Petrogenesis of Eagle Lake Granite and Its Associated Cu-Mo-Au Mineralization, Southwestern New Brunswick, Canada

The NE-trending multiphase Late Devonian Eagle Lake granite (ELG) in southwestern New Brunswick is mineralized, consisting of hypabyssal porphyritic stocks and dikes that intruded Silurian metabasic volcanic rocks; however, its various phases, ages, and associations with notable stockwork Cu-Mo-Au mineralization and alteration have yet to have been studied. The ELG suite is predominantly composed of phenocrysts and a microcrystalline groundmass of quartz, K-feldspar, and plagioclase, with minor biotite and accessory minerals. In situ LA ICP-MS U-Pb zircon dating of this pluton yielded 360 +/- 5 Ma (Late Devonian), so this pluton is considered part of the Late Devonian granitic series in southwestern New Brunswick. The isotopic analysis of two granitic samples yielded an initial Nd-143/Nd-144 of 0.512164 and 0.512184, initial Sr-87/Sr-86 of 0.70168 and 0. 70675, and initial Hf-176/Hf-177 of 0.282619 and 0.282631. The epsilon Nd (360 Ma) is 0.37 to +0.03, whereas the epsilon Hf (360 Ma) values are +2.1 and +2.5. Pb isotopic analysis yielded a Pb-206/Pb-204 of 18.49 and 18.72, Pb-207/Pb-204 of 15.62 and 15.63, and Pb-208/Pb-204 of 38.26 and 38.37, indicative of a relatively radiogenic source contaminating a primitive mantle melt. Potassic alteration and pyrite-quartz stockwork Cu-Mo-Au veining is evident in some parts of these porphyries. Petrographic and geochemical evidence indicates that this composite pluton is a low-T, I-type granite with zircon saturation temperatures between 720 degrees and 825 degrees C, with emplacement depths of 10.3 to 4.4 km. ELG was emplaced along a major structural trend manifested by contemporaneous faults and shear zones, i.e., the Belleisle Fault Zone in southern New Brunswick.