Discovery of sapphirine in Québec Far North:
a diamond indicator mineral?

Sandrine Cadéron
Direction de Géologie Québec

A fragment of breccia containing sapphirine has been discovered in the Troie Complex in the Archean Minto Subprovince, located in the northeastern part of the Superior Province. The area (Peters Lake, NTS 24M) was mapped by the Department during summer 2001 as part of the “Grand Nord” (Far North) project. Sapphirine, a rare mineral that had never been documented in the Canadian craton, is a deep blue ferromagnesian silicate which, when in equilibrium with quartz, characterizes rocks formed under ultrahigh pressure and temperature conditions.

Location of the Troie Complex in the Superior Province

Various geothermobarometers have been used to estimate the conditions under which the assemblages observed in the Peters Lake area were formed. They yielded high temperatures (755 - 1260 °C) and pressures (7,5 – 14 kbars), revealing that formation occurred at great depth (24 - 46 km). These new results, much higher than existing values, make it possible to propose new hypotheses for the tectonothermal evolution of the Archean crust in this part of the Superior Province. The presence of this high-pressure mineral in the Troie Complex could open the Peters Lake area to diamond exploration, since geochemical anomalies that could indicate the presence of kimberlites have already been mentioned in the Department's work.

Regional geology

The Troie Complex is dominated by synmagmatic felsic plutonic rocks containing volcanosedimentary inclusions, metamorphosed to granulite facies. The Douglas-Harbour Domain is composed of a tonalite-trondhjemite complex (Faribault-Thury) and is intruded into the 2740–2726 Ma Troie enderbitic complex. A breccia in the core of the Troie Complex contains a heterogeneous population of xenoliths in an enderbitic matrix. One of the xenoliths has sapphirine + plagioclase (and/or potassium feldspar) symplectite surrounding sillimanite and cordierite crystals.

Geological map of the lac Peters area

Microphotographs of textures
Sillimanite porphyroblast replaced by sapphirine + plagioclase symplectitic textures	Cordierite + biotite replaced by sapphirine + K-feldspar symplectitic textures

References

For more information about work carried out by the Ministère des Ressources Naturelles et de la Faune in this area and on the formation of sapphirine, please see the following article which has just been published in a special volume of the Canadian Mineralogist.

Truth and beauty in metamorphism: a tribute to Dugald Carmichael (Volume 43),
S. CADERON, W.E TRZCIENSKI Jr., J. H. BÉDARD & N. GOULET, An occurrence of sapphirine in the Archean Superior Province, northern Quebec, p. 463-478.