February 2009    Print this article

Emeralds in Québec?

Daniel Bandyayera and Geneviève Robert,
Bureau de l'exploration géologique du Québec

What is an emerald?

Emerald is a variety of beryl (Be3Al2Si6O18) and is the third most valuable gemstone, after diamond and ruby (Groat et al., 2008). Emerald's green color is caused by trace amounts of chromium (Cr) or vanadium (V) in the beryl structure, which normally contains none. Other gemstones also come from beryl, the best-known being the blue variety, aquamarine.

Why are emeralds rare?

Beryl is a relatively rare mineral because there is very little beryllium (Be) in the earth’s crust. It is found in small amounts in granites and pegmatites (Groat et al., 2005). Emerald, a variety of beryl, is rarer still. In addition to the presence of beryllium (Be), emeralds require a sufficient amount of Cr or V during crystallization. These two elements are nearly as rare as beryllium in the earth's crust and even rarer in granites. In addition, the three elements generally come from different magma sources (Marshall et al., 2003). Cr and V are normally found in mafic or ultramafic rocks and their metamorphic equivalents, that is, rocks very low in silica, while Be generally is found in rock known as "evolved," meaning very rich in silica, such as granites and pegmatites.

Conditions conducive to emerald formation

The classic model of emerald formation is exactly those conditions under which evolved rocks (granites and pegmatites) interact with mafic or ultramafic rocks (Groat et al., 2008). The necessary conditions for emerald formation are, however, extremely complex. The "interaction" between the two source reservoirs must occur through hydrothermal fluids. Hydrothermal fluids are hot water that can carry dissolved Be, Cr, or V from their source to the location of emerald crystallization. These fluids must generally be present when the granites, pegmatites, or mafic or ultramafic rocks are deposited, or else when they are metamorphosed. The fluids travel through naturally occurring pores in the rock or along flaws or cracks. As they do so they "wash out" the Be, Cr, or V contained in the rock and carry it elsewhere. When these elements all come together, the chances of emerald formation is increased.

There are also celebrated examples of emerald-rich deposits (e.g., in Colombia ) that differ from the classic model in that they occur without magmatic activity and use an alternative source of Cr (i.e., black shales instead of mafic or ultramafic rocks). These other types of deposits illustrate the complexity of the conditions under which emeralds can form, but also show that emerald exploration should not be limited by the conditions described above.

Emeralds in Canada

The Canadian Northwest hosts three occurrences of emerald and several of beryl (Groat et al., 2005), making the Yukon, the western Northwest Territories, and northern British Columbia good targets for beryl and emerald exploration. The only other occurrence of emerald recorded in Canada is the Ghost Lake occurrence in western Ontario. The beryl and emerald crystals at Ghost Lake are found in pegmatites intruded into a chlorite schist unit (metamorphic mafic rock) adjacent to an altered ultramafic intrusion (Groat et al., 2005). In the case of Ghost Lake, beryl and emerald formation is associated with host rock metamorphism, which released the elements necessary for their formation (Be, Cr, or V).

What about Québec?

Presently, there are no known emerald deposits in Québec, because little exploration for this type of mineral is conducted. However, geological systems conducive to beryl and emerald formation are known in Québec. The Réservoir Opinaca region, for example, hosts a thick sequence of metasedimentary migmatites (rocks formed through the partial fusion of sedimentary rocks—these rocks are generally evolved) as well as numerous pegmatites. The region also hosts several ultramafic intrusions of varying sizes. This juxtaposition of potentially large reservoirs of chromium (e.g., ultramafic rocks) and evolved rocks (e.g., migmatites or pegmatites) is a setting that can be conducive to emerald formation.

References

Groat, L.A. , Giuliani, G., Marshall, D.D., Turner, D. (2008). "Emerald deposits and occurrences: a review." Ore Geology Reviews 34 , 87–112.

Groat, L.A. , Hart, C.J.R., Lewis, L.L., Neufeld, H.L.D. (2005). "Emerald and aquamarine mineralization in Canada ." Geoscience Canada 32 , 65–76.

Marshall , D.D., Groat, L., Giuliani, G., Murphy, D., Mattey, D., Ercit, T.S., Wise, M.A., Wengzynowski, W., Eaton, W.D. (2003). "Pressure, temperature, and fluid conditions during emerald precipitation, southeastern Yukon , Canada : fluid inclusion and stable isotope evidence." Chemical Geology 194, 187–199.

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