Industrial Minerals of Vermont: 200 Years and Going Strong
The granites of Vermont are assigned
to the Devonian New Hampshire pluton series. The series extends the
entire length of the state from the Black Mountain pluton in Dummerston
to Averill in northeastern Vermont. The most prominent exposures of
granitic rock are located north and east of Barre. Although nearly all
of these plutons have been quarried at one time, the most active quarries
today are the Barre pluton at Graniteville and Websterville. While many
of the plutons of the series in New Hampshire represent distinctly different
episodes of intrusion, the granites of Vermont are roughly the same
age– emplaced following the deformation of the Lower Devonian
sediments during the Acadian Orogeny. Potassium/Argon (K-Ar) and Rubidium/Strontium
(Rb/Sr) ages for the pluton range from 380 to 330 million years old.
Graniteville (Photograph courtesy of Philip Greenspun)
The Barre pluton intrudes Silurian-Devonian metasediments
(A sediment or sedimentary rock that shows evidence of having been
subjected to metamorphism) of the Gile Mountain and Waits River formations,
and is estimated to be about four miles long, one mile wide, and ten
miles deep. Contacts with the metasediments are mildly to strongly
discordant (The contact is not parallel to foliation or bedding in
the country rock); xenoliths (a fragment
of country rock within a volcanic rock) are common near the contacts.
The Barre pluton, a gray medium to fine-grained granodiorite,
is very consistent across the unit in terms of composition. Its mineral
composition consists of 35 percent oligioclase feldspar, 9 percent
biotite mica, 27 percent
quartz , 21 percent microcline feldspar and 6 percent muscovite
and various accessory minerals.
Raw granite blocks not suitable for memorials are being promoted for use in levee construction, stream rehabilitation and and bank stabilization.
stone (stone quarried for blocks or slabs), either of granite or
marble, is the state’s leading mineral commodity in terms of value,
with granite number one in terms of tonnage. The Barre Granite Association,
the trade organization that represents 60 quarrying and manufacturing
companies in the Barre area, celebrated its 100th anniversary in 1989.
The companies employ more than 1,700 workers, with $25 million enhancing
the local economy through salaries and benefits. Rock of Ages and its
affiliates quarry about 1.5 million cubic feet per year from 10 active
quarries. More than 700,000 cubic feet in granite blocks are produced
from the Barre area annually, representing 25 to 40 percent of the quarried
stone, with a total recovery of about 15 percent.
VT DEC Geology and Mineral Resources Division 1 National Life Drive, Davis 2 Montpelier, VT 05620-3920
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