Jenna Rees, BSc

J. Rees¹, D. Lentz¹, F. Yousefi¹

¹Department of Earth Sciences, University Of New Brunswick, Fredericton, New Brunswick, Canada

In southwestern New Brunswick, eight NYF-type granitic pegmatite dykes intrude the Neoproterozoic Perch Falls Granodiorite. The steeply dipping dykes are narrow (<1 m) exhibit a consistent NE to E strike and locally contain a central quartz core. They contain quartz and feldspar, as well as magmatic epidote, and in some instances, allanite. Gamma ray spectrometry analyses exhibit variable radioelement concentrations (eTh = 3.6–75 ppm; eU = 0–6.8 ppm). Lithogeochemistry shows silica rich compositions (SiO₂ = 71.1–77.6 wt.%), moderate Al₂O₃ (12.3–15.6 wt.%), and elevated alkali elements (Na₂O = 2.65–4.12 wt.%; K₂O = 3.01–5.62 wt.%), with notable LREE contents (Ce = 8.9–33.6 ppm; La = 5.1–33.9 ppm) and quite variable Zr (23–420 ppm). Portable X-ray Fluorescence spectrometry (pXRF) data confirmed these results, which resemble those of the nearby Prince of Wales Granite, the youngest Neoproterozoic magmatic event in the area. Together, pXRF, gamma-ray spectrometry (GRS), and whole-rock geochemistry provide a petrogenetic framework for interpreting granitic magmatism and late-stage fluid modification in the Perch Falls system, as demonstrated by igneous and hydrothermal assemblages.

Petrographic and geochemical evidence indicate two stages of hydrothermal overprinting, both with copper. An early Fe–S-rich magmatic–hydrothermal event produced both disseminated to vein-like, Fe-rich domains and chloritization of ferromagnesian minerals, accompanied by epidote–chlorite alteration. A later Ca-rich hydrothermal pulse then occurred, partially retrogressed chlorite to secondary epidote, and promoted the precipitation of titanite along fracture networks. Zircon grains hosted within feldspar-rich domains exhibit very small Th–U–Si–O-rich inclusions, <10 µm in size, only revealed by SEM-BSE imaging. However, typical zircons have visible oscillatory magmatic zoning. As such, the visible textural distribution of these inclusions indicates decoupling of Zr from U–Th, which occurs when radioactive elements are redistributed and reprecipitated during post-crystallization fluid–mineral interactions. These features indicate the localized concentration and remobilization of radioactive elements during late-stage magmatic-hydrothermal processes. The preferential occurrence of altered zircon within feldspar-rich domains further suggests that these zones served as permeable pathways for late-stage fluids, facilitating micro-scale mobility of radioactive elements during hydrothermal overprinting. Ongoing U–Pb zircon geochronology will further constrain the timing of pegmatite emplacement and subsequent fluid-mediated alteration. Zircon-scale SEM-BSE imaging coupled with SEM-EDS analyses reveal zircon textures characterized by U–Th-rich inclusions within weakly zoned grains, indicating that radioactive element enrichment in these pegmatites reflects a combination of primary magmatic fractionation processes and subsequent fluid-mediated modification.