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.