The mineral riches of Mont St. Hilaire are known throughout the world. The reputation rests on the quality, quantity and diversity of minerals that have been found on the mountain. No fewer than 340 different minerals have been found there, ranking Mont St. Hilaire among the ten most mineral-rich sites in the world. Forty of the minerals were found and identified here for the first time. The alkaline nature of the magma that formed the mountain especially contributes to the occurrence of many minerals considered rare anywhere in the world.
Mont St. Hilaire originated in a series of three intrusions of magma into Ordovician-age sedimentary rocks. The three intrusions each have a distinct composition and chemistry and differ in age. The Sunrise intrusive group is the oldest (133 million years) and consists mostly of melagabbro, perkinite and pyroxenite. The intrusives of the Pain de Sucre group (120 million years old) are mostly gabbro, nepheline syenite, diorite and monzonite. Finally, the eastern side of the mountain (the 122-million-year-old East Hill intrusives) is mostly peralkaline nepheline syenites and porphyrites.
The magmatic intrusions that came up to form the core of Mont St. Hilaire penetrated layers of shale, limestone and siltstone that were melted and transformed by contact with the hot magma. This created a thin shell of fairly hard metamorphic rock around the igneous core of the mountain. Outside this shell and above the plutonic intrusion was the untransformed sedimentary bedrock, which is relatively soft compared to the igneous and metamorphic rock. Over millions of years, these softer rocks eroded away and on the upper portions of the mountains erosion and glacial action wore away even the somewhat stronger metamorphic shell around the igneous core. On the sides of the mountain, however, we can still find large areas of the metamorphic shell. These metamorphic rocks form a sort of crown around the mountain, oftenmost conspicuous as low cliffs.
It is possible to visit the Poudrette Quarry, where most of the most spectacular mineral finds have occurred. The quarry lies at the northern edge of the Gault Nature Reserve and penetrates deep into the igneous core of the mountain. Only organized visits are permitted and only at certain times of the year. These are usually coordinated through the Club de minéralogie de Montréal:
Club de minéralogie de Montréal (in French)
8300 de Teck (entrée par la porte de la rue St-Emile, angle de Teck)
Tel: (514) 353-0101
Le Club de minéralogie de Montréal,
C.P. 305, succ. St-Michel,
Montréal, Qué. H2A 3M1
Email: cmm [at] archambault [dot] net (Club de minéralogie)