New Mexico Mineral Symposium — Abstracts

Mont Saint-Hilaire is one of the Monteregian Hills, a series of igneous intrusions of Cretaceous age that form an arc across the Saint Lawrence Valley, a rift valley of Precambrian to Paleozoic age. The Monteregian Hills have been interpreted as resulting from a mantle "hot spot," though consensus about this hypothesis is lacking. Rocks relatively rich in silica are confined to the eastern end of the arc, while silica-poor rocks are typical of the western end. Mont Saint-Hilaire is near the western end of the arc, and is composed generally of syenites and other silica-poor igneous rocks. Oka, the west-most "hill," is composed primarily of carbonatites. Mont Saint-Hilaire was formed by several intrusive events, each of which influenced the mineralogy of the older intrusions. In addition, the rising magma passed through various sedimentary units, including limestones and evaporates, which altered the chemistry of the magma and yielded xenoliths to the rising melt.

The geological complexity of Mont Saint-Hilaire's origin has led to an extremely varied and unusual mineralogy, rich in rare earth elements, titanium, zirconium, beryllium, and boron. More than 400 mineral species have been described from the Poudrette quarry alone, and approximately 50 unknowns are yet to be described. A number of species are found only at Mont Saint-Hilaire, or are found only there as well-formed crystals. The sequence of intrusions produced conditions that favored a wide range of crystal habits among many minerals, as well as the development of pseudomorphs and epitactic relationships. An appropriate New Mexico comparison would be Point of Rocks, where rocks of similar chemistry but simpler genesis are found. Point of Rocks has 52 species (according to Mindat.org). All but four of them are also found at Mont Saint-Hilaire.

This talk will provide an overview of the geology and systematic mineralogy of Mont Saint-Hilaire, and illustrate selected species that are characteristic of the unusual minerals found there. It will not cover all 400 species!