INTERPRETING METASOMATISM AND REE ORE FORMING PROCESSES IN PERALKALINE GRANITIC SYSTEMS (Invited Presentation)

Extreme enrichment and hydrothermal mobilization of the rare earth elements (REE), Zr, and Nb have been reported for a number of peralkaline intrusions, including the world-class REE-Zr-Nb deposit at Strange Lake, Quebec, Canada [1]. These mineral deposits are commonly characterized by a complex interplay of igneous and hydrothermal processes, which need to be distinguished from the thin section to the ore deposit scale for their successful interpretation [1,2]. Using microtextures, element distribution maps and mineral chemistry, I will delineate snapshots of metasomatic processes associated to different evolution stages of the Strange Lake pluton. To put these observations in a broader perspective, I will further explore on how to combine microanalytical techniques with thermodynamic simulations of fluid-rock equilibria using GEM-Selektor to assess the nature of these metasomatic fluids.

The B-zone is a potential ore zone located in the northwest of Strange Lake, and contains numerous mineralized pegmatites hosted in peralkaline granite. This zone is characterized by an acidic alteration stage, which was responsible for a significant hydrothermal mobilization of the REE and Zr [1,2]. Microtextures reveal two distinct alteration substages: i) a high T alteration (Ca-metasomatism) characterized by pseudomorphic mineral replacement reactions (i.e., aegirine after arfvedsonite, and quartz +Ca-zirconosilicates after Na-zirconosilicates), and ii) a low T alteration (Ca-F-metasomatism) characterized by the formation of quartz/fluorite veins and dissolution/precipitation textures with the mineralization of hydrothermal zircon, gadolinite-group minerals and ferriallanite-(Ce) ±quartz/fluorite. Element distribution maps and backscattered photomicrographs show that mobilization of the LREE was associated to the high T acidic alteration, whereas mobilization of HREE and Zr was associated to the low T acidic alteration. These observations corroborate with thermodynamic predictions, which point to the importance of pH and temperature for the hydrothermal REE and Zr mobilization.

[1] Gysi, A.P., Williams-Jones, A.E., Collins, P., (2016) Econ. Geol., 111, 1241-1276. [2] Gysi A. P. and Williams-Jones A. E. (2013) Geochim. Cosmochim. Acta 122, 324-352.