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Famous mineral localities: The Lac d'Amiante mine Black Lake, Thetford Mines, Quebec.

The Lac D'Amiante mine is a huge asbestos mine in Quebec. Many other asbestos and chromite mines, all now closed, are located within a few square kilometers around the mine, in the municipality of Thetford Mines. During the last ten years the Lac D'Amiante mine has produced world-class green andradite crystals and the best suolunite specimens ever found.

INTRODUCTION

Since 2001 the village of Black Lake has been part of the municipality of Thetford Mines, which has a population of about 26,000 inhabitants. The village is about a three-hour drive east of Montreal and about 6 kilometers from the town of Thetford Mines, in the French-speaking Canadian province of Quebec, Canada. The Lac d'Amiante ("Lake Asbestos") mine is easily accessible via highway 112 connecting Disraeli with Thetford Mines. The belvedere of the mine, at point 41 on the area map (Fig. 2), offers a breathtaking view (Figs. 3 and 4).

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South and east of the village of Black Lake lie two huge asbestos (chrysotile) open pits, the Lac d'Amiante and British Canadian mines, as well as at least seven smaller asbestos mines and prospects (Edith, Murphy Hill-Sud, Union, Maple Leaf, Southwark, Poudrier, and Reed). In the same area there are eight mines and prospects (Standard, Ward-Ross, Old Greenshields, Greenshield, Provencal Hill, Lambly-Nadeau et Victoria, Frechette) which were worked for chromite at the end of the 19th century and during the two world wars. Except for the Lac d'Amiante mine, all mines in the Black Lake area are now closed, and their huge dumps have formed hills which dominate the landscape. The Lac d'Amiante mine, operated by the Lake Asbestos Bell (LAB) Chrysotile, Inc. has been worked for 4 months each year since 2003; the reduced activity is a consequence of the depressed asbestos market.

MINING ACTIVITY IN THE THETFORD MINES AREA

Around the town of Thetford Mines there were several other asbestos mines (Bell, Johnson, King, Beaver 1, Beaver 2, Bennett-Martin, Flinkote, Pennington, National) and chromite mines and prospects (Hall, Stewart, Lemelin), as well as a few quarries. In 2007 LAB Chrysotile decided to close the last of these mines, the Bell mine. This very large mine was begun in 1878 (just one year later than the Johnson mine, the region's first asbestos mine), and after 1951 it was exclusively an underground operation. The Bell mine, incredibly, produced only one known fine mineral specimen in so many years of activity: a cluster of green grossular garnets associated with white acicular diopside crystals.

Five additional asbestos mines are located in the few kilometers between Black Lake and the town of Coleraine; these are the Belmina, Windsor, Continental, Vimy Ridge, and Normandie mines. In the same area there are 14 chromite mines: Nadeau Hill, Huard, Gagne, Reed-Belanger, Caribou, Dumais, Vaillancourt, Noel, Bennett-Martin, Woolsey, Nadeau, Montagne Caribou, Parent and Montreal-Coleraine, also known as the Montreal chrome pit. Several other asbestos and talc mines were located around the nearby town of East Broughton; the last of these mines (Carey-Boston) was closed in 1986. This extensive mining activity over a span of 130 years places Thetford Mines among the elite of historic mining localities.

As already stated, there are two huge asbestos mines at the village of Black Lake: the British Canadian (B.C.) and Lac d'Amiante. The British Canadian mine has been the source of attractive clusters of lustrous green grossular crystals, with individuals around 2 or 3 mm, associated in some cases with white acicular diopside crystals. These specimens were collected in the 1990s and sometimes mislabeled in the international market as specimens from the Jeffrey mine at Asbestos (which is about an hour's drive west of Black Lake). The British Canadian mine, opened at the end of the 19th century, was closed in 1997 and is now flooded. The Standard and the Union mines, which lie very close to the British Canadian mine, have been abandoned for decades. These two mines were the source of the "colerainite" variety of clinochlore (named for the nearby town of Coleraine; Black Lake was in the Municipality of Coleraine at the beginning of 1900). Described as a new mineral in 1918, "colerainite" was discredited as a variety of clinochlore in 1970 (Horvath, 2003). The Montreal-Coleraine mine, also known as the Montreal chrome pit, was the source of an emerald-green, chromium-bearing variety of vesuvianite named "chrome-idocrase" in 1913 (Horvath, 2003). Superb specimens of emerald-green vesuvianite have been collected during the last few years at the Jeffrey mine at Asbestos (Amabili, Spertini and Miglioli, 2004; Amabili and Spertini, 2004).

