Structural controls on spodumene pegmatite emplacement along the Leinster Granite, SE Ireland: Implications for lithium exploration in a reactivated crustal-scale shear zone

Spodumene-bearing lithium–cesium–tantalum (LCT) pegmatites in SE Ireland are confined to the East Carlow Deformation Zone (ECDZ), a reactivated Mid-Ordovician shear zone at the eastern margin of the Leinster granite batholith. This study integrates district-scale structural field data with drill core observations and geophysical datasets from the Moylisha prospect to investigate the structural controls and palaeostress conditions governing pegmatite emplacement. Multiple generations of granitic and spodumene pegmatites show variable deformation styles, reflecting episodic intrusion during progressive cooling and strain localization. Spodumene pegmatites exhibit a consistent SW–NE strike and moderate NW dip, whereas granitic pegmatites show various orientations. Palaeostress reconstruction and Mohr circle analysis indicate that spodumene pegmatite emplacement occurred during sinistral transtension and NW–SE extension, under conditions where fluid pressure exceeded the minimum principal stress and promoted reactivation of favourably oriented fractures. We propose that spodumene pegmatite-forming melts were transported via transient, self-propagating mobile (hydro)fractures localized within the ECDZ shear zone, enabling rapid, channelized crustal-scale ascent with minimal loss of Li and other incompatible elements. The upward propagation of melts was arrested by moderately NW-dipping fractures that acted as structural traps. Electrical resistivity tomography , airborne magnetic and resistivity maps reveal a 400–600 m wide high-resistivity zone within the ECDZ, consisting of granite that is evidently interspersed with spodumene pegmatites. This zone marks a pegmatite-prospective corridor. The findings underscore the importance of inherited structures, directed stress fields, and lithological contrasts in pegmatite emplacement. Key Mineral Systems elements and important exploration vectors for lithium pegmatites in SE Ireland include crustal-scale shear zones serving as magma pathways and sites of strain localization, rheological boundaries facilitating dilation, and pre-existing structures and anisotropies as traps. The apparent absence of spodumene pegmatites outside the ECDZ highlights the need to target structurally controlled corridors with prolonged plutonic heat input. These findings establish a robust framework for LCT pegmatite exploration in orogenic terranes.