The Québec Asbestos Mining District
Thetford Mines, Québec

     I first visited the asbestos mines of Québec during the spring term of my Junior year, in 1974. It was the field trip for my Economic Geology course, taught by Richard E. "Dick" Stoiber. Dick had been taking students to Thetford Mines for more than 20 years to look at the asbestos mines and some base-metal massive sulfide mines in the area. The 1974 trip was radically different from all of his previous excursions to the area, however, because the area had just been reinterpreted in light of the relatively new concepts of Plate Tectonics. For the first time Dick was going to look at the area as an ophiolite.
     An ophiolite is a slice of oceanic lithosphere that has been incorporated into the continental lithosphere during the collision of two plates. The Thetford Mines ophiolite is one of a string of these pods that extend from Newfoundland to northwestern Georgia. They were emplaced about 490 million years ago during the Taconic Orogeny. In North Carolina, there are several well-known localities. The Webster-Addie district, near Sylva may be the best exposed.
     Ophiolites have a characteristic igneous stratigraphy that is apparently the result of fractional crystallization of a basaltic melt. They can typically be divided into an ultramafic layer, a mafic layer, and a sedimentary layer. The basic stratigraphy is shown below:
Sedimentary Layer Red radiolarian cherts are the predominant sedimentary rock. The strata indicate deposition in a deep marine environment.
Mafic Layer
Pillow Lavas Pillow lavas are only formed by underwater volcanic eruptions*.
Sheeted basaltic dike complex These vertical dikes fed the eruptions that created the overlying pillow lavas.
Gabbro Gabbro is the plutonic equivalent of the volcanic rock basalt. Instead of being erupted to the surface where it could quickly cool, this magma remained deep, cooling slowly, and crystallizing coarse-grained crystals.
*Unlike the ultramafic layer below, these layers do not repeat. Base-metal sulfide deposits are sometimes found in the pillow lava  layer.
Ultramafic Layer
Pyroxenite** Pyroxene minerals make up >95% of all pyroxenites.
Harzburgite** Pyroxenes and olivine are found together in harzburgites.
Dunite** Olivine makes up >95% of all dunites.
**These layers tend to repeat numerous times in a complete or broken bottom to top order. Chromite deposits may be associated with the ultramafic layer.

     The boundary between the crust and the underlying mantle, known as the Mohorovicic Discontinuity, or Moho, occurs between the ultramafic (mantle) and mafic (crust) layers. During metamorphism, hot water was flushed through the Thetford Mines ophiolite and altered the ultramafic layer. Olivine and pyroxene, the two major mineral groups in the ultramafic layer, are predominantly magnesium silicates. These chemicals react with the hot water to form the clay mineral known serpentine, a hydrated magnesium silicate. A major form of serpentine is chrysotile, more commonly known as asbestos.

    In 1984, I led a group of Colgate University students on a trip to the Thetford Mines area after consulting with Dick Stoiber to find out how to get to the key exposures. It was such an interesting trip that I led it several more times, both as a grad student at Dartmouth and as a professor at Norwich University. I haven't been to the area since 1991 but it is still my favorite field trip for students. The photos below show some of the highlights of the area. Most of these photos are from the 1984 trip. The late Half Zantop took the first shot.

Asbestos mining dominates the landscape around Thetford Mines. Tailings (mine waste) piles are everywhere. Most of the mines are deep open pits so the waste material is simply piled outside the mine. This neighborhood of Thetford Mines is surrounded by piles from a mining operation that closed decades ago (Photo by Half Zantop).

My favorite place to stay in Thetford Mines is the Hotel Balmoral on the northeast side of town. Enormous tailings piles loom over the hotel pool. Interesting cross-cultural exchanges usually take place at the hotel disco in the evenings.

The Black Lake Mine, just outside of Thetford Mines is a typical open pit mine in the area. For scale, the tires on the truck on the haul road in the lower left are about 2 m in diameter.

Dunites and harzburgites abound behind the houses in the town of Vimy Ridge. A tailings pile from the Vimy Ridge Mine is seen in the background.

A closer view of the Vimy Ridge Mine shows the mill and the conveyor system that deposits the waste rock on the tailings piles.

This is a fairly typical vista in the area with a tailings pile, mine and mill. Mt Ham is on the horizon in the background. It is one of the Monteregian Hills--a series of Cretaceous alkalic intrusive rocks.

During both World War I and World War II, the thin chromite bands at Caribou Lake were exploited for the war effort because German U-boats threatened the South African ore ships. They are too thin to compete with the enormous deposits of South Africa so as soon as the wars ended, the mine closed. The deposit ran down the middle of the trench seen in the photo.

The thin black Caribou Lake chromite band, shown here, is no match for the >2 m thick Bushveld Complex of South Africa. Apparent graded bedding within the chromite suggests it was deposited by gravity flow within the magma.

Several excellent exposures of pillow lavas are found in the Mt. Adstock area. These pillows vary between 0.5-1.5 m in diameter. Pillow lavas indicate that these lavas were extruded in a submarine environment.

One of my all-time favorite outcrops is the melange (tectonic breccia) exposed in the cemetery at Coleraine. Clasts from all strata of the ophiolite suite are present within the melange. See the next photo to get an idea of the scale.

The Colgate students in the front row are leaning against the face of the rock in the photo above. Wandering over the large roche moutonée behind the group gives an excellent idea of what a tectonic breccia is all about.

To the northeast of Thetford Mines is the small village of St. Pierre de Broughton. Just outside of town is a talc mine that is a favorite mineral collecting locality. Over the course of my visits here, students have found more than 30 minerals.

The climax of every trip I've taken to the area is the East Lake traverse. It is about a 2 km walk through woods and fields in which all layers of the ophiolite occur, in sequence, except for the pillow lavas which are apparently covered by an angular unconformity between the red chert and the underlying mafic layer. One can almost straddle the Moho--the boundary between the mantle and the crust. The last exposure of the ultramafic (pyroxenite) layer and the first of the mafic (gabbro) layer are only about 3 m apart.


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Last updated
October 10, 2005