In situ U-Pb geochronology, Lu-Hf and Sm-Nd isotope systematics of the Hoidas Lake REE deposit, northern Saskatchewan, Canada

摘要

The diopside-allanite veins and apatite breccia veins of the Hoidas Lake LREE deposit cross-cut the deformed Archaean and Paleoproterozoic granitoid gneisses of the Rae Subprovince, in northern Saskatchewan, Canada. The timing and source of this unusual and complex REE deposit are not well-constrained. Here we report on the results of LA-ICP-MS U-Pb geochronology of zircon and titanite from diopside-allanite veins and green apatite and monazite from apatite breccia veins, Hf isotopic data for zircon, and Sm-Nd isotopic data for titanite, apatite and monazite, in an attempt to constrain the age and duration of crystallization of REE-bearing phases, and provide implications on the source of the mineralization. Geological relationships and in situ U-Pb data indicate that the REE veins formed after peak metamorphism at ca. 1.9 Ga. However, there are zircon crystals in the diopside-allanite veins with concordant U-Pb dates as old as similar to 2350 Ma, the oldest of which may have been inherited from Arrowsmith Orogen granitoids. Zircon rims show a concordant cluster of U-Pb dates around 1911 Ma, which are interpreted to represent new zircon growth, based on textural features and their distinct initial epsilon Hf values (26.9 and 34.3), compared to the inherited zircon cores, which range from -9.5 to -2.0. Younger U-Pb Concordia ages of titanite (ca. 1860 Ma and 1830 Ma) and monazite (ca. 1843 Ma), and discordant apatite Pb-207/Pb-206 dates (ca. 1840 Ma), are considered to be the best constraints on the initiation of REE mineralization. The initial epsilon Nd values of titanite, green apatite and monazite (-15.2 to -10.7) are comparable to, but lower than the epsilon Nd values for the Martin Group alkali basalts (Beaverlodge Domain) and the ultrapotassic rocks of the Christopher Island Formation (Baker Lake Basin), both of which also yielded similar U-Pb dates to those of the youngest Hoidas Lake REE-bearing minerals. These regionally occurring alkali units originated from a similar source, most probably an ancient enriched lithospheric mantle reservoir. The melts and fluids, from which the Hoidas Lake deposit formed, then interacted with Rae craton crust as they utilized structures while they were transported from the source region to the depositional site.