The genesis of the Ashram REE deposit, Quebec: Insights from bulk-rock geochemistry, apatite-monazite-bastnäsite replacement reactions and mineral chemistry

Abstract

Growing demand for the rare earth elements, particularly the heavy REE (HREE), has fuelled a boom in mineral exploration and scientific research on carbonatite-hosted deposits, especially those with unusual HREE enrichment. The Ashram REE deposit is such a HREE-enriched carbonatite-hosted REE deposit. Magmatic processes were important in the genesis of the deposit, but hydrothermal mobilization was the main process responsible for the concentration of the REE to potentially economic levels and the fractionation of the HREE.The REE minerals in the Ashram deposit were precipitated from hydrothermal fluids. They comprise monazite-(Ce) and bastnäsite-(Ce), with lesser monazite-(Nd) and trace xenotime-(Y) and aeschynite-(Nd). This mineralization occurs as disseminations in breccia matrices, in veins, and as vug fillings. Monazite-(Ce) was the earliest mineral to form, followed by xenotime-(Y) and bastnäsite-(Ce). The composition of monazite-(Ce) varies with location in the deposit and is preferentially enriched in the light REE (LREE) with increasing distance from a large, irregular breccia body, underlying the zone of HREE enrichment. This body is interpreted to have been the main conduit for the mineralizing fluids which, on exiting, deposited monazite as a result of the cooling and pH buffering that accompanied their interaction with the adjacent carbonatites. Bastnäsite-(Ce) replaced monazite-(Ce) through ligand exchange (F−, CO32− for PO43−), preserving the original REE distribution. Interaction of a compositionally evolving fluid with host rocks of variable bulk composition and buffering capacity resulted in a deposit-scale fractionation and zonation of the REE.