Coupled U-Pb, Lu-Hf and trace element geochemistry of apatite reveals complex magmatic-hydrothermal Nb-REE remobilization of the Neoproterozoic Saint-Honoré carbonatite deposit (Quebec, Canada)

Abstract

Niobium and REE are critical resources that are predominantly associated with carbonatite and alkaline complexes. The Saint-Honoré ore deposit, located in Québec, is a major producer of Nb (7 % of world production) with reserves of 630 thousand tons Nb and major REE resources (1058 thousand tons inferred). This deposit is composed of an outer syenite, inner syenite enclaves, multiple Nb-rich carbonatite intrusions and, a central REE-rich carbonatite. The carbonatites exhibit several generations of apatite (Ca₅(PO₄)₃(F,OH,Cl)) including a magmatic ‘white’ apatite associated with pyrochlore ((Na,Ca)₂Nb₂O₆(OH,F)) and a late hydrothermal ‘red’ apatite. Trace element geochemistry of apatite reveals a very distinct chemistry for both apatite types. The magmatic carbonatite stage is associated with the main Nb mineralization and is dated at 577 ± 14 Ma (U-Pb) and 580 ± 13 Ma (Lu-Hf), with a typical magmatic carbonatite signature whereas the hydrothermal stage has a mixed signature between carbonatite and external fluids which remobilized Nb-(REE) mineralization at 564 ± 16 Ma (Lu-Hf). While the magmatic and hydrothermal stage ages overlap and are also coeval to our lamprophyre apatite age of 569 ± 29 Ma (U-Pb), the late hydrothermal stage is distinctively marked by higher U, Cl, Ba, Fe, Nb and K concentrations in apatite. The late hydrothermal event is linked to a carbonatite fluid mixed with crustal fluids which may have contributed to the emplacement of the REE mineralization in the late stage of the carbonatite complex. This study also highlights the first detailed data on apatite geochemistry from lamprophyre.