THE ST. LAWRENCE GRANITE AND ASSOCIATED FLUORITE MINERALIZATION

The St. Lawrence Granite (SLG) is best known for its association with economically significant fluorite deposits. It is located in the Avalon Zone of the Appalachian Orogeny and represents one of the highly evolved Late Devonian granites emplaced during the late stages of the Acadian Orogeny. Detailed sampling, petrography, geochemistry, geochronology, electron microprobe and SEM analyses of the granite and the fluorite veins revealed new details on the late- and post-magmatic processes that resulted in the fluorite mineralization.

The SLG is a highly evolved, A-type, peralkaline granite with an average fluorine content of 993 ppm, ranging up to 8760 ppm. The main primary minerals include quartz, K-feldspar, and plagioclase, with mafic minerals consisting of Na-rich amphibole and pyroxene, or biotite. Alteration ranges from weak to strong and includes albitization of feldspars, biotite altered to chlorite, arfvedsonite altered to quartz + hematite ± chlorite, magnetite altered to hematite, and quartz replaced by calcite.

The SLG is composed of several phases with varying degrees of fractionation and alteration. Economic quantities of fluorite are only associated with the S lobe, which is the most fractionated and the most strongly altered part of the SLG. The S lobe represents the upper part of the SLG, and the alteration and fluorite formation resulted from devolatilization of the granite. The volatiles included F, Cl, CO₂ and, in the later stages, H₂O. There is evidence for at least two different fluids in the SLG. One of them was initially a vapour phase rich in F, and played a significant role in the formation of the fluorite veins. Previous research indicates mixing of this magmatic fluid with external fluids to form the fluorite veins.

The other fluid was a denser liquid rich in C and Cl, but relatively poor in F. This liquid did not leave the granite due to its higher density, but migrated along it’s the upper contact towards the highest points, altering it and leading to extensive autometasomatism in the cupolas, where the rocks are composed of mainly albite and calcite. The origin and evolution of the two fluids are uncertain, but albitization and the relative lack of F in the denser brine suggest early volatile loss from the system through the F-rich fluid.