"Yardsticks" that measure the extent of reaction progress in hydrothermally altered rocks have been documented in a suite of intrusive rocks from Rico, Colorado. The degree of fission track partial annealing increases and the whole rock oxygen isotope ratios decrease systematically as a function of proximity to the system's heat source. The quantity of alteration products in the rocks decrease going away from the heat source in the peripheral portions of the district. Closer to the center of the system, reactant minerals are completely reacted, but the mineralogy of the products varies spatially as a function of position in the hydrothermal system. Compositions of product chlorite and plagioclase are variable but not functionally with respect to distance from the heat source. However, the degree of Al, Si order in the plagioclase varies with the An value of the exchanged plagioclase. The research proposed here will use the degree of partial annealing of the fission tracks in apatite and zircon to set constraints on the duration of the hydrothermal system. Kinetic models of mineral-mineral reactions and oxygen isotope exchange can be tested using the different reaction progress yardsticks and the fission track chronology in the Rico rocks. These data will provide a unique indication of applica- bility of kinetic constraints to hydrothermal water-rock inter- action: one that provides a real measure of time.
