Geochemistry and Geochronology of REE-Th-U PEGMATITES IN THE ALCES LAKE AREA, SASKATCHEWAN, CANADA, WITH A FOCUS ON MONAZITE AND ZIRCON

This paper presents an original petrochronological study investigating the genesis and time of emplacement of variable grade rare earth element mineralization of two highly mineralized zones (Wilson and Ivan) at the unique rare earth element Alces Lake deposit/prospect (Saskatchewan, Canada). The high-grade mineralized occurrences are hosted in monazites and are associated predominantly with felsic to mafic pegmatites, but also with biotite schists (glimmerites). Various analytical techniques have been utilized in order to characterize monazite and zircon grains, including optical microscopy study, X-ray fluorescence mapping of selected samples, scanning electron microscope, electron microprobe characterization, and laser ablation inductively coupled plasma-mass spectroscopy dating of monazite and zircon grains from two studied zones. The investigated monazite-bearing pegmatitic bodies display peraluminous signatures and are dominated by a quartz + K-feldspar + plagioclase + biotite (±muscovite) + rutile + monazite + zircon assemblage. The pegmatites are strongly enriched (well exceeding 10,000 times chondrites) in light rare earth elements and are depleted in heavy rare earth elements. The monazite forms homogenous to complexly zoned subhedral to fully rounded, corroded crystals variable in size and distribution and characterized by extremely high rare earth element and high Th and U contents. When present, the chemical zoning is characterized as (1) concentric (oscillatory growth zoning) and (2) patchy zoning. U-Pb dating of monazite grains (composite ages) at 1945.7 ± 13 Ma (Wilson Zone) and 1930.1 ± 12.7 Ma (Ivan Zone) and zircon (composite ages) at 1859.2 ± 13.9 Ma (Wilson Zone) and 1888.96 ± 5.8 Ma (Ivan Zone) imply that rare earth element mineralization at Alces Lake is related to high-grade thermotectonic events of the Taltson and Trans-Hudson orogenies, and possibly the Snowbird orogeny. Thus, the mineralized pegmatites at Alces Lake are interpreted to have formed by partial melting, melt (+xenocrystic and/or xenolithic restite) migration, and subsequent assimilation, magma mixing, and fractional crystallization during the thermotectonic events from 1946–1930 Ma to 1890–1860 Ma.