The geochemical, geochronologic, and isotopic constraints on U-ThREE pegmatites and related mineralization of south-central part of the the Wollaston Domain, Northern Saskatchewan, Canada
University of New Brunswick
In northern Saskatchewan, granitic pegmatites intrude Early Paleoproterozoic Wollaston Group metasedimentary rocks and interfolded granitoids that unconformably overlie Late Archean gneisses, all of which have been subjected to deformation during the protracted 1.86 to 1.77 Ga Trans-Hudson Orogeny. U-Th±REE-Y-Nb-pegmatite intrusions and fracture-controlled uranium mineralization characterizes the U-Th±REEY- Nb occurrences at Kulyk Lake, Eagle Lake, and Karin Lake mineral exploration properties in the south-central Wollaston Domain. The pegmatites are moderately to highly evolved, ranging from mineralogically simple- to complex-type pegmatites. The complex-type pegmatites are hybridized, due to bimetasomatic interaction with the host rocks. Saturation of U-Th±REE-Y-Nb occurs at the margins (predominantly border and wall zones) of the hybridized pegmatites, linking them genetically. These are rare-earth element class, NYF pegmatites (Nb-Y-F type), and are interpreted to have formed in a late syn- to post-collisional tectonic setting. The age constraints, textures and relatively high-T partial melting conditions (-750 to 800°C) confine the pegmatite's melt-forming conditions to an early deformational event dated between 1835 and 1805 Ma, followed by post-peak high-temperature retrograde metamorphism ca. 1770 Ma. Field relationships, textures, and geochemical variations between pegmatite groups provide strong evidence that U, Th, REE±Y- Nb phases in pegmatites from the Kulyk, Eagle, and Karin lakes continued to evolve in an open- to closed-system environment. Assimilation and/or fractional crystallization processes progressed during melt ascent, aiding in the exchange of U, Th, and REE accessory phases between continuous multiple pegmatite injections through various conduits during the Trans-Hudson Orogeny near peak metamorphic conditions. The mineralizing phases are progressively enhanced into the last stage of pegmatite evolution, when fluid fractionation would occur via infiltration of a volatile phase through partially crystallized portions of the pegmatite ( and eventually the host rocks) that will enhance metasomatic transfer, i.e., assist in the diffusion and advection of U, Th, and HFSEs (REEs and Zr) from the melt. Furthermore, mineralogical, textural, geochemical, geochronological, and isotopic data was used to expand on the fractionation model for these particular ascending NYF -type enriched melts, providing evidence for an immiscible phosphatic liquid during the evolution of the monazite showing at Kulyk Lake. These particular residual melts evolved via protracted fractionation that at a critical point will evolve into an immiscible phosphatic melt enriched in Fe, Ti, Th, Y and REE.