Mineralization of rare-earth elements (REEs) has been discovered in the Karrat Isfjord region of West Greenland (~72°N) at Niaqornakavsak on the island of Qeqertarssuaq along the southwestern margin of the Kangigdleq-Íngia Fjord gneiss dome complex. Mineralization occurs within a distinct ~30-m-thick horizon in an amphibolite host rock, which defines the upper boundary of the Paleoproterozoic Qeqertarssuaq Formation. REE mineralization extends across the entire Niaqornakavsak Peninsula (1.5 km) with a continuation of the unit 7 km east at Umiamako Nuna. Concentrations of whole-rock Y + REE oxide across 1-m sections in the mineralized horizon reach a maximum of 2.6 wt %, with an average of 1.0 wt %. The REE-enriched horizon is divided into two major units: (1) the upper unit (Calcium Carbonate Amphibolite) is characterized by abundant calcite, ankerite, and fluorite (>50% modal), with lesser amounts of grunerite, cummingtonite, magnetite, biotite, apatite, sphalerite, thorium silicate, fergusonite, bastnasite, allanite, and monazite; and (2) the lower unit (Biotite Layered Carbonate) contains modal biotite (>50%), with lesser amounts of calcite, ilmenite, magnetite, allanite, fergusonite, and monazite. Monazite formed within the mineralized horizon; the Calcium Carbonate Amphibolite unit yields a 207Pb/206Pb model age of 1859 ± 15 Ma. The combined attributes of the deposit (REE patterns, whole-rock chemistry, and mineralogy) suggest that the REE mineralization was produced by metasomatic alteration of the host amphibolites by a ferrocarbonatite-derived fluid. Monazite 207Pb/206Pb age results indicate that mineralization occurred during Paleoproterozoic continental convergence (1.95–1.80 Ma) between the North Atlantic and Rae cratons and emplacement of the Prøven Igneous Complex and potentially related carbonatite intrusions.