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

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 bastna center dot site-(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 bastna center dot site-(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. Bastna center dot site-(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.

Automated summary

The growing demand for rare earth elements, particularly heavy REEs, has led to increased exploration and research on carbonatite-hosted deposits. The Ashram REE deposit is rich in heavy REEs and was formed through a combination of magmatic processes and hydrothermal mobilization. The REE minerals in the deposit were precipitated from hydrothermal fluids, with monazite(Ce) and bastna center dot site-(Ce) being the main minerals present.