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 br ine salinity is of marine origin. Diagnostic marine properties include unif ormly elevated Br/Cl ratios typical of seawater concentrated beyond halite saturation, and Li/Br ratios (0.0254-0.0325) and delta(6)Li compositions (- 32.1 to - 36.3 parts per thousand) similar to seawater (- 32.3 parts per t housand). The mean delta(6)Li for all mine water samples of - 35.1 parts pe r thousand may reflect minor uptake of Li by secondary minerals. This inter pretation is supported by analyses of altered metabasalt from fault zones w hich is enriched in Li but depleted in delta(6)Li (-14.7 to - 15.6 parts pe r thousand) relative to the unaltered metabasalt (- 5.4 parts per thousand) . The mechanism responsible for concentrating the hyper-saline brine end me mber is not unequivocal as evidence exists to support both evaporative and cryogenic processes. On the one hand, the Devonian sedimentary record in th e Great Slave Lake region, in conjunction with Yellowknife brine isotopic c ompositions (delta(2)H and delta(34)S(SO4)) that are similar to various Dev onian fluids, support an evaporative origin. On the other hand, the Na/Cl-B r/Cl relationship in the brine strongly suggests a cryogenic mechanism. Reg ardless of the concentrative mechanism, the chemical data indicate that the Yellowknife parent brine was concentrated 28- to 30-fold relative to seawa ter. The extreme depletion of Mg and enrichment of Ca in the brines, accomp anied by Sr/Ca ratios similar to that of seawater, are accounted for by dol omitization of an aragonite-rich marine sediment by the brine before infilt ration into the crystalline basement rocks. Subsequent alteration of silica te minerals in the shield added additional Ca and Sr to the brine as indica ted by their radiogenic Sr-87/Sr-86 ratios (up to 0.7147), Based on mineral balance calculations, the major mineral products of the cryogenic and evap oritic concentration and evolution paths are significantly different. The c ryogenic evolution results in some 15% mirabilite, 60% hydrohalite, and 18% dolomite whereas the major minerals formed from the evaporitic evolutionar y sequence are 36% halite, 8% gypsum, 17% dolomite, and 30% albite. The gre at similarity between the calcium chloride brine from Yellowknife and other such Canadian Shield brines indicates that they may share a common marine origin. (C) 1999 Elsevier Science B.V. All rights reserved.