Browsing by Author "Beal, Kristy-Lee"

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    Geochronologic, petrographic, geochemical, and isotopic constraints on the origin of the uraniferous Lac Turgeon Intrusive Complex, Quebec
    (University of New Brunswick, 2014) Beal, Kristy-Lee; Lentz, David; McFarlane, Chris R.M.
    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.
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    The Zealand Station beryl (aquamarine) deposit, West-Central NB: mineralogic, geochronologic, and petrogenetic constraints
    (University of New Brunswick, 2007) Beal, Kristy-Lee; Lentz, David
    The Zealand Station deposit is located 30 km northwest of Fredericton, NB, along the north eastern cusp of the Hawkshaw Granite, previously dated at 411 ± 1 Ma by U-Pb titanite. A late-stage, SE-trending, sparsely porphyritic aplite dyke with coarse-grained perthitic feldspar has locally abundant beryl (aquamarine) in the aplitic groundmass. A pegmatitic phase of the dyke crops out 50 m east of the map area; it predominately consists of quartz and K-feldspar, with granophyric texture, and minor wolframite. Beryl is also found near altered greisen pockets and along quartz-rich veins in the granitic host rock; the veins show two predominant orientations: 135°/90° and 010°/75°W. The aplitic dyke has been dated at 400.5 ± 1.2 Ma using U-Pb TIMS on magmatic zircon. This isconsistent with the 404 ± 8 Ma using the U-Th-Pb CHIME method on magmatic monazite using the EPMA technique: these ages link the aplitic dyke to the Allandale Granite that was previously dated at 402 ± 1 Ma by U-Pb (monazite); it is the youngest and most evolved phase of the Devonian Pokiok Batholith. In addition, two monazite analyses yielded an age of 510 ± 20 Ma using EPMA, indicating a supracrustal xenocrystic origin for some of the monazite. The veins and greisen pockets have up to 5.61 wt. % BeO, and up to 20 vol. % beryl in a part of the aplitic phase of the dyke. Beryl samples (n=15) were analysed by EPMA. The average H20 content of the Zealand Station beryl is 1.53 wt. % calculated by (Na20 + 1.4829)/1 .1771 and includes ferric iron (up to 1.44 wt. %), which is the dominant chromophore. The dyke samples have a granitic composition and are predominantly magnesian, slightly potassic, and calc-alkaline with strong peraluminousity (1 .23-4.76). The apliteII pegmatite dykes are highly fractionated, based on their characteristic high incompatible elemental abundances, such as Th (1 l-33ppm), Rb (218-327 ppm), Cs (6-22 ppm), and Ta (3-14 ppm) and elevated HREE (Y, Yb, and Lu); they also have very low Nb/Ta (2.6-7.1) and very high Rb/Sr (4.1-15.8) ratios. The dykes were derived from a probably igneous in origin and had characteristics typical of an A-type source, the age and geological setting of this granite indicate that it is syn- to post- collisional with the Acadian Orogen. As this magma rises there seems to have been increased contamination by metasediments of the recently thickened crust, possibly explaining some of the S-type petrogeochemical characteristics. The dykes are related to the Allandale Granite phase of the Pokiok Batholith by their age, petrology, geochemistry and Nd/Sm isotope ratios (ε-Nd of -2.15 for the dykes whereas the Allandale has a value of -1.6). The pegmatite and aplite are a high-level rare-earth element pegmatite with both NYF and LCT-type characteristics. The beryl geothermobarometer indicated that the dyke crystallized between 600° to 660°C and atapproximately 2.5 kbar.