Lithium concentrations and isotopic compositions in the country rocks (amphibolites and schists) of the Tin Mountain pegmatite show systematic changes with distance to the contact. Both Li and δ7Li decrease dramatically along a ∼10 m traverse from the pegmatite into amphibolite, with Li concentration decreasing from 471 to 68 ppm and δ7Li decreasing from + 7.6 to − 19.9. Rubidium and Cs also decrease from the pegmatite contact into the country rock, but only within the first 2 m of the contact, after which their concentrations remain constant. Neither mixing between pegmatite fluids and amphibolite, nor Li isotope fractionation by Rayleigh distillation during fluid infiltration is a likely explanation of these observations, due to the extremely light isotopic composition required for the amphibolite end-member in the mixing model (δ7Li = − 20) and the similarly extreme isotopic fractionation required in a Rayleigh distillation model. Rather, these variations are likely due to isotopic fractionation accompanying Li diffusion from the Li-rich pegmatite (Li = 450 to 730 ppm) into amphibolites (Li = 20 ppm). The fact that other alkali element concentrations vary only within 2 m of the contact reflects the orders of magnitude faster diffusion of Li relative to heavier elements.