HISTORY OF THE LAC D'AMIANTE MINE

The Lac d'Amiante mine, now operated by LAB Chrysotile, Inc., is a huge open pit of about 1,875 X 1,560 meters, with a depth of 375 meters. Mining for chrysotile began in 1958. Looking now at the mine, it is difficult to imagine that until the mid-1950s the site was occupied by a lake, named Black Lake, surrounded by a forest of black pines and maple trees, with cottages sparsely scattered along the shore. In the summers people used to swim and fish in the lake, whose water looked black: hence the name of the village of Black Lake, officially founded in 1906. The lake was not large--1.75 km long, 1 km wide, and about 14 meters deep--but the layer of clay and other sediments on the lake bottom was 33 meters thick.

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By 1951, when the American Smelting and Refining Company (ASARCO) obtained from United Asbestos the mining rights for the region surrounding the lake, plans had already been made for the exploitation of the 60-million-ton asbestos orebody. In 1951 the geologist Stephen Ogryzlo of ASARCO studied the deposit and made 17 drill probes around the lake; the results regarding the potential of the deposit agreed with those of Theodore Koulomzine, who had done a magnetometer study of Black Lake for United Asbestos in 1949. Ogryzlo confirmed that beneath the lake there was an asbestos orebody of at least 60 million tons. In order to exploit this orebody, enormous and expensive preliminary work would have to be done: the lake's water and the clay lakebed would have to be removed, as would the river which fed the lake, and the provincial road which passed nearby. The cost of the operation would be $35 million.

The draining of the lake began on June 6, 1955, with an official ceremony attended by the president of ASARCO, C. Pryer. A machine called "Fleur de Lys" (named after the symbol of the province of Quebec), able to pump 5,000 gallons/minute, was the star of the operations. It took three and a half years to remove the 38 million gallons of mud and clean the lake's bottom. During the draining operations, ASARCO built a modern plant to process 5,000 tons of ore per day. The plant started operation in June 1958, and at once the Lac d'Amiante mine became the third largest producer of asbestos in the world (after the Asbest deposit in the Urals and the Jeffrey mine in the nearby town of Asbestos). Between 1958 and 1979 the Lac d'Amiante mine produced 60 million tons of asbestos ore.

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GEOLOGY

The rocks of the Quebec Appalachians, Cambrian and Ordovician in age, are grouped into three major tectonic domains (Saint-Julien and Hubert, 1975). From west-northwest to east-southeast, these are the Autochthonous domain, the External domain and the Internal domain. The Autochthonous domain rests on a Precambrian Grenville-like basement and contains three recognized lithostratigraphic assemblages: (1) sandstones and carbonates (the shelf sequence), (2) shale and mudstone (the flysch sequence) and (3) red and green shales (the regressive sequence). These rock units have experienced almost no folding.

The External domain is divided into two zones: an outer belt of thrust-imbricated structures very similar to the lithologic assemblages of the Autochthonous domain, and an inner belt of nappes emplaced by gravity and composed of flysch, limestone, feldspathic sandstone and shale. The typical units of the Internal domain are shale-melange, slate-sandstone-tuff, calc-alkaline volcanic assemblages and ophiolites. The ophiolitic complex lies structurally above the slate-graywacke assemblage of the Caldwell Group and is overlain by the rocks of the St. Daniel Formation, which is in contact on its southeast with the Beauceville Formation (Magog Group). All of these lithostratigraphic assemblages were shifted, slightly folded and metamorphosed 450 million years ago during the Taconian Orogeny, the first phase of the creation of the Quebec Appalachians. This event began at the end of the Lower Ordovician period and finished in early Silurian time (Saint-Julien and Hubert, 1975).

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A second phase in the evolution of the Quebec Appalachians took place during the Acadian Orogeny, 400 to 360 million years ago. Important granitic intrusions, 380 to 370 million years old, were emplaced during that period.

The asbestos deposits of southern Quebec are situated in a zone of mafic and ultramafic rocks called ophiolitic complexes. According to plate tectonic theory, ophiolites are remnants of oceanic crust trapped during orogenic events. These rocks are composed of marine sediments, basaltic pillow lavas, gabbro and diabase intrusions and ultramafic rocks of the earth's upper mantle. Ophiolites are rarely preserved as oceanic crust because they are swallowed in the subduction zones by the continuing movement of the oceanic plates. This is the case for the ultramafic formations associated with the folded rocks of the Sutton Notre Dame Mountains.

The ophiolite zone can be followed to the southwest, beyond the Quebec border and into Vermont's Green Mountains, and to the northeast as far as Newfoundland. The ophiolite marks the suture zone of a proto-Atlantic ocean named Iapetus. Working from K-Ar age determinations on hornblende and amphibolites from Asbestos, Laurent and Vallerand (1974) proposed an age of 550 million years for the ophiolitic rocks.

