In situ neutron activation of 35Cl within the rock and groundwater of geologic deposits that have elevated concentrations of uranium provides a hydrogeological tracer. We determine the production rate and mobility of 36Cl in the 1.3-billion-year-old Cigar Lake uranium ore deposit. Accelerator mass spectrometry was used to map the concentrations of 36Cl in the ore and in the groundwater that were up to 100 times greater than those encountered in unmineralized portions of the host sandstone aquifer. The residence time of this mobile anion in groundwater within the mineralized zone ranged from 14 to 280 kyr. These residence times are consistent with the hydraulic and geochemical data, suggesting significant control of Cl- and groundwater movement by the clay-rich matrix of the mineralized zone. ¿ American Geophysical Union 1996