Metasomatic High Field Strength Element, Tin, and Base Metal Enrichment Processes in Lithium Pegmatites from Southeast Ireland

Abstract

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.

Paragenesis and precipitation stages of Nb-Ta-oxide minerals in phosphorus-rich rare-element pegmatites (Buranga dike, Rwanda)

“…Subsequently, with HFSE still available in the melt, rutile followed a disequilibrium crystallization path. Local supersaturation at the crystal-liquid interfaces was possibly what generated the oscillatory zoning and Fe-Nb-Ta-rich rims (Beurlen et al 2008;Van Lichtervelde et al 2018;Ballouard et al 2020a;Kaeter et al 2021). Chemical quenching at the crystal-liquid interfaces due to consumption of P and Li (crystallization of phosphates) can also enhance this supersaturation, leading to a quick crystallization of rutile, as suggested by Kaeter et al (2021) in the case of tourmaline and Nb-Ta-rich rutile association.…”

“…Most of the large-scale melt differentiation and element enrichment can be explained by fractional crystallization in a residual melt segregated from granitic mushes (Raimbault et al 1995;Hulsbosch et al 2014;Stepanov et al 2014). However, the extreme elemental enrichment and fractionation of particular geochemical ratios (i.e., Nb/Ta) observed in some pegmatites cannot be attributed to a pure equilibrium fractional crystallization model (Van Lichtervelde et al 2018;Ballouard et al 2020b;Kaeter et al 2021).…”

“…Alternatively, it can be triggered by interactions with an aqueous fluid phase (Ballouard et al 2020b;Kaeter et al 2021), possibly due to the remobilization of the mobile major elements present in NTO (i.e., Fe and Mn) into the melt (McNeil et al 2020).…”

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

“…Occurrences are mostly within the ECDZ and thinner intrusions present complex interfingering relationships with their country rocks, which are primarily the Tullow Lowlands pluton and marginal granitic intrusions (Figure 2), but occasionally Ribband Group schists. Mineralized pegmatites typically have a mineral assemblage of 10-40% spodumene and varying quantities of K-feldspar, albite, quartz, muscovite and garnet, with accessory apatite, beryl, columbite-tantalite group minerals and sphalerite, among others [18][19][20]. They are usually unzoned bodies, but some thicker intrusions have quartz-rich cores.…”

“…Previously published mineralogical, petrographic and geochemical descriptions of Leinster pegmatites in general did not account for their interaction zones with wall rocks (e.g., [16,17]). Advances in the understanding of the internal processes and geochemical evolution in these pegmatites [18][19][20] now allow for better insight into the formation of exomorphic halos. Moreover, a recent and ongoing mineral exploration drilling campaign has provided an excellent opportunity for study.…”