In the Lac d'Amiante mine there are three major ore zones. As in most asbestos deposits found in ophiolites, asbestos orebodies occur about 800 meters from the fault contact separating the enclosing ultramafites from the older metasedimentary units. The host rock consists mainly of serpentinized peridotite (harzburgite). It still contains remnants of the original pyroxene, giving a coarse-grained mottled texture. Along the western margin of the deposit the rock is more intensely serpentinized; it is mostly glassy dark green or black serpentinite, with local alteration by granite dikes and sills.

The most prominent structural feature within the mine property is a faulted talc-carbonate zone striking nearly north-south and separating two orebodies. The footwall of the mining operations to the southwest could be a subsidiary of the main faulted talc-carbonate zone. The talc-carbonate unit is commonly highly foliated, with intercalated bands of felsic material. A second important structure is the series of intrusive dikes of granite and other felsic rocks, located on the western and northern sides of the mine. The dikes have a predominant 30[degrees] to 60[degrees] NE strike, and dip to the northwest. The collector-quality garnets, diopsides, vesuvianites, etc. are hosted by rodingite dikes that were formed by the hydrothermal alteration of some of the granitic dikes during serpentinization. The structure of the deposit is shown in Figure 5.

MINERALS

Andradite [Ca.sub.3][Fe.sub.2.sup.3+][([SiO.sub.4]).sub.3]

Opaque brown andradite crystals, most commonly rhombic-dodecahedral, are not uncommon in the mine, but the beautiful, gemmy green trapezohedral crystals of the demantoid variety are extremely rare. Most demantoid crystals from Lac d'Amiante are only translucent, or are filled with internal fractures.

All green andradite crystals from Lac d'Amiante vary in color depending upon the light source (a property called metamerism), appearing pale green to emerald-green in sunlight and under neon lamps, and pale bluish gray-green to bluish green under incandescent and halogen lamps. The composition of the andradite crystals has recently been analyzed by Federico Pezzotta, using an EDS microprobe in the laboratory of the Natural History Museum of Milan, Italy. The analysis confirmed the presence only of calcium, iron and silica in stoichiometric quantities for garnet, indicating a composition close to end-member andradite.

Matrix specimens showing sharp, lustrous demantoid crystals of lovely emerald-green color (in sunlight) up to 2.5 cm were collected around 1997 (Figs. 9, 10). The first discovery of similar crystals at Lac d'Amiante had been made only a few years earlier. The total number of specimens recovered in the late 1990s was very small. After that time, green andradite garnets of good quality have been found only once, in September 2006, when loose crystals to 3 cm were collected, a few of these making it to the 2007 Tucson Show (Moore, 2007). Most of these crystals have a beautiful emerald-green color (in sunlight), ranking them among the finest demantoid crystals ever found.

A few of the demantoid specimens are associated with small, dark, acicular crystals, probably of vesuvianite. One unique specimen (Figs. 6 and 7) is a 2.7 X 5.6-cm group of six sharp, lustrous, emerald-green andradite crystals to 3 cm, joined by a white albite blade; this specimen was brought to the 2007 Sainte Marie-aux-Mines show (Larson, 2007), and a photo of it was published in the same year in Le Regne Mineral (no. 76, page 17) and Rivista Mineralogica Italiana (vol. 31, no. 4, page 285). This is the only demantoid specimen recovered at that time which is not a single loose crystal.

Suolunite [Ca.sub.2][Si.sub.2][O.sub.5][(OH).sub.2]*[H.sub.2]O

Suolunite was described from a locality in Inner Mongolia (China), but the only collector-quality specimens known are from a single isolated occurrence at the Lac d'Amiante mine. About 30 cabinet-size specimens offered by a Canadian dealer at the 2002 Tucson Show display suolunite microcrystals packed into deep purple-blue to pale lilac mammillary crusts with glittering surfaces, mostly without matrix (Moore, 2002). The specimens came from a collecting site in the mine (now completely exhausted) discovered at some time between 1996 and 1999 by a young geologist from Thetford Mines, who originally classified the mineral as prehnite.

Some specimens from the find, considered not valuable, were sold to well-known collector and mineral dealer Charles Key, who had them analyzed. When Key sold his collection of Canadian minerals to the Royal Ontario Museum in Toronto, the occurrence of suolunite in the Lac d'Amiante mine became publicly known. In February 2001 the Royal Ontario Museum exhibited at the Tucson Show the just-acquired Charles Key collection, including suolunite specimens from Black Lake.

