| 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:
||Red radiolarian cherts are the predominant sedimentary rock. The strata
indicate deposition in a deep marine environment.
*Unlike the ultramafic layer below, these layers do not repeat.
Base-metal sulfide deposits are sometimes found in the pillow lava
||Pillow lavas are only formed by underwater volcanic eruptions*.
|Sheeted basaltic dike complex
||These vertical dikes fed the eruptions that created the overlying pillow
||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.
**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.
||Pyroxene minerals make up >95% of all pyroxenites.
||Pyroxenes and olivine are found together in harzburgites.
||Olivine makes up >95% of all dunites.
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.