SOLUTE CHEMISTRY OF INCLUSION FLUIDS FROM SPARRY DOLOMITES AND MAGNESITES IN MIDDLE CAMBRIAN CARBONATE ROCKS OF THE SOUTHERN CANADIAN ROCKY-MOUNTAINS

Middle Cambrian carbonate rocks of the southern Canadian Rocky Mountai ns are host to widespread units of white, sparry, hydrothermal, replac ement, 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 concentr ations of magnesite (Mount Brussilof mine). Results of studies of solu te chemistry of saline (18-25 equivalent wt.% NaCl) inclusion fluids r eveal distinctly low Na/Br (55-220) and Cl/Br (95-340) values. These v alues indicate that the brines which formed the dolomite originated fr om seawater that had deposited large amounts of halite in an evaporiti c environment. Low I/Br ratios for the dolomite-magnesite inclusion fl uids are consistent with their derivation from seawater and contrast s harply with the high I/Br ratios of Laramide-age fluids, which formed veins throughout the Rocky Mountains. Variations in F/Br ratios betwee n texturally early and late magnesites indicate the involvement of a s econd fluid in the formation of the late magnesites. Results of the st udy of solute chemistry of inclusion fluids from hydrothermal dolomite s, magnesites, and associated mineralization are consistent with a mod el of the pre-laramide formation of these materials from seawater that had undergone extensive evaporation and halite deposition. Distinct d ifferences in I/Br, total salinity, and delta D values between the dol omite-magnesite depositing fluids and Laramide-age vein-forming fluids clearly indicate the lack of the involvement of Laramide-age fluids i n the genesis of the dolomites, magnesites, and associated mineralizat ion.