Magmatic-hydrothermal evolution of the Hoidas Lake REE deposit, northern Saskatchewan, Canada
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The Hoidas Lake rare earth element (REE) deposit in northern Saskatchewan, Canada, was investigated through field observations, detailed petrographic studies, scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM-EDS) and electron microprobe analyses (EMPA), fluid inclusion microthermometry and in situ U-Pb, Lu-Hf and Sm-Nd isotopic studies, in order to unravel the magmatic-hydrothermal evolution of the deposit. The main aims were to evaluate the source and chemical variations of the fluids and melts that formed the mineralization and to provide constraints on the age of the deposit. The structurally controlled mineralization consists of diopside-allanite veins and apatite breccia veins emplaced along the Hoidas-Nisikkatch Fault. In the diopside-allanite veins the allanites, commonly intergrown with hyalophane, titanite and diopside, show chemical variations that reflect relative REE-depletion in the melt during allanite crystallization, and subsequent REE-enrichment, possibly due to open system behavior and a new influx of melt/fluid into the vein system. The later apatite breccia veins show multiple phases of crystallization and a shift from Ce-dominance in the earlier red and green apatite phases to Nd-dominance in the latest coarse red apatite phase, which reflects a transition from magmatic to hydrothermal growth. Interaction with hydrothermal fluids resulted in chlorite-hematite alteration, irregular REE zonation in allanite and apatite, and local redistribution of the REEs into secondary monazite, REE-carbonates and REE-Sr-carbonates. Late quartz-carbonate veins that represent this hydrothermal overprint occasionally contain allanite, interpreted to have formed through hydrothermal remobilization of the REEs. The paragenetic relationships of the REE veins with hyalophane-bearing pegmatite dikes and late lamprophyre dikes and the mineral chemistry of the REE-bearing phases indicate a mantle-derived, most probably carbonatitic source for the melts and fluids responsible for the mineralization. The various vein generations formed due to repeated influxes of the mineralizing melts and fluids into the vein system, and caused limited Ba-metasomatism and albitization in the wall rocks. Unusual LREE-rich primary graphic-textured inclusions in the apatite of the Hoidas Lake deposit were studied through integrated EMPA and SEM-EDS imaging, and show variable compositions between Ce2O3+SiO2(+ThO2)-dominant and La2O3+Nd2O3(+F)-dominant end members. These inclusions indicate rapid apatite growth and contemporaneous crystallization of REE-enriched phases from the boundary-layer melt phase at the apatite-melt interface or alternatively, trapping of a melt phase during apatite growth due to melt-melt immiscibility. The fluid inclusion microthermometric data and evaporate mound analysis of the apatite breccia veins and related hyalophane-bearing pegmatites and quartz-carbonate veins suggest that entrapment of the hydrothermal fluids at Hoidas Lake occurred below 310°C, and pressure was transient between 0.5 and 2 kbars. Evolution of the Hoidas Lake mineralization involved early entrapment of a carbonic fluid followed by introduction of mixed Na-Ca-K-(Ba-Mn-Mg-Fe-Sr) aqueous fluids that were responsible for the late alteration of the mineralized veins and local redistribution of the REEs into secondary phases. Combination of the aforementioned studies indicates that the Hoidas Lake REE mineralization is a distal magmatic-hydrothermal counterpart of the hidden carbonatitic or alkaline igneous source. Geological relationships and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb geochronology of various mineral phases indicate that the REE-bearing veins formed after peak metamorphism in the Hoidas Lake area, which occurred at ca. 1.9 Ga. However, zircon crystals with concordant U-Pb dates of ca. 2350 Ma are interpreted to be inherited from granitoids that formed during the Arrowsmith Orogeny, also reported from other parts of the southern Rae Subprovince. Zircon rims that show concordant U-Pb ages around 1905 Ma represent new zircon growth during the emplacement of the REE mineralization and have considerably different Hf isotopic compositions, compared to the inherited zircon cores. Concordia ages of titanite from two distinct samples are ca. 1900 Ma and 1830 Ma and two monazite U-Pb date groups were observed at ca. 1910 Ma and 1845 Ma. The U-Pb dates correspond to the estimated period of tectonic activity of the Black Bay Fault System. The Sm-Nd isotopic systematics of titanite, green apatite and monazite are comparable to those previously reported for the Martin Group alkali basalts in the Beaverlodge Domain and the ultrapotassic rocks of the Christopher Island Formation in the Baker Lake Basin, both of which also yielded similar U-Pb ages to those of the Hoidas Lake veins. These regionally occurring alkali units likely originated from a similar source, most probably an ancient enriched lithospheric mantle reservoir.
DegreeDoctor of Philosophy (Ph.D.)
CommitteeAnsdell, Kevin; Merriam, James; Pan, Yuanming; Holmden, Chris; Peak, Derek
Copyright DateApril 2015
rare earth elements, REE mineralization, magmatic-hydrothermal evolution, hydrothermal REE-mobilization, fluid inclusion microthermometry, radiogenic isotope geochronology