Late-magmatic processes in the St. Lawrence Granite: Implications for fluorite mineralization

Abstract

The St. Lawrence Granite (SLG), located in the Avalon Zone of the Appalachian Orogeny, is a peralkaline to metaluminous granite, best known for its association with extensive, and economically significant, fluorite deposits. Genetic models for the fluorite mineralization have been proposed, but major gaps and misconceptions still remain, such as the lack of fluorite in the country rocks, the source of Ca to form fluorite and the locally abundant calcite in the fluorite veins and the granite. Petrography, geochemistry and electron microprobe analysis revealed new details on the late-magmatic processes in the granite that played a significant role in fluorite mineralization.The SLG intruded at shallow levels and comprises several phases, each having different textures, mineralogy, and geochemistry. Most of the fluorite deposits are associated with the East lobe (E lobe), which is the only peralkaline and one of the most intensely altered phases of the granite. Alteration and fluorite mineralization are the results of devolatilization of the granite. The volatiles included F, Cl, CO2 and water. Early exsolution of a F-rich vapour, followed by a F-rich acidic fluid led to the fluorite mineralization. In the later stages, a F-poor and CO2-rich fluid was left and was unable to leave the granite, migrated along the upper contact of the granite and accumulated in the cupolas resulting in late carbonatization and extensive autometasomatism.Exsolution of a F-rich vapour, coupled with the shallow depth of intrusion of the SLG, allowed the F to leave the granite leading to fluorite mineralization in the country rocks, as demonstrated by the AGS fluorite deposit, which is partially hosted in sedimentary rocks overlying the granite; this encourages further exploration in the surrounding rocks. The source of the Ca is from the alteration of primary Ca-bearing feldspar and amphiboles in the granite, implying the lack of spatial correlation between fluorite mineralization and Ca-containing country rocks. This is significant for pursuing further exploration for fluorite in the St. Lawrence area, and elsewhere.