Soluble salts in a Canadian Shield granite as contributors to groundwater salinity

The presence of readily soluble Cl in the rock matrix of the Lac du Bonnet granite batholith, Manitoba, has been verified by crushing samples from various depths and leaching with deionized water. Unaltered grey granite has a leachable Cl content ranging from 30 to 48 mg/kg, and lower concentrations are found in pink, near-surface and fractured samples. A vertical profile of Cl content across a shallow, inclined fracture zone shows a clear decrease in leachable Cl in the altered rock of the zone, beginning ∼20 m above and ending ∼5 m below the zone. This suggests that Cl loss is caused by downward migration of dilute groundwater towards the fracture zone, with little recharge from below. Leaching of accessible salts is one mechanism that accounts for the geochemical evolution of groundwater in fracture zones in the granite.