The diopside-allanite veins and apatite breccia veins of the Hoidas Lake light rare earth element (LREE) deposit in northern Saskatchewan, Canada, represent a complex magmatic-hydrothermal system unrelated to its strongly deformed Archean and Paleoproterozoic host rocks in the Rae subprovince. The veins were emplaced along the Hoidas-Nisikkatch fault, a subsidiary of the deeply rooted Black Bay fault system, which likely provided pathways for the melts and fluids. Allanite in the diopside-allanite veins is concentrically zoned, reflecting rare earth element (REE) depletion, followed by REE enrichment. Later apatite breccia veins comprise multiple apatite generations. Graphic-textured inclusions in the earliest red apatite indicate the presence of a melt during apatite growth. The average total rare earth oxide (TREO) content increases from 1.5 wt % in the red apatite to 5.5 wt % in the green apatite, both dominated by Ce. In contrast, the latest generation, coarse red apatite, contains about 1 wt % TREO and is Nd dominant. This shift from Ce- to Nd-dominant apatite might reflect a transition from magmatic to hydrothermal growth. Interaction with hydrothermal fluids resulted in chlorite-hematite alteration and REE redistribution into secondary monazite, REE carbonates, REE-Sr carbonates, and allanite, the last occurring in late quartz-carbonate veins and containing up to 32.9 wt % TREO. The chemistry of the REE-bearing phases and their connection with hyalophane-bearing pegmatites and later lamprophyre dikes indicate a mantle-derived, most probably carbonatitic or syenitic source for the mineralizing melts and fluids.

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