Geochemical Halos

A broad geochemical dispersion halo has been identified with a direct link to the underlying Tara Deep deposit at Navan, Ireland. In situ laser S isotope analyses have been performed on petrographically well characterized samples from the halo. Four mineral assemblages have been identified. 1) In black shales, laminated pyrite comprising thin layers of framboidal low-delta S-34 pyrite with minor interstitial sphalerite. 2) Pyritized calcarenites are widely distributed and occur chiefly as biodebris replaced by low-delta S-34 pyrite. 3) A replacive assemblage occurs as late remobilizations exhibiting both crosscutting and bedding-parallel styles, overprinting the early laminated pyrite. It comprises mostly marcasite, with minor pyrite, sphalerite, chalcopyrite, galena, stibnite, arsenopyrite and pentlandite, with high delta S-34 values. 4) Hydrothermal cherts comprise thick microcrystalline quartz bands rimmed by dolomite, associated with marcasite, pyrite, sphalerite, chalcopyrite, galena, Ni-sulfosalts and stibnite with high delta S-34 values. These results indicate overlapping diagenetic and multi-phase hydrothermal sulfide mineralization. First, laminated pyrite and pyritized calcarenites suggests a bacterial origin within sediments during early diagenesis. Later, hydrothermal chert and replacive sulfides suggest hydrothermal exhalation during early-mid diagenesis. Similarities in mineralogy and S isotope compositions suggest genetic links between the halo and the underlying Tara Deep deposit

Correlated Petrographic and Isotopic Studies (S, Pb) of Carbonate-hosted Zn-Pb Mineralization: the Formation of the High-grade Island Pod, Lisheen

Irish Zn-Pb deposits are a type of carbonate-hosted mineralization, typically found adjacent to normal faults. Ore deposition occurred due to the replacement of Lower Carboniferous limestones, triggered primarily by fluid mixing. The Lisheen deposit (23 Mt at 13.3% Zn & 2.3% Pb) in the southern Irish orefield consists of several stratabound orebodies, which are strongly controlled by a left-stepping, ramp-relay fault array. The Island Pod (0.4 Mt at 20% Zn & 1.6% Pb) is a small high-grade orebody, found in the northern part of the Lisheen mine (Fig. 1). We present the first detailed petrographic and paragenetically constrained S-Pb isotopic study and mineral chemical analysis of the Island Pod mineralization. Homogenous Pb isotopic signatures in galena have been observed throughout the Island Pod, regardless of paragenetic stage. Sulfur isotope ratios vary but suggest a dominantly bacteriogenic source for S. Sulfide minerals have low trace element concentrations, below electron microprobe detection limits for most elements

A distal, high-grade Irish-type orebody: petrographic, sulfur isotope, and sulfide chemistry of the Island Pod Zn-Pb orebody, Lisheen, Ireland

Irish-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

Pyritic mineralization halo above the Tara Deep Zn-Pb deposit, Navan, Ireland: Evidence for sub-seafloor exhalative hydrothermal processes?

The Tara Deep Zn-Pb deposit, at Navan, Ireland, includes sub-economic pyrite-rich mineralization extending laterally for about 2 × 2 km within the overlying Lower Viséan calc-turbidites, known as the ‘New-Thin Bedded Unit’. Here, we investigate the genesis of this pyritic mineralization and its links to the limestone-hosted Zn-Pb deposit lying 100 m below it. Four mineral assemblages have been identified: 1) in black shales, laminated pyrite comprises thin framboid-rich layers with minor interstitial sphalerite, both showing variable but low δ34S values ranging from −37.4 to 3.3‰; 2) in calcarenite and calcsiltite layers, pyritized fossils are widely distributed and occur chiefly as biodebris replaced by low δ34S pyrite (mean values of −13.2‰); 3) a replacive assemblage occurs as late remobilizations exhibiting both crosscutting and bedding-parallel styles, overprinting the early laminated pyrite. It comprises mostly of marcasite, with minor pyrite, sphalerite, chalcopyrite, galena, stibnite and Co-pentlandite, with high δ34S values up to 24.5‰; and 4) in black shales, bedded sulfide-rich cherts comprising microcrystalline quartz lenses exhibit δ18O mean values of 25.3‰. Cherts are rimmed by dolomite, associated with marcasite, pyrite, sphalerite, chalcopyrite, galena, siegenite and stibnite also with high δ34S values up to 44.2‰. In general terms, pyrite shows a relatively high Co/Ni ratio > 1 and sphalerite Zn/Cd ratios vary from 268 to 364. Textural analysis indicates overlapping of early-diagenetic and multi-phase hydrothermal sulfide mineralization. Development of laminated pyrite and pyritized calcarenites suggests that this mineralization was generated during early diagenesis, close to the seawater-sediment interface in oxygen-poor conditions under the influence of low-temperature hydrothermal fluid circulation. Later hydrothermal cherts and replacive sulfides suggest discharge of relatively warm hydrothermal fluids during early to mid-diagenesis, presumed to be linked to movements of nearby normal faults. Similarities in mineralogy and S isotope compositions suggest genetic links between the sub-economic pyritic mineralization and the underlying Tara Deep deposit, and consequently, that the former represents a geochemical halo with direct applicability in exploration for Zn-Pb deposits

The Geology of Western Ireland: A Record of the ‘Birth’ and ‘Death’ of the Iapetus Ocean

This chapter reviews the evidence found in the remarkable and varied geology of western Ireland (Fig. 1) for the opening and the closing of the Iapetus Ocean adjacent to the Laurentian margin. The geology visited is briefly reviewed along with a short summary of its likely tectonic significance