1. Departement CG&M, Ecole Polytechnique, CP 6079 succ. Centre-Ville, Montreal, Quebec H3C 3A7, Canada.
2. Corresponding author.
3. Geological Survey of Canada, 615 Booth Street, Ottawa, Ontario K1A 0E9, Canada.
4. Fugro Airborne Surveys, 2191 Thurston Drive, Ottawa, Ontario K1G 6C9, Canada.
5. Present address: Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada.