Hypersaline brines of marine origin, deep within fractured crystalline rocks of the Canadian Shield, are characterized by calcium-chloride compositions with relatively low concentrations of magnesium and sulphate. Such chemistries among crustal fluids are very unusual, regardless of their origins, which makes it difficult to identify possible genetic associations between shield brines and other marine brines. Key conservative chemical and isotopic attributes of shield brines from the Yellowknife and Sudbury areas are similar, however, to those of fluid inclusions in sparry, late-stage hydrothermal dolomites in lower Paleozoic carbonates of the Western Canada Sedimentary Basin, suggesting that both fluid types originated from a residual seawater brine of probable Devonian age. This interpretation is supported by determination of a helium-4 model age for the Yellowknife brine that is consistent with a Devonian origin. Therefore, dolomitization was likely both a major source of Ca and a sink for about half of the initial Mg in the brine prior to its infiltration into the underlying basement rocks. Mass-balance considerations indicate that albitization of plagioclase, predominantly in basinal clastic sediments, was an equally important mineral source of Ca to the infiltrating brine, but interactions with carbonate and silicate rocks cannot account for the entire Ca inventory in the brine. A major contribution to the Ca budget must also have been initially provided by the evaporative concentration of seawater, which is only possible if it were enriched in calcium and depleted in sulphate compared with modern seawater, as was probably the case during Devonian time.