ZIRCON-RICH TA-NB-REE MINERALIZATION IN THE RADIOACTIVE MCKEEL LAKE PEGMATITE-APLITE SYSTEM, WELSFORD IGNEOUS COMPLEX, SOUTHWESTERN NEW BRUNSWICK

The Ta-Nb-REE-rich McKeel Lake syenitic pegmatite-aplite dyke swarm is hosted within the alkalic Silurian Welsford Igneous Complex (WIC, 422 +/- 1 Ma) near Browns Flats in southwestern New Brunswick. Discovered in 1979 during uranium exploration in the region, the radioactive McKeel Lake pegmatite-aplite system has been explored intermittently with recent drilling completed in the zone. It is made up of a steep easterly dipping series of aplite dykes ranging up to 40 cm wide with a trend of 340º. The dykes are closely spaced and interconnected by narrow branching dykelets that generally trend perpendicular to the overall strike direction. Previous studies of the coarse-grained syenitic WIC indicate that it has mantle A-type affinities and is characterized by the presence of Zr, Ta, Nb, and rare-earth-element (REE) containing minerals such zircon, allanite, pyrochlore, monazite, and uranothorite. XRF and INAA data indicate that the WIC syenites and later pegmatite-aplite dyke samples from surface (7 samples) and available drill core (2 holes; MKL-2002-01 (7 samples) and MKL-2002-02 (8 samples)) show elevated levels of Ta (74 to 220 ppm), Nb (750 to 3040 ppm), Zr (1450 to 36000 ppm), Y (540 to 3070 ppm), Th (170 to 870 ppm), U (60 to 250 ppm), Ce (430 to 1950 ppm) and Yb (110 to 290 ppm). Microprobe analysis of two samples identified Ta-Nb- and REE-rich phases: euxenite, fergusonite, aeschynite, (Y)-aeschynite, and a REE carbonate. From the analyses of the collected samples 2 separate aplite phases can be identified, an aplite that is non-mineralized which occurs at depth and a mineralized phase that occurs above the non-mineralized phase. The McKeel Lake pegmatite-aplite system has an age of 401 +/- 36 Ma, based on 14 zircon Th-U-Pb microprobe analyses, showing that the McKeel Lake system is cogenetic with the WIC. The McKeel Lake pegmatite-aplite likely represently a highly fractionated part of the WIC complex.