1. Geological Survey of Canada Natural Resources Canada Ottawa, Ontario, K1A0E8 Canada
2. Department of Earth Sciences Stellenbosch Unviversity Stellenbosch South Africa
3. Department of Earth and Atmospheric Sciences University of Alberta Edmonton, Alberta T6G 2E3 Canada
4. Anglo American Johannesburg South Africa
5. BHP Toronto, Ontario Canada
6. Earth and Planets Laboratory Carnegie Institution for Science 5241 Broad Branch Road NW Washington DC 20015, U.S.A.