Karrat Isfjord: A newly discovered Paleoproterozoic carbonatite-sourced REE deposit, central West Greenland

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.