Columbite-tantalite and cassiterite as indicator minerals for lithium pegmatites: implications from geospatial and mineralogical analyses of stream sediments in southeast Ireland

Hard-rock lithium exploration faces challenges due to less advanced deposit and exploration models compared to commodities like copper. Additional obstacles arise from the glaciated and densely vegetated terrains typical of much of Europe, which present additional obstacles to access and geological mapping. Our research focuses on a belt of LCT (Li-Cs-Ta-rich) spodumene pegmatites on the eastern margin of the Caledonian S-type Leinster Batholith, southeast Ireland. We analyse regional stream-sediment geochemistry data and mineralogy of selected stream-sediment samples. Our goal is to identify signatures of the Leinster LCT pegmatites and draw conclusions for lithium exploration. Our analysis reveals high tantalum and tin in several catchments containing known Leinster spodumene pegmatites before and after lithological background correction. Mineralogical analysis reveals that anomalous Sn and Ta in these stream sediments can be attributed to the presence of columbite-tantalite, tapiolite, microlite, and cassiterite. Through SEM-BSE/-EDS analysis and LA-ICP-MS chemical mapping, we show that these minerals exhibit similar textural and chemical characteristics to their counterparts in spodumene pegmatite drill cores and surface boulders and in similar deposits elsewhere, demonstrating that they are useful indicator minerals for these deposits. We conclude that regional lithium pegmatite and rare-metal granite prospectivity can be assessed by examining Sn-Ta associations in stream sediment data. If regional geology, tectonic setting, and stream-sediment geochemistry indicate prospectivity, more detailed local studies can be conducted. By analysing the mineralogy of regional stream sediment samples, prospective areas can be narrowed down to individual catchments. Local geochemical surveys can then follow to pinpoint mineral sources.