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 <1 to 7 m.
The freshest portions of the kimberlite consist of: abundant
(45 volume %, #5 mm) serpentinized olivine grains along with less common,
phlogopite (<5%, #5 mm) and picroilmenite (<2%, #5 mm)
grains.
Pyrope garnet, typically with kelypihtic rims, is common
and set in a fine-grained groundmass of serpentine, carbonate
and opaque minerals.
Paragenetically late deposition of hematite and corresponding reddening
of the kimberlite and country rocks is widespread. Locally, breccia zones are
observed at the structural top of the kimberlite and contain up to
90% rounded to angular country rock xenoliths in a carbonate
matrix.
Caustic fusion analyses on drill core from the sill has,
recovered promising macro diamond contents, the two largest
stones being larger that 2 mm in their longest dimension.
Electron microprobe data for garnets from the kimberlite reveals
a broad array of mantle-derived garnets with very common G9
(28.6 vol. %), G3 (19.5 %), G4 (15.6 %), rare G10 (1.8 %) and
G0 (< 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 11.771 Å (determined via FXRD) and similarly plot in
the gap between known garnet cell dimensions along the solid
solution (11.67-11.77 Å).
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.