COMPLEX VEIN RELATIONSHIPS AND ORIGIN OF THE HOIDAS LAKE REE DEPOSIT, SASKATCHEWAN, CANADA

The Hoidas Lake REE deposit consists of numerous veins along the Hoidas-Nisikkatch fault and hosted predominantly by Paleoproterozoic (?) felsic orthogneisses of the Zemlak Domain (Rae Province, northwestern Saskatchewan, Canada). The most significant is the JAK Zone, which consists of NE-trending steeply E-dipping anatomosing veins, with a combined measured and indicated resource of 2.56 Mt grading 2.4% total rare earth oxide (TREO), with LREE (Nd, Ce, La) dominating. The REE are hosted by allanite(Ce)-fluoroapatite, with minor amounts of monazite, bastnaesite, and chevkinite-perrierite. The JAK Zone is undeformed, and sharply crosscuts fabrics associated with peak upper amphibolite regional metamorphism. Peralkaline pegmatites and lamprophyres, both of which are undeformed, are spatially associated with the deposit, although there are no cross-cutting relationships. U-Pb geochronology is underway on these rocks, and vein minerals, but a published monazite age suggests an age of 1.87 Ga for mineralization.

From oldest to youngest, the vein generations consist primarily of: 1) diopside- allanite-(Ce) +/-chevkinite-perrierite, titanite, and hyalophane; 2) red apatite breccias; 3) green apatite breccias; and 4) coarse red apatite. Most apatite and allanite crystals are strongly zoned, and the earliest veins show significant alteration, reflecting chemical changes in the mineralizing system. The earliest apatite generations contain monazite inclusions, whereas later apatites are inclusion-free. The apatites in vein 3) are the most REE-rich (5.5% TREO), whereas those with the least are from vein 4) (>1.5% TREO). The high Sr content of the apatites is most similar to apatites from carbonatites, although high Y is consistent with an alkali igneous rock. Allanites typically have REE-poor rims and are altered to bastnaesite.

There is no textural or mineralogical evidence for replacement of a primary igneous rock. The complex alteration (and brecciation) of older vein minerals suggests that the veins were deposited from an evolving (magmatic)-hydrothermal fluid focused along an active fault structure. The presence of lamprophyres in the region is indicative of metasomatized mantle, from which an appropriate magmatic source for F-Cl-REE-Ba-Sr-P-bearing fluids may have been derived.