Mineral chemistry of metasomatic haloes in granite: constraints on fluid chemistry during spodumene pegmatite crystallization

Abstract

Crystallizing spodumene pegmatites exsolve hydrothermal fluids, which generate exomorphic metasomatic halos in their host rocks. Composition, timing and frequency of fluid release relate to the evolution of the pegmatite. At the Moylisha prospect of the Leinster pegmatite belt in SE Ireland, spodumene pegmatites intrude the granite of the Leinster Batholith. Narrow (~ 10 cm) visible halos developed at the contact, but halo mineralization may occur meters away, isolated from pegmatite contacts. Two halo assemblages were identified using petrography, in situ chemistry and imaging of metasomatic minerals and textures. A rutile-present assemblage (ferroan muscovite-siderophyllite-feldspar-quartz-rutile) has rare alkali-enriched mica and contains accessory rutile. In the rutile-absent assemblage (ferroan muscovite–quartz–apatite ± siderophyllite ± feldspar) micas are Nb–Ta-enriched and have lower Nb/Ta ratios; apatite is abundant. The metasomatic evolution of both assemblages is modelled with a progressive reaction model as the interaction of the host rock with two distinct fluids. The rutile-present assemblage fluid is mainly enriched in rare alkalis and likely expelled after the onset of orthoclase and spodumene crystallization in the pegmatite but before resorption of primary minerals. The rutile-absent assemblage fluid additionally carries Nb, Ta, K and P and likely corresponds to the late hydrothermal stage of pegmatite evolution after partial resorption of primary minerals. Isolated halo mineralization has potential implications for mineral exploration as it can be detected distal to the pegmatite. The (mineral) chemistry of the halo has potential for distinguishing pegmatites where spodumene is preserved, from pegmatites that have undergone widespread albitization and spodumene resorption.