The characterization of intragrain mineralogical and isotopic zoning patterns provides the basis for estimating the duration of fluid-rock interaction associated with mineral replacement reactions. In the Rico, Colorado, hydrothermal system, oxygen isotope ratios in reaction rims on partially reacted plagioclase feldspar exhibit some of the largest gradients yet reported for individual grains (to 15‰). The extent of rim formation and accompanying isotopic exchange vary across the system as a function of temperature, fluid isotope composition, and the local fluid/rock ratio. Distal feldspars show narrow rims with 18O enrichments relative to pristine feldspars. Feldspars intermediate or proximal to the system's center have wide reaction rims or are completely exchanged and show 18O depletions. Formation times of reaction rims and associated isotopic patterns have been estimated with a coupled reaction-diffusion model that suggests that hotter (∼250–350 °C) hydrothermal circulation was active for ∼100–300 k.y. in the center part of the system, perhaps only while the igneous heat engine was still magma. Cooler (∼150–200 °C) circulation was widespread, lasting for >1000 k.y.