Middle Cambrian carbonate rocks of the southern Canadian Rocky Mountains are host to widespread units of white, sparry, hydrothermal, replacement, and open-space filling dolomite. Contained within the dolomites are occurrences of talc and Mississippi Valley type Pb-Zn (the former Kicking Horse and Monarch mines) mineralization and economic concentrations of magnesite (Mount Brussilof mine). Results of studies of solute chemistry of saline (18-25 equivalent wt.% NaCl) inclusion fluids reveal distinctly low Na/Br (55-220) and Cl/Br (95-340) values. These values indicate that the brines which formed the dolomite originated from seawater that had deposited large amounts of halite in an evaporitic environment. Low I/Br ratios for the dolomite-magnesite inclusion fluids are consistent with their derivation from seawater and contrast sharply with the high I/Br ratios of Laramide-age fluids, which formed veins throughout the Rocky Mountains. Variations in F/Br ratios between texturally early and late magnesites indicate the involvement of a second fluid in the formation of the late magnesites. Results of the study of solute chemistry of inclusion fluids from hydrothermal dolomites, magnesites, and associated mineralization are consistent with a model of the pre-Laramide formation of these materials from seawater that had undergone extensive evaporation and halite deposition. Distinct differences in I/Br, total salinity, and delta D values between the dolomite-magnesite depositing fluids and Laramide-age vein-forming fluids clearly indicate the lack of the involvement of Laramide-age fluids in the genesis of the dolomites, magnesites, and associated mineralization.

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