Numerical models of two-dimensional heterogeneous reactive transport of H2O-CO2 fluid and oxygen isotopes were developed to investigate the heterogeneity of fluid flow and permeability attending contact metamorphism of siliceous dolomites. Comparison of results with observations from the Alta (Utah) contact aureole indicates that both mineral reactions and isotopic alteration record fluid flow through highly heterogeneous permeability. We interpret this extreme heterogeneity to be the result of positive feedback between reaction enhancement of permeability and flow-focusing. Similar distributions of mineral assemblages and stable isotopic alteration in other aureoles suggest that this is a common mechanism of permeability evolution during contact metamorphism of carbonate rocks. Flow-focusing requires fast reaction rates and slow compaction relative to fluid flow. More homogeneous permeability recorded in regional metamorphic rocks may reflect fundamental differences in the relative rates of fluid flow and compaction in regional and contact metamorphic environments.