Canadian Rockhound: Canadian Tourmaline

Gem-quality tourmaline has recently been discovered near O'Grady Lake in the Mackenzie Mountains, Northwest Territories, about 700 km west of Yellowknife. The deposit was initially discovered in 1994 by a research team from the University of British Columbia and the Canadian Museum of Nature (Groat et. al., 1995). In 1996 the Stargazer claims were staked to cover this deposit. Since then, limited exploration has outlined an area that has significant potential for hosting a large deposit of gem-quality coloured tourmaline. Canada does not have a history of gem tourmaline production, but the Stargazer claims could change that by becoming our country's first tourmaline mine.

Tourmaline, O'Grady
Tourmaline crystals from the Stargazer claim,
Northwest Territories, Canada.

The trench

Crystals of coloured tourmaline occur in open pockets called "miarolitic cavities" that are hosted within a large dyke of granite pegmatite/aplite which intrudes a megacrystic syenite. The coloured tourmaline is the Li-bearing mineral species elbaite, while black tourmaline, which is abundant only in the main body of the pegmatite/aplite, is the iron-bearing mineral species schorl (Groat et. al., 1995). Other minerals identified from the pockets include smoky quartz, microcline (white and green amazonite), albite (cleavelandite variety), lepidolite, danburite, hambergite, pollucite, ferroaxinite, and stilbite.

The tourmaline crystals are most often pink and brownish pink, although green, orange-brown, and blue colours also occur. Faceted gemstones up to 1.32 carats and cabochons over 12 carats have been cut. Most of the tourmaline found so far is suitable for cabochons, while facet grade material is uncommon. It is likely that facet grade tourmaline crystals will be more abundant once unweathered pockets are opened; only weathered and exposed pockets have been found to date. Some of the smoky quartz and danburite crystals are gem-quality and could be faceted.

Research by the Canadian Museum of Nature in collaboration with the University of British Columbia based on this summers' sampling and mapping program is expected be published in the near future. Additional exploration on the Stargazer claims is planned for next summer, and favourable results may put Canada on the map as a gem tourmaline producing nation.

References

Groat, L.A., Ercit, T.S., Mortensen, J.K. and Mauthner, M.H.F. (1995). Granitic pegmatites of the Canadian Cordillera: Yukon and Northwest Territories. Indian and Northern Affairs Canada, Northern affairs: Yukon Region, Open File 1995-14 (G).

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The preceding article was first published in the Canadian Gemmologist, Volume 18 (XVIII), No 4, Winter 1997, p.107. Reprinted in the Canadian Rockhound with permission from the author and the editor of the Canadian Gemmologist. The Canadian Gemmologist is published by the Canadian Gemmological Association.