Dolomite in an upper Ordovician sequence composed of the Irene Bay and Thumb Mountain Formations on Devon Island in the Canadian Arctic Archipelago is confined to burrows. Micron-sized dolomite crystals may have formed in burrows contemporaneous with deposition because of seasonal salinity changes in the Irene Bay-Thumb Mountain shelf lagoon. These penecontemporaneous dolomite crystals formed nuclei for the selective precipitation of late diagenetic dolomite from dilute subsurface solutions. Post-lithification crystal growth during late diagenesis caused idiotopic dolomite fabrics with intercrystalline micrite to become more coarsely crystalline xenotopic fabrics with no intercrystalline micrite. Dolomite crystal growth was accompanied by a progressive decrease in strontium and sodium contents and by a lowering of the amount of excess calcium. A possible rule regarding dolomite compositional variations is that high Mg/Ca solution ratios favor precipitation of more stoichiometric dolomite whereas low Mg/Ca solution ratios favour precipitation of calcium-rich dolomite with the exception that solutions of very low salinities will precipitate stoichiometric dolomite regardless of their Mg/Ca solution ratios.