Emerald and aquamarine mineralization in Canada.

Publication: Geoscience Canada

Publication Date: 01-JUN-05

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COPYRIGHT 2005 Geological Association of Canada

SUMMARY

This paper reviews the geology, mineralogy, and origin of the gem varieties of beryl, including emerald (green) and aquamarine (blue); it focuses on western Canada, especially the Yukon Territory, because this is where most of the recent discoveries have been made. However, emerald occurrences in Ontario are also considered, including Canada's first reported discovery in 1940. Beryl ([Be.sub.3][Al.sub.2][Si.sub.6][O.sub.18]) is relatively common and spatially associated with granites and granitic pegmatites, but emerald is rare because trace amounts of Cr and/or V are required (to replace Al in the crystal structure) and these elements generally do not occur in sufficient concentrations in granitic rocks. The geological conditions needed to bring Be into contact with Cr and/or V are briefly discussed, as are the factors to consider and techniques to use in exploring for gem-quality beryl.

SUMMAIRE

Le present article traite de la geologie, de la mineralogie et de l'origine de varietes gemmiferes de beryl (vert), dont l'emeraude et l'aigue-marine (bleue). Il traite principalement de l'Ouest canadien, particulierement du Territoire du Yukon, region ou la plupart des decouvertes ont eu lieu. Toutefois, des decouvertes faites en Ontario sont aussi consideres, incluant la premiere au Canada, en 1940. Le Beryl ([Be.sub.3][Al.sub.2][Si.sub.6][O.sub.18]) est relativement commun et associe aux granites et aux pegmatites granitiques, mais l'emeraude est rare parce qu'elle necessite le remplacement de l'Al dans la structure cristalline du beryl par du Cr et/ou du V, et ces elements ne se retrouvent generalement pas dans en concentrations suffisantes dans les roches granitiques. Les facteurs geologiques necessaires pour que le Be et le Cr et/ou le V soient mis en contact font l'objet de discussion, tout comme les facteurs a considerer et les techniques a employer dans l'exploration de gisements de beryls gemmiferes.

INTRODUCTION

Beryl, [Be.sub.3][Al.sub.2][Si.sub.6][O.sub.18], is a common, rock-forming cyclosilicate mineral, generally occuring within granites and granitic pegmatites. Gem varieties of beryl include emerald (green), aquamarine (blue), red beryl, goshenite (colourless), heliodor (yellow), and morganite (pink or peach). Of these, emerald is the most prized, and can be worth more than US$100,000 per carat. The colour of emerald reflects the trace amounts of Cr and/or V replacing Al in the crystal structure; it may be diminished by the presence of Fe which can add a greyish tint (Walton, 2004). Emerald is rare because Be and Cr are generally insoluble, and the geological conditions needed to bring Be into contact with Cr and/or V are, typically, incongruous.

There is debate over the difference between emerald and green beryl (see Conklin, 2002, and Schwarz and Schmetzer, 2002). However, a definition that appears to be attaining broad acceptance is that of Schwarz and Schmetzer (2002): "emeralds are yellowish green, green or bluish green, natural or synthetic beryls, which reveal distinct Cr and/or V absorption bands in the red and blue-violet ranges of their absorption spectra."

There are many classification schemes for emerald deposits. Most recently, Schwarz et al. (2002) and Grundmann (2002) divided emerald deposits into the following categories: pegmatites without schist, pegmatite and greisen with schist, schists without pegmatites, and black shales with veins and breccias. Most emerald deposits, and all of those described in this paper (with the possible exception of the Lened property) belong to the first three classes. The "black shale" category is typified by the Colombian deposits where emerald occurs in calcite + dolomite + pyrite [+ or -] albite veins in black shales and related rocks. In this type of deposit, the emerald is considered to have formed because of the thermochemical reduction of mesothermal brines by organic-rich black shales, which is effective at releasing Be, V, and Cr into solution. This model has most recently been espoused by Giuliani et al. (2000).

