The magmatic–hydrothermal transition in rare-element pegmatites from southeast Ireland: LA-ICP-MS chemical mapping of muscovite and columbite–tantalite

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 magmatic–hydrothermal 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. In stage II, polymerized silicic melt and depolymerized alkaline aqueous melt coexist as immiscible media, both of which influence muscovite and CGM crystallization. Stage II also marks the onset of phyllic alteration of primary mineral assemblages. Hydrothermal fluid release causes further resorption of primary minerals and eventual precipitation of fine-grained albite in stage III.

Muscovite and CGM both exhibit trace-element zoning, while CGM also show major-element zoning. Petrographic relationships and geochemical markers such as Ta# (=Ta/[Ta + Nb]) of individual mineral zones reveal that both mineral species crystallized contemporaneously over all three stages. Furthermore, Rayleigh fractional crystallization of muscovite is efficient in fractionating Ta from Nb. Tantalum and Nb are additionally fractionated by halogen-rich aqueous media, which remobilize both elements, but redeposit preferentially Ta-enriched oxides. Columbite–tantalite mineralization is therefore both magmatic and hydrothermal. Albite associated with stage-III muscovite and CGM was likely precipitated from a hydrothermal fluid when pH changed due to hydrolysis of primary minerals. The complex zoning and resorption of minerals indicates that bulk analyses and conventional LA-ICP-MS spot ablation analyses of muscovite in rare element pegmatites may lead to erroneous modelling of element partitioning and fluid evolution.

Combined petrographic and high-resolution geochemical analysis of two mineral species (which co-crystallize and incorporate the same elements of interest) is an effective tool to assess the complex processes of crystal–melt–fluid interaction. Our three-stage model may also be applicable to the still not well understood magmatic–hydrothermal transition from fertile granitic melts to formation of Sn-W veins and greisens or porphyry-type deposits.