The mineralogy, geochemistry and geochronology of the Wicheeda Carbonatite Complex, British Columbia

Abstract

Carbonatites are rare magmatic rocks composed of greater than 50% carbonate minerals. They are generally associated with continental rift-related tectonic settings and are commonly enriched in rare earth elements (REE), Nb, and P. The Wicheeda carbonatite complex, located 80 km northeast of Prince George in British Columbia, Canada, has been historically explored for its REE potential, but until recently there has been very little scientific study. The purpose of this study was to explore the geology, mineralogy, geochemistry, and geochronology of the Wicheeda Carbonatite Complex. The complex consists of a carbonatite plug with a number of carbonatite and potassic-syenite dykes and sills emplaced into the sedimentary rocks of the upper Cambrian and lower Ordovician Kechika Group. Sodic-fenitization is common around the carbonatites and the degree of alteration and abundance of syenite outcrops increases away from the carbonatite plug. The complex was mapped over an area of 1.45 km². The REE mineralogy of the Wicheeda carbonatite was defined through optical petrography (86 thin sections), scanning electron microscopy, electron probe microanalysis (371 points on 14 mineral species), and single-crystal X-ray diffraction (7 samples). The REE mineralogy is complex, with multiple stages of primary, late-stage, rapidly cooling crystallization. It consists of Ca-REE-fluorocarbonates, Ba-REE-fluorocarbonates, ancylite-(Ce), monazite-(Ce), euxenite-(Y), and allanite-(Ce); the majority of these minerals are LREE rich. Whole rock isotopic analysis was completed for the Rb-Sr and Sm-Nd systems. An isochron age of 316 ± 36 Ma was determined using the Sm-Nd system, giving values for eNdT and ⁸⁷Sr/⁸⁶SrT that range from -0.5 to 0.5 and 0.70526 to 0.70659, respectively. Evidence from the Wicheeda Carbonatite Complex along with comparisons with other worldwide carbonatites, suggests that the complex formed from a dominantly silicate, parental, mantle melt emplaced into the continental lithosphere. The lithosphere underwent metasomatism and, potentially, low degrees of partial melting and/or the incorporation of previously subducted sediments for carbonatite generation.