Recent discoveries of emerald in northwestern Canada have led to increased exploration expenditures; for example, in 2003, approximately 30% of exploration expenditures (>$3.5M) in the Yukon Territory were directed toward emerald exploration (mostly at True North Gem Inc.'s Tsa da Glisza property). Although it is uncertain if any of the existing properties will become producers, given the size of the Canadian land mass, the low population density, the diverse geology, and the (until recently) low level of exploration for coloured gems, there will almost certainly be more discoveries. This paper reviews the geology, mineralogy, and origins of the existing properties and suggests strategies for future exploration. It focuses on western Canada, especially the Yukon Territory because this is the location of most recent discoveries and exploration efforts.

REGIONAL FRAMEWORK

Beryl occurrences in Canada are mostly associated with either Archean pegmatites in the Canadian Shield or Mesozoic/Cenozoic granitoids in the Cordilleran. The late Archean Superior and Slave provinces host several examples of rare-element (i.e., beryllium) bearing pegmatites (Breaks et al., 2003; Tomascak et al., 1994). The pegmatites in the Superior Province are associated with S-type peraluminous granites and leucogranites which include the post-collisional Ghost Lake batholith (see below) and Separation Rapids pluton (~2650 Ma; U-Pb/monazite; Larbi et al., 1999).

In the Cordillera, Legun (2004) compiled information on all known beryl occurrences in British Columbia and produced a map showing a "beryl belt" running the length of the eastern side of the province, with a "principal area of emerald potential" in the north-central part of the province, extending to the B.C.-Yukon border (Fig. 1). Legun's "beryl belt" corresponds to the terranes that underlie the Omineca belt and the bordering eastern margin of the Intermontane belt, where Mesozoic and Cenozoic granite intrusions and associated pegmatites cut tectonic slices of ultramafic and oceanic rocks (Legun, 2004). In this paper, the belt is extended northward to encompass beryl occurrences in the Yukon Territory and it is referred to as the "Western Canadian Beryl Belt" or WCBB. The WCBB's eastern and northern boundaries mostly follow the margins of the Selwyn Basin, which is also the limit of Cretaceous granitoids; its western boundary is just west of the Tintina Fault and is dominated by the Yukon-Tanana Terrane but jogs as far west as the Teslin Fault in southern Yukon to encompass beryl occurrences associated with plutons in the Cassiar Terrane. The Tintina Fault is a large, through-going, dextral strike-slip fault with significant (430 km) displacement that is mostly early Cenozoic (D.C. Murphy, pets. comm. 2004).

[FIGURE 1 OMITTED]

TSA DA GLISZA, YT

In 1998, W. Wengzynowski discovered emerald at Tsa Da Glisza ("green stones" in the Kaska language; formerly Regal Ridge) in the southeastern Yukon Territory (61[degrees] 16.6' N, 130[degrees] 35.5' W, NTS 105G/7; Figs. 1 and 2). The geology, mineralogy, and origin of the occurrence are described in Groat et al. (2002), Marshall et al. (2003), Neufeld (2004), and Neufeld et al. (2003, 2004). The mineralization is associated with quartz-tourmaline veins and aplite dykes that intrude mafic metavolcanic rocks of the Yukon-Tanana Terrane. Pale green to green-blue to "emerald green" beryl crystals up to 4 cm in length occur in 12 mineralized zones within a 900 x 900 m area (Fig. 3 top). Chromium (average 3208 ppm) is the predominant chromophore (Fig. 4). Some of the smaller crystals, and sections of larger crystals, are gem-quality, and a number of small gems (up to 2.4 ct) have been fashioned from the Tsa da Glisza samples (Fig. 3 bottom). The property currently belongs to True North Gems Inc.

[FIGURES 2-4 OMITTED]

The Yukon-Tanana Terrane in the Tsa da Glisza area is mainly composed of pre-Late Devonian quartz-rich metaclastic and carbonate rocks, and Devonian and Mississippian metavolcanic and metaplutonic rocks, which are inferred to have formed in a continental magmatic arc (Mortensen and Jilson, 1985; Mortensen, 1992; Murphy and Piercey, 2000) and back-arc settings (Piercey et al., 2000). The oldest rocks are in the pre-Late Devonian to earliest Mississippian Grass Lakes succession. The Fire Lake Formation, composed mainly of chloritic phyllite of mafic metavolcanic composition (Murphy et al., 2002), is the...

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