Formation of rare earth deposits in carbonatites

Abstract

Carbonatites and carbonatite-related rocks are the premier source for light rare earth element (LREE) deposits. Here we outline a simplified ore formation model for LREE mineralised carbonatites, reconciling field and petrological observations with recent experimental and isotopic advances. REE can strongly partition to carbonatite melts, which are either directly mantle-derived or immiscible from silicate melts. As carbonatite melts evolve, alkalis and REE concentrate in the residual melt due to their incompatibility in early crystallising minerals. In most carbonatites, additional fractionation of calcite or ferroan dolomite leads to evolution into a mobile alkaline “brine-melt” from which primary alkali REE carbonates can form. These carbonates are rarely preserved owing to dissolution by later fluids and are replaced in-situ by monazite and alkali-free REE-(fluor)carbonates.