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UNIQUE GARNET COMPOSITIONS FROM THE MUD LAKE KIMBERLITE, SW SLAVE PROVINCE, NWT: AN OCCURRENCE OF RARE, HIGH Cr-Ca GREEN GARNETS

September 28, 2019

In 2007, I co-authored a paper on unique diamond indicator minerals, that was presented at the annual Yellowknife Geoscience Conference. Back then it was considered controversial by many diamond experts.

 

These green garnets fall outside the chemical compositions of what is considered to 

be important, at least back then, by diamond experts. They are formed in the same 

part of the upper mantle as diamonds and are nearly as rare.

 

Lately, some of those same experts that scoffed at our work are now writing about the importance of these diamond indicator minerals. It isn't easy to find on the internet and one of my friends asked if I can publish it because this is good stuff for future diamond explorers.

 

The second paper was published in the books of abstracts from the 9th International Kimberlite Conference in 2008. 

 

The papers on these unique diamond indicator minerals are very few in the literature for diamond exploration. These two are the only ones I'm aware of for the kimberlite we found. If it helps diamond explorers, I will be pleased.

 

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. Earlierhistorical 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.

 

An overview of the Mud Lake Kimberlite, SW Slave

craton, Northwest Territories, and implications of the

presence of high Cr2O3, CaO-rich green garnets

 

H. A. Sandeman(1), R.L. Barnett (2) and A. Barry Laboucan (3)

(1) Northwest Territories Geoscience Office, Box 1500, 4601 52nd Avenue, Yellowknife, NT, Canada X1A 2R3 (Hamish_sandeman@gov.nt.ca), (2) R.L. Barnett Geological Consulting Inc., London, ON, Canada N6P 1P2, (3) Snowfield Development Corporation, 508 - 675 West Hastings St. Vancouver BC, Canada V6B 1N2.

 

The discovery of diamondiferous kimberlite in the Lac de Gras region of the Slave

craton initiated an unprecedented diamond-driven staking rush, principally focused

in the central Slave, proximal to known diamondiferous deposits. With their discov-

ery throughout much of the craton, we now recognize that Slave kimberlites occur

in temporally and petrologically distinct “fields”, characterized by individual pipes

and pipe clusters. The Ordovician-Silurian (ca. 441-463 Ma) southwestern Slave field,

best known because of the ca. 25 ha, diamondiferous Drybones Bay kimberlite, has

received little attention compared to the central and southeastern Slave fields.

 

The Ticho Diamond Project, operated by Snowfield Development Corporation (SNO),

is located in the southwestern kimberlite field, ca. 50 km south-southeast of Yel-

lowknife on the eastern shore of Great Slave Lake. Initial exploration in the area was

largely driven by regional till sampling programs conducted by the Geological Survey

of Canada and prospector David Smith. Till samples collected in the early sampling

programs revealed elevated numbers of kimberlite indicator minerals including pyrope

garnet, chromite and picroilmenite. Follow-up investigations in 2003 (SNO) resulted

in the discovery of the Mud Lake kimberlite (MLK).

 

The MLK comprises a NNE-trending, SW-dipping, sill-like body generally contin-

uous along strike for ≥ 800 m and, although bifurcating, ranges in thickness from

< 1 to 7 m. The freshest portions of the MLK consist of: abundant (45 volume %,≤5 mm) serpentinized olivine grains along with less common, phlogopite (<5%,≤5mm) and picroilmenite (<2%,≤5mm) grains. Pyrope, typically with kelyphitic rims,is common and set in a fine-grained groundmass of serpentine, carbonate and opaqueminerals. Paragenetically late hematite and associated reddening of the kimberlite andcountry-rock is widespread. Breccia zones are locally observed at the structural top ofthe MLK, and contain≤90% rounded to angular country-rock xenoliths in a carbon-ate matrix. Caustic fusion analysis on drill core from the sill has recovered promisingmacro diamond contents, the two largest stones being≥2 mm in their longest dimen-sion. Snowfield Development Corporation has since removed the granite country-rockcap from a portion of the MLK sill and retrieved a 500 tonne bulk sample currentlybeing processed for diamond content.

 

We review historical, mineral industry assessment report information, and presentnew petrographical, electron microprobe and LAM-ICP-MS mineral chemical, X-raydiffraction and U-Pb geochronological data on samples of the MLK exposed duringthe recent bulk sampling, in order to better document the MLK. Comparisons with theadjacent Dry Bones Bay kimberlite and other pertinent global localities help providea better understanding of the origin of the enigmatic high-CaO, Cr2O3-rich garnets.

 

 

 

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