The Grant Brook deposit consists of a band of dolomite, at least 120 metres wide. The dolomite outcrops on Grant Brook, 3.3 kilometres east of the Grant Brook Station of the Canadian National Railway, and continues southeastward along the mountainside for at least 1.6 kilometres, 100 to 180 metres above the railway.
The area is underlain by a sequence of quartz-rich sandstones and coarser clastics, carbonates and pelites and other associated fine grained clastics from the Upper Proterozoic (Hadrynian) Miette Group through the Lower Cambrian Gog Group to the Middle Cambrian Chancellor Group. The Gog Group is subdivided into the lower McNaughton Formation, the Mural Formation and the upper Mahto Formation. The Resplendent fault and Moose Lake thrust are major structural elements that, in part, separate Cambrian stratigraphy from Hadrynian Miette stratigraphy to the southwest.
At the Grant Brook deposit, the entire dolomite bed is hosted in the Mural Formation of the Lower Cambrian Gog Group. The Mural Formation, in the immediate area, consists of limestone and dolomite with interbedded shale and sandstone. The dolomite bed, striking 114 degrees and dipping 30 to 90 degrees north, consists of fine grained, variably banded and mottled, pink, white and bluish white, impure dolomite. The dolomite is interbedded with layers of light green, talcose shale, 3 millimetres to 0.6 metres thick. Throughout the rest of the deposit, only a few streaks of blue and purple shale are present. The dolomite is thinly laminated to thinly bedded and massively but variably jointed, with most joints dipping westward. The rock is contaminated with minor crystals and veins of white quartz, a few chloritic streaks and a trace of pyrite. The deposit becomes more siliceous to the west. Several chip samples analyzed as follows (in per cent) (Canada Bureau of Mines Report 811, p. 217, Samples 96A, 96B, 96C):
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Sample CaO MgO SiO2 Al2O3 Fe2O3 Sulphur
96A 30.93 20.68 1.54 0.51 0.44 0.01
96B,96C 30.08 20.04 3.93 0.55 0.63 0.015
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Sample 93A was taken across 30.5 metres of white and bluish white dolomite exposed in a quarry. Assays for samples 96B and 96C have been averaged. The two samples were taken in succession across 30.5 metres of pink and white dolomite in the quarry.
The deposit was also investigated for its dimension stone potential. The stone, very hard and compactly crystalline, is generally white in color. The stone is strong, of low porosity, and should prove to have good weathering properties, however, it would be hard to work. The dolomitic character of the stone is shown by a high specific gravity and weight per cubic foot and by a slight loss under the corrosion test relative to the marbles from other localities. This test produced only slight etching and scarcely any color change. The specific values of test results on sample number 1249 were:
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Specific Gravity = 2.855
Weight per cubic foot = 177.32
Pore Space = 0.57
Ratio of Absorption = 0.150 (in one hour)
Coefficient of Saturation = 0.74 (in one hour)
Crushing Strength = 25,114 psi
Loss on Corrosion = 0.00586 grams per square inch
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This analysis reveals the high dolomitic character of the stone (Canada Bureau of Mines Report 452, p. 142-146, Sample 1249).
A small, unknown quantity of dolomite was quarried for marble by the Grant Brook Marble Company sometime before 1914.
