Dolostones of the Upper Proterozoic Society Cliffs Formation on northern Baffin Island are host to the Nanisivik lead–zinc sulfide deposits. Two distinct stages of dolomitization have occurred, distinguished by their petrographic, cathodoluminescence, and isotopic characteristics: (1) massive dolomitization of precursor carbonates, and (2) late-stage cementation. Sabkha facies of the lower member of the formation and laminated algal stromatolitic to massive dolostones of the upper member are isotopically similar: δ18O ranges from −1.5 to −6.5‰; δ13C from +1.7 to +3.1‰ PDB; and 87Sr/86Sr averages 0.70761. Petrographic and geochemical evidence indicates that these dolostones formed chiefly by seawater thermal convection beneath shale of the overlying Victor Bay Formation, although mixing of meteoric and marine water also could have occurred. Later fracture-filling dolomitic cements have consistently lower δ18O, but δ13C is similar to that in the host dolostones. Microtraverses of single fractures filled with dolomitic cements show progressive δ18O depletion through time; the last (innermost) generation is correlated with sparry gangue in the ore deposits and also has lower δ13C with respect to the Society Cliffs dolostones, due to mixing of bicarbonate produced from sulfate reduction. The cement was precipitated from warmer fluids, with partial replacement of earlier dolomite. Dolomitizing fluids responsible for later fracture and vug-filling cements may have been released by compaction of the underlying Arctic Bay shales, which also appears to have been the source of the base metals.