The Nature and Origin of the REE Mineralization in the Wicheeda Carbonatite, British Columbia, Canada

The Wicheeda carbonatite is a deformed plug or sill that hosts relatively high grade light rare earth elements (LREE) mineralization in the British Columbia alkaline province. It was emplaced within metasedimentary rocks belonging to the Kechika Group, which have been altered to potassic fenite near the intrusion and sodic fenite at greater distances from it. The intrusion comprises a ferroan dolomite carbonatite core, which passes gradationally outward into calcite carbonatite. The potentially economic REE mineralization is hosted by the dolomite carbonatite. Three types of dolomite have been recognized. Dolomite 1 constitutes the bulk of the dolomite carbonatite, dolomite 2 replaced dolomite 1 near veins and vugs, and dolomite 3 occurs in veins and vugs together with the REE mineralization. Carbon and oxygen isotope ratios indicate that the calcite carbonatite crystallized from a magma of mantle origin, that dolomite 1 is of primary igneous origin, that dolomite 2 has a largely igneous signature with a small hydrothermal component, and that dolomite 3 is of hydrothermal origin. The REE minerals comprise REE fluorocarbonates, ancylite-(Ce), and monazite-(Ce). In addition to dolomite 3, they occur with barite, molybdenite, pyrite, and thorite. Minor concentrations of niobium are present as magmatic pyrochlore in the calcite carbonatite. A model is proposed in which crystallization of calcite carbonatite preceded that of dolomite carbonatite. During crystallization of the latter, an aqueous-carbonic fluid was exsolved, which mobilized the REE as chloride complexes into vugs and fractures in the dolomite carbonatite, where they precipitated mainly in response to the increase in pH that accompanied fluid-rock interaction and, in the case of the REE fluorocarbonates, decreasing temperature. These fluids altered the host metasedimentary rock to potassic fenite adjacent to the carbonatite and, distal to it, they mixed with formational waters to produce sodic fenite.