Geochronologic, petrographic, geochemical, and isotopic constraints on the origin of the uraniferous Lac Turgeon Intrusive Complex, Quebec
University of New Brunswick
The Lac Turgeon Intrusive Complex (LTIC), along the north shore of the St. Lawrence Seaway in the Grenville Province of Quebec, is host to pegmatite-related uranium mineralization. The main zone has an inferred resource of 81.5 million tons U₃O₈, averaging 0.013% U₃O₈; isolated occurrences, including the Grandroy Zone, have revealed 20 m of 0.174% U₃O₈ in channel sampling and up to 0.213 % U₃O₈ over 5.4 m and 0.089 % U₃O₈ over 10.5 m in drill holes completed in 2009. The intrusion’s main lithologies include granite, pink pegmatite, and white pegmatite that contain biotite, muscovite, zircon, ilmenite, hematite, and magnetite with minor apatite, uranothorite, uraninite, monazite, and xenotime as accessory minerals. Previous mineralogical studies have also noted the presence of pyrochlore, churchite (YPO₄*2H₂O), zoned samarskite, allanite, titanite, bastnaesite, and titanobetafite. The texturally diverse complex has irregular and sharp contacts, highly radioactive late-stage felsic intrusives, local magmatic layering, and breccias. Mineral geochemistry and textures provide evidence of crystallization depths > 4-5 km in a fluid-rich environment and mineral saturation temperatures (monazite, zircon, and apatite) average between 686-894°C. The samples at the LTIC came from the same source with the granite less fractionated than the pegmatite phases (white pegmatite commonly more fractionated than the pink pegmatite) based on the fractionation factors of the large ion lithophile and high field strength elements. The granite and white pegmatite phases towards the outer rim of the complex reveal ages (U-Pb Monazite) of 969 Ma (±7Ma and ±6 Ma, respectively); a Double S Zone granite phase towards the center of the intrusion revealed a concordant age of 941 ± 2.7 Ma (U-Pb Monazite, Xenotime) confirming there are several fractionated intrusive phases present at the LTIC. The dates indicate the LTIC is a post-orogenic complex that crystallized at the current level of emplacement towards the end of the Rigolet orogenic phase but outside the area of penetrative Rigolet metamorphism. Geochemical evidence identify that the LTIC is a peraluminous granite-pegmatite intrusion with a complex mixture of previously melted, lower crustal and more juvenile crustal sources giving a crustal A-type (higher Ga, Nb and lower Ba) to S-type (high Rb, Al) affinity. Similar to other uranium deposits including the Limousin pluton in the French Massif Central, the complex increases in peraluminosity with a slight decrease in fractionation indicative of an L-type granite. L-type granites are the result of low degrees of partial melting of a Proterozoic, metasediment source (high ¹⁸O signature) that contains elevated uranium content. Batch-type melting would have commenced by adiabatic melting during uplift related to the orogenic collapse and reworking of normal-sense shear zones following the completion of the Ottawan orogenic phase of the Grenville Orogen. The U and Th values are highly variable (<2-4485 ppm and 0.7-620 ppm, respectively) with U/Th ratios ranging between 0.28 and 25. The average U/Th for the LTIC is 2.3 (+4.0/-1.5; based on log transform data). In most samples, uraninite is accompanied by increased accessory mineral content explaining the close relationship of uranium with La, Ce, Zr, and Y. The main zones associated with highly uraniferous phases, including the Double S, MA, and MB-zones, have a close association with hybridization that would have occurred during magma ascent or in the magma reservoir prior to ascent, as measured by higher CaO + MgO + FeOt. Hybridization is believed to be the main control for concentrated uranium mineralization for other post-orogenic Grenvillian pegmatites. Fractional crystallization is still a factor related to mineralization elsewhere in the intrusive complex including the J- and Lac Turgeon zones. The Grandroy Zone reveals evidence that albitization or sodium metasomatism (higher Na₂O and lower SiO₂ and K₂O and textural evidence) could play a role in uranium mineralization at that location.