The Hoidas Lake rare earth element (REE) deposit, located in northern Saskatchewan, Canada, is a structurally controlled vein-type LREE deposit with allanite-(Ce) and fluorapatite as the main REE carriers. The mineralized veins cut Archean and Paleoproterozoic orthogneisses and supracrustal rocks of the southern Rae Subprovince. The paragenesis includes hyalophane-bearing pegmatite dikes, REE-mineralized allanite-, diopside-, hornblende-, hyalophane-, and titanite-bearing veins, and later breccia veins that contain several stages of apatite. The mineralized veins record hydrothermal alteration with nucleation of monazite and REE-carbonate inclusions in apatite and allanite, respectively. Barren quartz-, carbonate-, and hematite-rich veins represent the final stage of hydrothermal activity. Samples from the hyalophane-bearing pegmatites, apatite breccia veins, and quartz-carbonate veins, used for fluid inclusion petrography and microthermometry, indicate four distinct fluid inclusion assemblages (FIAs): (1) carbonic inclusions showing LCO2 (liquid CO2) + VCO2 (CO2 vapor) phase composition with 20–40 vol.% VCO2 at 0 °C; (2) aqueous inclusions showing L (liquid H2O) + V (H2O vapor) phase composition with 90–100 vol.% V at 20 °C; (3) aqueous inclusions showing L + V + H (halite) phase composition with 15 vol.% V at 20 °C; and (4) aqueous L + V inclusions with 15–20 vol.% V at 20 °C. Type 1 inclusions homogenize to liquid CO2 with Th(CO2) from 3.3 to 30.5 °C. Type 2 V-rich inclusions have high salinities and contain salts other than NaCl. Type 3 L-V-H inclusions have different homogenization behavior in quartz of the hyalophane-bearing pegmatites (L+V+H → L+V → L) compared to quartz-carbonate veins (L+V+H → L+H → L), and Th (total homogenization) values range from 180 to 315 °C with salinities of 30–40 wt.% eq. NaCl. In type 4 inclusions Th ranges from 90 to 290 °C, but for specific samples and FIAs (fluid inclusion assemblages) there is a more limited spread (from 5 to 25 °C). Salinities range from 8 to 24 wt.% eq. NaCl, and the inclusions have variable Na/(Na + Ca). Evaporate mound analysis shows average normalized (to 100%) cation contents of 48% Na, 24% Ca, 6% K, 5% Ba, 4% Mn, 2% Fe, 2% Mg, and 9% Sr for quartz-hosted inclusions in quartz-carbonate veins, and 61% Na, 32% Ca, and 7% K for quartz-hosted inclusions in hyalophane-bearing pegmatite dikes. The thermometric and chemical data suggest that evolution of the Hoidas Lake mineralization involved two fluid types with early entrapment of a carbonic fluid followed by introduction of a mixed Na-Ca-K-(Ba-Mn-Mg-Fe-Sr) aqueous fluid with variable salinities that was responsible for the late alteration of the mineralized veins. Furthermore, the inclusion data provide constraints on entrapment temperature (<310 °C) and also indicate that the pressure was transient (0.5 to 2 kbars) based on the homogenization temperature data for the carbonic (type 1) and aqueous L-V-H (type 3) inclusions.

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