Processes controlling lithium isotopic distribution in contact aureoles: A case study of the Florence County pegmatites, Wisconsin

Li isotopes may be useful tracers of fluid flow in a number of geological environments and case studies of contact aureoles have highlighted the very large Li isotopic fractionation that can be generated in these settings. However, the amount of isotopic fractionation and the distance that Li travels into the country rocks vary greatly from place to place. Seeking to identify the parameters that govern Li distribution in contact aureoles, we apply a combination of Li isotope analyses, 1-D diffusion and 2-D advection-diffusion modeling to two country rock profiles adjacent to Li-rich pegmatite dikes from the Florence County pegmatite field, Wisconsin. Although less than ∼3 m thick, the pegmatite sheets have a large impact on the Li budget of the country rocks (amphibolites and schists); Li is enriched in adjacent country rocks by up to a factor of 20 over more distant amphibolites and schists. Li from the pegmatite has traveled more than 50 m into the country rocks, and Li isotopes are systematically fractionated with distance from the contacts (with δ⁷Li varying from +6 at the contact to-7 at 30 m from the contact in one case). These observations are consistent with diffusive fractionation of Li through an advecting grain-boundary fluid. Both one-dimensional diffusion and two-dimensional advection-diffusion models fail to reproduce the exact Li distribution in the profiles, suggesting that fluid advection, coupled with heterogeneous permeability, plays an important role in determining the final Li distribution within the contact aureoles.