Other Species

The Lac d'Amiante mine is listed in several classical mineral books as the source of huge natrolite crystals, not found recently. Moreover, the mine has yielded good, gemmy orange grossular crystals, paler than similar crystals from the Jeffrey mine at Asbestos. It has also produced (as mentioned) brown opaque andradite garnets, globular prehnite, colorless and transparent apophyllite-(KF), stilbite, stellerite, chabazite-Ca, dark vesuvianite in acicular crystals, zoisite (as the pink variety "thulite"), albite, clinochlore, antigorite, nephrite, chrysotile, rutile, galena, pyrite, magnetite, chromite, diopside, aragonite, fine yellow calcite and quartz (andradite, calcite and quartz are very rare at the famous Jeffrey mine at Asbestos).

Fine yellow calcite crystals associated with white stellerite were collected in 1999 (Fig. 12). The list of minerals could be expanded, but the mineralogy of the mine has not yet been accurately studied. The presence of minerals of the zeolite group is quite interesting, as well as the presence of both green andradite and orange grossular of gem quality. An interesting suite of minerals from the mine is owned by the Musee Mineralogique et Minier in Thetford Mines, but the world-class green andradite crystals can be seen only in a few private collections.

[FIGURE 12 OMITTED]

CONCLUSION

Unfortunately, the depressed market for asbestos has led to a cutback in mining at Black Lake. Future discoveries of minerals at the mine may have to wait for an increase in the demand for asbestos. During the current period of reduced, intermittent mining activity, which began in 2003, about 400 miners have been employed at the mine.

ACKNOWLEDGMENTS

The authors thank Dr. Federico Pezzotta of the Natural History Museum of Milan, Italy, for the analysis of the green andradite crystals.

REFERENCES

AMABILI, M., and MIGLIOLI, A. (2000) Ultimi ritrovamenti di granati nelle miniere di Asbestos e Thetford Mines, Quebec, Canada. Rivista Mineralogica Italiana, 24(2), 80-86.

AMABILI, M., and SPERTINI, F. (2004) La miniera Jeffrey, Asbestos, Canada. Rivista Mineralogica Italiana, 28(2), 66-76.

AMABILI, M., SPERTINI, F., and MIGLIOLI, A. (2004) Recent discoveries at the Jeffrey mine, Asbestos, Quebec, Canada. Mineralogical Record, 35, 123-135.

BONIN, G. (2001) Geologie des mines de chrysotile de la region de Thetford Mines, Quebec. Canadian Institute of Mining, Metallurgy and Petroleum, 53, 221-230.

FORTIER, C. (1983) Black Lake: Lac d'amiante, 1882-1982. Vol. 1 of Amiante et chrome dans les Appalaches: Cent ans d'histoire: Bibliotheque nationale du Quebec; National Library of Canada, 346 p.

GAUDARD, S. (1993) Voyage au Coeur des Appalaches. Musee mineralogique et miner Thetford Mines, Thetford Mines, Canada.

GRICE, J. D., and WILLIAMS, R. (1979) The Jeffrey mine, Asbestos, Quebec. Mineralogical Record, 10, 69-89.

HORVATH, L. (2003) Mineral Species Discovered in Canada. Mineralogical Association of Canada, Ottawa, Canada.

LARSON, B. (2007) What's new in minerals: Ste.-Marie-aux-Mines Show 2007. Mineralogical Record, 38, 474-478.

LAURENT, R., and VALLERAND, P. (1974) [Ar.sup.40]/[K.sup.40] isochron age for amphibolites of the ophiolitic complexes of the Appalachians of Quebec. Abstract, Geological Association of Canada, Annual Meeting, St. John's, Newfoundland, p. 53.

MOORE, T. (2002) What's new in minerals: Tucson Show 2002. Mineralogical Record, 33, 261-266, 273-275.

MOORE, T. (2007) What's new in minerals: Tucson Show 2007.Mineralogical Record, 38, 211-230.

SAINT-JULIEN, P., and HUBERT, C. (1975) Evolution of the Taconian Orogen in the Quebec Appalachians. American Journal of Science, 275-A, 337-362.

SPERTINI, F. (2001) La mine Jeffrey, Asbestos, Quebec, Canada. Le Regne Mineral, 37, 10-34.

Marco Amabili (1)

Francesco Spertini (2)

Marc B. Auguste (3)

Gilles Bonin (4)

(1) Department of Mechanical Engineering. McGill University, 817 Sherbrooke Street West, Montreal, PQ, Canada H3A 2K6. marco.amabili@mcgill.ca

(2) 212 Hutcheson, Asbestos, Quebec, Canada J1T 4J8

(3) 1283 Johnson, Thetford Mines, Quebec, Canada G6G 5W7

(4) 1000 Bellevue, Thetford Mines, Quebec, Canada G6G 5Y7
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Author:Amabili, Marco; Spertini, Francesco; Auguste, Marc B.; Bonin, Gilles
Publication:The Mineralogical Record
Article Type:Industry overview
Geographic Code:1CQUE
Date:Jul 1, 2009
Words:3027
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