The Fraser Lakes Zone B is a U–Th–REE deposit hosted by granitic pegmatite and leucogranite located in the Wollaston Domain of northern Saskatchewan, Canada, in close proximity to the uranium-rich Athabasca Basin. Here, these magmatic rocks are hosted within Paleoproterozoic metasedimentary gneisses of the Wollaston Group and the underlying Archean orthogneisses. The intrusive bodies at Fraser Lakes Zone B are interpreted to have formed from crustal melts generated by upper-amphibolite- to granulite-facies metamorphism during the ca. 1.8 Ga Trans-Hudson orogeny. Three pelitic gneiss host-rock samples with the least petrographic evidence of later alteration and suitable assemblages for P–T–t constraints were analyzed with an electron microprobe. Mineral assemblages in the pelitic gneisses, combined with chemical zoning in garnet, garnet–biotite and Ti-in-biotite geothermometry, and GBPQ geobarometry, suggest a peak T of about 750 to 780°C and a P of about 6 to 8 kbar, followed by isothermal decompression to a pressure of about 3 kbar. The low-P (retrograde) part of the P–T path is partially constrained by the presence of spinel in some pelitic gneiss samples. These constraints on temperature and pressure are consistent with partial melting, which would have generated significant amounts of melt via biotite-dehydration reactions. Evidence for this is in the form of abundant leucosome in the pelitic gneisses; however, these are generally not connected to the mineralized pegmatites. Instead, melt generated from similar rocks at slightly deeper crustal levels is believed to have crystallized within a structural trap at Fraser Lakes Zone B to form the U–Th–REE-mineralized granitic pegmatites and leucogranites.