Yellowknife 2007 Technical Program SS8: Advances in Kimberlite Geology and Diamond Genesis Organizers / Organisateurs: Hamish Sandeman (NTGO) and Thomas Stachel (University of Alberta) Room / Salle: Capitol 1 Presenter: Hamish A. Sandeman Unique garnet compositions from the Mud Lake Kimberlite, SW Slave Province, NWT: an occurrence of rare, high Cr-Ca green garnets Sandeman, H.A., Northwest Territories Geoscience Office, Box 1500, 4601 52nd Avenue, Yellowknife, NT, X1A 2R3 hamish_sandeman@gov.nt.ca, Barnett, R.L., R.L. Barnett Geological Consulting Inc., London ON N6P 1P2, Barry Laboucan, A., Snowfield Development Corporation, 508 - 675 West Hastings St. Vancouver BC V6B 1N2, Flemming, R., Department of Earth Sciences, University of Western Ontario, London, ON, N6A 5B7, and Tubrett, M., INCO Innovation Centre, Memorial University of Newfoundland, St. John’s, NL, A1B 1X5 The Ticho Diamond Project, operated by Snowfield Development Corporation, is located ca. 50 km south-southeast of Yellowknife on the eastern shore of Yellowknife Bay. Earlier historical work in the area includes regional till sampling programs conducted by prospector David Smith, industry and the Geological Survey of Canada. This work reports significant new data from the diamondiferous Mud Lake Kimberlite Sill complex. Regional till samples revealed elevated counts of kimberlitic indicator minerals and, during follow-up investigations in 2003, Snowfield discovered the Mud Lake kimberlite. The kimberlite comprises a NNE-trending, SW dipping sill-like body, generally continuous along strike for at least 800 m and, although bifurcating, ranges in thickness from < 0.1 %) garnets along with a major proportion of G12 (34.4 %) garnets. A minor proportion of these G12 garnets are green, high-Cr2O3 and high-CaO grains with CaO ranging from 12.83-21.47 wt. % with corresponding Cr2O3 from (7.01-17.80) and plot in the miscibility gap between ugranditic and pyralspitic garnets. Three of the green garnets have unit cell lengths of a= 11.700, 11.710 and 11.771Å (determined via µXRD) and similarly plot in the gap between known garnet cell dimensions along the solid solution (11.67-11.77Å). There is a remarkable correlation of these green garnets and diamonds in every diamond bearing kimberlite were green garnets have been identified. Green garnets with these compositions have been found in several diamond bearing kimberlites that have been mined including Premier in South Africa and Udachnaya in Russia. These unique diamond indicator minerals from the Mud Lake kimberlite represent the first publicly known green, high-Ca-Cr garnets in the Slave Province from a bedrock source.