The Genesis of Pollucite: Insights from the Lilypad Lakes Rare Element Pegmatite Group, Ontario, Canada

Abstract

Abstract Pollucite is the sole ore mineral of cesium, a metal deemed critical because of supply chain vulnerability. This study investigates the origin and evolution of pollucite in the Lilypad Lakes pegmatite field (Ontario, Canada), one of the few pollucite localities in North America. Textural observations and electron probe microanalyses from the Pollucite dike and Rubellite dike reveal three distinct pollucite (Pol) types: type 1 (massive Pol82), type 2 (veinlet-hosted Pol88), and type 3 (rim Pol>92). Type 1 pollucite is interpreted to have crystallized during the transition from magmatic to hydrothermal conditions, with Cs supplied by the exsolved fluid and with the Si-Al framework supplied by the coexisting melt. Subsequent separation and circulation of the fluid led to the precipitation of type 2 pollucite in veinlets crosscutting type 1 pollucite. This was accompanied by replacement of type 1 pollucite by Cs-enriched pollucite adjacent to the veinlets and by near-end-member type 3 pollucite at crystal margins, where metasomatism was most intense. The alteration of type 1 pollucite was driven by the coupled substitution Si4+ + H2O ⇌ Al3+ + (Cs,Na)+, in which the Cs/Na ratio of the fluid controlled the degree of Cs enrichment, whereas the increase in Al is charge balanced by a corresponding increase in alkalis. In the Rubellite dike, late-stage, low-temperature, Na-rich fluids induced analcimization through Cs+ ⇌ Na+ exchange. These findings challenge models that attribute pollucite formation to purely magmatic crystallization and its internal heterogeneity to subsolidus exsolution. Instead, the Lilypad Lakes data support a magmatic-hydrothermal origin and a subsolidus, fluid-driven, compositional evolution.