The origin and evolution of Canadian Shield brines: evaporation or freezing of seawater? New lithium isotope and geochemical evidence from the Slave craton

New chemical and isotopic data for deep seated calcium chloride brine from the Miramar Con gold mine, Yellowknife N.W.T., strongly suggest that the brine salinity is of marine origin. Diagnostic marine properties include uniformly elevated Br/Cl ratios typical of seawater concentrated beyond halite saturation, and Li/Br ratios (0.0254–0.0325) and δ6Li compositions (−32.1 to −36.3‰) similar to seawater (−32.3‰). The mean δ6Li for all mine water samples of −35.1‰ may reflect minor uptake of Li by secondary minerals. This interpretation is supported by analyses of altered metabasalt from fault zones which is enriched in Li but depleted in δ6Li (−14.7 to −15.6‰) relative to the unaltered metabasalt (−5.4‰). The mechanism responsible for concentrating the hypersaline brine end member is not unequivocal as evidence exists to support both evaporative and cryogenic processes. On the one hand, the Devonian sedimentary record in the Great Slave Lake region, in conjunction with Yellowknife brine isotopic compositions (δ2H and δ34SSO4) that are similar to various Devonian fluids, support an evaporative origin. On the other hand, the Na/Cl–Br/Cl relationship in the brine strongly suggests a cryogenic mechanism. Regardless of the concentrative mechanism, the chemical data indicate that the Yellowknife parent brine was concentrated 28- to 30-fold relative to seawater. The extreme depletion of Mg and enrichment of Ca in the brines, accompanied by Sr/Ca ratios similar to that of seawater, are accounted for by dolomitization of an aragonite-rich marine sediment by the brine before infiltration into the crystalline basement rocks. Subsequent alteration of silicate minerals in the shield added additional Ca and Sr to the brine as indicated by their radiogenic 87Sr/86Sr ratios (up to 0.7147). Based on mineral balance calculations, the major mineral products of the cryogenic and evaporitic concentration and evolution paths are significantly different. The cryogenic evolution results in some 15% mirabilite, 60% hydrohalite, and 18% dolomite whereas the major minerals formed from the evaporitic evolutionary sequence are 36% halite, 8% gypsum, 17% dolomite, and 30% albite. The great similarity between the calcium chloride brine from Yellowknife and other such Canadian Shield brines indicates that they may share a common marine origin.