Ann: Sinclair Mine Complete, First Caesium Shipment Scheduled, page-114

Field trip time!

"The Tanco pegmatite is a subhorizontal, essentially undeformed, bilobate, saddle-shaped body. The pegmatite is about 1520 m long, 1060 m wide, and up to ~100 m thick, thinning toward the edges. The Tanco pegmatite is situated about 60 m below Bernic Lake, and is accessible from surface either via a shaft or via a 400 m ramp with 20% decline."

https://www.manitoba.ca/iem/info/libmin/gacmac/OF2013-8_FT-C1.pdf

Given the ramp length and angle I would say it starts at about 136m deep from ground level. Luckily, PIO's pegmatite isn't located under a lake

Note: Pollucite is also found in the Upper intermediate zone.