The processes involved in the magmatic-hydrothermal transition in rare-element pegmatite crystallization are obscure, and the role of hydrothermal mechanisms in producing economic concentrations of rare elements such as tantalum remains contentious. To decipher the paragenetic information encoded in zoned minerals crystallized during the magmatichydrothermal transition, we applied SEM-EDS and LA-ICP-MS chemical mapping to muscovite-and columbite-group minerals (CGM) from a rare-element pegmatite of the albite-spodumene subtype from Aclare, southeast Ireland. We present a three-stage model for the magmatic-hydrothermal transition based on petrography, imaging and quantification of rare-element (Li, B, Rb, Nb, Sn, Cs, Ba, Ta, W, U) zoning, integrated with geochemical modeling and constraints from published literature. Stage I marks the end of purely magmatic crystallization from a peraluminous granitic melt.

Rare-element pegmatites have diverse chemical signatures and are important sources of strategic metals such as Li, Cs and Ta. The two main hypotheses to explain rare-element pegmatite formation are 1) residual magmas from granitic rocks' crystallization, and 2) partial melts from a relatively rareelement-rich source. In southeast Ireland, spodumene and spodumene-free pegmatite dykes occur along the eastern margin of the S-type Leinster Granite batholith. With indistinguishable emplacement ages around 400 Ma, the origin of the Li-rich pegmatitic fluids has been suggested to have resulted from extreme fractional crystallization of Leinster Granite granodiorite magma. To test this hypothesis, we used whole-rock geochemistry of pegmatite and granodiorite samples from drill cores and geochemical modeling of in situ crystallization and batch melting to investigate which process better explains the formation of pegmatites. Chemical signatures for pegmatites and granodiorite do not indicate a direct comagmatic relationship, as granodiorite has higher concentrations of many incompatible elements than the pegmatites (e.g. concentrations of Zr, Ti and Y). Concentrations of Li, Rb, Cs, Sr and Ba show no clear fractionation trends from granodiorite to pegmatite. The in situ crystallization model using the average granodiorite composition as initial magma generates a range of compositions that does not include pegmatites, so it is unlikely that they represent residual granitic magmas. Modeling of partial melting indicates that Leinster Granite granodiorite and pegmatite magmas could have been formed in separate events and from chemically different source rocks, with pegmatite magmas presumably formed in a younger event because pegmatites intrude granodiorite.

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Naturally CO2-rich mineral water springs (pouhons) in east Belgium occur in the context of the Rhenohercynian domain of the Variscan fold-and-thrust belt, mostly within the Cambro-Ordovician Stavelot-Venn Massif. The origin of the CO2 is still unclear, although different hypotheses exist. In this review study, we show pouhon waters are of the calcium bicarbonate type (~310 mg/l HCO3- on average), with notable Fe (~15 mg/l) and some Ca (~43 mg/l). Pouhon waters are primarily meteoric waters, as evidenced by H and O isotopic signature. The δ13C of CO2 varies from -7.8 to +0.8‰ and contains up to ~15% He from magmatic origin, reflecting a combination of carbonate rocks and mantle as CO2 sources at depth. Dinantian and Middle Devonian carbonates at 2–6 km depth could be potential sources, with CO2 generated by dissolution. However, carbonates below the Stavelot-Venn Massif are only predicted by structural models that assume in-sequence thrusting, not by the more generally accepted out-of-sequence thrust models. The mantle CO2 might originate from degassing of the Eifel magmatic plume or an unknown shallower magmatic reservoir. Deep rooted faults are thought to act as preferential pathways. Overall low temperatures of pouhons (~10 °C) and short estimated residence times (up to 60 years) suggest magmatic CO2 is transported upwards to meet infiltrating groundwater at shallower depths, with partial to full isotopic exchange with carbonate rocks along its path, resulting in mixed magmatic-carbonate signature. Although the precise role and interaction of the involved subsurface processes remains debatable, this review study provides a baseline for future investigations.

Compared to average crustal abundances, high field strength elements (HFSEs) including Zr, Nb, Hf, Ta, and U are commonly enriched in rare element pegmatites. Albite-spodumene pegmatites may show economic grades of these elements, along with Sn, primarily in oxide minerals. Processes leading to enrichment and precipitation of HFSEs in these rocks are not well understood. Here, we characterize the textures and geochemistry of minerals of HFSEs, tin, and base metals in the Leinster albite-spodumene pegmatites. We use these data to infer processes for enrichment and precipitation of these metals during pegmatite crystallization, especially subsolidus processes. The Leinster albite-spodumene pegmatites are located within the East Carlow deformation zone on the eastern flank of the Caledonian S-type Leinster batholith, southeast Ireland. The final crystallization stages of these pegmatites are characterized by autometasomatism and hydrothermal overprint leading to in situ greisenization and precipitation of massive, commonly replacive, albitites. Cassiterite and HFSE minerals (columbite-tantalite and zircon) crystallized predominantly during these late stages. Crystals of HFSE minerals that precipitated during the early magmatic stages commonly exhibit evidence of resorption and additional growth during later stages. Others, such as microlite and uraninite, only crystallized during metasomatism or from hydrothermal fluids. Base metal sulfides are among the last precipitates from these fluids. We present a detailed paragenetic sequence for the Leinster albite-spodumene pegmatites and show that late-stage aqueous fluids transported HFSEs, especially after all the melt had crystallized. Tantalum enrichment seems to have been controlled by processes affecting the entire crystallizing medium, as opposed to fractional crystallization of columbite-tantalite. The textures and parageneses described in the present and our previous work are well explained by element partitioning between coexisting liquids with characteristics similar to those described in published melt-melt-fluid immiscibility models for rare element pegmatites but do not exclude other models for early-stage pegmatite evolution. The chemical and textural features of columbite-tantalite and cassiterite in the Leinster albite-spodumene pegmatites are seen in similar rare element pegmatites and rare metal granites elsewhere, suggesting wide applicability of the processes interpreted for Leinster. Late-stage processes of the type that affected the lithium pegmatites at Leinster may either enhance or reduce economic potential: ore metal tenor may be increased because late-stage columbite-tantalite is generally richer in Ta, and/or ore metals may be lost from pegmatites to country rocks. Lithium pegmatites, including the ones at Leinster, are commonly associated spatially with Sn-W veins and greisens and share some geochemical and textural features, such as evidence of widespread albitization. We propose that lithium pegmatites are transitional products regarding the interrelated dimensions time, temperature, and depth in S-type granite-related Li-Sn-W mineralizing systems.